Elevator Tensioning Questions. Particularly for 1678

This is a photo from 1678’s 2018 robot CAD. I believe it is the rope tension system. I am wondering if anyone can explain. it looks to me like the drum behind the spring has something to do with tension but I don’t understand it. I would also like to see any photos of the real one of anyone some.

Also I would love to see photos of other peoples tension systems and ways of mounting the rope to the carriage and winch

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So the spring is NOT tensioning the cable/rope, the drum is a ratcheting winch that DOES tension the line. The purpose of the spring is to remove slack in the system while in motion as Spectra does stretch a little bit.


Isn’t “removing slack” another way of saying tensioning?

I am just trying to figure out how the spring and drum act together.

It removes slack created by dynamic events, after there is an initial tensioning via a drum to pull out bulk slack.

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Yes and no. While stationary the spring does nothing to keep the line taut. While the elevator is going up the spring does nothing. While the elevator is traveling down under power the spring makes sure the line stays on the pulleys by being able to remove about an inch of slack line. The spring does not tension the system but it does make sure it stays that way under dynamic load. The spring doesn’t carry any real load.


(emphisis mine)

Just curious…

Any idea how close you were getting to that ~1" margin in the real world? on what diameter line?

IK spectra is a different manufacturing process, but dynema has something like 1% stretch at 80% load or something (totally off the top of my head). Surely you were nowhere close to that ~1" margin…

In 2019 we used a continuous elevator. We found that if we did not add a tensioner spring to the pull down rope, that it would develop slack resulting in overrides on the down rope spool. We formed a bowline loop several inches from where the rope was tied off to the final stage and then attached a spring between that bowline loop and the final stage. When the elevator was all the way up, the spring saw fully relaxed creating slack in the length of the rope between the bowline loop and the final stage. When the elevator was all the way down, the spring would extend such that the rope between the bowline loop and the final stage was tight.

I’m not sure I can explain why this happened. The geometry alone did not seem to require it. But it was very clear that it was needed and without it, the elevator spools would not wrap and unwrap in a tidy fashion.

When we first started having the overwrap problems one of the students recalled that he had seem a sling tensioner on another robot in 2018, so that is why we tried it. We got it working on the first design iteration and never really messed with it after that.

I believe most of the extension of the spring occurred right near the bottom of the travel. But we did not do any experiments to try to quantify the effect.

We used much softer springs than what is shown in the CAD model in the OPs post.

Hey smtopps, I’m the 1678 alum who designed this system, you can thank @davepowers for referring me to this thread.

I had almost managed to forget that atrocious block of a markforged part in there…

Anyway @RoboChair was mostly spot on with his explanation, I just wanted to expand a little bit. The drum was the primary tensioning system. It captured the spectra line running down from the top of the elevator, and it was connected via hex shaft to a 1/2 ratchet we zip tied to the back of the 3D printed block. Once we had the parts dialed in it made tensioning extremely easy, all you did was grab a wrench and turn the shaft until it was tight enough. That being said, we had a lot of issues with the pulley rounding out or the cable crushing the printed material, this was definitely a case of 3D printed part abuse.

As Devin mentioned, the spring was for the most part unrelated to the drum. We put the spring on the return cable running down to the bottom of the elevator, so it never had to do any lifting. Exactly as @wgorgen did on their robot, we tied some cable connecting the top and bottom of the spring with enough slack that the spring could extend about an inch, then the cable would become tight and stop additional extension, to ensure we weren’t going to break the spring.

@Skyehawk iirc the spring would frequently reach maximum extension, but you’re right it most likely was not due to actual stretch in the dynema. I would guess it’s more related to slop in the entire system.


So when your robot climbed with another robot, was all the lifting force being applied to the spring and the 3d printed piece?

In 2018, we did not climb using the elevator, in order to keep everything on it lighter and easier. Instead, we had a separate mechanism to deploy a gate latch directly onto the bar, and used a winch directly on the base of our robot to crank ourselves up.