How are the actuation cables in multi-stage telescoping lifts tensioned?

[EricH]

Originally Posted by Robot360

Thanks for the quick reply, EricH. Did 330 do anything specific to ensure the wraps remained flat?

Nope, not that I can recall. I've seen some industrial solutions that involved grooved drums. I think we may have done the wrapping before stringing cable, but that's it.

Regardless, do you have a few pictures of the setup to share?

Not specifically. However...
http://www.chiefdelphi.com/media/photos/17610; Zoom in on the 330 machine, down low in front, and you'll see the winding drum.
http://www.chiefdelphi.com/media/photos/17599 is another of the same machine; similar location.
http://www.chiefdelphi.com/media/photos/12345; You might be able to make out the winch behind the green light.

There's another picture, from 2002, but it has a similar problem: The drum is obscured behind the robot frame, which happened to include the guide rails for what we were lifting. I can't recall us ever having an issue with slack cables, though, without some other major mechanical problems. (You'll also notice that 2004 is the last time 330 used a lift of that type in competition--2010 used a leadscrew, IIRC--but it had more to do with us finding single-joint arms to be at least as effective as single- or multi-stage lifts for easier design than with any problems with the lifts.)

Regarding your comment about the diameter changing with wrapping cable on itself: Even a wrap diameter increase of 0.5" is approximately an additional 1.5" of circumference, meaning with one drive pulley revolution, one cable will have fed in or out 1.5" further (assumes the up-feed and down-feed drive pulleys are driven off of the same shaft), which seems significant. Thoughts?

Consider this: Should your cables happen to be attached to the same moving object in the lift, which I consider highly likely, you'll have a retract full and an extend empty at lowest position. The extend cable speeds up as it wraps up; the retract cable slows down as it gets pulled out. Going down, the opposite is true. That's a setup that will generally keep tension in the system, no?