![]() The best means of preventing early fatigue of wire ropes is to use sheaves and drums of adequate size. The effect of fatigue on wires is illustrated by bending a wire repeatedly back and forth until it breaks. The repeated bending of the individual wires, as the rope bends when passing over the sheaves or drums, and the straightening of the individual wires, as the rope leaves the sheaves or drums, causing fatigue. It occurs when ropes operate over comparatively small sheaves or drums. The proper safety factor depends not only on the loads applied, but also on the speed of operation, shock load applied, the type of fittings used for securing the rope ends, the acceleration and deceleration, the length of rope, the number, size and location of sheaves and drums, the factors causing abrasion and corrosion and the facilities for inspection.įatigue Fatigue failure of the wires in a wire rope is the result of the propagation of small cracks under repeated applications of bending loads. It is not possible to set safety factors for the various types of wire rope using equipment, as this factor can vary with conditions on individual units of equipment. A wire rope with a strength of 10,000 pounds and a total working load of 2,000 pounds would be operating with a safety factor of five. Safety Factors The safety factor is the ratio of the strength of the rope to the working load. As the strength of a wire rope is determined by its, size, grade and construction, these three factors should be considered. For the most part, these stresses can be converted into terms of simple tension, and a rope of approximately the correct strength can be chosen. The stresses most frequently encountered are direct tension, stress due to acceleration, stress due to sudden or shock loads, stress due to bending, and stress resulting from several forces acting at one time. Strength Wire rope in service is subjected to several kinds of stresses. Ability to withstand distortion and crushing, otherwise known as abuse.Ability to withstand repeated bending without failure of the wire from fatigue.Strength sufficient to take care of the maximum load that may be applied, with a proper safety factor.A proper choice is made by correctly estimating the relative importance of these requirements and selecting a rope which has the qualities best suited to withstand the effects of continued use. Selecting Wire Rope When selecting a wire rope to give the best service, there are four requirements which should be given consideration. Wires used in these wire ropes are individually coated with a layer of zinc which offers a good measure of protection from corrosive elements. ![]() The lower cost is usually a consideration in the selection of galvanized carbon steel. Galvanized Carbon Steel This is used where strength is a prime factor and corrosion resistance is not great enough to require the use of stainless steel. Type 305 is used where non-magnetic properties are required, however, there is a slight loss of strength. Other types frequently used in wire rope are 304, 305, 316 and 321, each having its specific advantage over the other. The 18% chromium, 8% nickel alloy known as type 302 is the most common grade accepted due to both corrosion resistance and high strength. Stainless Steel This is used where corrosion is a prime factor and the cost increase warrants its use. ![]() In order to meet these requirements, wire rope is produced in a number of different materials. Materials Different applications for wire rope present varying demands for strength, abrasion and corrosion resistance.
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