Sunday, June 12, 2016

Principles of Motion Economy - Some More Details - R.M. Barnes

Use of the Human Body

1. The two hands should begin as well as complete their motions at the same time.

2. The two hands should not be idle at the same time except during rest periods.

3. Motions of the arms should be made in opposite and symmetrical directions and should be made simultaneously.

Barnes has written that the three principles can be examined or discussed simultaneously. He gave the example of bolt and washer assembly. In the old method bolt is picked up by the left hand and then a lock washer is picked up by the right hand placed on the bolt. Next, the right hand picks up a steel washer and placed on the bolt. Then a rubber washer is picked up the right hand and placed on the bolt. The completed assembly is disposed of in a container placed at the left side of the operator. As we can see two hands are moving simultaneously. One is holding the bolt and the other hand is doing picking and assembling work. In the revised method, a fixture is made that holds two bolts and has recess. The operator has 7 bins before him. He first pickups a rubber washer from bins numbered one on left and right. Actually the bottom of the bins slope toward the work area at a 30 degree angle so that materials are fed onto the work area by gravity. The operator slides the washers into the recesses of the fixture. Then steel washers are slided. The the lock washer is slided into position. Two bolts are picked and dropped into the fixture. Then both bolts are removed and disposed of in bins placed on the right side as well as left side. In the return motion they pick the new rubber washers.
You can visualize how all three principles are applied in the new design.
See video clip easier way produced by General Motors in 1946 illustrating these principles.

4. Hand and body motions should be confined to the lowest classification with which it is possible to perform the work satisfactorily.

Classification of Hand-motions
1. Finger motions
2. Wrist motions
3. Forearm motions
4. Upper arm motions
5. Shoulder motions (This class of motions results in disturbance of the posture)

Barnes highlighted that in one investigation it was found that finger motions were more fatiguing, less accurate, and slower than motions of the forearm. The evidence points out that th forearm is the most desirable member to use for light work, and that in highly repetitive work,motions about the wrist and elbow are superior to those of the fingers or shoulders.

Bending has physiological cost. A study made by Barnes et al. on picking bricks from a platform 5 inches above the floor and another platform 37 inches above the floor to place them on a bench 33 inches high showed that both energy expenditure and heart beat were high when picking up bricks from 5 inch high platform. These sort of experiments are to be conducted by industrial engineers in their motion studies to improve the comfort and health of operators.

5. 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.

In certain tasks, it is possible to employ momentum of the hand, the tool or the part being moved to do useful work.  Where a forcible stroke is involved, the motions have to be arranged such that the stroke is delivered with the greatest momentum. In the tasks where the momentum must be overcome by the worker's muscles, momentum must be reduced to a minimum by decreasing the weight of the tools and parts because it causes fatigue.

Barnes quoted Gilbreth's motion study. In laying a brick wall, if the bricks are conveyed from the stock platform to the wall with no stops, the momentum can be made to do valuable work by assisting to shove the joints full of mortar. If instead of being utilized, the momentum must be overcome by the muscles of the bricklayer, fatigue will result. The idea case is to move the brick in a straight path and make the contact with the wall to overcome the momentum.

Barnes quoted another example of candy dipping. He pointed out that  the piece tobe dipped was submerged under the surface of the melted sugar by the right hand at the end of a long return stroke of the hand and the momentum developed in the movement of the hand was employed in doing useful work instead of being dissipated by the muscles of the dipper's arm.

6. Smooth continuous curved motions of the hands are preferable to straight-line motions involving sudden and sharp changes in direction.

Barnes has given the examples of paper holding and dipping of candy to illustrate this point.

7. Ballistic movements are faster, easier, and more accurate than restricted or fixation or controlled movements.

Voluntary movements of the members of the human body may be divided into two general classes or groups: fixation movements and ballistic movements.

In the fixation or controlled movements, opposing groups of muscles are contracted, one group against the other. .

The ballistic movement is a fast, easy motion caused by a single contraction of a positive muscle group with no antagonistic muscle group contracting to oppose it.

The ballistic movement is initiated by an impulse given through the contraction of a muscle, once underway the muscles are relaxed and the course of the movement can not be changed.

The skilled carpenter swinging a hammer in driving a nail illustrates a ballistic movement.

It is not difficult to develop the free, loose, easy movements of the wrist and forearm.

8. Work should be arranged to permit an easy and natural rhythm wherever possible.

Rhythm can refer to the regular repetition of a certain cycle of motions by an individual.
Rhythm which is a proper sequence of motions, assists in making the operation practically an automatic performance - there is no mental effort on the part of the operator.

9. Eye fixations should be as few and as close together as possible.

The work place should be so laid out that the eye fixations are as few and as close together as possible. In one example given in Barnes, the author comments that, had the containers been placed directly in front of the operator, the head movements would have been eliminated entirely and he eye movements would have been greatly reduced.

