Elevator chain slipping

Just before bag, we noticed an issue with our elevator chain. When we run it upwards, the chain skips, causing the elevator to jitter and not move up. We took slow motion video of the issue:

When we tensioned the chain to a point we thought was too tense, the issue partially resolved itself, but it quickly appeared again. We also noticed the elevator crossbar on the top where the idler sprocket was mounted was bending down. Additionally, when we applied side load to the elevator (pushing the whole thing to the left) it seemed to fix the problem.

We will replace the crossbar with a thicker 2x1, but we also don’t think that’s the only problem. Based on the video (and pictures we can provide of the practice bot), is there anything else that could be the problem?


Make sure your rigging for sucecutive stages is correct and not trying to fight itself, also that whole system just looks extremely heavy for one what looks to be, number 25 chain to be taking, I would add some counterweight in the forum of constant force springs or the like, it’s possible that the whole system is just too heavy for even full engagement of the chain to not slip.

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The best thing to do, if at all possible, is to put the motor at the top and the idler at the bottom. That way gravity is helping you instead of fighting you.

However the fact that pushing everything to the left indicates there may be binding or excessive friction.


You probably have something like the 3rd gearbox, adding some support as the middle one should help.

As far as I can tell, the rigging is correct for the successive stages (when lifting manually, it cascades correctly).

On the note of system weight – the elevator weight is less than ~40lbs, would that be too much for #25 chain? How would heavier chain help the situation? I can see the advantage to CF springs taking the load off of the chain, but wouldn’t heavier chain have the same slipping issue? I think I’m missing something here.

I’m not sure I understand why putting the motor at the top and the idler at the bottom would make a significant difference for gravity help. Aren’t we going to have the same problem since the loaded part of the chain (connected to the next stage) will still want to go downwards?

Also, can you elaborate on the binding/excessive friction issue? Are you referring to binding between stages (friction between the rollers and the stages)?

@asaphfirst Here is what our gearbox setup looks like in CAD:

Are you saying we should support the end of the shaft rather than the middle? Wouldn’t our situation still match the simple load ratings because the sprocket is close to the gearbox?

I think strengthening up the crossbar and possibly adding some gussets to keep it from bending will definitely help with your problem. In the video the elevator also seems to be going up unevenly (maybe that’s why the side load helped as it made the elevator go up evenly?), so you should check to make sure there is no binding or added friction in between stages that could be causing excessive load on the chain. You could also try moving the location of your string on the other stages so it pulls in the center and not off to the side as this may help with the binding issue, though I wouldn’t think it would help a ton as it already seems pretty centered. Worst case you could try adding some c channel or something of the sort between the upper and lower sprocket on the back face to keep the spacing the same, thus not letting the chain slip. I also don’t think you need to go to #35 chain, #25 chain can hold quite a bit of load.

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With the driving gear at the top the top bar flexing and chain tension doesn’t really matter because gravity keeps the chain engaged on the teeth. Put it on the bottom and and any loss of tension, no matter the cause and gravity will keep the chain from being fully engaged in any teeth and allow the sprocket to spit the chain out of the 2 or 3 teeth that could have done the work.

In the video it looks like the left side is rising more than the right side. To me that suggests that there is something that is preventing that side from moving as easily as the left side. The other possibility is that the point where the chain attaches to the stage is offset and causing the uneven lifting.

You say your chain starts tight, and it isn’t broken or stretched, so I would say that the only explanation is that the center distance between the two sprockets is changing…something is flexing–either the top cross member or the bottom. If you could’ve run a rod directly between them, that should solve it.

But now your robot is looking at you through the bag.

It’s a problem exacerbated by putting the chain down the middle of the elevator. You’re farther from the actual supports.

I also think your elevator is probably heavier than it needs to be. But you’ve obviously thought that too…seeing the lightening holes.

As for 25 chain, it will skip and/or derail so much easier than 35, I avoid using it on our robots.

The people talking about gravity…I just don’t really see that.

It does help to use larger sprockets where you can–more teeth are engaged.

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40 lbs is a LOT for an elevator, I’m willing to bet that accounts for a good bit of your issues, but the flexing crossbar and making sure their is no sideways slop in your bearings is also important.

