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Tractor noise
operations, such as felling, de-limbing, cutting, dragging or walking at the work place, shows no significant variation. The only essential differences between the official statistics and the near accidents concerned 'kick-back of the power saw' and 'falling when walking'. Officially, about 50% of de-limbing accidents is due to kick-back, yet the study showed only 30%. Falling, however is much more frequent among the near accidents.
Vibration Agricultural and forestry conditions were also discussed in the paper by C. W. Suggs of North Carolina State University, USA, on the vibration of machine handles and controls and its propagation through the hands and arms. Intermediate and high frequency vibration between 25 Hz and 16 000 Hz is present in many power driven tools and machines. Workmen operating these devices come into contact with the vibration primarily through the hands and to a lesser degree through the feet. Vibration in this range is associated with a disorder in the hands. Motor-driven chain saws were chosen for study because particularly intense vibrational levels of 40 g r m s are commonly encountered in the handles of these tools. Although most saws operate at engine speeds of about 100 Hz (6 000 rpm), the most intense vibration is usually found at higher frequencies, often in the kHz range. Isolation of the handle from the body of the saw by an experimental rubber mounting block arrangement resulted in significant vibration reduction at all except the lower frequencies. Selection of mounting blocks to completely isolate the handles from the lower frequencies requires mounts with such low spring rates (soft) that directional control of the saw is jeopardized. Since various parts of farm tractors have been observed to vibrate, the steering wheel was also selected for study because operator's hands are exposed to this element for long periods of time. While the levels are not nearly as high as for chain saws, peak values of 5 g rms were common.
Tractor noise The risk of hearing loss by full time tractor drivers was examined by R. W. Tomlinson of the National Institute of Agricultural Engineering. By assessing typical noise levels with various tractor tasks, ranging from 86 dBA when haymaking to 92 dBA when ploughing, and the n u m b e r of hours spent driving or exposed to noise from other
machinery, it was estimated using the Equivalent Continuous Sound Level method that tractor drivers are exposed to a noise level of 88 dBA. Hearing loss for a given exposure time and noise level can be predicted. After making allowances for the facts that tractors are not run continually under loads and engine speeds leading to maximum noise, nor will drivers tolerate maximum levels and therefore often operate tractors below the theoretical maximum power, it was proposed that manufacturers should aim to achieve a test noise level of not more than 90 dBA.
Foot controls The session on motor actions included a paper by K. H. E. Kroemer of Wright-Patterson Air Force Base, USA, on the foot as an input device for a control operation. It included a comprehensive bibliography of work in such fields as perception and control of leg and foot motions, forces applicable to pedals, speed of activation of pedals, speed and accuracy of foot motions between pedals, and the effectiveness of pedal operation versus hand control operation. Despite the work listed, the author concludes that very little is k n o w n about the possibilities of using feet for control operations. Human engineering handbooks generally suggest that the feet are not as suitable for rapid or exact operation of controls as the hands. In industry, as if to support this, an operators feet are often idle or performing only simple tasks while his hands may be overburdened. However, the feet perform vital control operations in cars and aircraft. Fortunately some of the quoted experiments indicate that the feet can perform certain tasks accurately and fast, and Mr Kroemer thought that further research into the biomechanics of leg and foot control would lead to more control tasks being assigned to the feet. Lifting strength Some work on the prediction of the maximum load that can be lifted from a measurement of back muscle strength was described by E. Poulsen of the Danish National Association for Infantile Paralysis. Lifting should be performed with the back in a straight position to distribute the load evenly on the surfaces of the vertebrae and discs. The strength of muscles in the back limit the load that can be lifted with the hands in a forward inclined position. Back muscle strength can be expressed in the pull exerted on a dynamometer in an attempt to extend the back in a standing position. A
formula for load prediction is then: Load = 1"4 Force - 0"5 body weight. Seats and the spine Two papers were presented from Japan on seating design. Y. Yamaguchi and F. Umezawa of the Japan Institute for H u m a n Posture Research pointed out that modern life presents an increasing n u m b e r of sitting-hours. This has contributed to the weakening of the function of h u m a n skeletal and muscular systems and to the increase in low back pains. From an examination of the relationship of disc distortion with sitting, the author showed that with the seat - backrest angle within 105 ° , the discs did not show a neutral state whatever the seat inclination, and with the seat horizontal, disc neutral state was difficult to maintain at any seatbackrest angle. With the seat inclined at 5 ° and 10 ° and the seat-backrest angle at 115 °, the discs began to come into a neutral state. With the seat inclined at 15 ° and the seat-backrest angle at 110 ° , and with the seat inclination at 20 ° and the seat backrest angle at 105 °, the discs began to show a neutral state. The authors concluded, therefore, that seat inclination plays the most important part in minimising disc distortion. Seat height, length and angle, and a subjective assessment of comfort, were discussed by M. Oshima, (J.I.H.P.R.) who concluded from his experiment that a small backrest angle makes a short seat more comfortable, and that a low seat is more comfortable with a larger seat angle but there is less room for seat height variation. The best combination of values for comfort was: bacldength 435 mm, backrest angle 20 ° O seat angle 6 , seat length 420 mm.
