What is the D/d Ratio and How is it Calculated?

Normal wear occurs each time the rope is used as the wire rope bends around the winch drum or a sheave. The D-to-d ratio accounts for this wear and indicates if a wire rope will be excessively worn or damaged during normal operations. This ratio simply compares the winch drum diameter to the rope diameter.

The D-to-d ratio is calculated by adding the winch drum diameter (D) to the wire rope's size (d) and then dividing the sum by the wire rope size (d).

(D + d)

------

d

For example, if a winch has a 24 inch (D) drum diameter and a 1 inch (d) wire rope is used, the ratio is 25:1.

Higher D-to-d ratios put less stress on the rope. D-to-d ratios which are too low will over-stress and put excessive wear on the rope.

The minimum D-to-d ratio for lifting or lowering a load is 18:1, and the minimum recommended D-to-d ratio for moving personnel is 22:1.

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