Wire Rope Direct News

CONSTRUCTIONAL STRETCH IN WIRE ROPES

STRETCH IN WIRE ROPES

It is important to note the stretch in wire rope, 2 types of stretch occur,

Constructional (or permanent stretch) and Elastic stretch (or load stretch)

We have highlighted some key points in constructional stretch as a guide, to bear in mind when using wire rope, for your safety. ( Elastic stretch coming next week)

CONSTRUCTIONAL STRETCH (permanent stretch)

This type of stretch in wire ropes happens when the wires within the rope are bedding in.

The rate and degree will vary depending on the type of rope, how heavy and how often it is loaded and how much it is bent.

Any rope will stretch more when new, the rate of stretch will decrease throughout it's life, but may increase again as the rope begins to wear out at the end of it's life.

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.

REDUCING CONSTRUCTIONAL STRETCH IN WIRE ROPES by the means of prestressing and prestretching:

By regularly loading any rope, the above listed stretch factors can be reduced and in some cases almost eliminated. By removing the constructional stretch and applying the working 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.

Next week we will discuss elastic stretch or load stretch in wire ropes.

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