Lubrication of wire ropes
R. David Whitby | TLT Worldwide September 2020
The most effective method is to continuously apply the lubricant during operation.
Wire ropes are used for transmitting loads under tension when flexibility is needed. A wire rope consists of continuous strands of wire wound around a central core and twisted into a helix, forming a composite rope in a pattern known as laid rope. Larger diameter wire rope consists of multiple strands of laid rope in a pattern known as cable laid. There are many different ways in which the strands of wire can be twisted to form a wire rope.
The foundation of the wire rope is its core, which is made of materials that provide proper support for the strands under normal flexing and loading conditions. The main core materials are fibers, usually polypropylene, or steel, which can be either a strand or an independent wire rope. The outer strands are usually either steel or galvanized steel. Fiber core ropes have lower breaking strengths than steel core ropes.
Wire ropes are used in many applications. They are used in cranes used for construction and lifting, in ships’ winches for moving cargoes or containers, for cable cars and ski lifts, in suspension bridges, in mooring ropes for offshore drilling platforms and for exterior building maintenance. Smaller diameter wire ropes also are used in the automotive and aerospace industries, although these are more commonly known as Bowden cables.
They are typically wound around pulleys, drums or sheaves in order to transmit force. Depending on the application, wire ropes have to satisfy different requirements:
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Running ropes (stranded ropes) are bent over sheaves and drums and are stressed mainly by bending and tension.
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Stationary ropes (stay ropes) carry tensile forces and are loaded by static and fluctuating tensile stresses.
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Track ropes (full-locked ropes) act as rails for the rollers of cabins or other loads in aerial ropeways and cable cranes and are less stressed by bending.
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Wire rope slings (stranded ropes) harness various kinds of goods and are stressed by tensile forces and by bending stresses when bent over the more-or-less sharp edges of the goods.
When a wire rope is bent around a pulley, drum or sheave, the individual wires and strands move in relation to each other to adjust themselves to the curvature assumed by the rope. As it is bent, the top portion of the strands tend to be in tension while the bottom portion of the strands tend to be in compression. This causes the wires to move in relation to each other, which will eventually cause wear. In order to reduce or prevent the wear due to the metal strands rubbing against each other as they adjust themselves around the curvature, lubrication is needed.
Internal lubrication is essential to prevent excessive metal-to-metal rubbing, and external lubrication is needed to protect metal-to-metal contact with the outer surface of the rope and to minimize corrosion, as many ropes are used outdoors.
As a wire rope is being made, a suitable wire rope lubricant is used. The type and amount depend on the rope’s size and intended use. Ropes used in cranes and winches are subject to greater bending than those used in cable cars and bridges. The initial lubrication is intended to provide protection for a reasonable time if the rope is stored under proper conditions.
When the rope is being used, periodic or sometimes continuous lubrication will be needed. The lubricant should have sufficient adhesive strength to remain on the rope. It should have a viscosity that is low enough to be capable of penetrating the interstices between the strands but high enough for sufficient film strength to limit metal-to-metal rubbing between the strands and between the rope and other metallic surfaces. It should also be resistant to oxidation, corrosion and water wash-off.
The most efficient and effective method of field lubrication is to continuously apply the lubricant when the wire rope is in operation. Techniques include a continuous bath, dripping, pouring, swabbing or brushing. Automatic lubrication systems will usually apply a wire rope lubricant by dripping or pressure spraying. Before it is lubricated, dirt and other abrasive materials should be removed from the rope, ideally using an automatic system. Continuous cleaning and lubrication are appropriate for cranes and winches while periodic cleaning and lubrication are more suitable for cable cars and bridges.
David Whitby is chief executive of Pathmaster Marketing Ltd. in Surrey, England. You can reach him at pathmaster.marketing@yahoo.co.uk.