A Multilevel Approach to Manual Lifting in Manufacturing Industries

Transcription

1 INTERNATIONAL JOURNAL OF OCCUPATIONAL SAFETY AND ERGONOMICS 1996, VOL. 2, NO. 3, A Multilevel Approach to Manual Lifting in Manufacturing Industries Berman Kayis Kamal Kothiyal The University of New South Wales, Australia Musculoskeletal injuries are often the consequences of wrong postural configurations used during Manual Materials Handling (MMH). This eventually leads to a large payout of worker's compensation and loss of production time. A simulated study of back injury risks has been carried out on seven selected manufacturing industries to identify and evaluate harmful working postures. For each MMH task, Ovako W orking Posture Analyzing System (OWAS) codes have been identified w ith the help of motion study pictures. Also, Chaffin's biomechanical model was used to calculate L5/S1 load compression values on the spine during MMH activities. The m ultilevel approach adopted was a com bination of OWAS and Chaffin's biomechanical model. The application of a digitizer enabled us to identify the coordinates and it made a subsequent evaluation of the angles of each body link possible. manual lifting manufacturing back pain OWAS 1. IN T R O D U C T IO N Manual handling is considered the main cause of workplace injuries. In Australia, during , there were 14,478 new major manual handling claims as noted by Work Cover, New South Wales, Australia (Work Cover Authority of New South Wales, 1995). A new major claim is a claim that entered the insurers computer system in that time period and resulted in a fatality, permanent disability, or temporary disability where 5 or more working days were paid for total incapacity. These claims accounted for almost a quarter of all major claims and increased by 17% over the previous year. These claims accounted for over 33% of the total time lost (time lost is calculated for temporary disability cases only and does not include those resulting in 3 or more years off work) and almost 25% of the total gross incurred cost (GIC; this is the sum of all payments made to date, and for claims still active at the end of the year. It includes all estimates of future liability.) The total compensation payments for manual handling during were $189 million. This figure represents 14% of all payments for the year. Sprains and strains accounted for 87% of the manual handling claims. Table 1 summarizes the nature of injury and disease statistics of Over 66% of sprains and strains and over 68% of musculoskeletal problems involved the back (Table 2). Manual handling claims were most frequent from the manufacturing industry (23.4%) in Australia, followed by community services (21.6%) and retail trade (11%; Work Cover Authority of New South Wales, 1995). As a result, back pain is the greatest single cause of work time loss in Australian industrial organizations. Similar statistics have also been seen in other countries, including the United States and the United Kingdom. It is claimed that overexertion due to lifting, pushing, pulling, and carrying objects accounts for 27% of all industrial injury compensations and illnesses in the United States. Resulting strain or sprain injuries account for over 50% of compensation claims in many industries. Almost two thirds of these injuries involve lower back pain, with Correspondence and requests for reprints should be sent to B. Kayis, The School of Mechanical and Manufacturing Engineering, The University of New South Wales, PO Box 1, Kensington, NSW 2033, Australia

2 252 B. KAYIS AND K. KOTHIYAL TABLE 1. Injury and Disease Statistics in (%) Injury or Disease Males Females Persons (*) Sprains and strains Hernia Occupational Overuse Syndrome Musculoskeletal Fractures and Dislocations Other reported compensation and medical payments totaling well over 1,000 million dollars annually. An estimated 300,000 workers are affected in the United States each year, 5% to 10% of whom will have a permanent disability, often making them unemployable (Mathews, 1992). The reduction of the number of back injuries in workplaces has been a primary concern of ergonomists for the last several decades. The National Institute for Occupational Safety and Health (NIOSH, 1981) developed a work practices guide to manual lifting to recommend safe weight limits. This has now been revised and includes work situations involving twisting and so on (Waters, Putz-Anderson, Garg, & Fine, 1993). The National Occupational Health and Safety Commission (NOHSC, 1990) in Australia developed a National Standard for Manual Handling. It also published a Code of Practice for Manual Handling to provide a framework for risk identification and assessment in manual handling. The objective of the study carried out among several New South Wales, Australia, manufacturing companies was to provide a subjective and objective rating of MMH tasks resulting in harmful working postures. A multilevel approach was adapted by using a combination of OWAS (Karhu, Kansi, & Kuorinka, 1977) and Chaffin s biomechanical model (Garg & Chaffin, 1975) together with Borg s scale (Borg, 1978) in order to identify and evaluate harmful working postures in several manufacturing industries. Workplace and work task modifications for improvements were also proposed. TABLE 2. Bodily Location of Injury and Disease Statistics in Injury or Disease and Bodily Location Males Females Persons (*) Sprains and Strains Back Shoulders Upper Limbs (excluding shoulder) Lower limbs Other Occupational Overuse Syndrome Shoulders Upper Limbs Other Musculoskeletal Back Upper Limbs Other

