A: The relationship between the wire rope diameter as it is bent around the Drum diameter is expressed as a (D/d) ratio. The smaller the ratio the sharper the bend a wire rope must make as it spools around a drum. Imagine how a garden hose (fig.1) (small "d") would bend and kink if you tried to wrap it around the small diameter of a pencil (big "D").
Q: Why is the D/d ratio so important?
A: Using a smaller than recommended D/d ratio aggravates this bending motion thereby causing fatigue, irregular wear and accelerated deterioration (fig.2). This increased wear usually results in more frequent inspections and costly wire rope replacement in order to avoid unexpected failures. For this reason Ingersoll-Rand and most wire rope manufacturers conform to *ANSI/ASME which recommends a minimum of 15:1 (D/d) ratio for pulling/hauling applications and a minimum of 18:1 (D/d) ratio for lifting and lowering applications.
A tighly spiralled pig-tailed rope; this condition is a result of the rope being pulled around an object that has a small diameter
Drum crushing and spiralling in a winch line. This is caused by the small drums, high loads, and multiple layer winding conditions frequently found on winches.
Q: How do you calculate the D/d ratio?
A: Add the diameter of the drum barrel to the diameter of the wire rope you want to use. Then divide by the diameter of the wire rope.
Example: When using ½" wire rope on a 10.75" drum barrel.
10.75" + .5" = 11.25". 11.25 divided by .5 = 22.5:1 D/d ratio. This meets the *ANSI/ASME recommendation of 15:1 for pulling and the 18:1 for lifting applications.