In another example, Barnes highlights that with practice eye fixations come down and the time required also comes down accordingly. This example is related to the punch-press operation wherein, a part has to be placed in the die using a tweezer. Initially, three eye fixations are used. One for placing the parts in the die, one for taking the part from the left hand with the tweezer and one for taking the part in left hand from the plate having the parts. After 10,000  cycles, only 44 per cent of the time. eye fixation is used pick up the part from the plate having the parts. 56 per cent of the time, the part is being picked up by the left hand without an eye fixation.  The average time for the operation has come down to 0.0258 minutes from 0.0584 minutes. One of the reasons was the decrease in number of eye fixations.

Principles Related to the Work Place

10. There should be a definite and fixed places for all tools and materials.

Definite and fixed places for materials and tools aid the operators in habit formation. This helps in the development of automaticity. It is advantage when operators can perform the operations with the least conscious mental direction. When materials and tools are at fixed places, the hand automatically finds them without support of eyes and the eyes may be kept fixed on the point where the tools and materials are used.

11. Tools, materials, and controls should be located close to the point of use.

For an operators , sitting or standing, the comfortable working place is bounded by lines which are arcs of circles.
The maximum working area for each hand is determined by an arc drawn with a sweep of the hand across the table, with the arm pivoted at the shoulder. In the overlapping area work involving both hands can be done comfortably.

Each hand has its normal working area in the vertical plane as well.

Those tools and parts that must be handled several times during an operation should be located closer to the fixture or working position than tools or parts that are handled but once. For example  if an operation consists of assembling a number of screws into a metal switch plate, the containers for the screws should be placed closer to the fixture than container for the plates as only one plate is to be transported and several screws have to be transported in a cycle.

It is also important to emphasize that parts must be arranged in such a way as to permit the shortest eye movements, the fewest eye fixations, and the best sequence of motions, and to aid the operator in rapidly developing automatic and rhythmic movements.

Interesting example: A radio assembly consists of 260 separate parts/subassemblies. Moving the parts closer by 6 inches saved 34,000 hours per year. which means saving of 17 mandays.

Corollary: The machines, process apparatus, and equipment should be arranged so as to require the least movement on the part of the operator.

When worker operates several machines and when they are located in line along an aise, considerable walking between machines is required. A better arrangement is to have those machines located close together in a group so that the operator can load and unload each of the machines with little or no travel.

12. Gravity feed bins and containers should be used to deliver material close to the point of use.

Bins with sloping bottoms provide the parts at the bottom tray of the bin and operator need not dip into the bin to pick up the part. To provide many different parts, nested bins one above the other are used.
Bins and hopper for process shops -

13. Drop deliveries should be used wherever possible.

Arrangements are to be done to release the finished units from the position is was completed and deliver them to their destination by gravity.
There is significant amount of time involved in manually disposing the finished items. A study of disposing gauging small pins was conducted in this context. The study involed disposing of the pins into a tote box kept as 3 inches behind a fixture, 10 inches behind a fixture and 20 inches behind a fixture.
The time was least when the pins were tossed into the bin 3 inches near the fixture. The therbligs involved were transport loaded and release load. Eighteen percent more time was used when the bin was at 10 inches and 34 per cent more time was spent when the bin was kept at 20 inches distance.

14. Materials and tools should be located to permit the best sequence of motions.

The material required at the beginning of a task cycle should be place next to the point of release of the finished piece in the preceding task cycle.

In the example of assembly of the bolt and washers (points 1,2 and 3) the rubber washers were in bins located nearest to the chute into which assemblies were disposed in the last motion of the previous cycle.

The time for a motion changes based on the preceding or succeeding motion. For example, the time for the motion transport empty is likely to be longer when it is followed by the motion select than when it is followed a well defined motion such as a grasp of a pre-positioned part.

When the motion transport loaded is follwed by a position motion, it is slowed by the mental preparation for the position.

15. Provisions should be made for adequate conditions for seeing. Good illumination is the first requirement for satisfactory visual perception.

Adequate illumination means:
(1) light of sufficient intensity for the particular task,
(2) light of the proper color and without glare, and
(3) light coming from the right direction.

The visibility of an object is determined by the following variables.

# Brightness of the object
# Its contrast with its background
# The size of the object
# The time available for seeing
# The distance of the object from the eye and
# Other factors such as distractions, fatigue, reaction time, and  glare.

All of these factors must be above a limiting value and then a deficiency in one may be compensated by an augmentation of one or more of the others.

 16. The height of the work place and the chair should preferably arranged so that alternate sitting and standing at work are easily possible.

17. A chair of the type and height to permit good posture should be provided for every worker.
Design of tools and equipment

Principles of Motion Economy As Related to the Design of Tools and Equipment

18. The hands should be relieved of all work that can be done more advantageously by a jig, a fixture, or a foot-operated device.

Barnes has written that foot operated equipment is not utilized adequately in methods and tool design (p.223). He also mentions that one company saved 50 percent time on the operation of soldering a wire to the end of flat metal electric static shield by the use of a foot-operated soldering iron. The brief description of foot operated soldering iron was given in the book by Barnes. 

Procter and Gamble designed and built foot control units which rotate the pipe or tube when welder is cutting and welding pipes.