3 A MULTILEVEL APPROACH TO MANUAL LIFTING METHODOLOGY Manual handling tasks in several manufacturing industries were analysed by using the OWAS (Karhu et al., 1977) posture analysis system and the 2D Static Strength Prediction Program (Garg & Chaffin, 1975). The Borg (1978) scale was used to determine workers overall subjective perception of the level of risk involved. The analysis of the data gathered from an Occupational Disease hospital (Kayis & Kothiyal, 1992,1993) revealed that the percentage of male and female patients who suffered from back pain was 84.7% and 15.3%, respectively. Lifting was found to be the major cause of back pain among several MMH tasks in manufacturing. The results obtained from the analysis of hospital data helped us to select manufacturing industries where manual handling tasks had been a prime cause for back injuries. A total of more than 100 MMH tasks were considered and 25 tasks that involved excessive lifting activities were selected for the study. First, all MMH tasks were videotaped. These recordings were analyzed frame by frame to identify tasks that involved harmful or awkward postures, or both. Also, OWAS codes and action categories were determined for all tasks. The OWAS method is based on work sampling in which the time spent in each posture can be noted. The working postures are then sorted into a classified system: OWAS codes. The system is targeted at the postures of the back, upper limbs, lower limbs, and also force category. A code number is given for each of these body parts (Karhu et al., 1977). OWAS codes were obtained from video films of the task and also from Chaffin s biomechanical model output reports. Twenty-five MMH tasks were selected for further analysis. Each was simulated in the sagittal plane with retroreflectors (markers) fixed to the various joints on the participant s body. The setup in the laboratory had facilities for taking fast video films with reflectors on. High-speed video recordings were made of a participant performing symmetric two-handed lifts of various loads from floor level to a carrying position. These reflectors (markers) were then picked up by mouse on the VDU screen. Table 3 lists the shape and position of the markers. The position of reflectors on the necessary regions of the body served as identifiable spots on the computer screen for digitizing. The reflectors were used during the simulation stage. Digitizing is the process of finding out the coordinates of the reflector points with respect to a TABLE 3. Position and Shape of Markers Marker Position and Shape of Marker 1 Left and right big toe, using a single-sided box 2 Lateral side of the head of the fifth metatarsal on both feet, using a small sphere 3 Lateral side of the heel on both feet, using a small sphere 4 Lateral malleolus on both feet, using a small sphere 5 Head of fibula on both legs, using a large sphere 6 Lateral condyle of the femur of both legs, using a large sphere 7 Right and left greater trochanter, using a flat circle 8 Right and left anterior superior iliac spine, using a triangle box 9 Right and left posterior superior iliac spine, using a single-sided box 10 Immediately below the acromion process on both shoulders, using a large sphere 11 Lateral condyle of both elbows, using a small sphere 12 Right and left ear lobes, using a small flat circle 13 Forehead and back of head, using double-sided boxes placed on a head band

