Work Study
Lecture 1
Definition:
Work study may be defined as the
analysis of a job for the purpose of finding the preferred method of doing it
and also determining the standard time to perform it by the preferred (or
given) method. Work study, therefore, comprises of two areas of study: method
study (motion study) and time study (work measurement).
Role of Work Study
in Improving Productivity
In order to
understand the role of work study, we need to understand the role of method
study and that of time study.
Method study (also
sometimes called Work Method Design) is mostly used to improve the method of
doing work. It is equally applicable to new jobs. When applied to existing jobs
and existing jobs, method study aims to find better methods of doing the jobs
that are economical and safe, require less human effort, and need shorter
make-ready / put-away time. The better method involves the optimum use of best
materials and appropriate manpower so that work is performed in well organized
manner leading to increased resource utilization, better quality and lower
costs.
It can therefore
be stated that through method study we have a systematic way of developing
human resource effectiveness, providing high machine and equipment utilization,
and making economical use of materials.
Time study, on the
other hand, provides the standard time, that is the time needed by worker to
complete a job by the standard method. Standard times for different jobs are
necessary for proper estimation of
- manpower, machinery and
equipment requirements
- daily, weekly or monthly
requirement of materials
- production cost per unit
as an input to better make or buy decision
- labor budgets
- worker's efficiency and
make incentive wage payments.
By the application
of method study and time study in any organization, we can thus achieve greater
output at less cost and of better quality, and hence achieve higher
productivity.
Work Study and
Ergonomics
The work study and
the ergonomics are the two areas of study having the same objective: design the
work system so that for the operator it is safe, and the work is less fatiguing
and less time taking.
Historical
Developments
The Work of Taylor
Frederick W.
Taylor is generally considered to be the founder of modern method and time
study, although time studies were conducted in Europe many years before Taylor
's time. In 1760, Jean Rodolphe Perronet, a French engineer, made extensive
time studies on the manufacture of No. 6 common pins.
Taylor began his
time study work in 1881 while associated with the Midvale Steel Company in
U.S.A.. He evolved a system based on the “task”, and proposed that the work of
each employee be planned out by the management in advance. Each job was to have
a standard time, deter mined by time studies made by experts. In the timing process,
Taylor advocated dividing the work into small divisions of effort known as
"ele ments." Experts were to time these individually and use their
collective values to determine the allowed time for the task.
Early
presentations of Taylor 's findings were received with little enthusiasm,
because many interpreted his findings to be somewhat new piece-rate system
rather than a technique for analyzing work and improving methods. Both
management and employees were skeptical of piece rates, because many standards
were earlier typically based on the supervisor's guess or even sometimes
inflated by bosses to protect the performance of their departments.
In June 1903, at
the American Society of Mechanical Engineers meeting, Taylor presented his
famous paper, "Shop Management," which included the elements of
scientific management: time study, standardization of all tools and tasks, use
of a planning department, use of slide rule and similar timesaving implements,
instruction cards for workers, bonuses for successful per formance,
differential rates, mnemonic systems for classifying products, routing systems,
and modern cost systems. Taylor 's techniques were well received by many
factory managers, and by 1917, of 113 plants that had installed
"scientific manage ment," 59 considered their installations
completely successful, 20 partly successful, and 34 failures.
In 1898, while at
the Bethlehem Steel Company , Taylor carried out the pig-iron
experiment that became the most celebrated demonstrations of his principles. He
established the correct method, along with financial incentives, and workers
carrying 92-pound pigs of iron up a ramp onto a freight car were able to
increase their productivity from an average of 12.5 tons per day to between 47
and 48 tons per day. This work was performed with an increase in the daily rate
of $1.15 to $1.85. Taylor claimed that workmen per formed at the higher rate
"without bringing on a strike among the men, without any quarrel with the
men and were happier and better contented."
Another of Taylor
's Bethlehem Steel studies that became famous was on shovel ing work. Workers
who shoveled at Bethlehem would use the same shovel for any job—lifting heavy
iron ore to lifting light rice coal. Taylor designed shovels to fit the
different loads: short- handled shovels for iron ore, long-handled scoops for
light rice coal, and showed their usefulness in improving productivity.
Not as well known
as his engineering contributions is the fact that in 1881, he was a U.S. tennis
doubles champion. Here he used an odd-looking racket he had designed with a
spoon curved handle.
The Work of
Gilbreths
Frank and Lilian
Gilbreth are considered as the founders of the modern motion study technique,
which may be defined as the study of the body motions used in performing an
oper ation, for the purpose of improving the operation by eliminating
unnecessary motions, simplifying necessary motions, and then establishing the
most favorable motion sequence for maximum efficiency. Frank Gilbreth
originally implemented ideas into the bricklayer's trade in which he was
employed. After introducing meth ods improvements through motion study,
including an adjustable scaffold that he had invented, as well as operator
training, he was able to increase the average num ber of bricks laid from 120
to 350 per worker per hour.
More than anyone
else, the Gilbreths were responsible for industry's recogni tion of the
importance of a detailed study of body motions to arrive at the best method of
performing an operation that would increase production, reduce operator
fatigue. They developed the technique of filming motions for study, known as
micromotion study.
The Gilbreths also
developed the cyclegraphic and chronocyclegraphic analysis techniques for
studying the motion paths made by an operator. The cycle- graphic method
involves fixing small electric light bulb to the finger or part of the body
being studied and then photographing the motion while the operator is
performing the operation. The resulting picture gives a permanent record of the
motion pattern employed and can be analyzed for possible improvement. The
chrono- cyclegraph is similar to the cyclegraph, but its electric circuit is
interrupted regularly, causing the light to flash. Instead of showing solid
lines of the motion patterns, the resulting photograph shows short dashes of
light spaced in proportion to the speed of the body motion being photographed.
Consequently, with the chronocyclegraph it is possible to determine direction
and compute velocity, acceleration, and deceleration, in addition to study of
body motions.
The Work of Others
Carl G. Barth, an
associate of Frederick W. Taylor, developed a production slide rule for
estimating the most efficient combinations of speeds and feeds for cutting
metals of various hardnesses, considering the depth of cut, size of tool, and
life of the tool. He is also known for his work on estimation of allowances by
establishing the number of foot-pounds of work a worker could do in a day. He
developed a relationship in which a certain push or pull on a worker's arms was
equated with the amount or weight that worker could handle for a certain
percentage of the day.
Harrington Emerson
applied scientific methods to work on the Santa Fe Railroad and wrote a book, Twelve
Principles of Efficiency, in which he made an attempt to lay down
procedures for efficient operation. He reorganized the company, integrated its
shop procedures, installed standard costs and a bonus plan, and introduced
Hollerith tabulating machines for the accounting work. This effort resulted in
annual saving of $ 1.5 million and recognition of his approach, called efficiency
engineering .
In 1917, Henry
Laurence Gantt developed simple graph that would present performance while
visually showing projected schedules. This production control tool was adopted
by the shipbuilding industry during World War I. For the first time, this tool
demonstrated the possibility of comparing actual performance against the
original plan, and to adjust daily schedules in accordance with capacity, back
log, and customer requirements. Gantt is also known for his wage payment system
that rewarded workers for above-standard performance, eliminated any penalty
for failure, and offered the boss a bonus for every worker who per formed
above .standard. Gantt advocated human relations and promoted scientific
management in the back drop of an inhuman "speedup" of labor.
Motion and time
study received added stimulus during World War II when Franklin D. Roosevelt,
through the U.S. Department of Labor, attempted to establish standards for
increasing production. The stated policy advocated greater pay for greater
output but without an increase in unit labor costs, incentive schemes to be
collectively bargained between labor and management, and the use of time study
for setting production standards.
Lecture 2
Method Study
Method study, aims
to achieve the better method of doing work, and for this reason method study is
sometimes called Work Method Design.
Definition: Method study can be defined as the procedure for systematic recording,
analysis and critical examination of existing or proposed method of doing work
for the purpose of development and application of easier and more effective
method.
Method Study
Procedure
The following
general steps describe the procedure for making a method study.
- Select the job – on which
method study is to be applied.
- Obtain information and
record.
- Examine the information
critically.
- Develop the most
practical, economical and effective method by considering real limitations
of the situation.
- Install the new method as
standard practice.
- Maintain the standard
practice by regular follow up.
Let us consider
these steps in some detail.
Selection of Job
for Method Study
Practically, any
activity or a job is a potential project for improvement but as the work study
engineer is to sell his ideas and maintain his existence in the organisation,
he should always attempt to select those jobs for improvement which are
unpopular among employees or are considered “dirty” by them.
By improving such
jobs, he would earn goodwill from the employees as well as the management, and
can expect their full cooperation for other studies in the future.
Considerations may
be given to the following factors while selecting a job for method study
• Economic
Factors
• Technical
Factors
• Human
Factors
Economic Factors:
If the economic
importance of a job is small, it is not wise to start or continue a long study.
Priorities should be given to those types of job which offer greater potential
for cost reduction. Such jobs are easily identifiable, as they have
• High
labour content, i.e. they consume more time
• excessive
machine or man idleness
• higher
frequency of occurrence, i.e. they have large demand
•
bottlenecks in production line
• higher
proportion of accidents
• movement
of material or men over long distance
• high scrap
and reprocessing costs
• high
payment of overtime bills.
