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The asymmetrical lifting of postal sacks without handles was simulated. Type-2 and Type-3 postal sacks (those typically used by the U.S. Postal Service) were loaded and unloaded from and to different cart and conveyor levels. An adjustable table was used to simulate the different conveyor levels. There were six combinations of lift for each of twenty participants (ten male and ten female). A modified version of the psychophysical methodology was used to determine the maximum acceptable weight of lift. A significant difference was found between the weights lifted across cart heights. Specifically, the mean load lifted over the 96.5 cm (38.0 in) cart height was 15.337 kg (33.74 lb) and was significantly greater than the average load lifted across the 20.40 cm (8.0 in) and the 104.20 cm (41.0 in cart heights. However, this difference is not practically significant different (mean difference = 1.44 kg). There was no significant difference (p > 0.05). This lifted across table heights. A significant interaction occurred between cart height and conveyor height (p < 0.05). This interaction indicated that as the vertical distance between the cart height and the conveyor height increased, the acceptable load handling capabilities of the participants decreased. The mean MAWL for Type-2 postal sacks (14.5 kg, 31.91 lb) was similar to the mean weight lifted at the postal distribution center (16.4 kg, 36.0 lg). The mean MAWL for Type-3 sacks (14.5 kg, 31.9 lbs) was less than half of the mean weight lifted at the postal distribution center (24.6 kg, 54.1 lb). The results of this study have direct implication regarding the design of postal operations, the types of lifting and lowering tasks that should and should not be performed, as well as more practical weight limitations than are previously employed in these types of operations. |
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