6 wheel chain drive idlers and friction

I have a 6 wheel drive robot. And I use #25 chain to power all the wheels on each side.

Right now this is how I’m wrapping my chain (see attachment).

(Small sprocket is 22t, large sprockets are 66t)
The 66t sprockets are all anchored to large 10" wheels
The 22t sprocket is fixed onto the CIM gearbox shaft.

Without the yellow idler rollers, the middle sprocket would have very poor chain contact.

However, I am experiencing two problems using this setup:

  1. The chain still skips mainly due to the 22t sprocket being very small with few contact teeths. It stops if the chain is extremely tight, but that creates an enormous amount of friction. I am barley able to push the robot around when it is powered off due to the friction in the system.

  2. The chain eats away at the plastic rollers slowly. I tried metal rollers, and those are being eaten away too. Perhaps I should try idler sprockets?

Thanks in advance :slight_smile:

*note that this is a non-competition robot for fun. Hence all the custom non-legal parts

It is much better design practice to have separate loops of chain between adjacent sprockets. The motor would drive the first axle only. Then a separate loop of chain would run from the first axle to the second axle. And then another loop of chain from the second axle to the 3rd axle. This way, each chain wraps around ~180 degrees of the sprocket and you have many teeth being engaged with the chain.

This means that you need to stack 2 sprockets onto the first axle and second axle. However, you can potentially make those sprockets quite a bit smaller than the ones you show in that picture as you would not need the large sprocket for the gear reduction.

You can add idlers if you need to in order to add a little tension, but you will not need much tension to keep the chain from skipping.

Thank you. This makes a lot of sense!

I will try to widen the wheel sections and double stack the sprockets

I’ve worked with one drivetrain where we had a continuous chain, similar to what you are trying to do. Getting enough wrap of the chain around each sprocket is tricky. We had to use a bizarre chain routing to make it work: https://www.chiefdelphi.com/media/photos/30068 You might note that all of our idlers are just stationary plastic guides. Since there is adequate chain wrap on all of the sprockets, the chain doesn’t need to be over-tensioned and friction isn’t really an issue.

The rule of thumb that I’ve always heard is that you want at least 120 degrees of wrap around each driven sprocket to reliably engage, but more is always better. It looks like you have less than 90 degrees on the middle wheel. To make things worse, the middle wheel usually carries most of the load in a drivetrain! Not the one you want to have too little chain wrap.

I agree that adding additional sprockets to the wheels, so that you could have three chains on each side (motor to back wheel, back wheel to middle wheel, and middle wheel to front wheel) would probably be the easiest fix.

Can you confirm that the center sprocket is 66T like the other two?

Could be an optical illusion in the photo but it appears that all the axle holes are in a straight line. There also appears to be a lot of gap under the center sprocket between the chain and the bottom of the center sprocket. Thus my question about that center sprocket size…


Looks like the chain is sagging in the middle. The bottoms of the sprockets are in-line.

The idlers are going to wear in a bit, yeah. Ideally they’ll “wear happy” or wear to a sort of equlibrium point.

Your chain isn’t actually fully tensioned though, the bottom section has some sag. However if you fully tension it then you’ll have the chain contacting the middle sprocket, most likely out of phase with the teeth. Not great.

I would add a double sprocket if possible like everyone is suggesting. I think it’s too late to add one but a drop center (or raising the outer wheels) would help this thing turn a lot more efficiently too.

Finally - are you using live axles or dead? I would assume dead axles but I see keyways on the shafts and if those shafts are spinning in bare metal that’s bad news bears.

yeah, they are all 66t
Yes, the chain sags

the shafts are definately dead shafts!

Its too much hassle to lower the center drive for now, if the triple chain still doesnt work, i may have to try that

thanks for the help :slight_smile:

Have you tried driving it around or pushing it for more than a short distance? It might be tensioned in spots but not others if that makes sense. What I do when tensioning chain in similar setups is pull the chain near your wheel tight by hand as you turn the wheel, then tension the wheel.

Check the tension on your chains minus the long one on the bottom, I could be wrong but it may be you have your chains over-tensioned despite the large dip.

For the plastic roller you could try upgrading them to steel with a through hole and adding some bronze with oil on the shaft or some plastic bushings, or ball bearings if you felt like it.

Plus one on the multiple chain loops though, they are fair better than the single loop ones.

It’s been a while since I’ve anything other than c-c with chain so someone please feel free to correct me if I am wrong