Requirements of the disabled At a specially convened session, the problems of the disabled were discussed. This is a most important area for ergonomics work, meriting considerably greater attention. A. Isherwood of the B.R.A.D.U. Department of Health, L o n d o n , spoke of work in the design of wheelchairs. A prototype had been produced costing only 2 0 - 3 3 % of the price for conventional chairs. Special problems assessed included ability to climb steps or negotiate rough ground; stability, especially when traversing slopes; size adjustment; and seat comfort, especially to prevent the thighs being forced together by seat sag. He emphasised that a greater psychological approach to'the patients social needs was required. For example, a 'roller walker' under development consisted essentially of
Applied Ergonomics September 1970
253
Vibration Agricultural and forestry conditions were also discussed in the paper by C. W. Suggs of North Carolina State University, USA, on the vibration of machine handles and controls and its propagation through the hands and arms. Intermediate and high frequency vibration between 25 Hz and 16 000 Hz is present in many power driven tools and machines. Workmen operating these devices come into contact with the vibration primarily through the hands and to a lesser degree through the feet. Vibration in this range is associated with a disorder in the hands. Motor-driven chain saws were chosen for study because particularly intense vibrational levels of 40 g r m s are commonly encountered in the handles of these tools. Although most saws operate at engine speeds of about 100 Hz (6 000 rpm), the most intense vibration is usually found at higher frequencies, often in the kHz range. Isolation of the handle from the body of the saw by an experimental rubber mounting block arrangement resulted in significant vibration reduction at all except the lower frequencies. Selection of mounting blocks to completely isolate the handles from the lower frequencies requires mounts with such low spring rates (soft) that directional control of the saw is jeopardized. Since various parts of farm tractors have been observed to vibrate, the steering wheel was also selected for study because operator's hands are exposed to this element for long periods of time. While the levels are not nearly as high as for chain saws, peak values of 5 g rms were common.
Tractor noise The risk of hearing loss by full time tractor drivers was examined by R. W. Tomlinson of the National Institute of Agricultural Engineering. By assessing typical noise levels with various tractor tasks, ranging from 86 dBA when haymaking to 92 dBA when ploughing, and the n u m b e r of hours spent driving or exposed to noise from other
machinery, it was estimated using the Equivalent Continuous Sound Level method that tractor drivers are exposed to a noise level of 88 dBA. Hearing loss for a given exposure time and noise level can be predicted. After making allowances for the facts that tractors are not run continually under loads and engine speeds leading to maximum noise, nor will drivers tolerate maximum levels and therefore often operate tractors below the theoretical maximum power, it was proposed that manufacturers should aim to achieve a test noise level of not more than 90 dBA.
Foot controls The session on motor actions included a paper by K. H. E. Kroemer of Wright-Patterson Air Force Base, USA, on the foot as an input device for a control operation. It included a comprehensive bibliography of work in such fields as perception and control of leg and foot motions, forces applicable to pedals, speed of activation of pedals, speed and accuracy of foot motions between pedals, and the effectiveness of pedal operation versus hand control operation. Despite the work listed, the author concludes that very little is k n o w n about the possibilities of using feet for control operations. Human engineering handbooks generally suggest that the feet are not as suitable for rapid or exact operation of controls as the hands. In industry, as if to support this, an operators feet are often idle or performing only simple tasks while his hands may be overburdened. However, the feet perform vital control operations in cars and aircraft. Fortunately some of the quoted experiments indicate that the feet can perform certain tasks accurately and fast, and Mr Kroemer thought that further research into the biomechanics of leg and foot control would lead to more control tasks being assigned to the feet. Lifting strength Some work on the prediction of the maximum load that can be lifted from a measurement of back muscle strength was described by E. Poulsen of the Danish National Association for Infantile Paralysis. Lifting should be performed with the back in a straight position to distribute the load evenly on the surfaces of the vertebrae and discs. The strength of muscles in the back limit the load that can be lifted with the hands in a forward inclined position. Back muscle strength can be expressed in the pull exerted on a dynamometer in an attempt to extend the back in a standing position. A
formula for load prediction is then: Load = 1"4 Force - 0"5 body weight. Seats and the spine Two papers were presented from Japan on seating design. Y. Yamaguchi and F. Umezawa of the Japan Institute for H u m a n Posture Research pointed out that modern life presents an increasing n u m b e r of sitting-hours. This has contributed to the weakening of the function of h u m a n skeletal and muscular systems and to the increase in low back pains. From an examination of the relationship of disc distortion with sitting, the author showed that with the seat - backrest angle within 105 ° , the discs did not show a neutral state whatever the seat inclination, and with the seat horizontal, disc neutral state was difficult to maintain at any seatbackrest angle. With the seat inclined at 5 ° and 10 ° and the seat-backrest angle at 115 °, the discs began to come into a neutral state. With the seat inclined at 15 ° and the seat-backrest angle at 110 ° , and with the seat inclination at 20 ° and the seat backrest angle at 105 °, the discs began to show a neutral state. The authors concluded, therefore, that seat inclination plays the most important part in minimising disc distortion. Seat height, length and angle, and a subjective assessment of comfort, were discussed by M. Oshima, (J.I.H.P.R.) who concluded from his experiment that a small backrest angle makes a short seat more comfortable, and that a low seat is more comfortable with a larger seat angle but there is less room for seat height variation. The best combination of values for comfort was: bacldength 435 mm, backrest angle 20 ° O seat angle 6 , seat length 420 mm.
Requirements of the disabled At a specially convened session, the problems of the disabled were discussed. This is a most important area for ergonomics work, meriting considerably greater attention. A. Isherwood of the B.R.A.D.U. Department of Health, L o n d o n , spoke of work in the design of wheelchairs. A prototype had been produced costing only 2 0 - 3 3 % of the price for conventional chairs. Special problems assessed included ability to climb steps or negotiate rough ground; stability, especially when traversing slopes; size adjustment; and seat comfort, especially to prevent the thighs being forced together by seat sag. He emphasised that a greater psychological approach to'the patients social needs was required. For example, a 'roller walker' under development consisted essentially of
Applied Ergonomics September 1970
253