We are using climber in a box from andymark to climb this year because we’ve used them in the past and are familiar with them. The one thing we remember is that the rope will stretch (creep) over time which can affect your lengths. Is there a rope or cable that someone has used with success that will not stretch (creep) ?
The rope we usually use stretches a lot, so don’t use Dyneema/UHMWPE/whatever other brand name. But if you’re trying to do research or talk to rope experts, you’ll want to use the term creep not stretch. Stretch means temporary deformation like a rubber band, whereas creep is permanent deformation that requires a tensioner to take up.
1 Like
Iirc Dynema will stretch up to 2%. (Not sure about permanent deformation) . I am not sure how noticable that will be over a short distance.
What is definitely noticable is knots slipping or tightening down.
would (metal) wire rope be a better option?
Wire rope loves to tangle on a spool the moment it loses a bit of tension, and will fray over time as well; I wouldn’t recommend it in an application where it has to be spooled and unspooled repeatedly. It might work fine, right until you take a jostle or something comes loose and then you’ve got a tangled rat’s nest of a mess on your spool and insufficient time to deal with it before your next match.
In terms of “is there a type of rope/cable that won’t creep?”, the short answer is “no.” All materials will deform under tension, and the return to their original shape is never perfect. Some materials will creep more than others, but every rope or cable is going to creep.
@EmileH 's team gave us some very strong rope for use last year, and it worked great for us–I don’t remember what it’s called, but it’s for sailing or something. One of their mentors spliced loops into the end rather than tying knots, because that’s a lot stronger and we didn’t know how to do it. Hopefully he’ll see this and weigh in on what it was.
The way we have always dealt with creep is to have a limit switch on at least one end of the mechanism. Last year our arm would extend on springs (like a climber in a box), and we used the encoder on the pulley to make sure it extended “more than enough” (to its full length, for which there was a mechanical stop, and slack on the cable inside the arm). To retract, we didn’t use the encoder values, we had a limit switch set at the full retraction position that would (a) stop the winch motor, (b) enable the brake, and (c) reset the encoder value to zero. This switch could be mechanical, optical, a string encoder, LIDAR, whatever.
This setup let us use our arm consistently with every extension, despite creep.
That’s why you get coated wire rope.
Have used it on robots successfully but you do need the right tools, and a tensioner in the system if you can fit one. There are better options.
Okay, I’m looking at the details in the climber in a box page on andymark and it just says rope. It doesn’t specify what type it is while the page on Thrifty bot telescoping arm says Dyneema cable. Would this cable be better with less creep? I might take the cable from the TB arm we have in our storage unit and use it here.
We’re planning on 1/2" nylon webbing. Used it in 2022 on a thrifty bot climber and it worked well.
You can stitch loops into it with a sewing machine or by hand. We didn’t notice any creep but we weren’t really looking for it.
The one caveat is that as the webbing winds on to the spool you’re effective spool diameter changes so you have to make sure to account for that when choosing gear ratios.
Stretch shouldn’t really be a big deal here, if it’s stretching to the point where you can’t climb, for sure you got a knot slipping in there or some other major problem. I’m curious what exactly you’re trying to solve here?
A common problem I’ve seen is that the rope can spool inconsistently which can cause a few different issues. Aside from using a markforged “screw spool”, which is a little bit hard to make happen, I think Vector 8177 solved that problem in 2022 by using a ratchet strap as the string which would of course always spool the same way. That can be a nice solution, but keep in mind your ratio will slightly change depending on how spooled in you are.
Dyneema is a trade name for Ultra High Molecular Weight PolyEthylene or UHMWPE or just UHMW. We love this material in both rope and solid bar stock forms, but it take some getting used to. We have used Dyneema in every elevator we have built since 2018, usually 3mm, but it absolutly creeps and needs tensioning. We also us UHMW in block form as the bearings in our elevators, but that is a topic for another time.
Creep is very dependent on stress, so if you are using a rope that is rated to much more force than you are applying, you won’t get much actual creep, and Dyneema is very strong so a fairly small rope can be rated to very high loads. 6mm Amsteel Dyneema can be good for over 8,000lb. However if you are working near it’s tensile rating, Dyneema (UHMWPE) has some of the worst creep of any advanced fiber.
As Skyehawk mentioned, you also have to consider knots. When you apply a load to a knot a lot of length will pull out of the knot, this isn’t exactly creep, but it will cause many of the same problems of creep. Luckily the once the knot is fully tensioned it will stop “creeping”. When we tie a new knot we expect half of our turn buckles’ travel to be taken up tensioning the knot leaving only a fraction for actually maintaining tension as the rope creeps. Dyneema is also very slippery, and many types of knots will not hold. Bowlines are pretty much the bare minimum, with double bowlines and truckers half hitches proving more reliable for us. Even cutting Dyneema can be a pain, last year when ever we had to cut it in any kind of hurry a fresh scalpel blade was needed.
Most spools will also feed at slightly different rates as the effective diameter changes or the wraps moves side to side on the spool. In 2018 we had to put huge surgical tubing springs in our elevator to take up the slack resulting from variations in feed rates on the up and down ropes, that would otherwise cause the ropes to jump their pulleys. Since then we have eliminated spools by using just enough chain to drive systems with a sprocket instead of winding and unwinding spools. We have used mostly cascade elevator rigging and put chain only on half of the bottom stage loop using Dyneema rope on half the bottom loop and all other stages to minimize weight.
This topic was automatically closed 365 days after the last reply. New replies are no longer allowed.