Limit Switch Failure on a Gantry Crane
A semi-gantry crane runs on a rail along a wall and on a rubber-tyred leg on the open side. During a shift, the operator notices the long-travel limit switch does not stop the crane at the end of the runway; the crane overruns by about 300 mm before the operator applies the brake manually. The end stop is a welded steel buffer, and beyond it is a walkway used by maintenance staff. The crane is carrying a 4 t coil at the time, and the operator needs to finish three more coils before the truck leaves. The maintenance fitter is on lunch. The operator has not reported the fault yet.
1. What is the risk, and what must the operator do about the faulty limit switch?
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A failed long-travel limit switch means the crane can run into the end stop at speed with a 4 t load. The end stop is a buffer, not a normal stopping device; repeated impacts can damage the runway, the crane structure and the buffer, and a failure could let the crane overrun into the walkway where maintenance staff pass. With a suspended 4 t coil, an impact could cause load swing, rope slack then shock load, or derailment of the rubber-tyred leg.
Operator actions:
- Stop using long travel toward that end. Finish the immediate positioning only if it can be done without approaching the faulty end, and lower the coil to a safe resting place.
- Report the fault immediately to the supervisor and maintenance, and tag/isolate the crane as required by site procedure. Do not wait for the fitter to return from lunch.
- Do not continue production with a known safety device defeated. The three remaining coils are not a justification.
- Record the fault in the logbook and on the defect tag.
The crane should be taken out of service until the limit switch is repaired and function-tested by a competent person.
2. What interim controls are needed if the crane must be moved to a maintenance bay before repair?
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If the crane must be moved, treat it as a controlled maintenance move, not normal operation:
- Authorisation from the supervisor and maintenance, with a written or recorded instruction.
- A spotter at the faulty end to signal stop and watch the buffer and walkway.
- Clear the walkway and barricade the end-of-runway zone.
- Slow speed only, with the operator ready on the brake, and no load on the hook (land the coil first).
- Agreed stop point short of the buffer, marked on the rail.
- Isolation at the maintenance bay, with lock-out/tag-out before anyone works on the switch.
- Function test after repair: operate the limit switch at slow speed and confirm it stops the crane before the buffer, with the test recorded.
No production loads should be carried during the move.