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HOIST UPPER LIMIT SWITCHES
One of the most important safety features of a wire rope hoist is an upper limit switch. The upper limit switch prevents the hoist hook block from traveling higher than designed.
Without an upper limit switch, or in the event of a failed upper limit switch, the hook block can run into the drum (generally referred to as "two-blocking") and cause major damage to the hoist or worse yet, cause the wire rope to break and drop the load.
The US hoist standards and the European hoist standards differ substantially on the method to achieve this safety protection. US hoist manufacturers generally conform to ASME B30.16 "OVERHEAD HOISTS (UNDERHUNG)". ASME B30.16 states that you must have a method to prevent overtravel in the up direction and that if a geared limit switch is used it must have an additional independent method to prevent overtravel in the up direction.
To comply with ASME B30.16, US hoist manufacturers generally offer as standard equipment a weight or lever operated upper limit switch that contacts the hook block when it is full up and stops hoist motion. They then offer a geared limit switch as optional equipment. Examples of US hoist manufacturers that comply with this method are Acco, Detroit Hoist, Yale-Shawbox, ElectroLift, and Saturn.
The general method employed by European hoist manufacturers is to use a geared limit switch only as an upper limit switch. Examples of manufacturers that employ this method are Demag, Harrington, Street, Stahl, and R&M.
A few years ago we had an incident with a hoist built to European standards in which the geared limit switch failed. Luckily, the operator recognized that the hoist had gone past its upper limit and stopped the hoist motion before the hook block hit the drum.
In our opinion, the safest method to prevent hook overtravel is to use a hoist that conforms to ASME B30.16 and add an optional geared limit switch.
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