Don’t Let Wire Ropes Hang Dry
Ken Bannister | August 23, 2018
Fatigue, wear, corrosion, and/or core shrinkage destroy wire ropes. Extend rope life with a proper lubrication program.
Wire ropes, sometimes referred to as cables in gauges less than 3/8 in. dia., are connective elements used in static and dynamic work environments servicing industry and infrastructure needs. Common static wire-rope applications can be found in the main support systems for suspension bridges or as tensioned cable used to support tall, free-standing structures such as microwave cell towers. Popular dynamic applications include mining-cage hoists, elevator hoists, draglines, and cranes in which the wire rope moves under tension and load to lift, hoist, and transfer motion and power.
Originally developed in early 19th century Germany for the mining industry as a better alternative to existing metal chains and hemp rope—regular failure always proved catastrophic—wire rope is still manufactured in a similar manner from multiple strands of metal wire laid (wound) in a helical pattern around a center core. The center core can be made from hemp rope, plastic, fiber, or steel (specific to aircraft cable).
The multi-strand fabrication method provides tremendous tensile strength (100,000 to 350,000 psi, depending on the grade of wire steel) for lifting and hoisting while delivering flexibility of movement needed for traction and movement recovery over pulley or drum surfaces. In addition, stranded wire provides superior resistance to crushing and abrasion that can result from the extreme working conditions in which wire rope is often employed.
Wire ropes are gauged (sized) based on the number of strands surrounding the core and the number of wires used per strand. For example, an 8×19 wire rope will consist of 8 strands laid around the core, with each strand consisting of 19 wires. With 152 individual wires rubbing against each other as the rope moves over the drum or sheave pulley, friction and wear will result. That must be combatted with an effective lubrication strategy designed to maximize reliability and optimize life cycle.
In addition to load and movement, wire ropes are often subjected to weather and operating conditions that can introduce contamination and place heavy demands on the rope lubricant. Lack of a lubrication strategy can exasperate the situation and cause the wire rope to fail prematurely.
Dynamically loaded wire ropes typically fail from fatigue, wear, corrosion, and/or core shrinkage. Fatigue is a result of repetitive work cycles, subjecting the rope to constant bending, torsional twisting, and tension. These stresses eventually lead to broken wires within the strands. These same cyclic stresses also lead to high contact pressures between the wires that, in turn, set up a friction-and-wear cycle when the rope is inadequately lubricated. In addition, poorly lubricated ropes will allow solid contaminants between the wires as they open and close around the pulley. This contamination then sets up as three-body abrasion, causing accelerated wear inside the rope.
Because wire ropes are made from steel and can be subjected to outside elements such as moisture and acidic chemicals, unless a galvanized-steel rope is employed, successful corrosion abatement will rely solely on a quality lubrication program.
Core shrinkage starts to occur when the initial lubricant charge dries out, resulting in a reduced diameter and loss of support for the surrounding strands. This, in turn, can cause the strands to overlap one another, leading to nicked and cut wires.
In all four failure scenarios, effective lubrication can retard or eliminate premature failure of the wire rope.
New wire ropes are lubricated at the factory. The lubrication gradually depletes, once the rope is placed in service, at a rate that depends on the load and working conditions. When setting up a wire-rope lubrication program, the first rule of thumb is to ensure the field lubricant is compatible with the original lubricant charge.
Considering the four common failure modes, a good lubricant must be able to coat the outside of the wire rope and get in between the wires to provide a lubricant film between all of the moving wires. The lubricant also must provide adequate corrosion protection. Because most wire ropes fail from the inside, it is important to always use a penetrating lubricant first. Penetrating oils are petroleum-based and contain solvents that allow these lubricants to “creep” into the core and ensure the strands and core are fully coated with a heavy lubricating film. Once in place, the solvent eventually evaporates, leaving only the oil film.
In extreme operating environments, the wire rope may also require a coating oil to protect the outside surface, sealing out moisture and protecting the inner strands and core. This coating oil also protects the outer rope surface from wear and damage.
The correct choice of lubricant is based on the application, load, rope construction, and working environment, all of which will require expert assistance from your local lubricant supplier.
If a new lubricant is to perform correctly, a full wire-rope cleaning must take place prior to re-lubrication. Ropes tend to pick up dirt in service and old lubricants can harden on the rope exterior. Cleaning ranges from a wire-brush-and-solvent cleaning, if only light surface debris is present, to a full steam clean for heavily soiled wire ropes.
Wire-rope lubricant can be applied manually or automatically in the field. Most manual applications are performed with a brush, spray, or even a dip-tank process, depending on the length and size of wire rope. If the lubricant is to be manually applied, always strive to do so at a directional change point, such as a pulley sheave or drum, when the rope strands naturally open up to accommodate flexibility.
For automatic application, a device known as a pressure boot is used. It is clamped on a tensioned straight section of the rope and lubricant gently pressured into the wires as the rope passes through the boot.
A lot of responsibility rides on a wire rope; they are deserving of a diligent lubrication approach. EP
Contributing editor Ken Bannister is co-author, with Heinz Bloch, of the book Practical Lubrication for Industrial Facilities, 3rd Edition (The Fairmont Press, Lilburn, GA). As managing partner and principal consultant for Engtech Industries Inc., Innerkip, Ontario, he specializes in the implementation of lubrication-effectiveness reviews to ISO 55001 standards, asset-management systems, and training. Contact him at email@example.com, or telephone 519-469-9173.