appreciation of the zone of convenient reach by naive operators performing an assembly task

Industrial Ergonomics

ELSEVIER International Journal of Industrial Ergonomics 19 (1997) 187-199

Appreciation of the zone of convenient reach by naive operators performing an assembly task

John Lim, Errol Hoffmann * Department of Mechanical and Manufacturing Engineering, The University of Melbourne, Parkville, Victoria, 3052, Australicl

Received 11 February 1995; revised 10 October 1995

Abstract

Little research has been done relating the layout of an assembly workplace in terms of the 'zone of convenient reach' (ZCR) to occupational health risks to the operators. Four groups of ten subjects were asked to assemble hacksaws under four experimental conditions. In three conditions subjects were free to arrange their own workplace, with each condition representing an improvement in terms of using a jig for assembly and being given instructions on proper use of the jig. The fourth condition used an ergonomically designed workplace where all parts were kept within the ZCR. Subjects had no practical experience in assembly work and were from non-engineering backgrounds. A video recording system was used to record the performance times, hand movements, body posture and strategies in assembling the hacksaw. Results showed significant learning between the first trial and the rest of the 20 trials for all the experimental conditions. The results also showed that an improved layout of the workplace would potentially produce the following benefits: (i) Reduced o~cupational health risks to the operator from back injury problems through safer use of hand movements and body posture. (ii) Increased productivity for the employer through more economical use of hand movements.

Relevance to industry

An improved workplace design, achieved through use of an assembly jig, instructions on proper use of the jig and layout of components within the zone of convenient reach, can reduce work times and also reduce occupational health risks. Occupational health risks may be reduced by the operator having less contralateral movements and less use of two hands to pick single parts. Time is reduced through use of more simultaneous motions.

Keywords: Zone of convenient reach; Assembly times; Occupational health

1. Introduct ion

Assembly is the technical term used commonly in manufacturing industries for putting parts together in

* Corresponding author.

the final stages of production lines. The technology of assembly has been assuming an increasingly important role in manufacturing industries, prompted by constant drives for higher efficiency and productivity. Even though some processes in assembly lines have been automated to a varying degree, manual assembly is still indispensable in processes where a high degree of manual manipulation is required.

Research on assembly tasks has proceeded along

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188 J. Lim, E. Hoffmann / International Journal of Industrial Ergonomics 19 (1997) 187-199

three main paths: (i) motion and time studies (for example, Chung, 1983; Maynard et al., 1948), (ii) generation of assembly sequences (De Fazio and Whitney, 1987; Homem de Mello and Sanderson, 1989; and Lui, 1988), and (iii) cognitive models of assembling tasks (Baggett and Ebrenfeucht, 1988; Baggett and Ehrenfeucht, 1991; Fish, 1993; Van Santen, 1970; and Wilde, 1978). None of the research to date has measured how an assembler would set up his /her own workplace if given the opportu- nity and the productivity and occupational health implications of so doing.

Awkward working postures can lead to discomfort or injury. According to studies by Keyserling et al. (1993), working posture is determined by the interaction of many factors in the workplace such as the workstation layout, visual demands, hand tool design, anthropometric characteristics of the workers and the individual work methods. Low or extended reaches may involve significant trunk flexion while lateral reaches may require axial twisting (Corlett and Bishop, 1976;Grandjean, 1988; Keyserling et al., 1988). High or far reaches may involve significant elevation of the shoulder (Grandjean, 1988; Keyser- ling, 1986; Ulin et al., 1990). Little research has been done relating occupational health risks to the operators to the layout of the assembly workplace in terms of the zone of convenient reach (ZCR).

