What is the Strength of Wire Rope?

In the United States, wire rope strength is typically expressed in tons of 2,000 lbs and is represented as Minimum Breaking Force (MBF). This is a calculated value widely accepted in the wire rope industry. When tested on a tensile machine, a new rope will break at a value equal to—or higher than—the published MBF.

These published values apply to new, unused ropes. A rope should never operate at or near its minimum breaking force. To determine the maximum allowable load, the MBF must be divided by the design factor appropriate for the application. Over its service life, a rope gradually loses strength due to natural causes such as surface wear and metal fatigue.

What is Fatigue Resistance?

Fatigue resistance refers to a rope’s ability to withstand repeated bending stresses without failure. The wires in a rope must be capable of bending repeatedly—such as when a loaded rope passes over a sheave during operation.

Rope fatigue resistance is influenced by both wire properties and rope construction. In general, a rope made from many smaller wires has higher fatigue resistance than a same-sized rope made of fewer, larger wires. This is because smaller wires bend more easily as the rope passes over sheaves or drums.

To minimize fatigue, ropes should never bend over sheaves or drums with a diameter too small, which would excessively stress the wires. Standards for specific applications specify minimum sheave and drum diameters. All ropes experience gradual strength loss from bending-induced metal fatigue during normal operation.

What is Crushing?

Crushing occurs when external pressure distorts the rope’s cross-section, its strands, or its core—or all three—resulting in permanent damage. Crushing resistance is a measure of a rope’s ability to withstand such forces and is commonly used to compare different ropes.

When a rope is crushed, the wires, strands, and core cannot move or adjust normally during operation, which can significantly reduce performance. Generally:

• IWRC (Independent Wire Rope Core) ropes are more crush-resistant than fiber core ropes.
• Regular lay ropes are more crush-resistant than Lang lay ropes.
• 6-strand ropes have higher crush resistance than 8-strand or 19-strand ropes.
• Compacted strand ropes are more resistant than standard round-strand ropes.

How Does Rotation Resistance Work?

When a load is applied to a rope, torque is generated as wires and strands try to straighten. This is normal, and ropes are designed to operate under this load-induced torque.

However, this torque can cause rotation in single-part and multi-part hoisting systems. Rotation can be minimized using specially designed rotation-resistant ropes.

In standard 6- and 8-strand ropes, the torque generated by the outer strands and the IWRC is in the same direction, which adds together. In rotation-resistant ropes, the lay of the outer strands is opposite to that of the inner strands, so the torques counteract each other, reducing overall rope rotation.