Tracking scenery slippage

Hello folks,

I will have a dilemma. (it hasn't happened yet, but it's inevitable) I'm designing the scenery for "Joseph" and due to the lack of backstage space, there are no rolling units. Instead, we have a track system which cutouts of pyramids, sphinxes, sand dunes...etc will come on and off. I am trying to fabricate a design that has dual tracks and each is motivated by a hand winch on the offstage side. Chain and any motorized elements are out of the budget. Instead, I'm trying to find a method of creating a loop of 1/8" GAC that can be tensioned. Here's my concern: The sheaves will not provide enough friction to drive 20lb cutouts with the added friction of the track. Has anyone ever attempted moving horizontal loads with aircraft cable and a single-loop system? Also, I can't afford a cable drum which would help with the friction, but the horizontal shift of the wire would throw it out of alignment with the track anyways. Attached is a small screenshot of a concept sketch which may give you a better idea of my intentions.



Thanks in advance for all your help and advice!
Nik

Here's what we did once back in the dark ages for a similar situation:
The endless loop was half 1/8" GAC and half 3/4" manila. One 1/8" pulley at the (far) dead end. Two 1/8" muling block s mounted vertically just offstage. Overhead, a 3/4" pulley that was height adjustable (from the bottom of the floor fly to provide tension). The operator never touched the GAC, only the manila purchase line. I can't for the life of me recall how we transitioned from manila to GAC and back. Permanently attached to the GAC were "dogs" that the various scenic wagons hooked on to. Pull down on the onstage rope to move the piece away from you, pull down on the offstage rope to move the piece toward you. Kind of like an upside-down traveler track. Sorry for the lack of details, but it's been 30+ years .

Another option: "Some sort of pushrod"--this became a running joke in college, as the go to response for any time something needed to move a vista. A length, or more often several joined end to end with loose pin hinges, of square tube that pushed or pulled the scenic piece from storage to playing position and back.

Hope this provides some inspiration.

Hmm...Interesting. Sounds like an unique solution there. It comforts me to know that you seem to have had no problem with your GAC muling blocks slipping, yes? Do you by chance remember what and/or where you got the block? I'm torn between trying to find the perfect theatrical solution or spending a couple of bucks at Grainger to find something that works. For anyone else reading this, there are dozens of v-groove sheaves out there. Any noticeable difference between these and wire rope sheaves?

Nik

Nikgwolf said:

there are dozens of v-groove sheaves out there. Any noticeable difference between these and wire rope sheaves?

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For a non-overhead, short term solution, you can get away with using nearly anything. Wire rope sheaves have a groove profile that is matched to the size of GAC that will be running around them, properly supporting the GAC for minimal internal stress and wear as it bends around the sheave, v-groove sheave profiles are intended for rubber belts with matching profiles. Running GAC on a v-groove pulley will cause somewhat more wear on the cable and/or sheave face, but if your run of show is only a couple weeks, you'll never get to the point that it causes a problem.

At a community theater I used to work for, we had a couple of hand cranked cable drums we used for many shows. With a muling block or two, or enough distance back from the track edge to make the fleet angles work out, we never had any cable tracking problems with them, and they definitely were not rocket science designs.

we have found that when trying to do this it is best if you can go around the sheave twice with a spring loaded "tension" sheave to take up the slack. So the wire rope goes over the "headblock" around the tension sheave back over the "headblock" and out to the center...This give you more friction that you are looking for. it is not perfect but it helps a lot.

There's a type of winch frame you can make for endless loop GAC (often used for revolves) that use two sets of about 6 sheaves each on separate shafts. One set of sheaves is keyed to a drive shaft, which could be motor or crank driven, and the other set are idlers on the shaft that can be tensioned away from the drive shaft. The GAC loops back and forth between the shafts, creating enough friction to drive the cable. There are instructions in Focal Press's "Technical Design Solutions for Theatre" Volume II.

spribil said:

we have found that when trying to do this it is best if you can go around the sheave twice with a spring loaded "tension" sheave to take up the slack. So the wire rope goes over the "headblock" around the tension sheave back over the "headblock" and out to the center...This give you more friction that you are looking for. it is not perfect but it helps a lot.

Click to expand...

I trust this solution worked for your production, but I'd worry that the wire would bind on itself if wrapped twice around the same sheave.

kicknargel said:

There are instructions in Focal Press's "Technical Design Solutions for Theatre" Volume II.

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YES! I've read that article! I think I will take a stab at that solution. Thank you for the reminder.

Nik