One factor of importance in determining the operator's posture and use of hands is the design of the workplace and specially the layout of the parts and location of work within the workplace. Pheasant (1986) defined the zone of convenient reach as a space in which an object may be reached conve- niently without undue physical exertion. This is the maximum working area for the right and left hands, working separately, and for both hands working together (Barnes, 1980). The maximum working area for the fight hand is determined by an arc drawn with a sweep of the right hand across the table, with the arm pivoted at the right shoulder. The maximum working area for the left hand is determined in a similar manner by an arc drawn with a sweep of the left hand. The area formed within these two maximum arcs constitutes a zone beyond which two- handed work cannot be performed without causing considerable disturbance of posture, which may be accompanied by excessive fatigue. Within this maxi-

mum working area is a much smaller 'normal working area', described by a comfortable sweeping movement of the upper limb about the shoulder, with the elbow flexed to 90 degrees or a little less (Phea- sant, 1986).

The purpose of the present work was to determine operators' awareness of the zone of convenient reach in a simple assembly task in which they were required to design their own workplace, and to determine whether their personal designs would produce any physical problems due to excessive reaching and awkward body postures during the assembly work.

2. Method

2.1. Experimental design

Four conditions under which the assembly task was performed were designed for this experiment. These conditions are summarized in Table 1 and detailed in the following. The code for the condition is of the form xxx where each 'x ' represents, in order, the presence of an assembly jig, instructions on how to use the jig, the presence of an experimenter-designed component layout [Yes (Y) or No (N) for each condition]. 1. Condition NNN: the subjects did not use the jig

and could assemble the hacksaws anywhere on the surface of the workbench. No instructions were given on how to assemble the hacksaws. They were free to use their own methods and were free to lay out all the parts to their convenience for assembly work. For this, and the following two conditions, the subjects were allowed to change the position of components at any time during the 20 trials.

Table 1 The four experimental conditions used in the assembly experiment

Condition Was jig Was subject Were all the parts code used? instructed how ergonomically

to use the jig? arranged?

NNN No No No YNN Yes No No YYN Yes Yes No YYY Yes Yes Yes

J. Lira, E. Hoffmann/International Journal of lndustrial Ergonomics 19 (1997) 187-199 189

2. Condition YNN: the same as condition NNN c=) except a jig was provided. Subjects were told they handte must use the jig but were not told how to use it. ~ - - - ~ ~"

3. Condition YYN: the same as condition YNN except the subjects this time were told how to make proper use of the jig. Contours of components were drawn on the jig so that the subjects ..... knew quickly where to fit the components onto the jig.

4. Condition YYY: the same as YYN except all the co) components were placed within the normal working and zones of convenient reach of the I ~la~e [ areas

I I subjects: an attempt was made to provide the operator with an ergonomically designed set-up. All components were within the ZCR of the 5th percentile female population (using data from hanc~te

Pheasant, 1986). Thus for all subjects, the individual ZCRs were greater than the reach distance to any component.

2.2. Subjects

blade

I * *

?ra~e

b o l t .~'~gm~,

ml ~ ~ ~Qshel" //

?taMe

Fig. 1. (a) The hacksaw components. (b) A typical layout of the workplace for hacksaw assembly using a jig in condition YYN.

Twenty-two female and eighteen male subjects took part in the experiment. All subjects were right- handed (necessary because of the jig design), without any kind of practical experience in assembly work, from non-engineering background (no knowledge of ergonomics or the setup of assembly workstations) and aged from 18 to 42 years old. They all reported to be in good health and were without any physical disability. A different ten subjects took part in each of the four conditions; they were allocated to groups by the order in which they responded to advertise- ments for taking part in the experiment.

Measurements were made of the subjects' anthropometric data relating to calculation of their zones of

convenient reach and normal work areas. These were: (a) distance from the center of the hand to the underside of the elbow (center of the hand was determined by the subject gripping a small cylinder), (b) distance from the elbow to the acromion, (c) distance between shoulders (biacromial breadth). (a) and (b) were measured for both right and left arms. Table 2 shows the mean length and standard deviations of each measurement for male and female subjects. The Zones of Convenient reach of the subjects was measured experimentally; the data col- lected allowed verification of this ZCR and calculation of the Normal Work Area (NWA) as given in Pheasant (1986).

