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FAQ >> Crane Hoist Limit Switches - Pretty much failsafe, right?

Crane Hoist Limit Switches - Pretty much failsafe, right?

Wrong! A great many rope failures (dropped blocks) can be attributed to failure of the hoist limit switch, which can result in the hook block being hoisted into the bottom of the trolley frame structure (two blocking). In many cases, the failure stems from the fact that the limitswitch drop weight did not come into contact with the hook block in the high position. Typically, this weight is loosely wrapped around one or more of the equalized (non-moving) hoist ropes to insure contact with the hook block. Should this weight become dislodged from this rope(s), contact with the block may not occur, thus rendering the limit switch ineffective. Another possible cause for limit switch failure is the "freezing-up" of the make/break mechanism inside the switch. According to OSHA, the hoist limit switch must be operationally checked under no load at the beginning of each shift. The purpose of this check is two-fold. It not only verifies the operation of the switch, it also "exercises" the make/break mechanism, thus reducing the chances of it "freezing-up".

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