# Keeping #35 Chain Tight

My team is looking at developing a West Coast Drive platform during the off season so we have more knowledge about one before this year’s game is released. We pretty much have our CAD finished up but the issue we are running into is figuring out how to keep the chain tight. We thought of making sliding bearing blocks to tighten as the chain loosen but then we realized if each side isn’t stretching the same, and we adjust one side, then it may pull to one side. What do you guys do to keep the chain tight? Do you just do the math before hand to get it close enough and just remove links when it stretches too much?

My suggestion would be to calculate the exact center-to-center distance, then add 0.012".

If you do sliding blocks, (I wouldn’t be too worried about the drivetrain drifting to one side as long as everything is properly tensioned), I’d use the small cams from WCP so you’re not entirely relying on the friction between the block and tube to hold the block in place.

Chain doesn’t stretch all that much for FRC robots, especially #35 chain. So, if you have decent manufacturing tolerances you can just set it and forget it.

If you do want tensioners just in case, vex sells bearing blocks + cams that you can buy/imitate.

This is definitely not universally true, and shouldn’t be posted as a general statement. It is a function of the size of your chain (25 vs 35), size of your sprocket, size of your wheel, loading, run time, etc. With the small sprockets used in WCDs, “chain stretch” (really, chain link pin wear) is absolutely possible and should be accounted for.

You can do the math for exact centers beforehand, and you will initially find success doing that. Depending on the circumstances the chain may indeed “stretch” though, and you should be prepared to handle it. It’s not invalid to replace entire chain runs when they stretch too much to be usable, although it is a bit wasteful. But chain will stretch too much to be usable long before it stretches an integer number of links, so the “remove a link” method won’t solve your problem - if you want to use chain as it stretches you need a tensioner.

I don’t understand what you mean by “pull to one side” here. You generally chain the axles so that each axle is connected to a stationary axle (i.e. the gearbox output shaft) - that way, adjusting the axle won’t mess up the chain tension for any other axles and you don’t have to juggle anything.

Agreed. #35 is very toleranct on spacing, so even if it wears a little you will almost never throw a chain. I don’t think I’ve seen a #35 chain fail before, although I’m sure others have seen it.

I’ve blown links apart, popped it off sprockets, and sheared several barrels.

But all of that was caused by seriously flawed design, bad maintenance, and most of all, willful ignorance of the good practices and procedures for designing and maintaining a chain system.

I operate a small gentleman’s farm. Every chain on my equipment has a freewheel sprocket that is controlled with a spring type mechanism to take up slack - a tensioner if you will. You can try to tighten your fab tolerances all you want but you might as well try to shoot an arrow at a target while on horseback blindfolded. It’s just the hardest way to get to the result you want.

Look at your chain path and find a location where you can add a sprocket on a bracket that can have use a spring to tighten it. I suggest a spring over a piece of all thread with a nut tightener for this reason - any sudden increase in tension, the spring will give and you won’t (shouldn’t) shear a crucial piece of your system. A spring is more forgiving.

If a spring isn’t feasible, consider mounting one of your freewheel sprockets into a slot instead of a hole and leave room to have some sort of adjustment with all thread to slide that axle in the slot. Small turnbuckles lend themselves nicely to small adjustments but put a nut on each side of the turnbuckle to ‘lock it’ in place after you make your adjustments. Leave at least a 1/4" play in the chain when you press on it with your thumb (I know that’s a really precise test isn’t it…) You don’t want the chain taught or it will bind and pop. Based upon your question, you already know what is too loose…

Good luck.

Back in 2008 1747 tried that (sliding pillow blocks) and we had issues with drive axles getting cockeyed. Then again, we also had chains tight enough to play music on…

For what it is worth: #35 chain does not stretch. What it does is wear. Keep the chains clean and they won’t ‘stretch’. Or consider replacing them occasionally.