Table 2 Mean lengths and their standard deviations in mm of subjects' anthropometric data relating to reach envelopes

Female Male

Mean SD Mean SD

Right hand: centre of palm to elbow 309.3 21.0 339.7 35.4 Right hand: elbow to shoulder 303.4 26.2 335.0 29.6 Right shoulder to left shoulder 394.1 31.1 445.6 27.3 Left hand: elbow to shoulder 303.4 26.2 335.0 29.6 Left hand: centre of palm to elbow 309.3 21.0 339.7 35.4

2.3. Apparatus

The apparatus consisted of a video recording system, a work table (height 660 mm × width 750 mm x length 1220 mm) and an adjustable chair, 20 hacksaws and a jig specially designed for hacksaw assembly. An adjustable chair was necessary to sat- isfy the seating posture recommended by Grandjean (1988) for light assembly work that t he working height should be 50-100 mm below elbow height. The hacksaw components are shown in Fig. la. A

190 .L Lim, E. Hoffmann/International Journal of Industrial Ergonomics 19 (1997) 187-199

typical layout of the workplace, as set up by a subject, is shown in Fig. lb for a case where a jig was used in the assembly operation.

2.4. Procedure

The task was explained to the subjects before the start of each experiment and the experimenter went through a set of written instructions. Each subject performed only one condition as learning was being studied. The main instructions for all conditions were concentration on the task, work at maximum but accurate speed and use any order of assembly that the subject thought most suitable.

The subjects in conditions NNN, YNN and YYN were reminded to lay out parts of the hacksaw in such a way that it would be most convenient to them to assemble the hacksaw. All components were placed on a table away from the workbench so that the operator was required to bring them to the workbench and lay them out as he /she thought best, The locations of the parts could be changed at any time during the trial to suit their convenience. Position of the components selected by the subjects was drawn on the table top along with any changes in location made during the trials. Assembly times were measured by a digital clock recorded on the VCR.

The subjects in conditions YNN, YYN and YYY were reminded to use the jig specially designed for assembling the hacksaws. Each subject was seated in an upright position with elbows laying between 50- 100 mm above the working surface of the table (an adjustable chair was used). Each subject was required to perform 20 hacksaw assemblies for each experiment. The subjects were not allowed practice trials before the start of each experiment as the aim was to investigate the learning processes of each subject as a function of experimental trials.

At the beginning of each trial, the subject placed his /her hands at the center of the normal working areas and awaited an 'OK' signal from the experimenter. The task was then to assemble all the parts as rapidly as possible to form hacksaws identical to a sample displayed in front of the subject. The subjects

Table 3 Mean and standard deviations of assembly each experimental condition (conditions are

times of 20 trials for defined in Table 1)

Trial NNN SD YNN SD YYN SD YYY SD

l 156.2 125.2 239.1 180.5 128.0 59.3 116.8 49.1 2 76.7 22.3 90.6 43.8 74.3 32.5 76.7 47.6 3 73.3 39.2 6 5 . 1 22.0 60.8 9.5 61.6 26.6 4 49.8 9.9 46.0 11.5 57.2 18 .9 50.4 18.0 5 48.0 10.3 56.0 28.6 50.3 10 .3 4 4 . 1 11.3 6 44.4 10.6 46.0 1 3 . 0 43.3 9.4 41.6 11.0 7 39.7 7.6 44.4 7.7 46.8 8.5 42.8 8.0 8 39.7 8.4 5 5 . 1 25.8 45.3 10 .6 42.2 10.5 9 37.4 5.4 47.5 17.1 39.8 7.2 42.2 12.2

