Questions about an Idea for a drivetrain.

We had an idea for a new transmission to power train chain assembly, and we’re wondering if anyone’s ever used this before, and if so, has it worked? Thanks.

What are your thoughts on this design?
Why are you sharing it?
Why do you think this design is worthwhile?

Share some more of your thoughts - it will get you more feedback! :]


I’m no mechanical engineer, but by driving the front 2 wheels off the same chain, you are reducing the amount of sprocket teeth off the transmission “sunk” into the chain from about 1/2 to 1/6.

My opinion… I wouldn’t trust it. Those CIM motors are pretty torquey, and if you hit someone hard im not sure if you would break something.

But then again, it is #35 chain.

Ahh, the beauty of design decisions! :slight_smile:

We wanted, as we are a rookie team, to determine whether the lower amount of chain necessary for this design (as opposed to the split design with the transmission driving both rear wheels). We are especially concerned about the pressure on the transmission sprocket and the requisite % of the transmission sprocket that must be actively contacting the chain. Thanks for your advice.

I’m pretty sure our team used something similar last year with the same motor and wheel setup for a 6-wheel design, only we powered all 3-wheels on each side by a single chain. We used a roller between the middle wheels and wheels farthest from the motor to make the chain lower, that way the chain would catch on more teeth on the drive sprocket.

Two things on this–One is your tranny-to-wheel setup. The only thing I can say is: You’re pushing it. Be prepared to use the split setup, but test your robot hard ASAP to make sure. Two, using #35 on the back is OK, but using a lighter chain (say, #25) in front is iffy. We’ve had bad experience and now use it only for prototyping, if at all. You may be able to get away with it though. Once again–TEST! Don’t wait until Thursday at L.A. to find out that something doesn’t work.

What i would tell you to do is have 2 sprockets attached to your ourput shaft. One that goes to the lead wheel. And then one that leads to the middle wheel. Then from the middle wheel power your back wheel (as you have in the picture.) That way if one of the chains pops in a match you are still life with at minimium of 1 wheel on that side still driving, so you aren’t a sitting duck in the middle of the field. But overall its pretty sweet.

I wouldn’t trust it,
my team isn’t very good at lining things up perfectly, and if that things is not lined up perfectly it will probably give alot more easily than a setup which has 50% tooth contact.
Plus it is really not saving that much room or weight… maybe 1 sprocket.
what we did last year and this year is put 3 sprockets on the center wheel, (they don’t have to be all hubbed you can use plate sprockets). 1 sprocket connects to tranny, then 1 goes to front, and one to back. This is a lot more robust, plus if you break one chain you still have 4wd, (unless of course it is the chain from your drive to center…then you’re screwed)

From your drawing, you’re going to have problems off the bat with amount of chain gripped by the drive sprocket. It would end up being better to just have a small loop being the 35 chain, and that loop being back motor to back wheel. From there you could put a 25 chain size sprocket onto the back, middle, and front wheels. This would mean that the back and middle wheel would have 2 sprockets each. In order to cut down on weight, you can use 25 chain for the linking of the wheels, and only use 35 for motor to back wheel transmission.

As for the concern stated about 25 chain not being strong enough, We have used it on all of our drive trains for the last 2 years, and not had any problems, as long as you design the system correctly.

One other small tip: Don’t forget tensioners! Regionals are a lot more fun on the field, versus in the pit sixing a chain that popped off!

I like the weight saving idea of the #25 chain to the front wheels. The biggest concern I see is the wrap of the chain on the drive motor sprocket. I would not trust the design as shown in your sketch, but if you used two small idler sprockets on the outside of the chain you could kill two birds with one stone. If the idler sprockets are mounted just below the motor and kinda close to each other you should be able to get close to 180 degrees of wrap on the drive motor sprocket. Basically the chain going over the top sprocket needs to look more like an upside down ‘U’ instead of an upside down ‘V’. the other thing about the idler sprockets is they will make a good tensioning device. That’s just my $.02 overall a good weight saving design that just needs some massaging.

I would also be concerned about the small amount of wrap around the motor sprocket. Another possible way to improve that would be to put an idler sprocket just below the motor output sprocket. You could rout the chain 180° around the motor sprocket then over the top of the idler and around the idler and back to the other wheel sprocket. Maybe you could even use the idler sprocket as the chain tensioner.

Or you could eliminate chain all together. :wink: We have used no chain on our robots since 2003.

We have used a similar arrangement with good success. Some things we have learned:

I concur regarding drive sprocket chain wrap. Try to approach at 180 degrees. You won’t get that much, but shoot for wrap as a single pair. If you use a single chain for both middle and front wheels, you need an idler as suggested to increase drive wrap.

All 035, all the time. Even if you use a single CIM to drive (we use pairs), the 025 will be a thorn in your side. We learned that the hard way two years ago. It’s well worth the weight. A robot that can’t move isn’t a robot. It’s just furniture.

We opted to use a “live” center shaft to drive the center wheel last year, and slaved the front and back wheels to the center wheel. This made for a third chain and more complex machinery, but it let us put the transmissions where we wanted them. I think two chains with an idler wrap is a more efficient approach, much as you’ve drawn. A live shaft is also more complex to build and assemble.

It would be wise to keep the wheel sprocket size a little large, say 30 or 32 teeth. This tends to de-emphasize runout issues in the sprockets.

Good luck!

To illustrate this idea for you i used paint. This is what i would do if you were going to use a drive system like this.



This is the setup 229 ran last season. We used #35 chain for both sets, but the idler/wheel/transmission setup looked a lot like that. It worked out well for us; and I would probably do it again.


Aside from chain problems, it has a pretty high center of gravity, which argues against that layout. First, the idlers would probably weigh as much as another sprocket, and, second, this year keeping CG low is going to be a big problem – especially if you plan to climb the ramp. Move the motor down inside the frame rails, driving the primary chain to the center wheel, and then run lighter chains (if you want you could just use #35 chain instead) to the front and rear wheels. Titan Robotics (492) has used this VERY successfully for the last two or three years. In fact, this is a lot like what we are doing this year. Our transmissions are only going to stick up about an inch above the frame rails.

I REALLY like this approach. Nothing breaks your heart more than seeing a bot on the field, going in tight circles, and you can see a motor spinning the air and nothing else.

Are you steeling our design.

I think you idea is great and worked for us last year.

Our team started with a setup exactly like this (from Greg’s post - sorry I don’t know how to include his thumbnail in the reply) but quickly ran into issues on the ramp. When the bot would go over the peak at the top of the ramp the chains would drag. We have redesigned and are now using a setup similar to the previous post (black drawing). Watch out for you chain clearance on the peak of the ramp.

Your sketch show that the ‘light’ chain runs on smaller (less teeth) sprockets than the ‘heavy’ chain. Is that really what you intend or am I reading too much into it. No one else commented on this so I just thought I would mention it.

Yes you are right, if a chain is running 1 wheel we use #25 chain and if it is running 2 wheels we use #35 chain. The wheel on the right has #35 chain on it just to keep things symetrical in the gear box.