String or chain lift?

So we’ve been using chain to power most of our lifts. I’ve noticed that a large number of teams use a rigging of string or rope to power lifts. What are some advantages to doing that if there are any over a chain lift? Just wondering

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Weight is a big one.

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Porque no los dos?

We’ve had a good experience with using #25 chain off of ganged together versaplaneteries for the first stage, and the carriage rigged in a cascading fashion with the standard 1/8” rope.

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Yeah I think we used almost the same setup for our elevator, We still had some issues with the chain skipping without constant force springs. Thanks for the advice.

If I remember correctly, we used a rope elevator this year because we had so many issues with chains breaking or not working in general for whatever reason last year.
Result: Elevator broke because we used Delrin bearing blocks…

We had some of those issues in 2018, but that was due to a serpentine chain setup. This year, the VPs were geared together with the last in the stage also having a 22t sprocket. On our top stage crossbar, there’s a clamping bearing block and nautilus cam with an idler sprocket for tensioning to the side of the top sprocket. Tensioning is a much on long chain runs like this, especially if you’re relying on the chain to keep accurate positioning. The 2018 skips destroyed quite a few chains, and always in eliminations.

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Just an anecdote:. We started this year with a string driven continuous elevator, and ended with a chain driven cascade elevator. String (actually spectra rope) is lighter, but you have to be much more careful about guiding the route and keeping enough tension the spool, so that the spool doesn’t get tangled. For us, it was worth the weight penalty to get some easier reliability

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Last year, Team 8 went with a three-stage cascading elevator with the first stage driven by chain and the following stages connected with string. We didn’t like how the elevator would sometimes skip chain links and get tilted, so we decided to go with a completely 1/8" dyneema driven elevator this year.

We ended up finding that it was a lot more difficult than we expected – getting the rope to not wrap over itself (and lose tension) was very difficult, and we iterated on our spools several times to get it to work functionally. In retrospect, the benefit from using rope probably wasn’t worth the very late nights we needed to get it working.

If you’re designing your first lift I would probably go with chain. It’s much easier to tension with the spartan chain tensioners and you don’t have to worry about stuff like spooling. As others have stated the biggest difference is the weight because chain is very heavy compared to rope which is much less important than having a working elevator. We used a chain elevator in 2018 and the only time when it broke were when we were dumb and did stuff like run the elevator full speed onto the bottom stage and honestly it was nice having the chain break over something which couldn’t be fixed.

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We used a continuous rigged elevator that used dyneema rope.

We used the pulleys and gussets from the GreyT elevator kit but modified a few small things to work how we wanted. We used a piece of VexPro versatube for the rope drum and then used a turnbuckle at the pull up and pull down near the final inner carriage stage for tensioning.

If you spend the time to run things long enough and stretch the rope out and continually tension it… it’s very nice. Our entirely dyneema rope driven elevator was very reliable and we were able to control our elevator with significant automated precision at high speeds and under significant load. We haven’t had issues with keeping everything tensioned since our DCMP. We barely have had to do anything to it to keep it running how we want it.

The weight savings compared to what we used last season is massive. Our 2018 elevator used massive amounts of 35 chain and it was far more of a pain to work with.

I can post some pictures later if that helps. 254 was a big help to us early in the season. Their lead engineering student sent us some pictures of their spool on their 2018 robot to make it easy to understand how they fixed their rope. It was pretty trivial in the end.

Honest question. How to use string?

I’m guessing the mechanism was something similar to 118’s Flyby elevator “screw” for the strings.

Pictures and slow mo vids could help me.

We used chain for the first stage of our cascade the past two years. In 2018 we used 2 runs, this year was one run of #25. It was a really easy setup and didnt weigh a ton really. Our following cascading stages used rope. Since it wasnt wrapping around or being winched, there werent any concerns on how everything would work, it was just really simple.

This was one of our major takeaways as well.

There is a short section in our reveal video that has a decent video of the final version of the spools. https://youtu.be/v9dXf6EdstQ?t=7

This was probably the 5th iteration done on the spools in under a week, so we were pretty lucky that it ended up working out. The iterations went (approximately):

(1) No grooves (2" diameter)
(2) Light grooves (2" diameter)
(3) Deeper grooves (2" diameter PVC)
(4) First Version 3DP Spools
(5) 3.5" diameter spools 3DP with deeper grooves

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sounds like a full send to me.

We did chain in 2018 but ran into problems with slamming the lift destroying the gear box so this year we did belt which help a lot(never broke anything in that system) so think about chain too

Using string makes for a very lightweight mechanism, and you can get away with a pretty thin string if you get the right kind (Spectra/Dyneema). You can deal with the overlapping string on the spool problem by adding a tensioner instead of making a fancy spool. The tensioner also keeps the string from jumping off the pulleys. You can see the tensioner we used in 2019 in this pic, it is on the bottom path of the string so that it is not put under load when lifting the elevator, it just takes up slack. We were using ~500lb Spectra.

We had a fairly effective string elevator with no major issues relating to the rigging. While last year we chose to do a belt-driven cascading elevator, we found that there were major issues with the belt skipping, which shouldn’t happen with chain. This year, the driving factor behind using a continuous elevator was to maximize our lift’s travel height, since the two stages are not linked in stage travel.

Our elevator ended up being very light, at only around 15lbs for the entire mechanism, which I attribute to using Dyneema. We never had any issues with the string piling up on itself, and that is due to how we tied the string down far away from the pulley, forcing it to spool up on our long VersaRoller spool. Tensioning was all handled by a 40 lb/in spring on the up run. It was painful to tension it, but once you did it, you never had to touch it again. We never had to iterate on our elevator, since we found it was so reliable in the shop.

Some pics of our elevator:

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I couldn’t tell from the video clip, but how did your team make the grooves? And what stock/material does the spool consist of?

It’s important to consider your design/manufacturing resources. It’s pretty easy to minimize the gains in weight by adding additional components for the rigging and tensioning. I’m a big fan of having the first stage be chain and any additional cascaded stages being webbed straps or strings. You can see this setup in a number of bots such as 2056, 1114, 5406…

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