Question 2 of 4: Manual Materials Handling — Brake-Pad Boxes

Question 2: Manual Materials Handling — Brake-Pad Boxes (40 marks: a–10, b–6, c–6, d–8, e–10)

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.

Part (a) — RWL and Lifting Index

Given.

Find. $RWL_{\text{origin}}$, $RWL_{\text{destination}}$, and the corresponding Lifting Indices $LI_{\text{origin}}$, $LI_{\text{destination}}$.

Approach. Read each multiplier off the exam's own Appendix 1 tables at the given/assumed $H$, $V$, $D$, $A$, $F$ and coupling (interpolating where a value does not land on a printed row), then apply $RWL=LC\times HM\times VM\times DM\times AM\times FM\times CM$ and $LI=L/RWL$ once at each end of the lift, since $V$ (and therefore $VM$) differs between the conveyor pick and the pallet placement.

1. Read the multipliers off Appendix 1. Horizontal Multiplier: $H\approx15\,\text{cm} \le 25\,\text{cm}\Rightarrow HM=1.00$ (both ends). Vertical Multiplier: $V_o=80\,\text{cm}$ is an exact table row $\Rightarrow VM_o=0.99$; $V_d=95\,\text{cm}$ falls between rows $90\to0.96$ and $100\to0.93$, interpolating $VM_d=0.96+\frac{95-90}{100-90}(0.93-0.96)=0.945$. Distance Multiplier: $D=15\,\text{cm}\le25\,\text{cm}\Rightarrow DM=1.00$ (same value both ends – one continuous travel). Asymmetric Multiplier: $A=30^{\circ}$ is an exact row $\Rightarrow AM=0.90$. Frequency Multiplier: $F=4$ lifts/min, duration $>2\le8\,\text{h}$; both $V_o=80$ and $V_d=95\,\text{cm}$ are $\ge75\,\text{cm}$ (30 in), so the $V\ge30$ column applies at both ends $\Rightarrow FM=0.45$. Coupling Multiplier: Fair, $V\ge75\,\text{cm}$ column $\Rightarrow CM=1.00$ (Fair and Good read the same 1.00 in this column – only a Poor rating, $CM=0.90$, would change the numeric answer).
2. Compute the Recommended Weight Limit at each end. $$RWL_{\text{origin}} = 23 \times 1.00 \times 0.99 \times 1.00 \times 0.90 \times 0.45 \times 1.00 = \boxed{9.22\ \text{kg}}$$ $$RWL_{\text{destination}} = 23 \times 1.00 \times 0.945 \times 1.00 \times 0.90 \times 0.45 \times 1.00 = \boxed{8.80\ \text{kg}}$$ The destination value is slightly lower because $V_d=95\,\text{cm}$ is a little further from the NIOSH-ideal 75 cm knuckle height than $V_o=80\,\text{cm}$ is; every other multiplier is unchanged between the two ends.
3. Compute the Lifting Index at each end. $$LI_{\text{origin}} = \frac{L}{RWL_{\text{origin}}} = \frac{22}{9.22} = \boxed{2.39}$$ $$LI_{\text{destination}} = \frac{L}{RWL_{\text{destination}}} = \frac{22}{8.80} = \boxed{2.50}$$

Both lifting indices are well above 1.0, so this task exceeds the NIOSH recommended limit at both ends of the lift, with the pallet-side placement marginally the higher-risk point. The dominant penalty here is not the horizontal reach (the box is grasped close to the body, $HM=1.00$) but the combination of the load being close to the bare 23 kg load constant itself, the required 30° twist ($AM=0.90$) and, above all, the sustained 4 lifts/min frequency over a multi-hour shift ($FM=0.45$, more than halving the limit on its own).