Technical
Factors: The method study engineer must have the necessary
technical knowledge about the job to be studied. Only surface knowledge about
the subject may not lead to the right solution to the real problem. To
illustrate, consider that a particular machine tool in proving bottleneck. The
output from this machine is not reaching the assembly line in the required
quantity. Through a preliminary study, it is found that it is running at lower
speed and feed than that recommended for the pair of work and tool material
used. Just increase in speed or feed may not be the solution of this problem.
It may be possible that the machine itself is not rigid enough to operate at
higher speeds or take a deeper cut. Just increase in speed may increase the
output but the quality of job may be seriously affected. Technical expertise in
machine tools and metal cutting process would be essential to solve problem of
this kind.
Human
Factors: Emotional reaction of the workers to the method
study and changes in method are important considerations. If the study of a
particular job is suspected to cause unrest or ill feeling, it should not be
undertaken, however useful it may be from the economic point of view. It is
always better to take up first those jobs which are considered ‘dirty', unsafe,
unpleasant, boring, or highly fatiguing, and improvements brought about as a
result of method study. This would possibly ensure cooperative from the workers
for the other jobs as well.
After it is
recognized that a problem exists, the first step is to properly formulate it.
From the general statements like “Costs are too high“, “Increase the
production”, “Reduce shop floor accidents”, it is necessary to determine just
what the real problem is. After it is ascertained that the problem merits
consideration, it is decided whether this is the proper time to solve it, and
how much time can be spent in solving it. The problem may then be defined
broadly giving minimum constraints at this stage, as it will permit the use of
imagination and creativity in finding a solution. It may sometimes be desirable
to divide the complete problem into a couple of small problems and solve them.
Lecture 3
Information
Collection and Recording
Information
Collection Techniques:
The
accuracy of data about the method study problem is important for the
development of improved method. The following techniques are used for the
collection of information / data about the task under consideration. These are
not exclusive of each other, and for any particular method study problem, some
or all the techniques may be employed.
•
Observation. It is a common technique used for collecting information about the
present method or the existing problem. The method study person visits the site
where the work is currently being done and observes various steps in the method
being followed. There are many instances where all the data needed is obtained
by only observing the work or work site.
•
Discussion. Discussion with those who do or who supervise the work can
frequently provide information not obtainable by observation. The discussion
technique is commonly used where irregular work is involved or where one is
trying to analyze past work in order to improve efficiency of work to be done
in future.
Even
where observation by itself may accomplish the data collection task, discussion
may be used for developing good human relations.
•
Records. Valuable information can be obtained from past records concerning
production, cost, time, inventory and sub-contracts. For certain type of
information concerning the past practice, sometimes this is the only way to
obtain authentic data.
•
Motion Pictures or video Films. Accurate and most detailed information can be
obtained by taking motion pictures or video film. Information obtained by this
procedure can easily be transmitted / forwarded to all levels in the
organization and if needed, can be used directly for training purposes. The
film can be used to focus attention at particular point or motion in an
operation. For obtaining information concerning those types of work that
involve large crew size, it is probably the only procedure.
Information
Recording Techniques:
There are
three main types of information recording techniques. These are
•
Process Charts
•
Diagrams
•
Templates
A Process Chart is a graphic means of
representing the activities that occur during a manufacturing or servicing job.
There are
several types of process charts. These can be divided into two groups.
(i) Those
which are used to record a process sequence (i.e. series of events in the order
in which they occur) but do not depict the events to time scale.
Charts
falling in this group are
•
Operation process chart
•
Flow process chart – (man / material / equipment type)
•
Operator chart (also called Two Handed Process Chart)
(ii)
Those which record events in the sequence in which they occur on a time scale
so that the interaction of related events can be more easily studied. Charts
falling in this group are
•
Multiple activity chart
•
Simo chart
Diagrams.
A diagram gives pictorial view of the layout of workplace or floor on which
locations of different equipment, machines, etc. are indicated. The movement of
subject (man or material) is then indicated on the diagram by a line or a
string. The diagrams are valuable in highlighting the movement so that analyst
can take steps to simplify or reduce it and thus effect saving in time or
reduction in collisions / accidents.
Templates
and 3-D models:
Two-dimensional
cut outs made from thin card sheet representing machinery, furniture, etc. can
be used for developing new layouts and methods. The templates may have pieces
of permanent magnet attached to them, so that when used on iron board; they
remain glued on the board whenever placed.
A scaled
3-D model of a working area helps easy understanding of lighting, ventilation,
maintenance and safety aspects that may be important in a method. Such models
are often of great value in demonstrating the advantages of the proposed
changes to all concerned. However, their use is limited because of higher cost
involved. Some computer softwares are available which help in constructing the
layout and possibility of visualizing the working of process in a systematic
way.
Before taking
up descriptions of these charts or diagrams, it is necessary to know the
various elements of work.
Elements
of Work:
There are
five basic elements of work: Operation, Inspection, Transportation, Delay, and
storage. Table gives the definitions and symbols by
which these elements are represented. Also given in the Table are examples of
each element.
Sometimes,
more than one element occur simultaneously. It is shown as combined element
with combined symbol. Examples are “Operation in combination will inspection”,
and “Inspection in combination with Transportation”.
Operation
Process Chart:
An operation
process chart provides the
chronological sequence of all operations and inspections that occur in a
manufacturing or business process. It also shows materials used and the time
taken by operator for different elements of work. Generally a process chart is
made for full assembly, that is, it shows all the operations and inspections
that occur from the arrival of raw material to the packaging of the finished
product.
Flow
Process Chart:
A flow
process chart is used for recording greater detail than is possible in an
operation process chart. It is made for each component of an assembly rather
than for the whole assembly.
A flow
process chart shows a complete process in terms of all the elements of work.
There are two main types of flow charts: product
or material type , and the operator
type . The product type
records the details of the events that occur to a product or material, while
the operator flow chart details how a person performs an operational sequence.
An
important and valuable feature of this chart is its recording of non-productive
hidden costs, such as delays, temporary storages, unnecessary inspections, and
unnecessary long distances traveled. When the time spent on these non
productive activities is highlighted, analyst can take steps to minimize it and
thus reduce costs.
It is
also called Left Hand – Right Hand chart and shows the activities of hands of
the operator while performing a task. It uses four elements of hand work:
Operation, Delay (Wait), Move and Hold. Its main advantage lies in highlighting
un-productive elements such as unnecessary delay and hold so that analyst can
take measures to eliminate or shorten them.
Multiple
Activity Chart:
Worker-Machine
process chart and gang
process chart fall in the category of multiple activity charts. A
worker-machine chart is used for recording and analyzing the working
relationship between operator and machine on which he works. It is drawn to
time scale. Analysis of the chart can help in better utilization of both worker
and machine time. The possibility of one worker attending more than one machine
is also sought from the use of this chart.
A gang
process chart is similar to worker-machine chart, and is used when several
workers operate one machine. The chart helps in exploring the possibility of
reducing both the operator time and idle machine time.
Simo
Chart:
A Simo
chart is another Left-Hand
Right-Hand chart with the difference that it is drawn to time scale and in
terms of basic motions called therbligs. It is used when the work cycle is
highly repetitive and of very short duration.
Lecture 4
CRITICAL EXAMINATION
Critical examination
of the information recorded about the process in charts / diagrams is the most
important phase of the method study. In this, each element of the work, as
presently being done and recorded on the chart is subjected to a systematic and
progressive series of questions with the purpose of determining true reasons
for which it is done. Based on the reasons, improvements are found and adopted
into a new method, called better method. This examination, thus requires
exhaustive collaboration with everyone whose contribution can prove useful, and
also full use of all available sources of technical information. The use of
questioning technique reduces the possibility of missing any information which
may be useful for the development of better method.
A popular procedure
of carrying out critical examination uses two sets of questions: Primary
questions (answers to these show up the necessity of carrying out the
activity), and Secondary questions (answers to these allow considerations to
alternative methods of doing the activity). Selection of the best way of doing
each activity is later determined to develop new method which is introduced as
a standard practice.
A general-purpose set
of primary and secondary questions is given below:
Primary Questions:
1. Purpose. The need of
carrying out the activity is challenged by the questions-What is achieved? Is
it necessary? Why?
The answers to these
questions determine whether the particular activity will be included in the
proposals of new method for the process.
2. Means. The means of
carrying out the activity are challenged by the questions- 'How is it done?'
and 'Why that way'?
3. Place. The location
of carrying out the activity is challenged by the questions- 'Where is it
done'? and 'Why there'?
4. Sequence. The time of
carrying out the activity is challenged by the questions- 'When is it done'?
and 'Why then'?
5. Person. The level of
skill and experience of the person performing the activity is challenged by the
questions- 'Who does it'? and 'Why that person'?
The main object of
the primary questions is to make sure that the reasons for every aspect of the
presently used method are clearly understood. The answers to these questions
should clearly bring out any part of the work which is unnecessary or
inefficient in respect of means, sequence, person or place.