10 40.0 5.2 41.4 8.2 41.8 9.2 44.3 6.8 11 37.2 6.8 41.1 7.4 37.8 6.6 39.3 9.0 12 37.6 4.8 43.1 4.8 39.0 7.5 38.2 3.2 13 40.0 13.3 40.8 8.8 45.9 24.4 42.2 7.5 14 39.9 8.0 42.1 12.1 36.1 6.9 37.2 5.9 15 37.2 5.2 39.6 7.3 43.0 20.1 37.1 3.6 16 36.0 8.1 39.5 7.7 37.0 13 .3 37.0 8.6 17 40.1 9.3 37.2 7.4 35.6 8.9 36.6 6.1 18 35.3 6.9 39.0 7.2 33.8 4.9 36.6 7.3 19 36.3 6.3 37.1 6.6 34.5 6.4 37.8 9.0 20 44.8 18.0 36.7 6.8 36.0 8.8 39.8 9.0

were requested to say 'OK' once they finished assembling a hacksaw. The experimenter was seated behind the subject to videotape and monitor the experiment via a TV monitor throughout the approx- imately one hour session. The subjects were given a rest period of 5 minutes whenever it was requested. At the end of each experiment, the subject was asked to draw his /her own ZCR on his /her workplace by drawing two arcs with their right and left arms fully stretched and the torso upright.

3. Results

3.1. Assembly times

Table 3 shows the mean assembly times and standard deviations for each of the four experimental conditions. Panels (a)-(d) of Fig. 2 show the actual assembly times of the four experimental conditions and the learning that took place over the 20 trials.

Fig. 2. Assembly times for each of the four experimental conditions: (a) NNN, (b) YNN, (c) YYN, (d) YYY. Also shown are the MODAPTS times and the regressions obtained by fitting De Jong's model.

J. Lim, E. Hoffmann~International Journal of Industrial Ergonomics 19 (1997) 187-199 191

200

150

o'1

I - - 100

3E

50

0 0

200

150

~E I - - 100

50

(a)CONDITION NNN

M = 0 . 2 3 6 6 n = 0 . 7 4 3 - , = 2 6

"H-~+.

~ L , L ~ I , l , I ~ L L l , I , I L L 2 4 6 8 10 12 14 16 18 20

DE JONG M O D E L T IME ( S E C )

~EXPT +DEJONG + M o d o p f s Times

(c)CONDITION YYN

M = 0 . 2 0 6 1 n = 0 . 6 3 8 - , = 2 2

t -k

0.. . . . . . . . . . . . . . . . . . . . . . -0

I , I ~ I , I , I J t L I t I , I ~ I 2 4 6 8 10 12 14 16 18 20

DE JONG MODEL TIME ( $ E C ) ~EXPT +DEJONG ÷ M o d o p t s Times

250

200

u•" 150 v L,J

~ 100

50

2°° I 150

~- 100

50

0 0

(b)CONDITION YNN

M = 0 . 1 9 8 9 n = 1 . 1 3 0 u = 3 3

~- . . . . . . . . . . . . . . . . . . . . . -.4,

, I , I L I i q i I i I i I i I , I , i I 2 4 6 8 10 12 1i4 16 18 213

DE JONG M O D E L TIME ( S E C )

~EXPT +DEJONG + M o d a p t s Times

(d)CONDITION YYY

M = 0 . 3 0 4 5 n = 0 . 8 5 8 a = 3 1

$- . . . . . . . . . . . . . . . . . . . . . -4,

L I , I , I , I , 1 , I , [ , t , L , I

2 4 6 8 10 12 14 16 18 20

DE JONG M O D E L TIME ( S E C )

~EXPT -FDEJONG + M o d a p t s Times

192 J. Lim, E. Hoffmann/International Journal of Industrial Ergonomics 19 (1997) 187-199

Also shown is the time predicted by the predetermined motion time system MODAPTS (Heyde, 1975) for this assembly task. The MODAPTS time assumes that the operators are skilful in doing the assembly tasks.

A four-factor mixed-model ANOVA of assembly times, with subjects nested under assembly condition, was computed for 20 trials × 4 experimental

conditions X 10 subjects. The analysis of variance of assembly times showed significant effects of Trials (F(19,684) = 35.01, p < 0.01) and significant interaction between Trials and Experimental conditions (F(57,684) = 1.60, p < 0.01).

