Elevator rope tensioning HELP

My team is using rope and pulleys to lift a continuous elevator. We have noticed that the ropes pulling the elevator down are tight in the elevator’s lowest position and become loose as the elevator goes up. I have videos linked at the bottom and am going to describe how we tension it. Please help with advice or thought.

So far this is what we have been doing, we :

  • have the springs on both ropes that are bringing the elevator up and ropes bringing the elevator down; we start to tension by fixing the elevator at its highest point with pieces of wood in between stages
  • tie the ropes going down to the springs and tension them with a taughtline hitch
  • then we roll the winch as we bring down the elevator so that the rope stays tight (at that point elevator is in the bottom position with ropes pulling elevator down coiled on the winch)
  • then run the rope pulling the elevator up through our pulleys and tie them to the springs
  • tension them with the same taughtline hitch.

You can see the middle two ropes become loose near the winch on the bottom:

Here you can see the two ropes in the middle loose tension as the elevator goes down:

also, do you know better ways of tensioning then using a taughtline hitch?

It’s hard to tell, but it appears that you have a single winch winding and unwinding at the same time. Is that correct? If so, then note that the diameter of your winch is changing as you wind/unwind rope from it, just because you’re layering rope on top of itself.

I would suggest trying:

  1. Using turnbuckles to tension
  2. Use UHMWPE – it’s very inelastic and is thick (1.7mm will take a 650 lb load)
  3. Look at the stretch in your springs – if they are not stretched out BEFORE the point when your rope goes slack, they certainly won’t help alleviate the slack
  4. Increasing the core diameter of your winch (make sure your gearing can handle it). This has the effect of reducing the overlay effect.
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When running two lines to the elevator, you actually need to connect the two lines together at the carriage. Don’t fix the two “up pull” lines to the carriage. Connecting them together will move the slack around as needed so the lines have even tension.

Have each line go down to carriage, turn a 90 degree around a stud, then tie to the other line. Do the same for the “Down pull” lines. We have done this on our elevator this year, been a huge win. Here is a terrible drawing I made 2 months ago:

Hope this helps.

-Mike

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I hate to steal the thread, but are there any tricks to tensioning the cable between stages for a cascade rigged elevator?

  • Everett

For a rope drawn elevator I’d recommend using sailing clam cleats (attached picture)
image
These allow you to pull the rope tight and it won’t slip back. These were used on 1310’s elevator last year making it extremely easy to tension the ropes.

As for Everett asking about a cable drawn elevator, I’d second Mike’s recommendation for a turnbuckle, just find a spot out of the way (that won’t interfere with pulleys) to put it and you’re off to the races.

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We’re having a similar issue with our ropes becoming untensioned as the elevator lifts. The source of our problem is that the spools are only about 5/8" wide so the rope run must wrap over itself to lift the elevator to max height. We are running a cascade elevator so we only need to spool up about half as much rope. We’re using 1/8" dyneema though so there isn’t room for more than a few turns before the rope double wraps. Our pulleys are mechanically separated and creating a wider pulley to account for the extra wrap isn’t an option at the moment as we leave for our first regional on Wednesday. Our solution for now is to allow some extra slack besides what is already there into both the up and down runs. Then we plan on tensioning the ropes with surgical tubing attached elsewhere in our frame. The ropes will run through an eye bolt attached to the end of the surgical tubing and the tubing will be pulled tight enough to keep everything in line without adding significant load to the system.

Mike, I noticed that in your drawing there are two separated drums. Is this how 1678’s actual elevator is setup or was this simply just a concept drawing? Like I said above, our elevator is powered by two mechanically separated drums and we’re looking for any ideas on how we might keep them even with each other besides the encoder feedback we already have. Running the ropes around studs on the carriage and then connecting them looks like it might be the solution. I can post some pictures of our setup if that would help. Thanks!

Sorry for the delay. Our elevator has two drums, but they are mechanically linked via gearing, so they operate as a single drum. It just worked better for packaging to split up the drum.

If you have separately powered drums I’m not sure linking the cables makes sense. Anytime you have two uncoupled systems that are supposed to be synchronized, you are going to run into challenges. Is there any way to mechanically couple the two drums together?

Best of luck!

-Mike

No problem at all, thanks for the reply!

My thinking was that with the linked cable, even if one side was running slower than the other, the elevator would still lift and the whole cable would stay taut. There isn’t an apparently easy way to link the spools mechanically as a mechanism needs to sweep through the center of the robot where a connecting shaft would be. Our first stage is rigid enough that it shouldn’t rack, and so far we haven’t had any issues with keeping the motors timed to each-other. Is there a separate element I’m missing?

We’re looking at switching the whole system out for a more conventional full-width drum at a different location in the robot at our second regional. We unfortunately already had too much work cut out for us at Rock City, so we shelved the replacement project temporarily. We decided we would be more successful with the temporary tensioning system for now.

Thanks a ton for your help.

I don’t think you are missing anything, sounds like you have a firm grasp of the problem. I agree that system would stay taunt in your example, but I would worry that if the cable shifts too much, the spring/turnbuckle would slide to a corner, or hit some other mechanical interference.

If you can figure out a way to unify the drum, I think that’s the go-to option.

-Mike

Thanks for your input. I’ll see what we can do. Good luck competing this weekend!

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