Wednesday, May 13, 2009

Cable Bending Radius

Wire rope, like all flexible cables, has a limit as to how sharply you can bend it before it is permanently deformed. This is called the d:D ratio. It is the ratio of the diameter of a sheave to the diameter of the cable. It depends upon the stranding and lay of the rope, and it is usually in the range of 20:1 to 40:1. This ratio is for maximizing the life of the cable as it flexes repeatedly while it transitions from straight to curved and back to straight as the cable passes over the sheave.

For a 1/8" diameter cable, not typically used for stage rigging, but frequently used for operating smoke vent hatches, this would typically use a 4" diameter sheave (32:1 d:D ratio). The picture below is a 1/8" diameter cable that operates a smoke vent. You can see that it is bent under tension over the sharp edge of a steel I-Beam with no pulley or block to smooth-out it's travel around the corner. You can also see where the cable strands have broken due to repeated operation.

So, why is this important? The smoke vents should be configured to allow both manual opening and closing and for the automatic release in coordination with the Fire Alarm and the Fire Curtain systems. It is very typical (but shouldn't be) that the Smoke Vents are rigged by someone other than the Stage Rigging contractor. When that happens, the manual operation system is either non-existent (so you have to climb up on the roof in a rain storm to close the hatches), or is 'user hostile' (isn't convenient for the staff to use on a day-to-day basis).

Dysfunctional mess (left). Neatly marked and operated with a RED handline (right). Sign reads: NOTICE - Smoke Vent Operation Line Inside". Sign is on both sides of hinged protective screen panel. Panel in front of line set is painted red.

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