Secondary Questions:
The aim of secondary
questions is to arrive at suitable alternatives to the presently used method:
1. Purpose. If the answer
to the primary question 'Is the activity necessary"? is convincingly
'Yes', alternatives to achieve the object of carrying nut the activity are
considered by the question— 'What else could be done'?
2. Means. All the
alternative means to achieve the object are considered by the question— 'How
else could it be done'?
3. Place. Other places
for carry ing out the activity are considered by the question— 'Where else
could it be done'?
4. Sequence. The secondary
question asked under this heading is— 'When else could it be clone'?
5. Person. The
possibilities for carrying out the activity by other persons are considered by
asking the question- 'Who else should do it' ?
This phase involves
the search of alternative possibilities within the imposed restrictions of
cost, volume of production, and the like. For this the method study man uses
his own past experience with same or similar problems or refers to text books,
handbooks, etc.
The answers to the
following questions are then sought through evaluation of the alternatives.
'What should be
done'?
'How should it be
done'?
'Where should it be
done'?
'When should it be
done'? and
'Who should do it'?
These answers form
the basis of the proposals for the improved method. The evaluation phase
requires the work study man to consider all the possibilities with respect to
the four factors—economic, safety, work quality and human factors—the economic
factor being the most important in most situations.
Economic
considerations to any alternative refer to determination of 'How much will it
cost'? and 'How much will it save'? The purpose of evaluating safety factor is
to ensure that the alternative selected shall not make the work less safe. The
evaluation of quality factor shall determine whether the alternative selected
shall make for better product quality or quality control.
And lastly human
factors considerations shall ensure that the new method will be interesting,
easy to learn, safe, less monotonous and less fatiguing to the operator.
Figure shows a sample sheet used for critical
examination the use of which can be quite helpful in this phase of method
study.
Lecture 5
Developing Better Method:
With the present
method or procedure for the job in mind, the application of ‘critical analysis'
highlights the essential part of the job, for which alternative ways for its
carrying out are developed .
When developing
alternative ways for doing a task the following may be considered.
• Where and
how to use ‘man' in the process?
• What
better work procedure be adopted?
• What
better equipment be used?
• What
better layout of work station, shop or factory be used?
In deciding
whether a particular element of work (operation, inspection, or transportation)
be carried out manually or with the help of a device, method study engineer
must be well aware of things which man cannot do or does in inferior fashion
than machine. Examples of such things are:
- Exert large amount of
force, as needed in metal cutting.
- Exert force precisely or
smoothly at a fixed rate as needed in metal forming.
- Do high speed
computations of complex nature.
- Perform repetitive tasks
without suffering from side effects like boredom, fatigue, etc.
- Move at high speeds for
hours together.
- Carry out several tasks
simultaneously.
- Respond fast to
frequently changing control signals.
- Perform satisfactorily in
an environment where conditions relating to cold, heat, noise, dampness,
etc. are extreme.
In contrast,
machines prove inferior generally when for carrying out a task it is necessary
to
a.
Think creatively
or inductively
b.
Learn
c.
Generalize
d.
Cope will
unexpected events.
In most cases, the
relative roles of man and machine vary from one extreme end in which entire process
is manual to the other extreme in which the process is completely mechanized
with the presence of man only for monitoring, trouble shooting, maintenance,
and the like.
Man is readily
available and extremely flexible tool, who has the capability of doing a large
number and type of tasks with learning and practice that is generally less
expensive than the cost of creating devices for the same purpose. Man is
therefore considered a strong competitor for low, medium and even some high
volume production tasks.
When an activity
is decided to be carried out manually, the best work procedure is determined by
considering the principles of Motion Economy.
Equipped will the
various alternative ways of carrying out essential elements of task, method
study engineer has now to choose the best alternative method. He decides upon
the criteria, which may be additional fixed costs involved, running cost,
production rate, operator's fatigue, operator learning time, and the like. The
weight to each criterion is fixed and performance is predicted of each
alternative with respect to each criteria. The one which gets the maximum
points is selected for adoption as a standard method.
Detailed
specifications of this method are prepared with the description of procedure,
workplace layout and material/equipment to be used. This is important for
•
Communication of the proposed work method to those responsible for its approval
•
Communication of the proposed method to those concerned with its installation,
for example instructors and supervisors who are actually responsible for
instructions to operators and setting up the machinery and work place layouts.
• Official
record of the work method.
Installation of
Improved Method:
When the proposals
of the improved method for a job are approved by the management of the company,
the next step is to put this method into practice. Installation of method
requires necessary prior preparation for which the active support of everyone
concerned is very important.
The activities of
the installation phase include:
1. Gaining
acceptance of the change by the workers involved and their representatives. The
method change may affect the routine and paper work of wages, costs, planning,
and even purchase department. It may require displacement of staff from one
section to another of the organisation. Adjustments of this type need to be
carried out very carefully so that the least possible hardship or inconvenience
is caused.
2. Retraining the
workers. The extent to which workers need retraining will depend on the nature
of the job and the changes involved. It is much more for those jobs which have
a high degree of manual dexterity and where the workers have been doing the
work by traditional methods. The use of films demonstrating the advantages of
new method as compared to traditional one are often very useful in retraining
the workers.
3. Arranging the
requirements of the new method. This involves -
(i) arranging the
necessary plant, tools and equipment at all the workplaces,
(ii) arranging
building-up of necessary stocks of new raw materials, and running-down of old
stocks,
(iii) checking up
the availability and continuity of all supplies and services, and
(iv) arranging any
clerical records which may be required for purposes of control and comparison.
4. Taking other
necessary actions. These will depend upon situation to situation. For example,
if changes in working hours are involved, necessary instructions should be
passed on to auxiliary services such as transport, canteen, water supply, etc.
If change in wages is involved, information concerning the date of installation
must reach the costing department. Necessary instructions should be passed on
to every one concerned about the time table for the installation of the change
in method.
5. Giving a trial
run to the new method. It is important that all departments affected by the
change are represented at the rehearsal. It is often advantageous to conduct
the rehearsal while the old method is still operating. It should usually take
place outside normal working hours; say at week-end or at holiday time so that
there is no interference with normal production. The suggestions for minor
variations in the proposed method if they are worth while and cost effective
should be accepted and incorporated.
It is obvious that
the method analyst has to be extra tactful and keep restraint throughout the
period of installation. The installation is considered complete when the new
method starts running smoothly.
Follow-up:
The work of method
study man is not complete with the installation of the improved method; the
maintenance of the new method in its specified form is also part of his
activities. The main aim of maintenance of the new method is to ensure that the
workers do not slip back into old method, or introduce elements which are not
part of the proposed method.
For effective
maintenance it is important to define and specify the new method very clearly.
An operator chart giving adequate details of the tools, equipment, and
workplace layout and operator-motion pattern is often helpful.
The workers have
tendencey to drift away from the method laid down. The purpose of the
method-maintenance is to check this tendency. But if it is found that the
change from the method specified is in fact an improvement which can be made in
the method, this should be officially incorporated.
Lecture 6
Motion Study
Motion study is a technique of analyzing the body
motions employed in doing a task in order to eliminate or reduce ineffective
movements and facilitates effective movements. By using motion study and the
principles of motion economy the task is redesigned to be more effective and
less time consuming.
The Gilbreths pioneered the study of manual motions
and developed basic laws of motion economy that are still relevant today. They
were also responsible for the development of detailed motion picture studies,
termed as Micro Motion Studies, which are extremely useful for analyzing highly
repetitive manual operations. With the improvement in technology, of course,
video camera has replaced the traditional motion picture film camera.
In a broad sense, motion study encompasses micro
motion study and both have the same objective: job simplification so that it is
less fatiguing and less time consuming. While motion study involves a simple
visual analysis, micro motion study uses more expensive equipment. The two
types of studies may be compared to viewing a task under a magnifying glass
versus viewing the same under a microscope. The added detail revealed by the
microscope may be needed in exceptional cases when even a minute improvement in
motions matters, i.e. on extremely short repetitive tasks.
Taking the cine films @ 16 to 20 frames per second
with motion picture camera, developing the film and analyzing the film for
micro motion study had always been considered a costly affair. To save on the
cost of developing the film and the cost of film itself, a technique was used
in which camera took only 5 to 10 frames per minute. This saved on the time of
film analysis too. In applications where infrequent shots of camera could
provide almost same information, the technique proved fruitful and acquired the
name Memo Motion Study.
Traditionally, the data from micro motion studies
are recorded on a Simultaneous Motion (simo) Chart while that from motion
studies are recorded on a Right Hand - Left Hand Process Chart.
Therbligs
On analysing the result of several motion studies
conducted, Gilbreths concluded that any work can be done by using a combination
of some or all of 17 basic motions, called Therbligs (Gilbreth spelled
backward). These can be classified as effective therbligs and ineffective
therbligs. Effective therbligs take the work progress towards completion.
Attempts can be made to shorten them but they cannot be eliminated. Ineffective
therbligs do not advance the progress of work and therefore attempts should be
made to eliminate them by applying the Principles of Motion Economy. Table gives different therbligs along with
their symbols and descriptions.
