Repeated Comb-Impact Stops and an Overheating Handrail at a Transit Station
At a busy transit station, a 30-degree escalator with a 5 m rise and a rated speed of 0.5 m/s stops repeatedly during the morning rush. The stop is logged as a comb-impact switch trip. The steps show polishing at the leading edge, the comb teeth are worn, and the left handrail slips and feels warm after 20 minutes. Maintenance records show the step chain was last measured two years ago. The station manager wants the unit back in service immediately because the adjacent elevator is also out. A visual check shows a small stone lodged near the comb and a gap between the skirt panel and the step. The mechanic must decide whether to reset and run or keep the unit out of service.
1. What are the credible causes of the comb-impact trips, and how should the mechanic test them without exposing passengers to risk?
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Treat the comb-impact trip as a symptom, not a nuisance. Credible causes: debris or a stone at the comb (the immediate finding), worn or misaligned comb teeth, a step that is not level with the comb, an excessive skirt-panel gap, or step-chain elongation that changes step-to-comb timing. The correct sequence is to stop the escalator, lock out, barricade both landings, remove the debris, then inspect the comb for broken teeth and correct clearance, check step level and skirt gap at several points, and measure step-chain elongation against the manufacturer's limit. Run the unit unloaded with the comb-impact circuit monitored; do not reset and load it with passengers to 'see if it happens again.' The fact that it stopped repeatedly in the rush means the switch is doing its job; repeated resetting without correction is the unsafe choice.
2. How should the handrail slip and heating be evaluated, and what mechanical conditions could explain it?
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The warm, slipping left handrail points to the handrail drive, not the handrail surface alone. Check handrail tension and length or stretch, the drive roller pressure and wear, the drive chain tension and lubrication, and the guide rollers that keep the handrail in its track. If the drive roller slips, the handrail slows relative to the steps; the friction generates heat and the slip worsens. A handrail that is too long or too tight can also drag and overheat. Compare handrail speed with step speed and inspect the underside for glazing or embedded debris. Adjust or replace the drive roller and chain, correct tension, and replace the handrail if it is stretched or damaged. Do not apply a temporary dressing to stop the slip; that masks the cause and can contaminate the drive.
3. What return-to-service criteria and records are appropriate, and how should the station manager's request be handled?
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Return to service only after the comb area is clean and undamaged, the comb clearance and step level are within limits, the skirt gap is correct, step-chain elongation is within the manufacturer's limit (or the chain is replaced), the handrail drive is repaired and both handrails run at the correct speed without heating, and all stop switches, comb switches, and skirt switches test correctly. Record measurements before and after, parts replaced, and the test run. The station manager's request cannot override this. If the adjacent elevator is out, the correct response is to arrange accessible alternatives, keep the escalator barricaded, and give the manager a written status with an estimated repair time. Returning a unit with a known comb or handrail defect because of passenger pressure converts a maintenance issue into a foreseeable injury.