4 254 B. KAYIS AND K. KOTHIYAL fixed origin. A black screen with grids in the background helped in better reflection. The entire simulation had been done on a platform right in front of the grid. A 1,000-watt spotlight placed by the side of the camera provided distinct identification of the reflectors. The coordinates were known from the from the computer hooked to the video camera. These coordinates were located with respect to a known origin. From the coordinates, the angles made by each limb were evaluated and these became the input values for Chaffin s biomechanical model. Reflectors of various shapes were used because of the ease with which they can be fixed to different parts of the body. Because the video camera was fixed on a tripod stand and it filmed from the right side (sagittal plane), mainly the reflectors fixed to the right side of the participant s body were picked up. Actual weights were not used in the simulation because of the difference in participant characteristics. The stature of the participants varied from 153 cm to 188 cm with a mean of 174 cm (±11.2) for males, and 161 cm (±6.5) for females. Body weight also varied from 48 kg to 99 kg with a mean of 79 kg (± ) for males and 61 kg (± 10.1) for females. Three out of 25 participants were females. The simulation was done for posture with dummy weights and in Chaffin s biomechanical model, the actual weight lifted was entered in Newtons. The Chaffin s biomechanical model reveals the severity of the effect of load on males and females. One of the most important results of the calculation procedure is the compression loading of the spine at L5/S1, which enables an estimate of injury risk, based on experimental values of spinal segments. The model takes into account the masses of body segments, allows for different body sizes, and indicates strength limitations at each joint. The following details follow each particular lift: 1. Lifting posture of the person lifting the load. 2. Calculation of lifting parameters (horizontal distance between the center of the ankles and the center of gravity of the load, hand distance to lumbar region (L5), vertical location of the hands at the start of the lift). 3. Back compression values. 4. Percentage capability of lifters with respect to the elbow, shoulder, L5/S1 joint, hip, knee, and ankle. A total of more than 100 MMH tasks were videorecorded from several manufacturing industries according to the results of the study carried out by the authors (Kayis & Kothiyal, 1993). Several cycles of each task were observed and 25 of those considered harmful were selected for further analysis. Brief task descriptions of the tasks are given in Table RESULTS From Table 5, it is clear that the tasks under OWAS, Action Category IV, immediate action constituted 64% of all tasks. Action Category III and II accounted for 20% and 16% of the tasks, respectively. Detailed analysis of the postural analysis made using the OWAS technique is also given in Table 6. A subjective rating scale, called the Borg Scale Rate of Perceived Exertion was used to note the subjective difficulty of each tasks (Borg, 1978). The range of the scale is between 6 and 20. Six corresponds to a heart beat of 60 beats/min and 20 to 200 beats/min. The tasks are rated as follows: 6 No exertion at all 7-8 Extremely light 9-10 Very light Light Somewhat hard Hard Very hard 19 Extremely hard 20 Maximal exertion

