Question 2 of 5: NIOSH Lifting Analysis of a Repetitive Tray-Unloading Task

Question 2: NIOSH Lifting Analysis of a Repetitive Tray-Unloading Task (20 marks: a–5, b–5, c–15)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Recommended Weight Limit and Lifting Index

Given. Origin lift: horizontal distance $H=50\ \text{cm}$; vertical hand height $V=60\ \text{cm}$; vertical travel distance $D=60\ \text{cm}$; asymmetric twist $A=45^{\circ}$ (turn to place on conveyor); frequency $F=4\ \text{lifts/min}$, duration 8 h ($>2$ but $\le 8$ h band); load $L=10\ \text{kg}$; load constant $LC=23\ \text{kg}$.

Find. $RWL$ and $LI=L/RWL$ for the task as measured.

Approach. Apply the revised NIOSH lifting equation $RWL=LC\times HM\times VM\times DM\times AM\times FM\times CM$ with the table-derived multipliers above, then compare the 10 kg load against $RWL$ via $LI=L/RWL$.

1. Recommended Weight Limit. $$RWL = 23 \times 0.50 \times 0.96 \times 0.85 \times 0.86 \times 0.45 \times 0.95 = \boxed{3.45\ \text{kg}}$$
2. Lifting Index. $$LI=\frac{L}{RWL}=\frac{10}{3.45}=\boxed{2.90}$$

(b) Is the Task Safe? Risk Factors and Potential Injuries

The task is not safe. A Lifting Index of 2.90 means the worker is lifting nearly three times the recommended weight limit for the posture, frequency and duration measured; NIOSH classifies $LI>1$ as elevated risk and $LI$ in the 2–3 range as representing a substantially increased risk of low-back injury for a large fraction of the working population, well beyond the "acceptable to nearly everyone" zone the RWL is calibrated to.

The risk factors driving this result are visible directly in the multiplier table: a large asymmetric twist ($AM=0.86$, from turning 45° under load instead of turning the feet or re-orienting the workstation), a sustained high frequency over a full 8-hour shift ($FM=0.45$, the single most punishing multiplier here), a moderate horizontal reach ($HM=0.50$), and a fair (not ideal) coupling. Combined, these compound multiplicatively rather than merely adding, which is why the resulting RWL falls to under 15% of the 23 kg load constant. The worker is therefore exposed to cumulative loading of the lumbar spine under a twisted posture, repeated roughly 1,920 times per 8-hour shift (4/min × 60 × 8) – a classic profile for cumulative low-back disorder. Likely injuries include low-back strain/sprain, lumbar disc injury (twisting under load is particularly implicated in disc herniation, since torsion combined with compressive loading is more damaging to the annulus than pure compression), and general musculoskeletal fatigue-related soreness that can progress to a chronic disorder if the exposure is not reduced.

(c) Solutions to Reduce Risk

Three independent redesigns are evaluated below, each targeting a different multiplier in the RWL equation; a fourth combined option is noted briefly.

1. Solution 1 – eliminate the twisting motion. Reorient the workstation (turn the outfeed of the oven to align with the conveyor, or add a small rotating table) so the worker faces the conveyor directly and no longer twists to place the tray, taking $A:45^{\circ}\to 0^{\circ}$ ($AM:0.86\to 1.00$). $$RWL_1 = 23\times0.50\times0.96\times0.85\times1.00\times0.45\times0.95=\boxed{4.01\ \text{kg}},\quad LI_1=\frac{10}{4.01}=2.49$$ Advantages: removes the single most injurious component of the lift (torsional loading on the spine) with no change to equipment throughput; a purely layout-based fix with low capital cost. Disadvantages: only improves $LI$ from 2.90 to 2.49 – still well above 1 – because twisting was not the dominant multiplier; requires physical re-plumbing of the oven/conveyor line, which may not be feasible in an existing facility footprint.
2. Solution 2 – reduce the horizontal reach distance. Redesign the tray hand-off point (a chute, roller table, or lowered oven-exit shelf) so the tray is transferred closer to the worker's body, taking $H:50\to 25\ \text{cm}$ ($HM:0.50\to 1.00$, the maximum value for $H\le 25$ cm). $$RWL_2 = 23\times1.00\times0.96\times0.85\times0.86\times0.45\times0.95=\boxed{6.90\ \text{kg}},\quad LI_2=\frac{10}{6.90}=1.45$$ Advantages: the largest single-change improvement of the three (LI falls from 2.90 to 1.45, close to the acceptable boundary) because $H$ enters the exam's table as a near-linear penalty and 50 cm was already well past the 25 cm "ideal" threshold. Disadvantages: requires physical modification to the oven exit and possibly conveyor position, and may increase the vertical or frequency demands elsewhere in the line if not carefully engineered (a genuine layout-integration effort, not a quick fix).
3. Solution 3 – reduce the effective lifting frequency. Introduce a short accumulating buffer (a small gravity roller or powered mini-conveyor) between the oven and the main conveyor so the worker transfers trays in less-frequent batches, taking $F:4\to 2\ \text{lifts/min}$ ($FM:0.45\to 0.65$). $$RWL_3 = 23\times0.50\times0.96\times0.85\times0.86\times0.65\times0.95=\boxed{4.98\ \text{kg}},\quad LI_3=\frac{10}{4.98}=2.01$$ Advantages: directly targets the frequency multiplier, which was the most punishing single factor in the base case ($FM=0.45$); does not require the worker to change posture or reach, so retraining need is minimal. Disadvantages: requires added buffering equipment and floor space, and does not reduce total shift throughput – the same number of trays must still be moved, only in a less continuous pattern – so it only partially resolves the risk ($LI$ still $\approx 2$) unless combined with another change.