The Math

Recognizing the hazards, using manufacturers recommendations, company policies and how OSHA and ANSI work into fall protection and making sure you have done your math properly.

Strength Loss of Wire Rope Over Stationary Sheaves or Pins

Rope breaking strength is determined in a standard test wherein fittings are attached to the ends of the rope and the rope is pulled in a straight line.

If, however, the rope passes over a curved surface (such as a sheave or pin) its strength “is decreased.” The amount of such reduction will depend on the severity of the bend as expressed by the D/d ratio. For example, a rope bent around a pin of its own diameter will have only 50% of the strength attributed to it in the standard test. This is called “50% efficiency” (Fig. 38). Even at D/d ratios of 40, there may be a loss of up to 5%. At smaller D/d ratios, the loss in strength increases quite rapidly.

The angle of bend need not be 180º, 90º, or even 45º; relatively small bends can cause considerable loss.

All discussion of strength pre-supposes a gradually applied load not to exceed one inch per minute.