Post-hoc tests, using the Newman-Keuls procedure, showed that there was a significant difference between the first trial and the rest of the 20 trials,

100

L.d

(~ I_d

I--

>- --J n,"a

i , i t n

.<

80

6O

I 40

20 lo.~

0 0

(NnJI~LGI 0~111L.~rr 1o 'rmALI)

~27.e ( A ~ 01111 III tlIIALII)~

I 1~I

20 40 60 80 100

TRIALS

Assembly Times-'Pred. Times(MODAPTS)

Fig. 3. Times for assembling hacksaws. This subject achieved the minimum assembly time predicted by MODAPTS at his 84th trial.

j. Lirn, E. Hoffmann / International Journal of Industrial Ergonomics 19 (1997) 187-199 193

implying the need for clear instructions at the beginning of each experiment especially for subjects in condition YNN as seen in panel (c) of Fig. 2. When asked to do extra trials, one subject in condition YYN achieved the assembly time predicted by MODAPTS at his 84th trial (Fig. 3).

The averaged assembly times of the last five trials showed that an improved layout of workplace de- creased the assembly times realtive to condition N N N from 0.6 seconds (for condition YNN) to 3.1 seconds (for condition YYN). A one-way A N O V A of the last five trials showed that these differences were not statistically different ( p > 0.05).

The learning that took place in each condition was quantified using the De Jong (1957) model given by,

T s = T , M + T,(1 - M ) / S ~ = a + b / S "

in which T s is the time at the Sth trial, T 1 is the time for the first trial, M is the 'factor of incompressibil- ity' (a = T I M is the minimum time after extensive learning), n is the exponent related to the rate of

60

z bJ

cd

40

20

NNN YNN

,oo 80

/ / / / / / /

i / / 1 f /

f i i f /

I t / / / i / / / /

I I /

t t / /

I / /

i i /

i I / / /

" I J

YYN YYY

EXPERIMENTAL CONDITIONS

IIHandle E:::lFrarne I~]Blade

Fig. 4. The percent of parts laid outside the personal ZCRs by the subjects across the four experimental conditions for heavy and long parts (handle, frame and blade).

w o_

1 0 0

80

60

40

20

5 i J J J i i f i i I i

i if'A

I

5 i I

I I I I I I I f

i I I

i

I i

i I

f /

t / / -

NNN

f f

i

I

i /

YNN YYN YYY


IHandle I~lFrarne ~181ade

Fig. 5. The percent of parts outside the ZCR of 5th percentile female population by the subjects across the four experimental conditions for heavy and long parts.

learning. The regressions for the four conditions are shown in panels (a)-(d) of Fig. 2 for the different experimental conditions. In these regressions, the asymptotic value was found by trial and error as that value which gave the maximum r 2 value for the regression. The results were:

NNN: T s = 26 + 8 3 . 9 / S ° 743; r 2 = 0.82,

YNN: T s = 33 + 133/S113; r 2 = 0.90,

YYN: T s = 22 + 8 4 . 8 / S ° 6 3 8 ; r 2 = 0.91.

YYY: T s = 31 + 7 0 . 8 / S ° 858; r 2 = 0.91.

Tests were made to determine whether the amount and rate of learning the task was different for the four conditions. These t-tests showed that the potential amount of learning for each condition (Tj - a )

194 J. Lim, E. Hoffmann~International Journal of Industrial Ergonomics 19 (1997) 187-199

was significantly different between the YNN condition compared with the YYN and YYY conditions ( p < 0.05). This was due to the very high initial assembly times for the YNN condition. The rate of learning, expressed by the exponent in De Jong's equation, was significantly different between a number of the conditions. Most rapid learning occurred with the YNN condition (for which initial times were longest); least rapid with the YYN condition. Note that the subject groups were different in each case. The following exponents were significantly different: NNN < YNN ( p <0.01) ; YYN < YNN (p < 0.001); YYN < YYY ( p < 0.01) and YYY < YNN (p < O.O2).

