Limit Switch
A switch that detects by mechanical contact that a moving part has reached a predefined position and changes its contact. On a crane it prevents mechanical overloading and rope breakage by cutting the movement at the upper and lower hook limits and at the trolley and bridge end limits.
When the switch hits a trigger such as a roller or a pin, the contact position changes. In safety applications a normally closed (NC) contact is used: if the cable breaks or the switch fails, the circuit opens and the movement stops. A limit built with a normally open contact silently drops out when the cable breaks, and that is unacceptable.
- Position: the switch must trigger early enough to cover the braking distance needed for the movement to stop; the inertia of the load is taken into account.
- Type: on dusty and damp sites, an enclosure with a suitable protection class and a trigger matching the approach angle are chosen.
- Maintenance: free rotation of the trigger roller, the body mounting not working loose, and a clean contact transition are tested periodically.
A radio remote control system does not replace the limit switch; limit protection must be built into the power circuit independently of the radio link.
The context in which this term is used: Wireless Crane Remote Control Selection Guide