5 A MULTILEVEL APPROACH TO MANUAL LIFTING 255 TABLE 4. Task Descriptions of Selected Manual Materials Handling Tasks Task Code Task Descriptions A, AA, AA1 D ie polishing. (O n e person w orks with two hands, lifting the die sets with frictional grip. T h e difference between A, AA, A A 1: the loads vary in each of these tasks, the operation is th e s a m e (21 kg s A A A s 36.8 kg s AA1 s 50 kg). The task also involves pulling and pushing trolleys with 12 to 15 die sets.) B, B 1,B 2 D ie assem bly for rollers. (For task B, 7 3 kg is the weight o f the die set lifted for a fraction of a second during die assem bly. For task B1, a die weighing approxim ately kg w as rolled for 6-8 m on the groun d.) C D E Rectifier straightening operation. Cutting and examining. T h e cutting and examining of long alum inium pieces. (Tw o persons, one at eac h end o f the long piece, carried out the task all day.) F, FA, FB Die assem bly with different harm ful postures but with the s am e load (W e ig h t lifted during these tasks is approximately 36.8 kg. Tasks are carried out by a single person.) G H, I J N 6 m rolled length packing. D ie rew orking. (T a s k is carried out by a single person. W eight lifted is approxim ately 2 9 kg. T he difference between H and I is in posture.) Press 5, heavy manual lifting, lowering. Aluminium strips loaded onto cartons. (T w o persons w ork, one holding at each end o f th e strip, all day. Load lifted is approximately 15 kg at each end.) 0 Loading o f alum inium strips for anodising. (Load lifted by a person is 15 kg. It is an all-day continuous task.) M M A C able roll packing. (Task is perform ed by one person. Fasteners are being screw ed in the bent posture. Drilling operation by hand drill gun is involved for the big wooden wheel.) M MB Rolling of cable roll. (Task is carried out in the bent posture of the trunk, by a single person.) M M C Stencilling the address of the m akers on the side o f the cab le drum. NOR-B N O R -C N O R -D N O R -E Pouring powder into a hopper. (A single person working with two hands lifts 50 kg.) Lifting. (The task involves lifting a hollow shaft of outer diam eter 100 cm, and 1500 cm long.) Lifting. (This task involves the rem oval of individual abrasive belt rolls from the spindle. W eight lifted is approximately 25 kg.) Abrasive belt production. Accordingly, the results of the study revealed that the lowest rate of perceived exertion is 13 and the highest is 19; 32% of the tasks have been rated 19 on the Borg Scale; 16% and 20% are rated 18 and 16, respectively. The average Borg Scale rating is 17, which is very high (SD = 2.2). The back compression values are given in Table 7. The range of values vary between 499 N (Task 0) to 8,897 N (Task Bl). The average back compression value is 4,266 N (SD = 1,901) for males and 3,033 N (SD = 1,838) for females. As seen in Table 7, a wide range TABLE 5. Percentage Split Up of OWAS Action Categories Total Number of Tasks Studied Number of Tasks Under Different OWAS Action Categories I II III IV % 0% 16% 20% 6 4 %

6 256 B. KAYIS AND K. KOTHIYAL TABLE 6. OWAS Codes, Action Categories, and Borg Scales for Each Task T a s k T im e (m in :s ) T r u n k (T ) U p p e r L im b (U ) O W A S C O D E L o w e r L e g F o rc e (L ) (F ) B o rg S c a le O W A S A c tio n C a te g o r y A 1 1 : A A 1 1 : AA1 1 1 : B 0 8 : B1 0 8 : B : C 0 7 : D 0 5 : E 0 2 : F 0 4 : F A 0 4 : FB 0 4 : G 0 2 : H 0 0 : I 0 3 : J 0 3 : N 0 4 : : M M A 1 0 : M MB 03: M M C 0 4 : N O R -B 0 2 : N O R -C 0 4 : N O R -D 0 5 : N O R -E 0 5 : Average SD of loads is being handled under MMH. Any back compression value in excess of 3,500 N crosses the action limit (AL). AL is a calculated value for the mass of a load that can be safely lifted in given circumstances by at least 99% of men and 75% of women (NIOSH, 1981). The AL is exceeded in 48% of the tasks analyzed and 12% of the tasks exceed the maximum permissible limit (MPL) of 6,500 N. Only 25% of men and 1% of women are estimated to be able to regularly withstand this force without injury (NIOSH, 1981). A detailed analysis of each lift provided the lifting parameters, such as horizontal distance between the center of the ankles and the center of gravity of the load (H), hand distance to lumbar region (L5-hand distance), and vertical location of the hands at the start of the