3.2. Layout of components relative to personal zone of convenient reach and that of the 5th percentile female

In designing a workplace such as this where it might be expected that a large range of the poulation

20

15

F3

~ 1 o ~" k,J

~- 4:'1 N:.I

Nt.L

ti:: F , NNN YNN YYN YYY


nScrew L-'qBoH []Washer [--IWingnuf

Fig. 6. The percent of parts laid outside the personal ZCRs by the subjects across the four experimental conditions for small and light parts (screw, bolt, washer and wingnut).

100

bJ o_

80

60

40

0 NNN

I YNN YYN YYY


[]Screw k'~Bolf ~:~Washer r-]Wingnuf

Fig. 7. The percent of parts outside the ZCR of 5th percentile female population by the subjects across the Ibur experimental conditions for small and light parts.

could be used as an assembler, the layout would be ideally arranged for use by the small woman. Only with this layout could it be expected that the majority of workers would be comfortable with the body motions required for assembling the various components. Thus in this work we studied the subject's layout relative to two criteria: that in relation to their own zones of convenient reach and that for the smallest assembly worker. The latter was of interest in that it was possible to place all components within the ZCR of the 5th percentile female (data from Pheasant, 1986) and hence to reduce the amplitude of the movements even for the worker of larger stature. It might be expected that with such a design the times for reaches and moves by the larger worker would be reduced.

Fig. 4 compares the percentage of large and heavy components (handle, frame and blade) laid outside their personal ZCR by the subjects across the four experimental conditions. Note that for the YYY condition, the workplace was designed so that all corn-

J. Lira, E. Hoffmann/International Journal of Industrial Ergonomics 19 (1997) 187-199 195

ponents were within the 5th percentile female ZCR and hence, for this group of subjects, were all within their personal ZCR. Fig. 4 also shows that the use of the jig had an important effect in keeping parts within the ZCR. Although many of the components were placed within the personal ZCR, most were placed outside the ZCR of the 5th percentile female (as calculated from the data in Pheasant, 1986), indicating that the subjects set up the workplace for their own comfort of reaching, but using more of the available space than was necessary (Fig. 5); smaller reaches for components could have been utilised in the overall personal design of the workplace.

A similar pattern is seen for the placement of the smaller and lighter components (screw, bolt, washer and wingnut). As these components were in small boxes, it was easier to place them close to the assembly jig and again the presence of the jig tended to reduce the number of components that were placed outside of the personal ZCR (Fig. 6). Many of the components were however placed outside the ZCR of the 5th percentile female population (Fig. 7),

NNN YNN YYN YYY


mHandle []Screw ~:~lBlade

Fig. 8. Increasing percent of using left hands for picking the LH-orientated parts with the improved layout of workplace.

.<

100

80

60

40

20

0 NNN YNN

EXPERIMENTAL

/

/

/ / 1

J

/ --1 /

- " i:il!

/

5 ~ :1 /

/ i.iJ /

J . 4 / - 4 /

/ ~ iii

/

/ "

/

/ "

YYN YYY

CONDITIONS

I~Frame k~Bolt FlWasher r-JWingnuf

Fig. 9. All the right-handed subjects had the tendency to pick up the RH-orientated parts with their right hands for small and light parts.

illustrating the importance of using ergonomic data when designing workplaces for manual assembly.

3.3. Hand use Jbr right- and left-hand orientated parts

The correct use of both hands could have far-reaching effects on both productivity and the occupational health of the operator, especially for the workplace where repetitive manual tasks are common. These would be achieved through the decrease in amplitude of reaches and through less twisting of the torso in order to reach components when the correct hands are used for obtaining components.

The arrangement of the jig for fight-handed operators, combined with what appeared to be a logical order of assembly, indicated that the left-hand orientated components were the handle, screw and blade. Fig. 8 illustrates that the presence of the jig, use of instructions and ergonomic design of the workplace

196 J. Lim, E. Hoffmann~International Journal of Industrial Ergonomics 19 (1997) 187-199

each contributed to the proper use of the left hand for picking the left-hand orientated parts.

