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How to Choose the Right Mechanical Cable Pulleys

Wednesday, March 25, 2020

Manufacturers can make pulleys for mechanical cables with a variety of materials, such as steel and industrial plastics. Steel pulleys can be made of plated, carbon steel and stainless steel, while the plastic materials used to make pulleys, include Delrin, nylon, PEEK and Teflon. In addition to the material comprising the pulley, designers must also consider its diameter when selecting a pulley for a particular application.

Pulley Size

A smaller mechanical cable pulley requires the cable to make a sharper bend, thus increasing the bending stress with which the cable is subjected. Larger cables will experience greater stresses when passing over the same pulley as a smaller cable, all other factors being equal. Each application, therefore, has a wire rope pulley specification based on the size and construction of the mechanical cable.

Cables are constructed of a number of strands wrapped around each other, which are themselves composed of a number of individual wires. Cable construction is, therefore, a function of the number of strands per cable and the number of wires per strand. Its design is typically expressed as a x b, where a is the number of strands per cable and b is the number of wires per strand.

Both the cable diameter and its construction determine the size of the pulley appropriate for the application. Industry testing over the years has created recommended minimum groove diameters to be used with various cable constructions. For example, the groove diameter of a pulley for a 7x19 cable should be at least 25 times the diameter of the cable. Similarly, a 7x7 cable should use a pulley with a groove diameter at least 40 times that of the cable, while a 7x49 would be recommended at 15 times the diameter of the cable. In general the more flexible the cable, the smaller the recommended groove diameter would be to maximize the cable life.

Trade-Offs

The primary tradeoff in pulley size is that a smaller pulley will wear out the cable faster, while a larger pulley may be too big for that application. Since the remaining disadvantage of using a pulley greater than the recommended size deal largely in costs, designers should use the largest size pulley that will fit that particular application. In practice, this means that most pulleys are still too small. Too often designers allow insufficient space for properly sized pulley selection, which will limit the cable’s lifespan.

Lubrication

Lubrication can help extend a cable’s life, especially in applications with small cables or pulleys. Sava lubricates all of its commercial cables that are less than 2 millimeters in diameter, which can double or even triple the cable’s life when the pulley is too small. All cables made to military specification (Mil-Spec) standards are lubricated.

When possible, selection of pulleys and cables should involve avoiding metal-to-metal contact, because such combinations will lead to premature cable failure due to abrasion. Extruded, or coated, cable should be used in applications with metal pulleys, while plastic pulleys may be used with bare cable. Of course, these guidelines are established to maximize cable life. If a given application does not require high cycle counts, such applications are afforded more deviations from conventional rules.

Contact Us

Browse Sava’s wire rope and pulley catalog, which contains small cables as well as fittings, pulleys and tools. Sava also provides engineering and sales expertise to meet all your mechanical cable needs. Contact us today!