Well those are the facts. Of course it is long gone now, but last season’s robot had chain so loose by the end of the season that barely engaged the teeth on the idler sprocket. It was so loose that you could make the chains bypass each other. However it never skipped at all, even when it got bound up due to other parts failing. You are on the right track that it is all about having more teeth engaged, but they need to be fully engaged, putting the drive at the top means gravity will insure that every tooth that can be engaged will be fully engaged.

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I can’t tell due to the video quality - but do you have a spring tensioning the chain, or a turnbuckle? If its a spring, that could cause some of your problem.

Also, based on your cad - it looks like your drive sprocket/motor is mounted via 2 long bolts. These bolts, or what they are mounted to could be flexing as well.

Could be multiple things flexing a little bit causing the problem.

If it’s your top shaft flexing, changing from Aluminum to Steel may help. 3946 used cold roll 1018 1/2" hex from online metals to solve a similar problem a few years back.

40lb is a bit of an overestimate, but I’m curious as to how teams get lower than that with a 3 stage elevator. Everything on the elevator is 1/16" or .05" VersaFrame (until we replace that one crossbar) .

I think I understand your point now, but I don’t think it’s feasible for us to move the gearbox to the top at this point.

It’s the AndyMark #25 chain turnbuckle.

The bolts are 1/4" grade 8 steel bolts, I don’t think those are flexing. We will check the 2x2 beam that it’s mounted to.

We’ll check that, thanks! It’s currently 7075 aluminum ThunderHex, do you think that’s likely to be the issue?

Based on what we saw (and most of the comments here) I’m fairly sure it is primarily the crossbar bending. I am still unsure as to how switching chain thickness will impact this issue – as far as I can tell, the chain is not about to snap or anything, it’s just looser below the gearbox than it should be. Will the chain thickness make a significant impact to this?

A 40 lb load is well within the capabilities of 25 chain for this size of sprocket. This is not going to be the sole cause of failure in a situation like this.

If you are relying on gravity to hold your chain to the sprocket teeth it is engaging with, your chain is extremely undertensioned. Hundreds of FRC teams have driven elevators from the bottom for many years - this is absolutely doable.

One possible issue may be related to the chain slack. You can tension the turnbuckle all you want, but if your chain slack is on the other side of that sprocket at the bottom, you’re not actually tensioning the chain fully. You have to be sure that all of the slack in your chain is on the side with the turnbuckle before tensioning. Sometimes slack can hide “underneath” that bottom sprocket…

If you are bending the crossbar, you are either applying too much tension to one side, or there are structural problems with your elevator design. Can’t really tell from the video, can you take pictures?

Have you tried disconnecting the chain and moving the elevator by hand? From the short video it appears that the elevator is jamming or rubbing on the right hand side. It looks like that is what is causing the chain to skip as you are binding somewhere. When the elevator starts to move you will see the left side of the elevator start to move up but the right side stays stationary, causing a jam.

My suggestion would be to remove the chain and try to move each stage independently and see if they all move freely.

How would you suggest we ensure this? We pulled the chain tight under the sprocket before connecting the turnbuckle, but apparently it’s still not tight enough.

The crossbar is more twisting than bending (we can see the 2" side walls of the 2x1 bending). Unfortunately, I can’t take pictures right now (not at build site with practice bot).

We’ll check this and post when we do, thanks!

After more testing, the team found that removing the rope on the right side connecting the first to second stage resolves the issue. This seems to indicate that reducing load on the elevator does help fix the problem, but I don’t think it’s the root cause – if it was, then pushing in the chain at the bottom wouldn’t have resolved the issue (I forgot to mention this earlier, but when we manually pushed in the chain at the bottom, ensuring more contact with the sprocket, the elevator was able to move up).

We could add a constant force spring to the elevator, but it would be difficult to integrate it at this stage (especially since we’re competing week 1). Are there other solutions here?

EDIT: Also, the team tried checking the tension while the elevator was propped up, but in this case, there was no slack under the chain mount to remove. Also, when they helped the elevator up by lifting a little with it, the gearbox was able to move it.