The frame, bolt, washer and wingnut of the hacksaw were considered to be right-hand orientated according to the design of the jig. Since all the subjects were right-handed, it is not surprising to see (Fig. 9) that these parts were picked up mostly by the right hands of the subjects. Note however that there is an assumed best order of assembly for the hacksaw in this calculation; if the subject developed a different strategy, then the component may have been correctly used by the left hand in the particular arrangement used.

3.4. Simultaneous hand motions

In designing tasks, industrial engineers emphasize the importance of using simultaneous hand motions for economy of motion in obtaining components. In this experiment, subjects were not instructed as to how they were to move and thus it was interesting to see whether they did use the strategy of obtaining several components at the same time.

50

40

50

~" 20

o ~ NNN YNN YYN YYY


mHandle [~:;]Frame ~]Blade

Fig. 10. The occurrence of simultaneous motion for heavy and long parts.

30

25

2O

15

10

0 NNN

~J

~J o_

YNN YYN YYY


[ ]Screw k--~ Boll [~Washer ]-]Wingnuf

Fig. 11. The occurrence of simultaneous motion for small and light parts.

Subjects were found to use more simultaneous motions for the heavy and long parts (the handle and the frame as seen from Fig. 10) in the NNN condition where no jig, instruction nor design of ergonomic workplace were present. The availability of more workspace in the absence of a jig could have facilitated the subjects to carry out the simultaneous motion of picking up two parts at one time. It would appear that instruction on method of assembly is important to make use of more economic motions.

On the other hand, subjects were found to use more simultaneous motions for the small and light parts for the YYY condition, as shown in Fig. l l, but the percentage of such movements is low. Again, this result illustrates the importance of instructions on the most economical method of assembly.

3.5. Uneconomical hand motions

Subjects were often found to use both hands to pick up a single component, even though the racks

J. Lira, E. Hoffmann~International Journal of Industrial Ergonomics 19 (1997) 187-199 197

40

30

~- 20

lO

o ,

NNN YNN YYN YYY


~Handle E]Frame ~Blade

Fig. 12. The occurrence of uneconomical motion across the four experimental conditions for heavy and long parts.

holding the parts were designed so that they could easily use either of their hands to pick up. Using two hands to pick up a single component under the experimental condition was considered to be both uneconomical for productivity and unhealthy to the operator's body, due to the twisting of the torso required for such reaches.

Fig. 12 shows that the subjects were more likely to carry out this type of uneconomical motion under the NNN condition for the heavier and longer parts. It is interesting to note that such a problem did not exist for the smaller and lighter parts.

3.6. Contralateral movements of the hands

Frequently, the subjects were found to use their right hand to pick up parts on their left-hand side and to use their left hand to pick up parts on their right-hand side. These contralateral movements could expose the subjects to serious back health problems over a long term, due to excessive trunk rotation, as

well as reducing productivity. The subjects leaned forwards and backwards as well as twisting their body to the left and right during these movements.

Fig. 13 indicates that, in particular, the use of a jig helped to reduce the contralateral movements for handling the heavier and longer parts. Fig. 14 shows that the use of instructions reduced greatly the subjects' contralateral movements for handling the smaller and lighter parts. The reason for the improvement is unknown at this stage. However, a marked increase in contralateral movements was found in the YYY condition. This is likely to be due to the fact that these small parts were placed directly in front of the subjects in an effort to keep all parts inside the ZCR of the 5th percentile female; the subjects could have found it easier and more convenient or even more economical to exercise contralateral movements rather than having to move their elbows backwards in order to pick up the parts directly in front of the body. This indicates a deft-

6O

50!

40 /

/

30 ~

20 -" i

I I

J

I I I

NNN YNN YYN YYY


~Hondte E3Frarne I~lBlade

Fig. 13. The occurrence of contra-movements for heavy and long parts.

198 J. Lira, E. Hoffmann~International Journal of Industrial Ergonomics 19 (1997) 187-199

50

40

3O

Z L~

W

~- 20

It i E7

NNN 'NN YYN YYY


[]Screw k'qBolt [~Washer [-IWingnuf

Fig. 14. The occurrence of contralateral hand movements for small and light parts.

ciency in the so-called ergonomic design of the workplace.