SIMO Chart
It is a graphic representation of an activity and
shows the sequence of the therbligs or group of therbligs performed by body
members of operator. It is drawn on a common time scale. In other words, it is
a two-hand process chart drawn in terms of therbligs and with a time scale, see Figure.
Making the Simo Chart. A video film or a motion
picture film is shot of the operation as it is carried out by the operator. The
film is analyzed frame by frame. For the left hand, the sequence of therbligs
(or group of therbligs) with their time values are recorded on the column
corresponding to the left hand. The symbols are added against the length of
column representing the duration of the group of therbligs. The procedure is
repeated for the right hand and other body members (if any) involved in
carrying out the operation.
It is generally not possible to time individual
therbligs. A certain number of therbligs may be grouped into an element large
enough to be measured as can be seen in Figure.
Uses of Simo Chart
From the analysis shown about the
motions of the two hands (or other body members) involved in doing an
operation, inefficient motion pattern can be identified and any violation of
the principle of motion economy can be easily noticed. The chart, therefore,
helps in improving the method of doing an operation so that balanced two-handed
actions with coordinated foot and eye motions can be achieved and ineffective
motions can be either reduced or eliminated. The result is a smoother, more
rhythmic work cycle that keeps both delays and operator fatigue to the minimum
extent.
Lecture
7
Cycle graph and Chrono cycle graph
These are the
techniques of analyzing the paths of motion made by an operator and were
originally developed by the Gilbreths. To make a cycle graph , a small electric
bulb is attached to the finger, hand, or any other part of the body whose
motion is to be recorded. By using still photography, the path of light of bulb
(in other words, that of the body member) as it moves through space for one
complete cycle is photographed. The working area is kept relatively less
illuminated while photograph is being taken. More than one camera may be used
in different planes to get more details. After the film is developed, the
resulting picture (cycle graph) shows a permanent record of the motion pattern
employed in the form of a closed loop of white continuous line with the working
area in the background. A cycle graph does not indicate the direction or speed
of motion.
It can be used for
- Improving the motion
pattern, and
- Training purposes in that
two cycle graphs may be shown with one indicating a better motion pattern
than the other.
The chrono cycle
graph is similar to the cycle graph, but the power supply to the bulb is
interrupted regularly by using an electric circuit. The bulb is thus made to
flash. The procedure for taking photograph remains the same. The resulting
picture (chrono cycle graph), instead of showing continuous line of motion
pattern, shows short dashes of line spaced in proportion to the speed of the
body member photographed. Wide spacing would represent fast moves while close
spacing would represent slow moves. The jumbling of dots at one point would indicate
fumbling or hesitation of the body member. A chrono cycle graph can thus be
used to study the motion pattern as well as to compute velocity, acceleration
and retardation experienced by the body member at different locations. Figures
show a cycle graph and a chrono cycle graph.
The world of
sports has extensively used this analysis tool, updated to video, for the
purpose of training in the development of form and skill.
Principles of
Motion Economy:
These principles
can be considered under three different groups.
• Those
related to the use of the human body.
• Those
related to the workplace arrangement, and
• Those
related to the design of tools and equipment.
1. Principles
related to the use of human body:
(i) Both hands
should begin and end their basic divisions of activity simultaneously and
should not be idle at the same instant, except during the rest periods.
(ii) The hand
motions should be made symmetrically and simultaneously away from and toward
the centre of the body.
• Momentum
should be employed to assist the worker wherever possible, and it should be
reduced to a minimum if it must be overcome by muscular effort.
• Continuous
curved motions should be preferred to straight line motions involving sudden
and sharp changes in the direction.
• The least
number of basic divisions should be employed and these should be confined to
the lowest practicable classifications. These classifications, summarized in
ascending order of time and fatigue expended in their performance, are:
• Finger
motions
• Finger and
wrist motions.
• Finger,
wrist, and lower arm motions.
• Finger,
wrist, lower arm, and upper arm motions.
• Finger,
wrist, lower arm, upper arm motions and body motions.
• Work that
can be done by the feet should be arranged so that it is done together with
work being done by the hands. It should be recognized, however, that it is
difficult to move the hand and foot simultaneously.
• The middle
finger and the thumb should be used for handling heavy loads over extended
periods as these are the strongest working fingers. The index finger, fourth
finger, and little finger are capable of handling only light loads for short
durations.
• The feet
should not be employed for operating pedals when the operator is in standing
position.
• Twisting
motions should be performed with the elbows bent.
• To grip
tools, the segment of the fingers closed to the palm of the hand should be
used.
2. Principles
related to the arrangement and conditions of workplace:
• Fixed
locations should be provided for all tools and materials so as to permit the
best sequence and eliminate search and select .
• Gravity
bins and drop delivery should be used to reduce reach and move times.
Use may be made of ejectors for removing finished parts.
• All
materials and tools should be located within the normal working area in both
the vertical and horizontal plane ( see Figure ),
and as close to the point of use as possible.
• Work table
height should permit work by the operator in alternately sitting and standing
posture.
• Glare-free
adequate illumination, proper ventilation and proper temperature should be
provided.
• Dials and
other indicators should be patterned such that maximum information can be
obtained in minimum of time and error.
3. Principles
related to the design of tools and equipment:
• Use
colour, shape or size coding to maximize speed and minimize error in finding
controls.
• Use simple
on/off, either/or indicators whenever possible. If simple on/off indicator is
not sufficient, use qualitative type indicator, and use quantitative type
indicator only when absolutely essential.
• All
levers, handles, wheels and other control devices should be readily accessible
to the operator and should be designed so as to give the best possible
mechanical advantage and utilize the strongest available muscle group. Their
direction of motion should conform to stereo-typed reactions.
• Use quick
acting fixture to hold the part or material upon which the work is being
performed.
• Use stop
guides to reduce the control necessary in positioning motions.
• Operating,
set-up and emergency controls should be grouped according to the function.
Design of Workplace
Layout
The design of
workplace layout involves the following
- Determination of work
surface height
- Design of operator chair
(if work is to be done in sitting posture), or allowing the use of
antifatigue mats for standing operator
- Determination of location
of tools, materials, controls, displays and other devices.
We shall consider
these briefly.
Work Place Height
This should be
decided from the standpoint of comfortable working posture for the operator.
Generally, it is equal to the elbow height of operator whether work is done in
standing or sitting posture. However, for work involving lifting of heavy
parts, it is useful to lower the work surface height by as much as 20 cm. This
would reduce the fatigue to the trunk of operator. Similarly, it may be useful
to raise the work surface height when work involves visual examination of
minute details of fine parts. This would reduce the eye fatigue to the
operator. Alternatively, the work surface may be inclined by 15 degrees or so.
Work surface height may also be made adjustable in situations where operator is
permitted to do work in alternatively sitting and standing postures.
Design of Operator
Chair
A seated posture
is better than standing posture from the standpoint of stress reduction on the
feet and the overall energy expenditure. A well-designed seat should
- Provide trunk
stabilization so that a good posture is maintained,
- Permit change of posture,
and
- Not unduly press the
thigh tissues.
This requires the
use of ergonomic considerations and anthropometric dimensions of operator so
that appropriate dimensions are chosen for the following features of chair
(i) Seat Height
(ii) Seat Depth
(iii) Seat Width
(iv) Seat
Inclination
(v) Arm Rests
(vi) Back Rest
(vii) Foot Rest
It is necessary to
provide adjustability, particularly with respect to seat height, in order that
the same seat (or chair) is useable by many operators doing same job.
Standing for long
periods of time on a cemented floor is fatiguing. If operator has to work only
in standing posture, it is essential to provide resilient anti-fatigue floor
mats. Such mats allow small muscle contractions in the legs and force the blood
to keep circulating.
Determination of
location of tools, materials, controls, displays and other devices.
We all know that
greater the distance through which operator moves his body member while doing
work, larger is the requirement of muscular effort, control and time. This
means that all tools, materials, controls, etc need to be located within close
reach of the operator. In this context, two areas can be identified: normal
working area and maximum working area. Figure identifies
these areas in horizontal and vertical planes.
Within these
areas, all tools, materials, controls, displays and other devices must be
located on the basis of any of the following principles.
(i) Importance
Principle. According to this principle, the most important item or group of items
is first located within the normal area in the best position. The next
important component item or group of items is then selected and located in the
best location within the remaining area. In this way, all the items are
located.
(ii) Frequency of
Use Principle. According to this principle, the item with the greatest
frequency of use has the highest priority for location at the optimum position.
From within the remaining items to be located in the remaining area, the same
principle can then be applied repetitively.
(iii) Functional
Principle. This principle provides for grouping of items according to their
function. For instance, all controls that are functionally related may be
grouped together and located at one place.
(iv) Sequence of
Use Principle. According to this principle, items are located according to
sequence of their use. For illustration, let us consider the case of assembly.
As we know, an assembly is made by assembling the sub-assemblies in a specific
order. From motion economy or production efficiency point of view, it would be
better if sub-assemblies and other items are located in the sequence in which
they are to be used in assembly.
