Page 56 - The Complete Rigger’s Apprentice
P. 56

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
                                                 1
             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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