Drive Chain Slippage

This year, my team ran an 8 wheel west-coast tank drive robot with three neos per side. We used 35 chain as our drive power transmission method, but even though the chain was very tight, we would still occasionally have issues with slippage. My mentor believes that this is partially because we used 12t sprockets on all of our wheels, and there weren’t enough teeth being engaged at one time. Could the sprockets have been part of the problem, or should I look elsewhere to try and find the issue?

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Absolutely, 12 tooth sprockets contributed to your problem. Consider the diameter of the sprocket compared to something like a 26 tooth sprocket. The torque on the smaller sprocket will be much higher for a given wheel load. So greater torque plus fewer teeth will make it much more difficult to avoid skipping.

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So what size sprocket might be better for a similar drivetrain? Should we look at stepping up to something huge like 26t, or would a smaller increase suffice?

Properly tensioned chain shouldn’t skip. If the chain is too loose, then yes you can see the chain skip if you put a high load on it. If the chain is as tight as you describe though, that shouldn’t be the problem. In fact, your chain was probably over-tightened which would give a lower efficiency but not contribute to skipping.

As @Tom_Line said, having smaller sprockets will make it easier to over-torque the sprocket, but you’ll still need the chain to be under-tightened. Unlike belts, chain doesn’t (significantly) instantaneously stretch under shock loads. So for any reasonable load, a properly tightened chain won’t skip even for undersized sprockets. It will just break. What effects were you seeing that lead you to believe that the chain was skipping?

Take a look at the chain’s working load (Vex lists them on their website). Make sure the wheel torque divided by the sprocket radius doesn’t exceed that value. That will keep the chain from breaking. If that chain is then tensioned properly, it shouldn’t skip either.

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We use something in the 20 range, yes.

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What effects were you seeing that lead you to believe that the chain was skipping?

It wasn’t see, so much as hear. If you listened to the matches when we would play heavy defense, you could hear our chain skipping clear as day even in the stands.

I would try putting the robot against the wall and driving into it to see if it really is the chain that is skipping. It is rather unusual for chain to skip repeatedly but not be thrown off the sprocket. It seems more likely that you have a hub or gear slipping than the chain.

If the chain is skipping, it would be good to try to figure out why - perhaps the sprockets are moving relative to each other, which might happen if a sprocket were mounted at the end of a cantilevered shaft, or in the unsupported middle of a long aluminum shaft. If it is a shaft flex issue, you may be able to improve things by replacing the aluminum shaft with steel, as steel bends less under the same load, or by moving the sprockets closer to a support bearing.

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Keep in mind he is specifying #35 chain. A 12 tooth sprocket for a 35 chain is closer to an 18-22 tooth sprocket in #25 chain. A 26 tooth #35 sprocket is huge.

That said, using bigger sprockets when possible decreases the force going through the chain and the tension required. It helps to go big when you can, as long as you mind your weight.

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Have you checked that it’s not the gears in your gearbox slipping you’re hearing? Just a thought. Were you hearing it when pushing against someone, or when changing directions? Depending on your programming, I think especially with the Neos, sudden direction changes (full forward to full reverse) can cause slipping also.

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For what it’s worth, we use 8WD with 3 Neos, 2 speed gearbox, #25 chain on 17T magic sprocket from Andymark inside 2" aluminum tubing. We experience no slippage because of the tight fit on the tubing, but experience sprocket wear… to a point where we even lost some chain in some matches… good news is that drivers barely felt it…

We did have problems with gear slippage in the evo slim gb, but I believe we determined that the chain was slipping as well, given wear patterns on the sprockets.

To make it easier to see if your sprocket is turning or not, put a mark on it near the edge with a Sharpie.

We’ve used 12 tooth sprockets on our drivetrains since at least 2015, and haven’t had issues with chains skipping. Assuming nothing weird is going on in the design, there will be (for all intents and purposes) a 50% chain wrap on these 12 tooth sprockets in a west coast drive, which is more than enough to prevent skipping.

Can you provide any pictures of your drivetrain? Better yet, can you provide video of the chain skipping?

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Yes and Yes please.

OP, you are not running a traditional WCD with gearbox direct driving one wheel.

https://imgur.com/G2xlZT4

Looks like the Evo Slims are mounted backwards over the rails, which means chain runs are connecting all the wheels. I expect something isn’t stiff enough, bends under load, and allows one or more of the runs to skip.

If you really want to see what’s going on… Put the robot on blocks, stick a bar through a wheel to get a hard stop, and run the gearbox against that. You’ll find your bending, maybe at the cost of a chain, unlikely at the cost of a sprocket.
Wear your glasses! :wink:

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Unfortunately I’m out of the lab until 7/10, but I’ll try and take some pictures and video when I get back.

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