Steel Wire Rope Elongation
When a steel wire rope is loaded it becomes longer. This elongation consists of two types of elongation - construction elongation (permanent) and elastic elongation. Elongation due to overloading (yielding) or due to rotation are not dealt with here.
Constructional Elongation
When a new steel wire rope is subjected to a load, the strands and the core decrease in size (are compacted). In addition, the strands are squeezing more tightly around the core. The construction settles. This means that the steel wire rope’s dimension becomes slightly smaller, causing the steel wire rope to become longer. This elongation is known as constructional elongation and remains in place until the steel wire rope has been subjected to loads several times in normal operation. If the steel wire rope is at a later date subjected to a greater force than that experienced under normal operating conditions, the steel wire rope will probably become a little longer.
Constructional elongation is dependent on:
- Type of core
- Steel wire rope construction
- Elevation (the length a strand passes to wrap once around the core)
- Material
- Load
Steel wire ropes with steel cores have less constructional elongation than steel wire ropes with fibre cores. Since the construction elongation of steel wire ropes is dependent on a number of factors, it is not possible to give a clear definition of construction elongation. Table 4 is intended to provide guidelines.
Elastic Elongation (Modulus of elasticity)
Elastic elongation is not only dependent on the load on the steel wires, but also on the construction, which is why steel wire ropes do not follow Young’s modulus. It is therefore not possible to produce an unequivocal Modulus of elasticity for steel wire ropes. Table 5 is intended as a guide only.
The elastic elongation in a steel rope is calculated according to the following formula:
Elastic elongation (mm) = W x L / (E x A), where
W = Load (N)
L = Length of steel wire rope (mm)
E = Modulus of elasticity (N/mm²)
A = Steel area (mm²)
If a more accurate Modulus of elasticity is required, it must be measured in the actual steel wire rope in question.
Heat Expansion
A steel wire rope will change its length when the temperature changes. Changes in length are according to the following formula:
Change in length (m) = a x L x Dt, where:
a = linear heat expansion coefficient = 11 x 10-6 m/m per °C in area 0 to approx. 100°C
L = Length of steel wire rope (m)
Dt = Change in temperature (°C)
When the temperature drops, the steel wire rope will become shorter, whereas it will become longer if the temperature rises.
