Posted by David Sherman on Friday, August 13, 2004 at 4:33PM :
In Reply to: Winch cable posted by Brian in San Mateo on Friday, August 13, 2004 at 1:35PM :
If you look at the wire rope tables in a good rigging handbook, they will tell you the smallest diameter sheave that can be used with any given size of rope and type of rope construction. These tables are based on the fatigue of the wires from repeated bending in continual use. Basically, if the rope takes a set when winding on the drum, you've exceeded the yield strength of the edges of some of the wires and they will eventually get fatigued from continual use.
What surprised me when I first saw such tables is that the diameters of the winch drums and snatch blocks we normally use with our trucks are always smaller than the recommended minimum bending radius of the ropes. I'm sure this is because a wire rope on a self-recovery winch is not used enough (compared to say, a construction crane or a sawmill carriage) for fatigue to be an issue, and winch ropes are more likely to get ruined by rust or abuse such as wrapping them around a sharp object long before they fail from metal fatigue.
Since it's clear from the tables that even under the best conditions, a truck winch drum is really too small for the rope that's wound on it, I would never use a larger diameter rope than the one it was designed for, or one of different construction (e.g. steel core vs hemp core) that was less flexible than a standard one. This is where it helps to go to a real industrial rigging shop and ask their advice.