Lifting Task on a Packing Line
A packing-line worker lifts 25 lb boxes from a conveyor at a vertical height of 75 cm, 25 cm in front of the ankles, and carries them 50 cm to a pallet. The lift involves 30° of asymmetry, occurs 4 times per minute for a 2-hour shift, and uses a fair coupling. Use the NIOSH lifting equation: RWL = 51 × HM × VM × DM × AM × FM × CM, where HM = 25/H (capped at 1.0), VM = 1 − 0.0075|V − 30|, DM = 0.82 + 4.5/D, and AM = 1 − 0.0032A, with H, V, and D in cm. For this frequency and duration, FM = 0.72; for fair coupling, CM = 0.95.
1. Compute the recommended weight limit and the lifting index. Interpret the result.
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Compute each multiplier:
- HM = 25/25 = 1.00 (at the cap)
- VM = 1 − 0.0075|75 − 30| = 1 − 0.3375 = 0.6625
- DM = 0.82 + 4.5/50 = 0.91
- AM = 1 − 0.0032(30) = 0.904
- FM = 0.72; CM = 0.95
RWL = 51 × 1.00 × 0.6625 × 0.91 × 0.904 × 0.72 × 0.95 ≈ 19.0 lb
LI = Load / RWL = 25 / 19.0 = 1.32
An LI above 1.0 indicates the task exceeds the recommended limit for a majority of workers. The task should be redesigned; it is not merely a matter of telling the worker to use better technique.
2. Which multiplier is most limiting, and what change would produce the greatest improvement?
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The multipliers are HM = 1.00, VM = 0.6625, DM = 0.91, AM = 0.904, FM = 0.72, and CM = 0.95. The vertical multiplier VM is the smallest, so the low origin height is the dominant limitation. Raising the pick-up point from 75 cm to about 30 cm would make VM ≈ 1.0 and raise the RWL to roughly 19.0 / 0.6625 ≈ 28.7 lb, giving LI ≈ 25/28.7 = 0.87. That single change would bring the task below the recommended limit. If the conveyor height cannot be changed, reducing frequency or adding a lift assist should be evaluated next; the frequency multiplier (0.72) is the second-largest opportunity, and improving the coupling from fair to good would also help modestly.
3. If the origin height cannot be changed, what lift frequency would be needed to bring the lifting index to 1.0 or less?
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With all other multipliers fixed, the required RWL is at least 25 lb. The current RWL at FM = 0.72 is 19.0 lb, so the required FM is:
- FM_required = 0.72 × (25 / 19.0) = 0.947
For a 2-hour duration, an FM near 0.95 corresponds to roughly 1 lift per minute, a large reduction from 4 per minute. Reducing frequency that much may not be practical on a packing line, so the better engineering solution is to raise the pick-up height or provide a vacuum lift assist. Administrative rotation alone does not change the NIOSH multipliers and is a weaker control.