Effects on load handling of restricted and unrestricted shelf opening clearances.

Changes in psychophysical lifting capacity with reductions in shelf opening clearances were experimentally determined. Three levels of shelf opening clearances were studied: unrestricted shelf opening clearance, loose shelf opening clearance (a clearance of 15 mm between the box width and the shelf opening), and tight shelf opening clearance (a clearance of 3 mm between the box width and the shelf opening). Two levels of lifting frequency (1 and 4 lifts/min.) and two lifting heights (floor to 0.81 m and 0.81 m to 1.52 m) were included. The metabolic energy expenditure rates and heart rates of the subjects (8 males and 8 females) at psychophysically acceptable weights of lift were also recorded to assess the physiological burden. The results indicated a decline in the psychophysical lifting capacity of the subjects as the shelf opening clearance became narrower. The decline in psychophysical capacity, compared to unrestricted shelf opening clearance for males was 9% when the shelf opening clearance was 15 mm. A further decline of almost 4% was recorded when the shelf opening clearance narrowed down to 3 mm. For females, the corresponding declines were approximately 10% and 4%. The effects of lifting frequency and height of lift were found to be consistent with previous findings.