Quote:
Originally Posted by EricH
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.
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.) |
Thanks for the pictures!
Quote:
Originally Posted by EricH
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?
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Yes, the feed rates change as the lift extends and retracts, but anytime the cables are not at the same feed rate, there will be cable slack (or binding) in the system.
For example, when lowering the lift, the up-feed cable's feed rate will start out being relatively fast since its drive pulley is fully wrapped, and then slow down as the cable unwraps. The down-feed cable's feed rate will start out being relatively slow, and then speed up as the cable wraps. The result will be initial cable slack. When the lift passes through its mid-portion of its stroke, the wraps on both drive pulleys will be relatively equivalent, so the feed rates will be relatively equivalent (thus, the slack will still be present, but not increasing or decreasing). As the lift approaches being fully lowered, the down-feed cable's feed rate will now be faster than the up-feed cable's feed rate since the down-feed cable's drive pulley has more wraps. The result is that the system will take the slack back up. However, during the stroke, the system will have slack (for the reasons mentioned above). Thus, unless you have a system that keeps the cable wraps flat, it seems a tensioner would be needed to keep the cable tight, right?