Operator Safety vs. Throughput on a Guarded Press
A stamping cell runs a mechanical press with a two-hand control and a light curtain. The press cycles in 1.8 s; load/unload takes 3.0 s, so the operator's hands are in the die area during part exchange. The light curtain is mounted 250 mm from the hazard; the press stopping time is measured at 180 ms and the light-curtain response is 30 ms. The operator's hand speed is assumed at 1,600 mm/s. A production engineer proposes raising the press speed to cut cycle time by 15% and moving the light curtain 100 mm closer to the die to shorten the operator's reach. The plant runs two shifts with a 30-min changeover; the press has a foot-pedal option that is currently disabled. Management wants output up and has asked whether the changes are acceptable.
1. Evaluate the light-curtain relocation using the safety-distance calculation.
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Minimum safety distance D = K × T + C, where K = hand speed (1,600 mm/s), T = total stopping/response time, and C = additional distance (for a light curtain, commonly ~8 × (resolution − 14) mm or a fixed allowance depending on the standard used; use the applicable standard's formula).
T = 0.180 s (press stop) + 0.030 s (curtain response) = 0.210 s. K × T = 1,600 × 0.210 = 336 mm. Adding the C term (e.g., for a 14 mm resolution curtain the C term is small; for coarser resolution it grows), the required distance is at least ~336 mm plus C - well beyond the current 250 mm.
Conclusion: the current 250 mm mounting is already below the K×T minimum (336 mm) before any C term, meaning the curtain is too close. Moving it 100 mm closer (to 150 mm) makes it substantially worse and is not acceptable. The curtain must be moved farther from the hazard, not closer, and the calculation must use the actual stopping time (re-measured after any speed change) and the curtain's resolution.
2. Assess the 15% speed increase and the foot-pedal option.
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Speed increase: stopping time typically scales with speed; a 15% faster press may raise stopping time, which increases the required safety distance. Re-measure stopping time at the new speed and re-run the safety-distance calculation. If the required distance exceeds the available mounting space, the speed increase is not permissible without additional safeguards (e.g., a faster brake, a different safeguarding device, or a hold-out/restraint device). Do not approve on throughput grounds alone.
Foot pedal: a foot pedal removes the two-hand control's protective function (it lets the operator initiate the cycle while hands are in the die). Enabling it without equivalent safeguarding (light curtain correctly positioned, or a hold-out device) would defeat the existing protection and is not acceptable. Keep it disabled unless a full risk assessment and compliant safeguarding redesign are done.
3. What would you recommend to raise output safely?
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- Fix the safeguarding first: relocate the light curtain to at least the calculated safety distance (≥ K×T + C), re-measure stopping time, and verify with a stop-time test. Do not reduce distance.
- Reduce load/unload time through fixturing, part presentation, or a rotary/transfer mechanism that keeps hands out of the hazard - this raises output without touching press speed.
- Cut changeover (SMED) to recover the 30 min per changeover.
- If speed increase is pursued, re-measure stopping time at the new speed, re-run the safety-distance calc, and add safeguards if needed; document the risk assessment.
- Verify compliance with the applicable machine-safeguarding standard and keep two-hand control active; any foot-pedal use requires a documented risk assessment and equivalent protection.