Enhancing Productivity

Enhancing productivity of Repetitive Manual Assembly operations - Use of Principles of Motion Economy and Low Profile Automation --- S.V.Prabhu A lot of progress has been made in Automating various manufacturing operations in the last few years resulting in increased productivity . This increase has been done at a cost which does not give uniform rate of returns when the same technology is used in different places, because of differences in wages and other input costs. Most of these technologies have been cost effective where wages are high or where manpower constraints are present. They are less cost effective in low wage areas or where manpower is available in plenty. The other benefits of such productivity increasing technologies like consistency of quality, lesser wastage , fringe benefits like better use of power resources , better deliveries etc. are therefore denied to those who are unable to take advantage of these technologies. The human component in any manufacturing is a very important one. No level of automation will be able to displace the human component fully. Nor it is desirable that this happens, since , one important aim of any manufacturing activity is to generate employment. A lot of efforts have been made in designing automation systems . Most of these systems have been carefully engineered to increase parameters like space utilization, reliability, and to reduce energy usage and unnecessary time consuming activities like movements ( of materials etc.). This diligent effort , over the last number of years, has resulted in the development and availability of a large range of generalized automation devices and modules like Industrial robots, pick and place units, Programmable controllers, various types of Pneumatic accessories etc. which can be used to develop systems for specific applications. Using these modules for specific applications needs a careful analysis of the operations which are sought to be either fully automated or partially automated. The aim of any such exercise would be to increase the productivity of any man-machine system as well as to get some of the other benefits accruable . Most of the bench type manual operations found in the assembly of products like electronic items, electrical switchgears, small consumer products etc. are unfortunately difficult to automate fully as they have a large content of ‘skill’ elements which require the dexterity and flexibility of the human hand. Quite a number of these operations also require a decision making ability which only a human has ( though systems like Camera inspection have tried to reduce the human involvement in areas like inspection ). This article is an attempt to develop some operational strategies to be used in enhancing the productivity of such bench type repetitive manual operations. Some basic Industrial …. Cont Pg 2

Page 2 Engineering principles have been found to be very useful in understanding the nature of such operations. Application of these principles has been also found useful in deciding the extent of automation to be incorporated in order to obtain a cost effective system.

NATURE OF BENCH TYPE REPETITIVE OPERATIONS: Bench type repetitive operations are typically operations which are completed in a few minutes and have to be repeated over and over again . An example is the assembly of 2 screw with a retaining washer on a cover for a motor starter. This operation is repeated maybe several times for the same starter depending upon the number of screw assemblies required per starter. The entire operation may take a couple of minutes. A process analysis with an adhoc workstation layout as in Fig 1 for this operation will be :

1. 2 3 4 5 6 7

Pick up screw, Pick up starter cover, Insert screw in cover hole, Pick up retaining washer, Press retaining washer onto screw, Repeat operations 1-5 for all the screws required for the cover, Place cover with screw and retaining washers aside for the next operation.

An examination of the activity elements involved in a typical assembly operation , like the one above, will show that most of the elements can be segregated into the following categories: 1. TRAVEL elements like REACHING to an object like say the Cover at 1, MOVING an object like the cover to a position to perform further operations, 2. a) HOLD elements like holding the cover in hand while the other operations are performed on it. Such a HOLD is the simplest form . b) HOLD element where some PRESSURE is applied like when the operator has …..cont.Pg 3

Page 3 to exert pressure while pressing the screw into the retaining washer, 3. SKILL elements like GRASPING the screw or washer to get a hold on it or POSITIONING the screw in the hole of the cover. These elements require some amount of finger dexterity , 4. DECISION elements like when the operator has to decide whether to use a screw with some defects. Distances through which the objects have to be moved in the course of the assembly operations also contribute to the cycle time. For some typical light engineering repetitive bench type assembly operation the broad break up of the above elements would be : 1. TRAVEL elements : REACH & MOVE ……. App. 54 %, 2. SKILL elements : GRASP ……………….. App. 23 %, POSITION ……………. App. 17 %, 3. OTHERS ………………………………………. Rest ( From samples taken on light electrical switchgear assembly operations. The composition will change for other products , but, the pattern will remain the same ) Improving productivity of such operations would therefore entail reduction of the time taken for these micro elements . CHART 1