4. Discussion

Even though none of the subjects had practical experience of assembling hacksaws (or any other assembly task), they, nevertheless, learned fairly quickly. All subjects settled down to a constant time by the tenth trial even though other measures (not discussed in this paper) suggested that they were still searching for the best sequence or method to complete the assembly task. An improved layout of the workplace brought about a marginal decrease in assembly times implying higher productivity.

The more relevant issues to address in this paper are the layout of the parts by the subjects in each experimental condition and the use of hands in performing the assembly tasks. These two issues are important for the occupational health of the operators and for achievement of higher productivity.

The results suggested that the operators did not appreciate the concept of the ZCR when they were allowed to design the layout of their workplace. The majority of them decided, for reasons known only to them, to place the heavier and longer parts outside their ZCR without realising the adverse consequences on their health and productivity of their work. Their lack of knowledge and lack of appreciation of the concept of the ZCR meant that they had to fully stretch out their arms. Very often, they also had to lean their body forward to reach and pick up the parts and then move their body backwards to resume normal body working posture for the assembly task on the jig.

In the real-world situation where the operators are doing manual repetitive assembly tasks for continu- ous mass production, the idea of keeping most parts, if not all, within the ZCR is not a luxury but a necessity. If unchecked, the operators could cause themselves serious back problems in the long run, such as development of local muscle fatigue, ten- donitis and other cumulative trauma disorders (Keyserling, 1986). The results suggested that the solution to the above problem is to use a jig, tell the operators how to use the jig correctly and to keep all parts if possible within the ZCR of the 5th percentile female population. In the event that not all parts can be placed inside the ZCR, it is better to place the heavier and perhaps longer parts inside the ZCR as this layout would bring about a reduction of work in transporting mass, as well as reducing the uneconomical use of two hands for the heavier parts outside the ZCR.

The second advantage brought about by the improved layout of the workplace was the more economical use of right and left hands as well as the lesser risk of back injury. The results suggested that an improved workplace would enable the operators to use both right and left hands safely and more economically. The subjects would use their right hand only to pick up parts placed on the right-hand side and use their left hand only to pick up parts placed on their left-hand side.

The third benefit from the improved workplace was the reduction of the uneconomical motion of using two hands for picking up a single component when the component could have been picked up by a single hand, while the other hand could have picked

J. Lim, E. Hoffmann~International Journal of lndustrial Ergonomics 19 (1997) 187-199 199

up other nearby parts to achieve s imul taneous mo-

tion. The results showed that the subjects, as a whole, did not appreciate the economica l advantage of exercis ing s imul taneous mot ion even though there

were opportunit ies for them to do so. The fourth benefi t f rom the improved workplace

was the reduct ion of contralateral movemen t s which are less product ive and which could potential ly cause

back injury problems to the operators such as back pain and local muscle fatigue (Keyser l ing et al.,

1993),

5. Conclusion

The results of the exper iment clearly indicated

that an improved layout of the workplace in which a proper j ig was used, instruct ions about use of the j ig

were g iven and all parts kept wi thin the zone of conven ien t reach, was ins t rumenta l in reducing the potential occupat ional health risks to operators. As well, there was an increase in product ivi ty relative to the control condi t ion where there were none of these

improvements .

An improved layout of workplace enabled the operators to use their hands correctly and safely by

reducing unheal thy lengthy reaches and contralateral movements , thus prevent ing the operators f rom potential serious back injury problems.

An improved layout of the workplace also en-

abled operators to reduce the uneconomica l practice of us ing two hand to pick single parts. It also

benefi ts the employer f rom reduct ion in contralateral movemen t s and increase in s imul taneous mot ions for p icking the smal ler parts. It encourages correct use

of both fight and left hands in terms of economy of

mot ion where the right hand should pick the parts on the r ight-hand side, and vice versa, all of which

would eventua l ly lead to an increase in productivi ty as well as saving employers f rom undesi rable worker compensa t ion bills.

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appreciation of the zone of convenient reach by naive operators performing an assembly task

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