Further, for
better productivity, it is important that location of all tools, materials and
controls be fixed so that their "search" and “select" is
minimized.
Lecture 8
Work Measurement
Work measurement refers to the
estimation of standard time for an activity, that is the time allowed for
completing one piece of job by using the prescribed method. Standard time can
be defined as the time taken by an average experienced worker for the job with
provisions for delays beyond the worker's control.
There are several techniques
used for estimation of standard time in industry. These include time study, work
sampling, standard data, and predetermined motion time system.
Applications:
Standard times for operations
are useful for several applications in industry, like
• Estimating material,
machinery, and equipment requirements.
• Estimating production
cost per unit as an input to
- Preparation of budgets
- Determination of selling price
- Make or buy decision
• Estimating manpower
requirements.
• Estimating delivery
schedules and planning the work
• Balancing the work of
operators working in a group.
• Estimating performance
of workers and using that as the basis for incentive payment to those direct
and indirector labor who show greater productivity.
We will study some of the
popular techniques of work measurement.
TIME STUDY. It is the most
versatile and the most widely used technique of work measurement.
Definition:
Time study is a technique to
estimate the time to be allowed to a qualified and well-trained worker working
at a normal pace to complete a specified task by using specified method.
This technique is based on
measuring the work content of the task when performed by the prescribed method,
with the allowance for fatigue and for personal and unavoidable delays.
Time Study Procedure:
The procedure for time study
can best be described step-wise, which are self explanatory.
Step 1: Define objective of the study. This involves statement of the use of the
result, the precision desired, and the required level of confidence in the
estimated time standards.
Step 2: Verify that the standard method and conditions exist for the operation and
the operator is properly trained. If need is felt for method study or further
training of operator, the same may be completed before starting the time study.
Step 3: Select operator to be studied if there are more than one operator doing the
same task.
Step 4: Record information about the standard method, operation, operator, product,
equipment, and conditions on the Time Study observation sheet.
Step 5: Divide the operation into reasonably small elements, and record them on the
Time Study observation sheet.
Step 6: Time the operator for each of the elements. Record the data for a few
number of cycles on the Time Study observation sheet. Use the data to estimate
the total number of observations to be taken.
Step 7: Collect and record the data of required number of cycles by timing and
rating the operator.
Step 8: Calculate the representative watch time for each element of operation.
Multiply it by the rating factor to get normal time.
Normal time = Observed time x
Rating factor
Calculate the normal time for
the whole operation by adding the normal time of its various elements.
Step 9: Determine allowances for fatigue and various delays.
Step 10: Determine standard time of operation.
Standard time = Normal time +
allowances
Selection of job for Time
Study
Time Study is conducted on a
job
• which has not been
previously time-studied.
• for which method
change has taken place recently.
• for which worker(s)
might have complained as having tight time standards.
Selection of Worker for Time
Study
The selection of worker for
time study is a very important factor in the success of the study. If there is
only one person on the job, as usually is, then there is no choice. But if more
than one person is performing the same operation, the time study man may time
one or more of the workers. If all the workers are using the same method for
doing the job and there is different in the rate of their doing it, it is
necessary to select a suitable worker for the study. The worker on which time
study should be conducted must
- have necessary skill for the job.
- have sufficient experience with the given
method on the job (that is, he should have crossed the learning stage).
- be an ‘average' worker as regards the speed of
working.
- be temperamentally suited to the study (those
who can't work in normal fashion when watched, are not suitable for the
study).
- have knowledge about the purpose of study.
Time Study Equipment
The following equipment is
needed for time study work.
• Timing device
• Time study observation
sheet
• Time study observation
board
• Other equipment
Timing Device. The stop watch ( see Figure )
is the most widely used timing device used for time study, although electronic
timer is also sometimes used. The two perform the same function with the
difference that electronic timer can measure time to the second or third
decimal of a second and can keep a large volume of time data in memory.
Time Study Observation
Sheet. It is a printed form with spaces provided for
noting down the necessary information about the operation being studied, like
name of operation, drawing number, and name of the worker, name of time study
person, and the date and place of study. Spaces are provided in the form for
writing detailed description of the process (element-wise), recorded time or
stop-watch readings for each element of the process, performance rating(s) of
operator, and computation. Figure shows
a typical time study observation sheet.
Time Study Board. It is a light -weight board used for holding the observation sheet and
stopwatch in position. It is of size slightly larger than that of observation
sheet used. Generally, the watch is mounted at the center of the top edge or as
shown in Figure near
the upper right-hand corner of the board. The board has a clamp to hold the
observation sheet. During the time study, the board is held against the body
and the upper left arm by the time study person in such a way that the watch
could be operated by the thumb/index finger of the left hand. Watch readings
are recorded on the observation sheet by the right hand.
Other Equipment. This includes pencil, eraser, device like tachometer for checking the
speed, etc.
Dividing Work into Short
Elements
Timing a complete task as one
element is generally not satisfactory. For the purpose of time study the task
is normally broken
into short elements and each element is timed separately, for the following
reasons:
into short elements and each element is timed separately, for the following
reasons:
(1) To separate unproductive
part of task from the productive one.
(2) To improve accuracy in
rating. The worker may not work at the
same speed throughout the cycle. He may perform some elements faster and
some slower. Breaking of task into short elements permits rating of each
element separately which is more realistic than just rating once for the complete
cycle.
same speed throughout the cycle. He may perform some elements faster and
some slower. Breaking of task into short elements permits rating of each
element separately which is more realistic than just rating once for the complete
cycle.
(3) To identify elements
causing high fatigue. Breaking of task into short elements permits giving
appropriate rest allowances to different elements.
(4) To have detailed job
specifications. This helps in detection of any variation in the method that may
occur after the time standard is established.
(5) To prepare standard data
for repeatedly occurring elements.
The following guidelines
should be kept in mind while dividing a task into elements.
(1) The elements should be of
as short duration as can be accurately timed. (This in turn, depends on the
skill of the time study man, method of timing and recording, and many other
factors. Generally, with the stop watch, elements of duration less than 0.03 to
0.05 minute are difficult to time accurately. The elements should not normally
be longer than 0.40 min.).
(2) Manually performed
elements should be separated from machine paced elements. (Time for machine
paced elements can be determined by calculation). Machine elements are not
rated against a normal. This rule also helps in recognition of delays.
(3) Constant elements should
be separated from variable elements.
(Constant elements are those elements which are independent of the size, weight,
length, or shape of the workpiece. For example, the time to pick screw driver
from its place and bring it to the head of a screw is constant, whereas the time
to tighten or loosen the screw is a variable, depending upon the length and
size of the screw).
(Constant elements are those elements which are independent of the size, weight,
length, or shape of the workpiece. For example, the time to pick screw driver
from its place and bring it to the head of a screw is constant, whereas the time
to tighten or loosen the screw is a variable, depending upon the length and
size of the screw).
(4) The beginnings and endings
of elements should be easily distinguishable. These should preferably be
associated with some kind of sound.
(5) Irregular elements, those
not repeated in every cycle, should be separated from regular elements. For
example, if the jig is cleaned off after every ten parts produced,
"cleaning" is an irregular element, and its time should be spread
over ten cycles.
(6) Unnecessary motions and
activities should be separated from those considered essential.
(7) Foreign or accidental
elements should be listed separately. Such elements are generally of
non-repetitive type.
Number of cycles to be timed.
The following general
principles govern the number of cycles to get the representative average cycle
time.
(1) Greater the accuracy
desired in the results, larger should be the number of cycles observed.
(2) The study should be
continued through sufficient number of cycles so that occasional elements such
as setting-up machine, cleaning of machine or sharpening of tool are observed
for a good number of times.
(3) Where more than one
operator is doing the same job, short study (say 10 to 15 cycles) should be
conducted on each of the several operators than one long study on a single
operator.
It is important that enough
cycles are timed so that reliable average is obtained.
Following techniques are used
to determine the number of cycles to be timed.
(i) Use of Tables: On the consideration of the cost of obtaining the data and the desired
accuracy in results, most companies have prepared their own tables for the use
of time study people, which indicate the number of cycles to be timed as a
function of the cycle time and the frequency of occurrence of the job in the
company. For example, one Company uses the Table for
such purposes.
(ii) Statistical
methods: On the basis of the requirements of the particular
situation involved, accuracy and confidence
level are decided (An accuracy of a confidence level of 95% is
considered reasonable in most cases). A preliminary study is conducted in which
some (say N) cycles are timed. Standard deviation o of these (N) observations
is calculated as
(iii) Mundel Method: In this method the following steps are followed.
Step 1. Take a few good watch readings of the work cycle. (Generally, 10 readings
are taken if cycle time is less than 2 minutes, otherwise 5 readings).
Step 2. Find the ratio , where H and L are respectively
the highest and the lowest value of the leading.
Step 3. Corresponding to the value of the ratio, determine the number of
observations from the Table
Normal Performance
There is no universal concept
of Normal Performance. However, it is generally defined as the working rate of
an average qualified worker working under capable supervision but not under any
incentive wage payment scheme. This rate of working is characterized by the
fairly steady exertion of reasonable effort, and can be maintained day after
day without undue physical or mental fatigue.
