CRANECRITIQUECORNER
NYLON SHEAVES This month’s topic:
WHAT ARE THE BENEFITS? by: Dave Barnhouse
A monthly crane and rigging informative column for all personnel directly or indirectly involved with crane safety. Each month we will attempt to explain a different technical issue pertaining to crane operations here on Guam, addressing the sometimes overlooked or misunderstood topics by management and operators alike, by Dave Barnhouse Nylon sheaves are increasingly found on a wide variety of lifting equipment. They often replace sheaves made of cast iron or steel. Today, nylon sheaves can be found on nearly any type of crane including gantry, tower, rough terrain, crawler, truck, and pedestal cranes. They are also used in equipment for the wire drawing and cable stranding industries, on fork-lifts, telehandlers, manlifts, mobile drill rigs and in many other mechanical systems. EXTEND WIRE ROPE LIFE Manufacturer’s of nylon sheaves claim several advantages of nylon over steel, such as extending wire rope life. Nylon sheaves are very wear resistant and do not stress the outer strands of the rope nearly as much as steel sheaves. Standard wire rope rests in the groove of a steel sheave on point contacts only, resulting in high specific loads between the outer wires of the rope and the groove. Premature wire rope failure due to the breaking of individual wires in the outer strands can occur. This does not happen with sheaves made of cast nylon. The elasticity of nylon sheaves results in a larger contact area between wire rope and sheave groove, providing a cushion in the groove contact area. The load bearing contact area on a nylon sheave is 10 to 11 times larger than on a steel sheave. The specific loading is greatly reduced, and the wire rope is under less stress. Crown breaks occur when a cable contacts a material of similar or greater hardness than itself. During this contact, tremendous pressure is exerted on the individual wires, in time these wires will break. One solution is to provide a material which is softer than wire rope and able to handle the extreme pressures. Nylon with its inherent resiliency provides an excellent solution. As the rope bends around the sheave, the sheave material deforms slightly to provide increased contact area for the individual wires. This lowers the pressure on the wires and lessens the likelihood of crown breaks. Wear resistance, reduced specific loading and elasticity make nylon sheaves
28 | APRIL2012
Destroyed nylon sheave after load testing. This was due to previous two-blocking damage resulting in hairline cracks propagating to hub.
extend the life of the wire rope by up to 300% or more. Because of the longer life of the outer wire strands, rope failure begins with breaking of the internal strands. These breaks cause the rope diameter to shrink. Therefore, periodic rope inspection requires a careful measurement of the rope diameter to determine remaining service life instead of counting visible breaks in the outer strands. U.S. Steel conducted tests to confirm the structural integrity of nylon sheaves before purchasing for their own equipment. What they found was after loading the wire rope until failure, there was no permanent damage to the sheave. This can be reassuring since the safety factor of the wire rope is 3.5:1, and the wire rope failed before the sheave, there should never be sheave failure if the wire rope is not over loaded. Note sheave failure in photo above. This is an example why structural components must be visually inspected
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periodically. Though this sheave was not over loaded, it failed when loaded to capacity due to existing damage from a two-blocking incident. The softer nylon properties contribute to the individual wire grooving found in cast nylon sheaves during sheave inspection. This should not be a major concern unless there is obvious excessive wire groove depths noted in the sheave groove. Since the sheave groove pressure should not be over 1,000 psi, any deflection in the grooves will be elastic and fully recover once the load is removed. LIGHTER WEIGHT The low weight of cast nylon sheaves can also increase their operational efficiency. The weight of cast nylon sheaves is one-seventh the weight of steel sheaves. In heavy cranes with multiple reeving, the weight savings adds up quickly. The total axle load on a large mobile crane (which may use as many as 18 sheaves) can be reduced by almost 2,200 lbs by using cast nylon sheaves. The effect of the weight savings on the boom tip is magnified at low
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