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.

Beam me up, Scotty!

I think this one speaks for itself . . . . Don't use homemade beam clamps!

When you do use commercial beam clamps, please install them correctly. These are reversed - the U-Bolt should be nominally parallel to the load, not tilted-in. Also notice the Bent Washer and the open S-Hook.

This is how they are supposed to be oriented:

Report it where?

Part of a Theatre Safety Program is Hazard Recognition and Reporting. There should be a clear and concise manner to report safety issues (unlike this sign, which was carefully placed in an entrance-way door, on both sides, but with none of the required information provided). Anonymous reporting should be available so that there is no fear of retribution.

Nuts!

The nut above may be tight, but it does not fully engage the threads on the bolt. Good practice is to always leave at least 1/2 to 1 full bolt diameter protruding from the nut. Like this:

Washers need attention, too! A washer is a device to distribute the load from the nut evenly across the bolted surface. If the surface is flat and the backing hole is just barely larger than the bolt coming through it, then a non-hardened washer may do, but if it is subject to bending or is suspending a load overhead, then the washer should be graded the same as the bolt and nut (i.e. SAE Grade 5, or 8, or ASTM 305, as determined by the engineer certifying the system).

Swab the decks, Matey!

Swab it with white paint, that is! And throw-on a bit of that traction grit for floors, too, while you're at it. Its primarily psychological, but walking across a white gridiron deck just makes you feel safer. Note the OSHA Hazard Warning marker tape along the loft wells, too.

Walking the dog

Fortunately, common sense and better engineering criteria have become more prevalent in stage rigging, however, I still find many brand-spankin'-new facilities with equipment suspended on dog chain.

Some of the stuff I see is really scary. This batten at the left is just a smidgen from falling out of the rigging. Fortunately some one put a (pot-metal) screw through the bent wire chain so it would (hopefully) stay-put.














Here we see a dog-chain wrapped around an old loft block rope sheave. It too, secured with a pot-metal bolt.


This is a good example of multiple failure points all in series:
Bolted joint (at least this one was an SAE Grade 5 Bolt).
Open ended hook (if the batten snags on something and the line goes slack, then this can disengage).
Pot-metal Turn-Buckle with closed eye at bottom, but not a one-piece forged eye (it could bend open under load).
Bent wire S-Hook connected to track (no load rating for this either).
"Dog chain" is nothing more than a piece of bent solid wire. It is not load-rated for overhead lifting or suspension of loads over peoples' heads. Only Grade 63 or Grade 8 alloy steel chain is rated for that purpose. The next time you see bent metal parts holding up your rigging, or find open eye hooks, or non-graded bolts, or pot-metal turn-buckles, you might think about getting that fixed ASAP.

Ladders on skateboards

How many of you have one of these? Raise your hands . . . now, take them to the dumpster. You can't add casters to a ladder. I know, it makes moving that ladder a lot easier to put it on wheels. It also makes it a lot deadlier. Why do you think that ladder manufacturers DON'T sell ladders like this? Because they can't sell an obliously dangerous product.

When you modify a ladder (i.e. cut it, drill holes in it, or otherwise change it) you have absolved the manufacturer of any liability for any accidents that happen after that. It's not just about lawyers, lawsuits, and insurnace -- its about doing the right thing.

Loose the casters.