Page 56 - The Complete Rigger’s Apprentice

rope takes in passing over a sheave. Standard three- 10 percent of the tackle’s power at typical working
strand and braided rope is happy on a sheave four loads. In addition, too small a sheave for the rope
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to six times the rope’s diameter—i.e., a ⁄2-inch size used causes excessive resistance, especially,
(13 mm) line needs a sheave 2 to 3 inches (52 mm of course, if the rope rubs against the walls of the
to 78 mm) in diameter. Wire rope should have a block mortise. A larger sheave, because of its gen-
sheave at the very least 20 times the wire diameter. tler curve, produces less surface friction as the rope
To get an idea of how much difference this ratio passes over it. Furthermore, axle and thrust-bear-
can make, take a look at Figure 2-17, which shows ing friction is relative to the ultimate strength of the
the effect of sheave diameter on wire-rope life (graph
courtesy of the MacWhyte Wire Rope Co.). Wire life Figure 2-17. The effect of sheave diameter on wire-
is almost doubled in going from a ratio of 15:1 to a rope life. Fiber rope is generally less sensitive to diam-
ratio of 20:1, and more than tripled between 15:1 eter, but it is recommended that the sheave be six to
and 25:1. Friction enters into it, too. The resistance eight times rope diameter. (Source: MacWhyte Wire
of each sheave turning on its axle robs from 2 to Rope Company handbook)

Relative service life

Ratio of sheave diameter
to rope diameter

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