It is also possible to use two foot pedals to actuate different parts of a jig, fixture, or machine by the operator. This arrangement will be similar to the automobile where there are pedals for accelerator, brake and clutch.

19. Two or more tools should be combined wherever possible.

Develop and use two ended tools. It is usually quicker to turn a small two-ended tool end-for-end that it is to lay one tool down and pick up another.  Tack hammer and tack puller, two-ended wrench, and pencila and erasure are good examples. The designer of telephone handset used this idea only when in one unit he arranged both the transmitter and receiver.

At a mid-western electric company two combination tools were developed. One the screw driver and tweezers. The other is a wrench and screw driver.

The multiple-spinder air-operator nut runner for automobile wheels is another good example of combination tool.

20. Tools and materials should be prepositioned whenever possible.

Pre-positioning refers to placing an object in a predetermined place in such a way that when next needed it may be grasped in the position in which it will be used.

Tools are kept in specified holders in the form of socket, compartment, bracket or hanger all the time when there are not in use. They are returned or kept in the same position by the operator after they are used. The design of the holder is to be such that the tool is quickly released into its place. Also, it should facilitate quick grasping for use.

It is important to state again that, the holder of the tool should be designed in such a way that, the tool can  be grasped in the same manner in which will be held while being used.

21. Where each finger performs some specific movement, such as in typewriting, the load should be distributed in accordance with the inherent capacities of the fingers.

22. Levers, hand wheels and other controls should be located in such positions that the operator can manipulate them with the least change in body position and with the greatest speed and ease.

Unless a machine is fully automatic, the amount of work that it will produce depends to some extent upon the performance of the operator. The time taken by the operator to handle levers, hand wheels and other controls has an impact on production quantity.  

The operator should not be required to leave his normal working position to operate his machine. The controls of machines should be placed in such a way that he need not bend over or twist his body in an uncomfortable manner when manipulating  them.

Exhaustive studies were done to indicate good location for levers and hand wheels.

Example: Barnes have given the example of Jones and Lamson CNC Lathe. It was provided with a small control center, located at the front of the machine with an alpha numeric keyboard for instant on-line commands and editing, plus a CRT display. The operator has a greater access to all working areas.

(Source: Ralph M. Barnes, Motion and Time Study Measurment of Work, John Wiley & Sons, New York, 1980, pp.174-236)

All these motion economy principles are included in the book Toyota Kaizen Methods: Six Steps to Improvement  By Isao Kato, Art Smalle in page numbers 93 - 94

Remarks on Textbooks

Marvin Mundel (Motion and Time Study) did not discuss the principles of motion economy exclusively in his book.
In Work Study of ILO, a simple listing of principles as given Barnes is given. But no special discussion was given.
In Nadler, Motion and Time Study, list of 15 principles of motion economy (given by Gilbreth) were given in Table 12-2 by not much discussion was there.


Principles of Motion Economy - Videos

More Detailed Coverage of Variable of Motion Study

A research paper on Principles of Motion Economy
Arm Motions in the Horizontal Plane
A I I E Transactions
Volume 1, Issue 4, 1969
Stephan A. Konza, Carl E. Jeansb & Ranveer S. Rathorec
pages 359-370
From Papers of Gilbreth Library Purdue - Principles and Accompanying Material - 77 pages

Motion Reductions in a Paper Mill

 Workstation Improvements in a paper mill

    *       A machine feeder reduced arm motions from 5000 per day to zero, and output increased from 5000 pieces per day to about 15,000 (300% increase).
    *       Improvements in a paper counting task reduced finger motions from 45,000 per day to near zero, and productivity doubled.
    *       A unique device to tie ribbons eliminated much fastidious hand motions and sustained pinch grips, plus increased output over 30%.
    *       Unconventional tables for a precise, hand-intensive task enabled employees to alternate between sitting and standing, plus eliminated reaches and motions. Modifications in hand tools reduced grasping force.
    *       Mechanical changes and automation in a packing operation reduced hand motions from approximately 32,000 per day to 3200.


Additional Sources

(presentation slides in pdf format of various examples of Barnes)

Classification of human motions by Gavriel Salvendy, 2004 published paper. but was made 35 years back.

Interesting abstract

Contrasting approaches to the analysis of skilled movements.
Hartson, L. D.
Journal of General Psychology, 20, 1939, 263-293.

It is a literature review. In the paper there is a section entitled "Principles of motion economy in the light of movement analysis." Here postural factors, the speed and precision of ballistic movements, the use of gravity and momentum, the advantages of cursive over angular movements and of rhythmical over arhythmical forms, and the importance of emphasizing form in training are discussed. A bibliography of 118 titles is included in the paper.

See the discussion regarding motion economy in (Cabinet making: Start to Finish)

Software for Motion Economy

Generating Economic Motion Plans for Manual Operations - Masters Thesis (Computer Engineering)

Originally Posted by me on Knol 2utb2lsm2k7a/ 2364

Updated  12 June 2016,  26 Nov 2013

1 comment:

  1. So my husband says that only lock washers are truly vibration proof washers. And Isn't that important in making any type of object that will move about?