7 A MULTILEVEL APPROACH TO MANUAL LIFTING 257 TABLE 7. Force Parameters, Posture Data, and Back Compression Values of Tasks Force parameters Posture Data/Body Link Angle Back Compression (N) Task Magnitude (N) Direction n Lower Arm Upper Arm Torso Upper Leg Lower Leg Males Females A AA AA B B E F FA FB H N O MMA MMB MMC NOR-B NOR-C NOR-D NOR-E lift (V; Section 2.0). Posture data/body link angles for the lower arm, upper arm, torso, upper leg, and lower leg are also given (Table 7). L5-to-hand distance (cm) is a very important contributor to back compression and is directly proportional to it. Hence, this distance should be as short as possible but with a caution that the posture should also be simultaneously such that there is not much bending of the trunk. Referring to task B l, although distance of L5-to-hand is 39 cm, the back was bent to a great extent (angle of 79 to vertical or 11 to horizontal), which resulted in high low-back compression. Minimum and maximum L5-to-hand distance was found as 18 cm (Task B2) and 81 cm (Task N), respectively. The L5-to-hand distance was greater than 40 cm in 60% of the tasks (Table 8). The horizontal distance (H) between the center of ankles to center of grasp must be small for low-back compression. Back compression is directly proportional to it. Task MMA had the largest H value of 117 cm. In this task, the operator s trunk is almost horizontal (18 to ground, Table 8). The vertical distance (V) of 75 cm the location of hands at the start of lift is accepted as ideal for low-back compression. But, as seen in Table 8,65% of the tasks have low V values, which forces the bending of the trunk while lifting the load. Tasks B2, F, MMB, and MMC are especially impossible to carry out without tremendous bending (Table 8). The data for body link angles along with the human figure in Chaffin s model helps in easy understanding of the posture. Torso angle, if it is toward zero, contributes a maximum to increasing back compression. For the 25 MMH tasks investigated, the torso angle ranged from 0 (Task MMB) to 82 (Task 0). The least back compression may be achieved with a magnitude of force as low as possible if: The lower arm tends to -90. The upper arm tends to -90. The torso tends to 90.

8 258 B. KAYIS AND K. KOTHIYAL TABLE 8. Lifting Parameters of the Manual Materials Handling Tasks (cm) Task H L5-to-Hand V A AA AA B B B C D E F FA FB G H I J N MMA MMB MMC NOR-B NOR-C NOR-D NOR-E The upper leg tends to 90. The lower leg tends to 90. The angles are all measured with respect to the pivot about which a particular link rotates and anticlockwise direction is positive (Figure 1). A sample angle calculation is given in the Appendix. a rm Lower arm angle angle Upper leg angle 9 2 ' Figure 1. Posture of Alcan task A.

9 A MULTILEVEL APPROACH TO MANUAL LIFTING 259 TABLE 9. Comparison of OWAS, Borg Scale, and the Biomechanical Approach for Risk Assessment of Tasks Task OWAS Borg Biomechanical A Risky Risky Notrisky AA Risky Risky Risky AA1 Risky Risky Very risky B Risky Very risky Very risky B1 Very risky Very risky Very risky B2 Very risky Very risky Risky C Not risky Notrisky Not risky D Not risky Notrisky Not risky E Not risky Not risky Risky F Very risky Very risky Risky FA Very risky Very risky Risky FB Very risky Very risky Risky G Not risky Not risky Not risky H Very risky Risky Risky I Risky Risky Notrisky J Notrisky Not risky Notrisky N Very risky Risky Risky O Very risky Risky Notrisky MMA Very risky Very risky Risky MMB Very risky Very risky Risky MMC Very risky Very risky Risky NORB Very risky Very risky Risky NORC Very risky Very risky Risky NORD Very risky Very risky Risky NORE Very risky Very risky Not risky In the case of Tasks H, FB, and O, the torso angles are 3, 4, and 0, respectively, which indicates that the torso is almost horizontal, causing a high moment or leverage detrimental to the spine (Table 7). The lower leg angle among all tasks has been a maximum of 90 for Task NOR-C and a minimum of 28 for Task AA1. For stability, this angle should not exceed 90. If it is 180 or more (Task MMC), a squatted working posture is observed (Table 7). The upper arm under normal conditions is at -90. Anything other than -90 creates a condition of abduction resulting in static loading. Maximum abduction is in Task MMA (-34 ) and minimum abduction is in Tasks NOR-D and NOR-E (-134, Table 7). The lower arm angle at anything other than 90 is an abducted condition. The minimum is in the case of Task E (-85 ) and the maximum is 46 (Task A, Table 7). A comparison of the results obtained using the OWAS technique, the Borg Scale, and the biomechanical approach is summarized in Table 9. As seen in Table 9, the majority of the results of risk assessment (80%) are in agreement with each other. Different risk assessments are obtained for five tasks (A, E, I, O, NOR-E). The OWAS and Borg Scale techniques, which are subjective techniques, are in agreement with each other for each task but differences occur between the subjective and the biomechanical approaches. This may mean that biomechanically the tasks are safe, but in the experts subjective assessment, they need corrective action. 5. RECOMMENDATIONS The multilevel approach used in the study has given both subjective and objective ratings of a posture for a particular task. The results were discussed at the workplaces and teams were formed for necessary long- term improvements aiming toward zero-lifting. Some of the proposals made to the teams included the following:

