MOTION STUDY - MOTION ANALYSIS
1. Motion study Motion study is involved in studying and recording the motions and movements of body parts which are necessary in a manufacturing process or an operational step. These body parts will almost always be fingers, hands and arms, but may also be head, feet, legs and trunc. For recording these motions we must agree on a notation, letters, characters or symbols, in order to be able to read for ourselves and for others somewhat later in time, what the motions and movements were and exactly how the process or operation was performed. Using this notation we record extensively all the motions and movements of all body parts and their sequence. In the analysis we record the motions and movements which determine the optimal working method, which must be pursued.
2. Motion analysis Motion analysis is a very suffisticated and detailled form of a process analysis or an operation analysis, often even a (small) part of it. For writing down the motion analysis, symbols or letters are used of an internationally recognized system of Predetermined Elemental Motion Time Systems (PEMTS) viz. WORK-FACTOR. Basis for working with Predetermined Elemental Motion Time Systems is the analysis - and documentation method of Frank Bunker Gilbreth (1881 - 1924). In order to perform motion studies Gilbreth divided work into the smallest elements possible, even no more than a simple single (finger) movement. He found that only a small number of catagories (18) were necessary to divide all work. He called these catagories "standard elements" and came up with some simple symbols to denote them, called Therbligs, and be able to write them down very quickly (as is done in stenography). Others used letters to denote these standard elements, and after a period of time almost every country or nation used its own letters to denote the standard elements. In international communications it is custom to use the Therbligs or to use the English letters.
Definition A standard element is a part of an action small enough to be repeated in the same form and shape in various, but all different patterns of work. The WORK-FACTOR Council have adopted this kind of registration technique, though the number of standard elements was slightly reduced and the therbligs were somewhat simplified. These standard elements are given below. The notation, symbols or letters, for the different standard elements is as follows:
1. È É Ê Ì Í Î Ï Ë 2. Õ 3. Ó Ô 4. Ð Ñ Ò
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Manual elements: Transportation Grasp Preposition Assemble Disassemble Release Machine times and process times: Process time Improductive elements: Wait Hold (in principle waiting times!) Mental process MP. amongst which: Eye focus Inspect React
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Reach, R Move, M Gr Pp mechanical assemble, Asy surface assemble, Sasy Dsy Rl PT W V Fo Ins Rn
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Reach
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Move
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Grasp
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Preposition
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Assembly mech.
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Surface assembly
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Disassembly
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Release
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Use
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Wait
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Vice
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Focus
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Inspect
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Reaction
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In total there are 8 standard elements, 3 basic elements, 1 process time and 2 non-productive times, while using 14 therbligs. (You will not see the therbligs on the left if you did not install one of the Work-Factor fonts)
3. Hands analysis Motion analysis of one hand or two hands. A foot, leg or knee may, again, be analyzed in the same way as a hand. In general in the following pages and sites the notation with therbligs will be used. To even shorten the notation more, often the following abbreviations and shortenings are used:
L
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left
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1st thr
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searching for 1st thread
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R
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right
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restr
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restricted
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LH
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left hand
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lm
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leg movement
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RH
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right hand
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t.
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to, towards
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BH
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both hands
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f.
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from
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WA
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work area
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bitf
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bringing inside the fingers
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WS
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work station
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botf
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bringing outside the fingers
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WP
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work place
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(de)palm
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palming/depalming
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W
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weight/resistance
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d.r.
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diameter ratio
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Using 2-hands analysis the movements and ‘waiting’ or ‘vice’ periods of each of the 2 hands are recorded very precisely, while continuously assessing the purpose ánd the efficacy/efficiency of the movements, as to disregard superfluous (parts of) the movements in the analysis. Do not analyse what you see, but analyse what you should see idealiter. Be aware of “automatic” analyses of filmed tasks. It must be noted that with “hand” also foot or leg could be meant and can be analyzed as such.
Below the results of an example is shown of a 2-hands analysis which was executed using therbligs-symbols. Each therblig represents 1 unit of time.
Example On a table stands an empty bottle, tapered corks along side on a heap and a measuring glass filled with a liquid, ready to use. The action is as follows: “Take the measuring glass and check the correct level of the fluid, pour the liquid into the bottle and close the bottle with a tapered cork. Put glass and bottle on the table again.”
a. b. c. d. a. b. c.
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Firstly, this is done using 1 hand, the RH, and the bottle is situated in the middle of the work area. Secondly, this is done again, but the other hand, the LH, is helping the RH to take the empty botttle and keep ready, so seemingly 2-handed. The 3rd time, it is executed “really” 2-handed: both hands do as many movements as possible on the bottle. The 4th time, both hands will work simultaneously filling 2 bottles: genuine working 2-handed.
A 2-Hands analysis shows the following In this case we conclude that “19 units” were needed to execute the work “fill 1 bottle”. It is also concluded that this work can be executed by a person with just one arm or hand. Mind the social work areas and the like, for the disabled workers. Filling 1 bottle with 2 hands, while the LH is just holding the bottle and acts like a vice, shows that the RH is still doing all the work; so, still “19 units” were needed to fill 1 bottle. Now, the LH will do as much as possible, so “really” 2-handed. The job can now be executed in “14 units”, but still fill 1 bottle.
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We established that the optimal working method must be a 2-handed execution of the process. Because moving both hands will result in a better balance and posture. Also, this way of working results in less time needed, therefore results in a higher productivity and a higher ouput. Introducing the LH in case b), it will be noticed that the total performance of the process in a) is a mere 50%, viz. half of the total number of “standard elements” consist of “waiting elements”, of the LH. In case b) it is noticed that 13 of the 38 elements are ‘waiting’ elements: so the performance is just 65%. In case c), the real 2-hands execution of the task, it is noted that 'waiting’ still occurs 5 times out of a total of 28 elements: so a performance of 82%. This leads to the idea to fill a bottle also with the LH.
d. So, as in case a), letting the LH also fill a bottle, will result in filling 2 bottles in “about 19 units”, so filling 1 bottle in “about 10 units” representing 100%.
It will be noticed that the savings in time are about 45 à 50%.
It can be stated that work processes that had no industrial engineering attention for the last couple of years on methods, on means, circumstances and organization, can often be improved easily by 25% in terms of time and consequently in output.
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4. Work-Factor In the courses and trainings on one of the systems of WORK-FACTOR, it is assumed that the participant is already familiar with 2-hands analysis as a part of work study. If needed and on request this work study part will be discussed and trained preceding the WORK-FACTOR course.
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