-------- Original Message --------
Subject: Re: [TowerTalk] Tram rope
Date: Tue, 19 Dec 2006 23:24:13 -0500
From: Eric Scace K3NA <email@example.com>
Yes, theater staging does use rigging techniques common to what is
needed in other industries.
Careful on choice of block. There are several considerations:
-- The sheave radius affects how sharply a wire rope must bend around
the sheave. Small sheaves cause the individual strands to break from
fatigue due to accumulated bending and straightening. Rope strength
also decreases if the bend is too sharp. The ratio of D/d, where "D" is
the sheave tread diameter (diameter to the bottom of the sheave groove)
and "d" is wire rope diameter, governs here. For 6x19 wire rope, the
suggested D/d ratio is 51. For 1/4" wire rope, the suggested sheave
tread diameter would therefore be 0.25*51 = 12.75 inches. The MINIMUM
D/s is 34, corresponding to a tread diameter of 8.5 inches. Make sure
your pulley exceeds this sheave tread diameter.
-- To properly support the wire rope, the sheave groove must be smooth
and just slightly larger in "pitch diameter" (the diameter of a round
object that bottoms out in the groove) than the nominal rope diameter.
If the pitch diameter is larger than the rope diameter, the rope will
tend to flatten because its sides are not supported by the sheave. This
accelerates bending fatigue of the strands and causes premature
failure. Conversely, if the pitch diameter is smaller, the rope will be
jammed under load into the groove, causing it to be pinched. Both the
rope and the sheave will wear rapidly, and working friction will be higher.
For wire rope of this size, the pitch diameter would be about 1/64
inch larger than the rope diameter. If the block is labeled as
servicing 1/4" diameter rope, then you are OK. If the block is for
5/16" rope, that's too wide.
-- Sheave material: For your one-time weekend installation with 100 lb
riding on the tram, I wouldn't worry about the aluminum sheave. But,
for your next investment, get a material as hard or harder than the rope
material. With soft materials, sheave life and rope life are shortened
because the sheave takes on an impression of the rope, leading to
scoring and damage. Save the aluminum sheave for fiberous ropes.
And now I'm SURE that people have heard enough from me about wire ropes,
-- Eric K3NA
on 06 Dec 19 Tue 22:04 Dick Green WC1M said the following:
>> I saw another nasty failure when a plastic-sheaved pulley (suitable for
>> fibrous rope, but not wire rope) was used on wire rope.
> Yikes! Thanks for the warning. I used a plastic-sheaved pulley to tram the
> 40-2CD on the nylon rope tram line. In fact, I got it from our esteemed
> moderator's website (Steve, if you agree with Eric, you should put a warning
> on the website not to use the pulley with wire rope. If you disagree, let's
> hear it.) Anyway, I have a much better pulley for the job -- an aluminum
> split-side pulley with a large diameter sheave, plenty wide enough for wire
> rope. Rated for 5000 lbs. I got it several years ago from an interesting
> website: Tools for Stagecraft (www.toolsforstagecraft.com, click on
> Carpenter/Rigging/Safety). Theater stage hands need to do a lot of rigging
> similar to what we do.
> 73, Dick WC1M
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