Wire Rope Technical Information
Wire Rope Strength Design Factors
The rope strength design factor is the ratio of the rated strength of the rope to its operating stress. If a particular rope has a rated strength of 100,000 lbs. and is working under an operating stress of 20,000 lbs., it has a rope strength design factor of 5. It is operating at one-fifth or 20% of its rated strength.
Many codes refer to this factor as the "safety factor" which is a misleading term since this ratio obviously does not include many facets of an operation which must be considered in determining safety.
Wire rope is an expendable item -- a replacement part of a machine or installation. For economic and other reasons, some installations require ropes to operate at high stresses (low rope strength design factors). On some installations where high risk is involved, high rope strength design factors must be maintained. However, operating and safety codes exist for most applications and these codes give specific factors for usage. When a machine is working and large dynamic loadings (shockloadings) are imparted to the rope, the rope strength design factor will be reduced, which may result in overstressing of the rope. Reduced rope strength design factors frequently result in reduced service life of wire rope.
Wire Rope Physical Stretch Properties
The following discussion relates to conventional 6 or 8 strand ropes that have either a fiber or steel core. It is not applicable to rotation-resistant ropes since these constitute a separate case.
Wire rope is an elastic member; it stretches and elongates under load. This stretch is derived from two sources:
When a load is applied to wire rope, the helically-laid wires and strands act in a restricting manner, thereby compressing the core and bringing all of the rope elements into closer contact. The result is a slight reduction in diameter and an accompanying lengthening of the rope. Constructional stretch is influenced by:
Ropes with a WSC or IWRC have less constructional stretch than those with a fiber core. The reason for this is steel cannot compress as much as the fiber core. Usually, constructional stretch will increase at an early stage in the rope's life. However, some fiber core ropes, if lightly loaded (as in the case of elevator ropes), may display a degree of constructional stretch over a considerable portion of their lives. A definite value for determining constructional stretch cannot be assigned since it is influenced by several factors. The Constructional Stretch table gives some idea of the approximate stretch as a percentage of rope under load.
Elastic stretch results from recoverable deformation of the metal itself. Here again, a quantity cannot be precisely calculated. However, the equation shown below can provide a reasonable approximation for many situations.
In actuality, there may be a third source of stretch -- a result of the rope's rotating on its own axis. Such elongation, which may occur either as a result of using a swivel, or from the effect of a free turning load, is brought about by the unlaying of rope strands. This type of stretch is undesirable and may lead to rope failure.
Changes in length (ft.) = ( Change in load (lbs.) x Length (ft.) ) / ( Area (inches^2) x Modulus of Elasticity (psi) )