The terms wire rope and steel cable are often used interchangeably, but there is a difference between the two. Wire ropes generally have diameters larger than 3/8 inch (10 mm). Sizes smaller than this are designated as cables. The major difference between the two, however, is in their construction.
Wire ropes are made up of multiple strands of wires twisted together. Each strand consists of several wires, and the strands are then twisted around a core (which can also be made of wires or other materials) as shown on the right. The construction allows for flexibility and strength, making wire ropes suitable for heavy lifting and rigging applications such as cranes, elevators, mine hoists and marine applications.
Cables are much simpler in design and normally consist of a single strand of wire or a few wires twisted together. They do not have the same multi-strand construction as wire ropes. A typical example is the brake cable on a bicycle.
Wire ropes wear from the outside, as well as from the inside. The outside wears as the wire rope moves over sheaves and pulleys and around winches or retaining drums. Along with dirt contamination, the rope is subjected to abrasive wear. The inside wears due to the inner strands rubbing and scuffing against each other as the rope is flexed and bent.
The extent to which wires move in a rope when it bends, is illustrated by the example of what actually happens when you wrap a 1 inch (25 mm) rope over a 30 inch (760 mm) sheave. The circumference of a 32 inch circle is slightly more than 6 1/4 inch longer than that of a 30 inch circle. Since the rope only touches half of the sheave at any time, the length differential which the rope must accommodate is 3 1/8 inch – almost 80 mm. This change of dimension is achieved by the sliding and adjusting of the strands in relation to one another, and a similar sliding and adjusting of the individual wires within each strand.
In addition to external and internal mechanical wear the rope also wears due to rust and corrosion.
Traditionally bitumen-based lubricants were used to protect wire ropes. These products must be heated before they can be applied since they are hard and thick at ambient temperatures. Some formulations contain a diluent or solvent to allow easier application. In low temperature conditions bitumen-based lubricants become very hard and brittle and fling off the rope. Environmental impact is another critical aspect of rope lubrication. Bitumen-based lubricants, when exposed to fire, release toxic fumes which pose significant health risks.
Nowadays more and more grease type wire rope lubricants are being used. Greases used for this application generally have a soft to semifluid consistency within NLGI grades 00 to 1. Wire rope greases typically offer the following benefits:
- Good covering properties
- They are water-repellent, water-resistant, and not emulsifiable
- Are not subjected to significant embrittlement
- Do not contain grit, abrasives, water, chlorine and impurities
- They are free from additives or compounds which can form corrosive products caused by water contamination or additive degradation.
Last, but not least, grease-based wire rope lubricants penetrate readily into the core of the wire rope when applied with a high-pressure lubricator.
If you require more information about wire rope lubrication, simply phone 011 462 1829 or email us at info@bcl.co.za. Our lubrication experts are at your disposal and ready to provide you with advice and answer any questions you may have.