The level of normal
performance differs considerably from one company to another. What company a
calls 100 percent performance, company B may call 80 percent, and company C may
call 125 percent and so on. It is important to understand that the level that a
company selects for normal performance is not critical but maintaining that
level uniform among time study persons and constant with the passage of time
within the company is extremely important.
There are, of course, some
universally accepted benchmark examples of normal performance, like dealing 52
cards in four piles in 0.5 minute, and walking at 3 miles per hour (4.83
km/hr). In order to make use of these benchmarks, it is important that a
complete description about these be fully understood, like in the case of card
dealing, what is the distance of each pile with respect to the dealer,
technique of grasping, moving and disposal of the cards.
Some companies
make use of video films or motion pictures for establishing what they consider
as normal speed or normal rate of movement of body members. Such films are made
of typical factory jobs with the operator working at the desired normal pace.
These films are found to be useful in demonstrating the level of performance
expected from the operators and also for training of time study staff.
Lecture 9
Performance Rating
During the time
study, time study engineer carefully observes the performance of the operator.
This performance seldom conforms to the exact definition of normal or standard.
Therefore, it becomes necessary to apply some 'adjustment' to the mean observed
time to arrive at the time that the normal operator would have taken to do that
job when working at an average pace. This 'adjustment' is called Performance
Rating.
Determination of
performance rating is an important step in the work measurement procedure. It
is based entirely on the experience, training, and judgment of the work-study
engineer. It is the step most subjective and therefore is subject to criticism.
Performance Rating
can be defined as the procedure in which the time study engineer compares the
performance of operator(s) under observation to the Normal Performance and
determines a factor called Rating Factor.
System of Rating
There are several
systems of rating the performance of operator on a job.
These are:
• Pace
Rating
•
Westinghouse System of Rating
• Objective
Rating
• Synthetic
Rating
A brief
description of each rating method follows.
Pace Rating
Under this system,
operator's performance is evaluated by considering his rate of accomplishment
of the work. The study person measures the effectiveness of the operator
against the concept of normal performance and then assigns a percentage to indicate
the ratio of the observed performance to normal or standard performance.
In this method,
which is also called the speed rating method, the time study person judges the
operators speed of movements, i.e. the rate at which he is applying himself, or
in other words "how fast" the operator performs the motions involved.
Westinghouse
System of Rating
This method
considers four factors in evaluating the performance of operator: skill,
effort, conditions, and consistency.
Skill may be
defined as the proficiency at of an individual in following the given method.
It is demonstrated by co-ordination of mind and hands. A person's skill in a
given operation increases with his experience on the job, because increased
familiarity with work brings speed, smoothness of motions and freedom from
hesitations.
The Westinghouse
system lists six classes of each factor. For instance the classes of skill are
poor, fair, average, good, excellent and superskill, as given in a Table .
Each class has further two degrees. The time study person evaluates the skill
displayed by the operator. And puts it in one of the six classes and also
decides the degree in that class, higher or lower, i.e. 1 or 2. As equivalent %
value of each class of skill is provided in the Table, the rating is translated
into its equivalent percentage value, which ranges from +15 % (for super skill
of higher degree) to -22 % (for poor skill of lower degree).
In a similar
fashion, the ratings for effort, conditions, and consistency are given using
the Table for each of the factors. By algebraically combining the ratings with
respect to each of the four factors, the final performance-rating factor is
estimated.
Objective Rating
In this system,
speed of movements and job difficulty are rated separately and the two
estimates are combined into a single value. Rating of speed or pace is done as
discussed earlier, and the rating of job difficulty is done by selecting
adjustment factors corresponding to characteristics of operation with respect
to (i) amount of body used, (ii) foot pedals, (iii) bimanual ness, (iv)
eye-hand co-ordination, (v) handling requirements and (vi) weight handled or
resistance encountered. Mundel and Danner have given Table of
% values (adjustment factors) for the effects of various difficulties in the
operation performed.
For an operation
under study, a numerical value for each of the six factors is assigned, and the
algebraic sum of the numerical values called job difficulty adjustment factor
is estimated.
The rating factor
R can be expressed as
R = P x D
Where: P = Pace
rating factor, and
D = Job difficulty
adjustment factor.
Synthetic Rating
This method of
rating has two main advantages over other methods. These are (i) it does not
rely on the judgment of time study person and (ii) it gives consistent results.
The time study is
made as usual. Some manually controlled elements of the work cycle are
selected. Using a PMT system (Pre-determined motion time system), the times for
these selected elements are determined. The times of these elements as
determined are compared with the actual observed times and the performance
factor is estimated for each of the selected elements.
Performance or
Rating Factor, R = P / A
Where P =
Predetermined motion time of the element, and
A = Average actual
observed time of the element.
The overall rating
factor is the mean of rating factors determined for the selected elements. This
is applied uniformly to all the manually controlled elements of the work cycle.
Example
A work cycle has
been divided into 8 elements and time study has been conducted. The average
observed times for the elements are given in the following Table:
Element No.
|
1
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
Element Type
|
M
|
M
|
P
|
M
|
M
|
M
|
M
|
M
|
Average actual time
(minutes)
|
0.14
|
0.16
|
0.30
|
0.52
|
0.26
|
0.45
|
0.34
|
0.15
|
M = Manually
Controlled, P = Power Controlled
Total observed
time of work cycle = 2.32 min.
Suppose we select
three elements 2, 5 and 8 (These must be manually controlled elements). By
using some PMT system, suppose we determine the times of these elements as
Elements No.
|
2
|
5
|
8
|
PMT System times (min)
|
0.145
|
0.255
|
0.145
|
Rating factor for
element 2 = 0.145 / 0.16 = 90.62 %.
Rating factor for
element 5 = 0.255 / 0.26 = 98.08 %.
Rating factor for
element 8 = 0.145 / 0.15 = 96.66 %.
The mean of the
rating factors of selected elements = 95.12 % or say 95 % is the rating factor
that will be used for all the manual elements of the work cycle.
The normal time of
the cycle can than be calculated as.
Element No.
|
1
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
Element Type
|
M
|
M
|
P
|
M
|
M
|
M
|
M
|
M
|
Average actual time (min)
|
0.14
|
0.16
|
0.30
|
0.52
|
0.26
|
0.45
|
0.34
|
0.15
|
PMT system time (min)
|
|
0.145
|
|
|
0.255
|
|
|
0.145
|
Performance Rating Factor
|
95
|
95
|
100
|
95
|
95
|
95
|
95
|
95
|
Normal Cycle Time
=
0.95(0.14+0.16+0.52+0.26+0.45+0.34+0.15) +1.00(0.30)
=1.92+0.30
=2.22 minutes
It is to be noted
that power controlled (or machine-paced) elements are always given 100% rating.
Allowances
The readings of
any time study are taken over a relatively short period of time. The normal
time arrived at, therefore, does not include unavoidable delay and other legitimate
lost time, for example, in waiting for materials, tools or equipment; periodic
inspection of parts; interruptions due to legitimate personal needs, etc. It is
necessary and important that the time study person applies some adjustment, or
allowances, to compensate for such losses so that fair time standard is
established for the given job.
Allowances are
generally applied to total cycle time as some percentage of it, but sometimes
these are given separately for machine time as some % and for manual effort
time some other %. However, no allowances are given for interruptions which may
be due to factors which are within the operator's control or which are
avoidable.
Most companies
allow the following allowances to their employees.
• Constant
allowances (for personal needs and basic fatigue)
• Delay
Allowance (for unavoidable delays)
• Fatigue
Allowance (for job dependent fatigue)
• Personal
Allowance
• Special
Allowance
Delay Allowance
This time
allowance is given to operator for the numerous unavoidable delays and
interruptions that he experiences every day during the course of his work.
These interruptions include interruptions from the supervisor, inspector,
planners, expediters, fellow workers, production personnel and others. This
allowance also covers interruptions due to material irregularities, difficulty
in maintaining specifications and tolerances, and interference delays where the
operator has to attend to more than one machine.
Fatigue Allowance
This allowance can
be divided into two parts: (i) basic fatigue allowance and (ii) variable
fatigue allowance. The basic fatigue allowance is given to the operator to
compensate for the energy expended for carrying out the work and to alleviate
monotony. For an operator who is doing light work while seated, under good
working conditions and under normal demands on the sensory or motor system, a
4% of normal time is considered adequate. This can be treated as a constant
allowance.
The magnitude of
variable fatigue allowance given to the operator depends upon the severity of
conditions, which cause extra (more than normal) fatigue to him. As we know,
fatigue is not homogeneous. It ranges from strictly physical to purely
psychological and includes combinations of the two. On some people it has a marked
effect while on others, it has apparently little or no effect. Whatever may be
the kind of fatigue-physical or mental, the result is same-it reduces the work
output of operator. The major factors that cause more than just the basic
fatigue includes severe working conditions, especially with respect to noise,
illumination, heat and humidity; the nature of work, especially with respect to
posture, muscular exertion and tediousness, and like that.