Part (b) — Safety Commentary and Risk Factors

An $LI$ of 2.4–2.5 places this task in NIOSH's elevated-risk band: a substantial fraction of the working population performing this exact lift, at this exact rate, would be expected to be at increased risk of low-back injury, and repeated daily exposure compounds that risk over time (cumulative loading, not just single-lift overload). The main risk factors, several of which compound rather than acting alone, are: (1) load magnitude relative to the population limit – 22 kg is close to the unadjusted 23 kg load constant before any penalty is applied; (2) repetition/frequency – 4 lifts/min sustained for hours is the single largest multiplier penalty ($FM=0.45$) and the dominant driver of the elevated $LI$; (3) trunk twisting – the stated 30° turn on every transfer adds asymmetric loading on the spine on top of the vertical lifting force, a combination NIOSH treats as materially worse than either component alone; (4) coupling – no integral handles force a side/underhand grip on a rigid box, increasing grip effort and the chance of an awkward, last-second regrasp; (5) bidirectional height change – the box moves from 70 cm to 85 cm (plus the worker's own vertical excursion bringing it to/from waist height), so the posture and moment arm are not constant through the lift.

Part (c) — Problems with Lifting-Technique Training

• Training cannot fix a task that exceeds the population's physical capacity. Part (a)'s $LI\approx2.4$–$2.5$ means the load/rate/geometry combination is intrinsically over the recommended limit for most workers – no technique, however correct, changes the physics of a 22 kg load lifted with a twist at 4 lifts/min; training addresses HOW a worker lifts, not whether the task itself is liftable safely, so it treats a symptom of a design problem, not the cause.
• Skills decay without reinforcement and under fatigue/time pressure. "Correct" lifting technique (close load, neutral spine, leg-drive) is a trained motor skill, not a permanent trait; under the repetition and pace demanded here (4/min, sustained), workers revert to faster, less-controlled habitual patterns as fatigue accumulates through a shift, so a one-time training session's effect erodes well before the task itself changes.
• A single generic training program does not transfer to every box/orientation encountered. The 30° twist and the specific no-handle coupling of this box are task-specific hazards; generic "bend your knees" training does not necessarily teach the worker how to manage an asymmetric, poor-coupling transfer specifically, so the trained behaviour may not generalize to the actual failure mode most likely to cause injury here.

Part (d) — Abdominal Belt: Impacts and Recommendation

Recommendation: do not rely on an abdominal belt as the company's primary control for this task. Per the standard hierarchy of controls, a belt is, at best, a personal-protective/administrative-adjacent measure sitting below elimination, substitution and engineering controls – and the evidence above shows it may worsen risk-taking behaviour precisely where Part (a)'s numbers already show the task is over the recommended limit. The recommendation is to pursue the engineering/administrative fixes in Part (e) first; if a belt is issued at all, it should be framed explicitly to workers as a postural reminder, not a lifting-capacity enabler, and paired with real technique training, not substituted for it.

Part (e) — Other Ergonomic/Human Factors Recommendations

• Reduce lift frequency via batching or an assist conveyor. $FM=0.45$ is the dominant penalty in Part (a); a powered roller or belt extension that brings boxes closer to the pallet, or splitting the 4/min rate across two rotating workers, raises $FM$ directly and is the single highest-leverage fix available (e.g. halving the effective per-worker rate to 2/min raises $FM$ from 0.45 to 0.65 in the same $V\ge30$/>2≤8h row – a roughly 44% increase in $RWL$ from frequency alone).
• Equalize conveyor and pallet height. Raising the pallet (or lowering the conveyor discharge) to match heights removes the 15 cm vertical travel and pulls both $V_o$ and $V_d$ toward the NIOSH-ideal 75 cm, improving $VM$ at both ends without needing any mechanical lift-assist device.
• Add integral hand cut-outs to the shipping box design. A single design change (die-cut hand holes) upgrades the coupling from Fair toward Good and removes the awkward no-handle grip identified as a risk factor in Part (b); at the current $V\ge75\,\text{cm}$ geometry this specific change alone would not move the tabulated multiplier (Fair and Good both read 1.00 in this column), but it materially reduces grip effort and slip risk, so it remains worth doing for injury factors NIOSH's own multiplier table does not fully capture.
• Reduce the required twist. Reorienting the pallet to be in-line with the conveyor discharge, or adding a rotating turntable at the transfer point, removes or reduces the stated 30° twist, raising $AM$ toward 1.00 and removing the asymmetric-loading risk factor entirely.
• Job rotation and scheduled micro-breaks. Given the sustained multi-hour duration assumed in Part (a), rotating workers off this specific station periodically and building in short recovery breaks reduces cumulative exposure even before any engineering fix is installed – a bridging measure, not a substitute for the fixes above.