Wire Rope Direct News

Stretch In Wire Rope

All wire rope has a small amount of stretch, especially when new, the rate of stretch in the rope decreases the more it is used, however sometimes the stretch will begin to increase again as the rope starts to wear out at the end of its working life.

There are 2 main types of stretch which affect wire rope, they are; constructional stretch and elastic stretch.

Constructional stretch in wire rope takes place when the wires within the rope are bedding in, i.e when they are first used from new. The degree of stretch varies between the different types of rope construction. It will also depend on how heavy the load is, how frequently it is loaded and if the rope is bent, and by how much.

Constructional stretch in wire rope can be significantly reduced by pre-stretching / pre-stressing the rope before use.

Regular loading of a wire rope can reduce  or sometimes eliminate the stretch factors; By removing the constructional stretch and applying the load to the wire rope, accurate measurements can be marked on the rope to enable accurate fixing of terminations either before or after installation.

Today wire rope can be bought already pre-stressed, for applications such as stay cables in bridges etc.

Total constructional stretch in wire ropes is difficult to predict. Below are guidelines as to what can be expected.

STRANDED ROPES

 FIBRE CORE

LOAD               TYPICAL F OF S            % STRETCH

Light                         10:1                             0.25

Medium                   5:1                               0.5

Heavy                       3:1                               0.75

STEEL CORE                                         as above ÷ 2

MANY BENDS                                     as above x 2

SINGLE STRANDED ROPES        these generally stretch 0.25% in total.

ELASTIC STRETCH in wire ropes takes place when the weight of the load at the end of the rope is changed; the rope will increase in length when the load is attached yet will decrease when removed and return to its original length. Again this varies between the different constructions of rope, but will be approximately 0.25% - 0.6% at the working load limit. Elastic stretch can turn into permanent stretch when the wire rope is loaded above 55-60% of its breaking strength or 2.5 -3 x its working load limit.  

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