Reeving failures have been a leading cause of accidents in our trade. They were thirty years ago when Wilbert A Lucht wrote his article, “Sheave Design vs Wire Rope Life” (reprinted in the June issue of Wire Rope News, page 26), and are still today.
Sheaves and Wire Rope Life
– Wilbert A. Lucht was So Right in 1992!
by Dennis J. O’Rourke, CSP
A Crane is a combination of two simple machines. One is a lever, and the other is a block and tackle. Both increase the machinal advantage of a crane’s lifting capability. However, the block and tackle (sheaves and wire rope, referred to as reeving) are “used-up” as the crane works.
The crane’s reeving system is a significant percentage of the overall maintenance cost, especially in duty cycle operations. Some cranes make ten lifts in a month. Other cranes move thousands of tons of concrete an hour. The difference in stress on the reeving system is noticeable. Weather and atmospheric variations contribute to wear on the system and can be seen.
The sheave differs from a “pulley” or roller importantly. Both change the direction of the rope and increase the mechanical advantage — pulling power. However, the sophisticated sheave does the same but also maintains the cross-section shape of the rope, critical to reducing wear, and the rope strength — if properly sized.
A Sheave gauge is being used to check the groove size by a crane inspector in Fig. 1. These gauges have the maximum size of the groove recommended for the rope that is installed on the crane.
An oversize or worn-out sheave acts just like a pulley, not preserving the rope’s shape, Fig. 2. Instead of the load being supported by at least three strands at the bottom of the groove, all the load is concentrated on one. With less support, increased pressure equals more significant sheave wear. There is another terrible consequence of improper groove support — premature wire breaks and internal wire wear. When the unsupported rope is bent over a sheave and loaded, the strands must adjust by sliding along each other. Sliding causes strand to strand nicking and “valley” wire breaks, a severe reduction in the rope’s life, Fig. 3.
The consequence of sheaves not correctly sized to the wire rope installed on them is accelerated wear and broken wires. How important is this to the crane owner depends on how many hours of operation per inspection interval. Some cranes aren’t used much, and owners aren’t too concerned with groove size, just so the grooves are “kind of” smooth — wrong. Current standards allow only one valley break, as shown in Fig.3.
The point is, to achieve a predictable rope life — the sheave must support about 150 degrees of the rope at the bottom of the groove, or the rope will flatten when loaded and be weaken — got that!
About the Author
DENNIS J. O’ROURKE, CSP, is the Director of National Crane Services, Inc. He has over sixty years’ experience in the industrial, maritime, and construction fields working with heavy equipment and material handling devices. As a safety engineer, Mr. O’Rourke has developed and/or presented hundreds of safety-training programs for all representative elements of government and industry. (firstname.lastname@example.org)