10 260 B. KAYIS AND K. KOTHIYAL A redesign, the use, or both, of the components, trolleys, dies, and tools. A possible elimination of frictional grip, using pockets if possible. The layout of the plant, especially the location of die reworking and assembly sections to minimize pulling and pushing trolleys. Training workers on safe MMH, and problem identification and solution. APPENDIX: SAMPLE ANGLE CALCULATIO N TASK A TABLE 10. Lower Leg Angle to Ground (With Respect to Malleolus Pivot) TABLE 11. Upper Leg Angle to Ground (With Respect to Femoral Condyle Pivot) Coordinates of Row Column Coordinates of Row Column Malleolus Femoral condyle Head of fibula Greater trochanter Difference VL = HL = Difference VL = HL = 3.59 TABLE 12. Upper Arm Angle to Ground (With Respect to Acromion Pivot) TABLE 13. Lower Arm Angle to Ground (With Respect to Elbow Pivot) Coordinates of Row Column Coordinates of Row Column Acromion Elbow Elbow Wrist Difference VL = HL = Difference VL = HL = TABLE 14. Torso Angle to Ground (With Respect to Psis Pivot) Coordinates of Row Column Psis Acromion right Difference VL = HL = REFERENCES Borg, G.A.V. (1978). Subjective aspects of physical and mental load. Ergonomics, 21, Garg, A., & Chaffin, D.B. (1975). A biomechanical computerised simulation of human strength, A IIE Transactions, 7(1), Karhu, O., Kansi, P., & Kuorinka, I. (1977). Correcting working postures in industry: A practical method for analysis. Applied Ergonomics, 8, Kayis, B., & Kothiyal, K. (1992). Analysis of MMH tasks in various manufacturing industries I. In E. Hoffman & O. Evans (Eds.), Proceedings o f the 28th Annual Conference o f the Ergonomics Society o f Australia. Melbourne (pp ). Kayis, B., & Kothiyal, K. (1993). Application of revised NIOSH guideline and OWAS to MMH tasks in a cable manufacturing industry. Ergonomics in a Changing World, Perth, ESA,

11 A MULTILEVEL APPROACH TO MANUAL LIFTING 261 Mathews, J. (1992). Health and safety at work. Sydney: Pluto. National Institute for Occupational Safety and Health. (1981). Work practices guide fo r manual lifting (DHHS Publication, No ). Akron, OH: American Hygiene Association. National Occupational Health and Safety Commission. (1990). National standard fo r materials handling and national code o f practice fo r manual handling. Canberra: Australian Government Printing Service. Waters, T.R, Putz-Anderson, V., Garg, A., & Fine, L. (1993). Revised NIOSH equation for design and evaluation of manual lifting tasks, Ergonomics, 26, Work Cover Authority of New South Wales. (1995). Workers compensation statistics. Sydney: Author.