Frequently Asked Questions
What is the Strength of Wire Rope?
Wire rope strength in the United States is typically shown in tons of 2,000 lbs. The wire rope strength is shown as minimum breaking force (MBF). This is a calculated strength that has been accepted by the wire rope industry. When tested on a tensile machine, a new rope will break at a value equal to- or higher than – the minimum breaking force shown for that rope. The published values apply to new, unused rope. A rope should never operate at – or near- the minimum breaking force. The minimum breaking force of the rope must be divided by the design factor required for the application to determine the maximum load allowed on the rope. During its useful life, a rope loses strength gradually due to natural causes such as surface wear and metal fatigue.
What is Fatigue Resistance?
Fatigue resistance involves fatigue of the wire used to make up a rope. To have high fatigue resistance, wires must be capable of bending repeatedly under stress – for example, as a loaded rope passes over a sheave during operation. Increased fatigues resistance is achieved in a rope design by using a large number of wires. It involves both the wire properties and rope construction. In general, a rope made of many wires will have greater fatigue resistance than a same – size rope made of fewer, larger wires because smaller wires have a greater ability to bend as a rope passes over a sheave or around drums. To overcome the effects of fatigue, ropes must never bend over sheaves or drums with a diameter so small as to bend wires excessively. Standard for specific applications contain requirements for minimum sheave and drum sizes. Every rope is subject to metal fatigue from bending stress while in operation, and therefore the rope’s strength gradually diminishes as the rope is used.
What is Crushing?
Crushing is the effect of external pressure on a rope, which damages it by distorting the cross-section shape of the rope, its strands or core -or all three. Crushing resistance therefore is a rope’s ability to withstand or resist external forces, and is a term generally used to express comparison between ropes. When a rope is damaged by crushing, the wires, strands and core are prevented from moving and adjusting normally during operation. In general, IWRC ropes are more crush
resistant than fiber core ropes. Regular lay ropes are more crush resistant than lang lay ropes. 6-strand ropes have greater crush resistance than 8-strand ropes or 19-strand ropes. Compacted strand ropes are more resistant than standard round-strand ropes.
How does Rotation Resistance work?
When a load is placed on a rope, torque is created within the rope as wires and strands try to straighten out. This is normal and the rope is designed to operate with this load-induced torque. However, this torque can cause both single part and multiple part hoisting systems to rotate. Load induced torque can be reduced by specially designed ropes. In standard 6 and 8- strand ropes, the torques produced by the outer strands and the IWRC are in the same direction and add together. In rotation-resistant ropes, the lay of the outer strands is in the opposite direction to the lay of the inner strands, thus the torques produced are in opposite directions and the torques subtract from each other.