Rotation trand
In the following graph the rotation of the rope depending on the load is represented.

It is clear that non rotating wire ropes, like Flexpack, show a much lower rotation than the non rotation resistant versions.
Rotation stability

Wire rope rotation stability is related to the capability to generate reduced torque or rotation when subjected to an axial force.

The maximum recommended fleet angle value is 2 degrees, 4 degrees for plastic impregnated ropes.

Each wire rope is composed of a certain number of wires having a helix closing, therefore they have the tendency to untwist to reach a more stable configuration and to allow a higher elongation value.

The torque generated by a wire rope depends on the rope diameter, the applied force and the characteristic torque factor, which depends on the rope construction.

Six strand ropes with steel core have a high torque factor, while non rotating wire ropes have a low torque factor.

In a capstan the rope is subjected to several bends and fleet angle and moreover its tension changes from the maximum pulling force to the back tension provided by the storage winch.

At the “load side” and at the “winch side” the rope generates torques depending on the type of rope, its size and the applied tensions (loads).

Usually the back tension is very low compared with the pulling force, therefore the torque has different values along the capstan.

The change of the torque is compensated by the rope rotation, but the free rotation of the rope is prevented by the friction of the rope itself over the capstan sheaves, therefore the rope rotation takes place only when it leaves the capstan (at the “winch side”).

This rotation forces a geometrical distortion of the rope and it can create kinks on the rope if the back tension is not high enough to prevent them.

The usual short distance between the capstan sheaves does not allow the proper distribution of the stress along the rope and this creates uneven stress conditions of the rope components resulting in a reduction of the life (usually for A/R application this is not a major problem).

When the rope moves from a groove to the next one, it is subjected to a lateral deflection and this implies that the rope reaches the sheave groove on its side absorbing an additional torque.

Such torque is added to the internal torque created by the rope tension (load). A rope with a high rotational stiffness has a small geometrical distortion but a high residual torque due to its rotation. In this case some problems may occur if the torque is not properly managed.

A rope with a low rotational stiffness has a high geometrical distortion but a small residual torque due to its rotation. In this case the geometrical distortion may be of such entity as to affect the rope breaking force and its performance in service.

A non rotating rope construction ensures a high control of the rotation of the load, but requires a capstan and a storage winch and very skilled personnel.

A standard rope construction doesn’t require a special machine, but it could be permanently damaged after the first operation.