It is true that in
modern industry, heavy manual work, and thus muscular fatigue is reducing day
by day but mechanization is promoting other fatigue components like monotony
and mental stress. Because fatigue in totality cannot be eliminated, proper
allowance has to be given for adverse working conditions and repetitiveness of
the work.
Personal Allowance
This is allowed to
compensate for the time spent by worker in meeting the physical needs, for
instance a periodic break in the production routine. The amount of personal
time required by operator varies with the individual more than with the kind of
work, though it is seen that workers need more personal time when the work is
heavy and done under unfavorable conditions.
The amount of this
allowance can be determined by making all-day time study or work sampling. Mostly,
a 5 % allowance for personal time (nearly 24 minutes in 8 hours) is considered
appropriate.
Special Allowances
These allowances
are given under certain special circumstances. Some of these allowances and the
conditions under which they are given are:
Policy Allowance:
Some companies, as a policy, give an allowance to provide a satisfactory level
of earnings for a specified level of performance under exceptional
circumstance. This may be allowed to new employees, handicap employees, workers
on night shift, etc. The value of the allowance is typically decided by
management.
Small Lot
Allowance: This allowance is given when the actual production period is too
short to allow the worker to come out of the initial learning period. When an
operator completes several small-lot jobs on different setups during the day,
an allowance as high as 15 percent may be given to allow the operator to make
normal earnings.
Training
Allowance: This allowance is provided when work is done by trainee to allow him
to make reasonable earnings. It may be a sliding allowance, which progressively
decreases to zero over certain length of time. If the effect of learning on the
job is known, the rate of decrease of the training allowance can be set
accordingly.
Rework Allowance:
This allowance is provided on certain operation when it is known that some
percent of parts made are spoiled due to factors beyond the operator's control.
The time in which these spoiled parts may be reworked is converted into
allowance.
Different
organizations have decided upon the amount of allowances to be given to
different operators by taking help from the specialists / consultants in the
field and through negotiations between the management and the trade unions. ILO
has given its recommendations about the magnitude of various allowances, as
shown in Table.
Example:
In making a time
study of a laboratory technician performing an analysis of processed food in a
canning factory, the following times were noted for a particular operation.
Run
|
1
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
9
|
10
|
11
|
12
|
Operation time (sec.)
|
21
|
21
|
16
|
19
|
20
|
16
|
20
|
19
|
19
|
20
|
40
|
19
|
Run
|
13
|
14
|
15
|
16
|
17
|
18
|
19
|
20
|
21
|
22
|
23
|
24
|
Operation time (sec.)
|
21
|
18
|
23
|
19
|
15
|
18
|
18
|
19
|
21
|
20
|
20
|
19
|
If the
technician's performance has been rated at 120 percent, and the company policy
for allowance (personal, fatigue, etc.) stipulates 13 percent,
• Determine
the normal time.
• Determine
the standard time.
Watch readings
falling 50 % above and 25 % below the average may be considered as abnormal.
Ans:
Lecture 10
Work
Sampling
Work
Sampling (also sometimes called ratio delay study) is a technique of getting
facts about utilization of machines or human beings through a large number of
instantaneous observations taken at random time intervals. The ratio of
observations of a given activity to the total observations approximates the
percentage of time that the process is in that state of activity. For example,
if 500 instantaneous observations taken at random intervals over a few weeks
show that a lathe operator was doing productive work in 365 observations and in
the remaining 135 observations he was found 'idle' for miscellaneous reasons,
then it can be reliably taken that the operator remains idle (135/500) x 100 =
27 % 0f the time. Obviously, the accuracy of the result depends on the number
of observations. However, in most applications there is usually a limit beyond
which greater accuracy of data is not economically worthwhile.
Use of
Work Sampling for Standard Time Determination
Work
sampling can be very useful for establishing time standards on both direct and
indirect labor jobs. The procedure for conducting work sampling study for
determining standard time of a job can be described step-wise.
Step 1 . Define the problem.
•
Describe the job for which the standard time is to be determined.
•
Unambiguously state and discriminate between the two classes of activities of
operator on the job: what are the activities of job that would entitle him to
be in 'working" state.
This
would imply that when operator will be found engaged in any activity other than
those would entitle him to be in "Not Working" state.
Step 2. Design the sampling plan.
•
Estimate satisfactory number of observations to be made.
•
Decide on the period of study, e.g. two days, one week, etc.
•
Prepare detailed plan for taking the observations.
This will
include observation schedule, exact method of observing, design of observation
sheet, route to be followed, particular person to be observed at the
observation time, etc.
Step 3. Contact the persons concerned and take them in confidence
regarding conduct of the study.
Step 4. Make the observations at the pre-decided random times about
the working / not working state of the operator. When operator is in working
state, determine his performance rating. Record both on the observation sheet.
Step 5. Obtain and record other information. This includes
operator's starting time and quitting time of the day and total number of parts
of acceptable quality produced during the day.
Step 6. Calculate the standard time per piece.
We will
now briefly discuss some important issues involved in the procedure.
Number of
Observations
As we
know, results of study based on larger number of observations are more
accurate, but taking more and more observations consumes time and thus is
costly. A cost-benefit trade-off has thus to be struck. In practice, the
following methods are used for estimation of the number of observations to be
made.
(i) Based
on judgment. The study person can decide the necessary number of observations
based on his judgment. The correctness of the number may be in doubt but
estimate is often quick and in many cases adequate.
(ii)
Using cumulative plot of results. As the study progresses the results of the
proportion of time devoted to the given state or activity, i.e. Pi from the
cumulative number of observations are plotted at the end of each shift or day.
A typical plot is shown in Figure.
Since the accuracy of the result improves with increasing number of
observations, the study can be continued until the cumulative Pi appears to
stabilize and collection of further data seems to have negligible effect on the
value of Pi.
(iii) Use
of statistics. In this method, by considering the importance of the decision to
be based on the results of study, a maximum tolerable sampling error in terms
of confidence level and desired accuracy in the results is specified. A pilot
study is then made in which a few observations are taken to obtain a
preliminary estimate of Pi. The number of observations N necessary are then
calculated using the following expression.
The
number of observations estimated from the above relation using a value of Pi
obtained from a preliminary study would be only a first estimate. In actual
practice, as the work sampling study proceeds, say at the end of each day, a
new calculation should be made by using increasingly reliable value of Pi
obtained from the cumulative number of observations made.
Determination
of Observation Schedule
The
number of instantaneous observations to be made each day mainly depends upon the
nature of operation. For example, for non-repetitive operations or for
operations in which some elements occur in-frequently, it is advisable to take
observations more frequently so that the chance of obtaining all the facts
improves. It also depends on the availability of time with the person making
the study. In general, about 50 observations per day is a good figure. The
actual random schedule of the observations is prepared by using random number
table or any other technique.
Design of
Observation Sheet
A sample
observation sheet for recording the data with respect to whether at the
pre-decided time, the specified worker on job is in 'working' state or
'non-working' state is shown in Figure.
It contains the relevant information about the job, the operators on job, etc.
At the end of each day, calculation can be done to estimate the percent of time
workers on the job (on an average) spend on activities, which are considered as
part of the job.
Conducting
Work Sampling Study
At the
predecided times of study, the study person appears at the work site and
observes the specific worker (already randomly decided) to find out what is he
doing. If he is doing activity which is part of the job, he is ticked under the
column 'Working' and his performance rating is estimated and recorded. If he is
found engaged in an activity which is not a part of job, he is ticked under the
column 'Not Working'. At the end of day, the number of ticks in 'Working'
column is totaled and average performance rating is determined.
The
observed time (OT) for a given job is estimated as
The
normal time (NT) is found by multiplying the observed time by the average performing
index (rating factor).
Where = is
average rating factor to be determined as , Figure
The
standard time is determined by adding allowances to the normal time.
Example
A work
sampling study was made of a cargo loading operation for the purpose of
developing its standard time. The study was conducted for duration of 1500
minutes during which 300 instantaneous observations were made at random
intervals. The results of study indicated that the worker on the job was
working 80 percent of the time and loaded 360 pieces of cargo during the study
period. The work analyst rated the performance at 90 %. If the management
wishes to permit a 13 % allowance for fatigue, delays and personal time, what
is the standard time of this operation?
Ans:
Here,
total study period = 1500 minutes
Working
fraction = 80 percent
Average
rating = 90 percent
Number of
units loaded = 360
Allowances =
13 %
Advantages
and Disadvantages of Work Sampling in Comparison with Time Study.
Advantages
Economical
•
Many operators or activities which are difficult or uneconomical to measure by
time study can readily be measured by work sampling.
•
Two or more studies can be simultaneously made of several operators or machines
by a single study person. Ordinarily a work study engineer can study only one
operator at a time when continuous time study is made.
•
It usually requires fewer man-hours to make a work sampling study than to make
a continuous time study. The cost may also be about a third of the cost of a
continuous time study.
•
No stopwatch or other time measuring device is needed for work sampling
studies.
•
It usually requires less time to calculate the results of work sampling study.
Mark sensing cards may be used which can be fed directly to the computing
machines to obtain the results just instantaneously.
Flexible
6. A work
sampling study may be interrupted at any time without affecting the results.
