7/17/2019 Musica

http://slidepdf.com/reader/full/musica-56909d5905f02 1/1

DESIGN AND TREATMENT OF HOISTING HOOKSThe construction and treatment of a hook intended for a crane or hoist involvesa problem deserving of careful consideration. This fact has impressed itself upon the writer from his experience in lawsuits arising from damages caused by thefailure of a defective hook.

To make a hook safe for the purpose for which it is intended two important requisites must be fulfilled1. Correct design.2. Proper treatment.

The design is guided to a large extent by the service which the hook is to perform; that is, whether the hook will be subjected to high or low, frequent or rare stresses, and whether the hook is likely to be loaded above its normal capacity.

Hooks on small cranes and hoists, of about 2 to 3 tons' capacity, may be loadedto full capacity several times every day; while hooks on cranes of 50 tons' capacity may carry full load at remote intervals only. Due to the repeated stresseson small hooks, occurring, possibly, under extreme temperatures, the hook may become fatigued and liable to break. It is, therefore, advisable to keep the stresses low in comparatively small hooks to provide a factor of safety.

This precaution can readily be observed with hooks for small loads, as the size

would scarcely render them unwieldy. But as the size of the hook increases it becomes necessary to increase the stress; that is, the load per square inch or cross section, in order to avoid the construction of a clumsy hook. A high stress is permissible with high loads because they are applied to the hook less frequently than in the case of small hooks and light loads. We may consider a stress of15,000 lbs. per sq. in. as safe for a 50-ton hook as a stress of 10,000 lbs. per sq. in. on a 10-ton hook.

The material for a hook may be ordinary steel, cast steel or wrought iron; depending on the load the hook is to carry. For small loads where a hook of ample size, yet not bulky, can be constructed, cast steel may be used. But for heavy loads a ductile material, having practically the same elastic limit for compressionand tension, should be selected.