BASIC

ELEMENTS

TRAVEL

LINEAR

CURVI-

GRASP

LINEAR

DECISIO N

HOLD with Pressure

SKILL

POSITION

With LOAD

W/O LOAD

With LOAD

W/O LOAD

SLIDING

OTHERS

PRECISE

LOOSE

A1

A2

B1

B2

C1

C2

D1

D2

….cont..Pg 4

E

F

Page 4

METHODOLOGY TO ENHANCE PRODUCTIVITY A strategy to improve productivity of such operations would therefore rely on the following principles: 1. MINIMISE distances through which the hand has to move to get parts and through which they have to be moved, 2. Use of HOLDING fixtures to free the hand to perform other operations, 3. Use of SIMULTANEOUS motions ( both left & right hand) , 4. Use of CURVILINEAR motions in lieu of straight line movements with mechanical stops where a motion has to be ended, 5. Use of NORMAL ergonomic working area to prevent overstretching , 6. Use of LOW PROFILE AUTOMATION (LPA) devices to perform some of the SKILL elements. Chart 1 above shows the broad break up of the various motions found in such operations. PRE-DETERMINED MOTION TIME SYSTEMS ( PMTS) have been generated and have been widely used to understand the nature of these motions. PMTS systems are basically tabulated values of normal time to perform the above basic motions. The time values have been decided based on a large number of observations and are decided by the following factors : 1) The distance traveled between 2 points while performing any motion, 2) The position and nature of the target point, 3) The load amount to be moved along with its orientation at the start and end of the motion, 4) The state of the motion at the beginning and end. These systems have been successfully used to: 1) Design new work stations, 2) Get an estimate of the cycle time for an operation before the start of the activity, 3) Analyse existing operations with a view to improvising. Of all PMTS systems used MTM has proven to be universally applicable . PMTS , used as an analytical tool, will be able to give a micro picture of the operation under study. This picture can be very useful to decide the strategies required for improvisation. Referring to the chart above the following course of actions become fairly evident: Case A1 & A2-Where possible eliminate the linear travel motions and convert to Curvilinear motions ( Case B1 & B2) using simultaneous motions to maximize use of both the hands. Use of Low Profile automation devices like pneumatic transporters, pick up arms can then be made. ….Cont..Pg 5

Page 5 Case C1 & C2—Convert complex Grasps to simple Sliding grasps using designed Work station storage bins. Use simultaneous motions . Use of LPA can then be explored. It should be noted that use of LPA for automating the SKILL elements pose more Engineering problems and can be costlier. Case D1 & D2—Using LPA devices for pre-positioning will make final manual Positioning easier. Case E- This element puts more fatigue on the operator as manual strength is called for . In the long run it is better to use some LPA devices to perform this motion. Case F- To develop automaticity or rhythm in manual work it is always useful to Minimize Decision elements. These can be a source of stress on the Operator, thereby effecting output.

The following diagrams illustrate the progression of the station layout ( Fig 1) to a layout where the strategies given above have been used. Stage 0: ( Fig 1) The cycle time for the operations as per the Adhoc layout given in Fig 1 based on PMTS analysis is app. 10 sec. Stage 1: (Fig 2) The layout given in Fig 2 has no LPA devices. The layout has been rearranged to minimize distances Grasping of the retaining washers has been simplified. Gravity chute has been added to carry the sub-assembly to the next station. The cycle time drops to app 6 sec. Stage 2: (Fig 3) The layout given in Fig 3 has been evolved from Fig 2 by adding Vibratory feeders to orient the Screws for easier grasp. The cycle time drops to app.5 sec. Stage 3: (Fig 4) Pneumatic cylinders have been added to the layout in Fig 3 to give layout as in Fig 4. The task of pressing the screws through the retaining washers is made easier. The cycle time comes down to app 3.8 sec. ……Cont. Pg 6

Page 6

…..cont pg 7

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CONCLUSIONS: Proper engineering of Work station layouts , using appropriate productivity improvement aids can contribute appreciably to making manual work more productive. Some of the benefits like consistency in quality, better space utilization, and better control over output can be achieved with a studied approach to designing work stations. Redesigning existing work systems involve change. Change is normally resisted. However change , for resistance to be minimal, has to be gradual to allow time for assimilation. Such ‘evolutionary’ changes have to be planned in stages with time in between for assimilation and with time in between for proper trouble shooting of problems and hurdles that may come about. Use of PMTS along with the Principles of Motion Economy , as can be seen from the above example, can give nearly 40 % reduction in cycle time. This phase is the cheapest with maximum benefit and can be easily engineered. Further improvements with LPA tends to be comparatively costlier with lesser benefit.

S.V.Prabhu Mob:9820002950

E.mail: [email protected]

B.Tech From I.I.T.-Kanpur 15 years in Larsen & Toubro in various departments. Free lance consultant for productivity improvements since 1989. Designer & manufacturer of Special Purpose Equipments

With LOAD