7. Operators
are not closely watched for long period of time. This decreases the chance of
getting erroneous results for when a worker is observed continuously for a long
period, it is probable that he will not follow his usual routine exactly during
that period.
Less
Erroneous
8. Observations
may be taken over a period of days or weeks. This decreases the chance of
day-to-day or week-to-week variations that may affect the results.
Operators
Like It
9. Work
sampling studies are preferred to continuous time study by the operators being
studied. Some people do not like to be observed continuously for long periods
of time.
Observers
Like It
10. Work
sampling studies are less fatiguing and less tedious to make on the part of
time study engineer.
Disadvantages
•
Work sampling is not economical for the study of a single operator or operation
or machine. Also, work-sampling study may be uneconomical for studying
operators or machines located over wide areas.
•
Work sampling study does not provide elemental time data.
•
The operator may change his work pattern when he sees the study person. For
instance, he may try to look productive and make the results of study
erroneous.
•
No record is usually made of the method being used by the operator. Therefore,
a new study has to be made when a method change occurs in any element of
operation.
•
Compared to stop watch time study, the statistical approach of work sampling
study is difficult to understand by workers.
Computerized
Work Sampling
Use of a
computer can save as much as 30 to 40 percent of the total work sampling study
cost. This is because too much clerical effort is involved in summarizing work
sampling data, e.g. in determining the number of observations required,
determining the daily observations required, determining the number of trips to
the area being studied per day, determining the time of each observation,
calculating the accuracy of results, plotting data on control charts and like
that. Computers can be used for mechanization of the repetitive calculations,
display of control charts and calculation of daily as well as cumulative
results.
Lecture 11
Predetermined Motion Time System
A predetermined
motion time system (PMTS) may be defined as a procedure that analyzes any
manual activity in terms of basic or fundamental motions required to perform
it. Each of these motions is assigned a previously established standard time
value and then the timings for the individual motions are synthesized to obtain
the total time needed for performing the activity.
The main use of
PMTS lies in the estimation of time for the performance of a task before it is
performed. The procedure is particularly useful to those organizations which do
not want troublesome performance rating to be used with each study.
Applications of
PMTS are for
(i) Determination
of job time standards.
(ii) Comparing the
times for alternative proposed methods so as to find the economics of the
proposals prior to production run.
(iii) Estimation
of manpower, equipment and space requirements prior to setting up the
facilities and start of production.
(iv) Developing
tentative work layouts for assembly lines prior to their working in order to
minimize the amount of subsequent re-arrangement and re-balancing.
(v) Checking
direct time study results.
A number of PMTS
are in use, some of which have been developed by individual organizations for
their own use, while other organizations have developed and publicized for
universal applications.
Some commonly used
PMT systems are:
- Work factor (1938)
- Method Time Measurement
(1948)
- Basic Motion Time (1951)
- Dimension Motion Time
(1954)
Important
considerations which may be made while selecting a PMT system for application
to particular industry are:
- Cost of Installation.
This consists mainly of the cost of getting expert for applying the system
under consideration.
- Application Cost. This is
determined by the length of time needed to set a time standard by the
system under consideration.
- Performance Level of the
System. The level of performance embodied in the system under
consideration may be different from the normal performance established in
the industry where the system is to be used. However, this problem can be
overcome by 'calibration' which is nothing but multiplying the times given
in the PMT Tables by some constant or by the application of an adjustment
allowance.
- Consistency of Standards.
Consistency of standards set by a system on various jobs is a vital factor
to consider. For this, the system can be applied on a trial basis on a set
of operations in the plant and examined for consistency in the so obtained
operation times.
- Nature of Operation. Best
results are likely to be achieved if the type and nature of operations in
the plant are similar to the nature and type of operations studied during
the development of the system under consideration.
Advantages and
limitations of using PMT systems
Advantages
Compared to other
work measurement techniques, all PMT systems claim the following advantages:
- There is no need to
actually observe the operation running. This means the estimation of time
to perform a job can be made from the drawings even before the job is
actually done. This feature is very useful in production planning,
forecasting, equipment selection, etc.
- The use of PMT eliminates
the need of troublesome and controversial performance rating. For the sole
reason of avoiding performance rating, some companies have been using this
technique.
- The use of PMT forces the
analyst to study the method in detail. This sometimes helps to further
improve the method.
- A bye-product of the use
of PM times is a detailed record of the method of operation. This is
advantageous for installation of method, for instructional purposes, and
for detection and verification of any change that might occur in the
method in future.
- The PM times can be
usefully employed to establish elemental standard data for setting time
standards on jobs done on various types of machines and equipment.
- The basic times
determined with the use of PMT system are relatively more consistent.
Limitations
There are two main
limitations to the use of PMT system for establishing time standards. These
are: (i) its application to only manual contents of job and (ii) the need of
trained personnel. Although PMT system eliminates the use of rating, quite a
bit of judgment is still necessarily exercised at different stages.
Physiological
Methods for Work Measurement
The physiological
cost to an operator of performing any given physical work results from the
activities of the muscles of arms, legs, back and other parts of the body and
is, therefore, affected by the number and type of muscles involved in either
moving the body member(s) or controlling antagonist contraction.
The activities of
body muscles cause changes in oxygen consumption, heart rate, body temperature,
lactic acid concentration in blood, 17-ketosteroid excretion in urine,
pulmonary ventilation, and other factors. Studies have shown that some of these
factors are only slightly affected by muscular activity. The important factors
which have linear correlation with the physiological cost of work performed by
an individual are oxygen consumption, heart rate, and pulmonary ventilation.
Increase in Heart
Rate
When a person is
at rest, his heart rate is at a fairly steady level (generally at about 70
beats/minute). Then when he starts doing some muscular work his pulse rate
increases rapidly to about 110 beats/minute and remains near to this level
during the working period. When work ends, the recovery begins and his heart
rate drops off and finally returns to the original resting level ( Figure )
.
The increase in
heart rate during work has been used as an index of the physiological cost of
the job. Some physiologists have also proposed the use of 'the rate of recovery
immediately after work stops' for the evaluation of physiological cost of
certain types of work. It is to be noted that the total physiological cost of a
task consists of the energy expenditure during work and the energy expenditure
above the resting rate during the recovery period. It is generally agreed that
the optimum limit of industrial performance is reached when the average pulse
rate during the work lies 30 beats/minute above the resting pulse rate.
Measurement. With every heart beat, a small electric potential is generated. This signal
can be picked up by placing silver electrodes on either side of the chest, and
transmitted to a receiver, where these can be counted directly or recorded
continuously on a ruled graph paper or integrated over time to measure in units
of beats per minute with the help of a cardiotachometer.
Another method of
getting the heart beat signals is through the use of an ear lobe unit, which is
a photo duodiode with a light source. This unit is mounted on an ear of the
subject in such a way that the duodiode is on one side and the light source is
on the other side of the ear. As the capacity of the ear lobe changes due to
the blood surges through the ear with beats of the heart, impulses are created
which are transmitted and recorded.
A simple method to
get the heart beat rate is through the use of a stethoscope and stop watch.
Studies have shown that the data obtained in this manner are fairly reliable
and also easy to obtain.
Oxygen
Consumption. It may be defined
as the volume of oxygen which a person extracts from the air he inhales.
Increase in the rate of oxygen consumption from the resting level to the
working level is also taken as a measure of the physiological cost of the work
done. The oxygen consumption per unit time is usually measured indirectly. To
do this the volume of air exhaled by a person in a certain time is collected
and the oxygen content of this air is determined. For this, use is made of a
portable respirometer. It is a lightweight gas meter which is worn on the back
of the subject. A mask is fitted on the face of the subject, and the exhaled
air is collected in the respirometer through a rubber tube. The respirometer
directly shows the volume of exhaled air in litres.
A sample of the
exhaled air is taken out at random intervals into a rubber bladder and an
analysis is carried out of its content. Comparison is then made between the
oxygen content of the two samples-drawn from the exhaled air and another from
the room air. For each litre of oxygen consumed by the human body, there is an
average energy turnover of 4.8 Kcal.
Table gives
the general values of oxygen consumption, lung ventilation, rectal temperature
and heart beats at the different work loads
.
Physiological
measurements can be used to compare the energy cost to the operator on a job
for which no time standard exists, with the energy cost to the same operator on
a similar operation for which a satisfactory time standard already exists. By
this comparison it is possible to establish the time standard on the job for
which it does not exist already. For the sake of illustration, consider a job
of lifting boxes weighing 2-3 kgs. from the floor level and placing it on a
conveyor belt. For this job a time standard of 6 seconds (10 boxes/min.) is
being used. When energy measurements were taken, it was found that to Mr.
Singh, the operator on the job, the energy cost of this job was 300 W. Let us
suppose now that there is another jab, similar to the first one, with the
difference that here, the weight of the boxes is 5-6 kgs. If it is required to
establish the t ime standard for this job, we need Mr. Singh to do this job of
handling 5-6 kg. boxes at various speeds. From the energy cost data collected
on him, we can select the speed of working that gives an energy cost of 300 W.
So, by keeping the energy cost of the two jobs same, the time standard (the
number of 5-6 kg. boxes/min.) can be determined.