Chain Drive Question

[Grommit]

Originally Posted by ahecht

I'll add in another plug for the adjustable sprocket-box that 190 has used the past few years:

The black line is the chain, the two larger sprockets are idlers, and the smaller sprocket is attached to the motor. On the bottom of the image, you can see the chain returning. By tightening or loosening the bolt, we can adjust the tension. Also, this design assures that we have a full 180 degrees of wrap around the motor's sprocket (this image isn's quite accurate).

The other nice thing is that by flipping this design around on the other side of the robot, and driving the lower part of the chain, we can have both motors driving in the same direction when we drive forward:
EDIT: Just to clarify, on each side, there are two identical lexan plates, one on either side of the sprocket. Therefore the adjustable sprocket is actually supported by two bolts, which need to be adjusted together.

That's a great way to keep the chain tensioned! I was curious about your bolt: did you have trouble with the bolt loosening in the competition? Did you use some kind of solution inside to increase friction so the bolt wouldn't slip? Or was that not a problem anyways? Also, was the sprocket shaft on a sliding slot or was there some other way to move the sprocket along?

We might consider something along those lines next year, especially if we added another pair of wheels. It would avoid using three sprockets on a shaft, one for drive and one to each other wheel: we could drive off one of the other two without sacrificing the 180 degrees of chain contact.

Very well implemented idea, thanks for sharing it.

[ahecht]

Originally Posted by Grommit

That's a great way to keep the chain tensioned! I was curious about your bolt: did you have trouble with the bolt loosening in the competition? Did you use some kind of solution inside to increase friction so the bolt wouldn't slip? Or was that not a problem anyways? Also, was the sprocket shaft on a sliding slot or was there some other way to move the sprocket along?

We didn't have any trouble with the bolt loosening, but if it is a concern, you can coat the bolt with a very thin layer of locktite before inserting it (make sure the locktite is completely dry before inserting, as the solvent will melt the lexan). In 2k3 we had trouble with our chains popping off, but that was due to our flexible frame and flaws in our manually milled (not CNC) 80 tooth sprocket, not our chain tension.

The adjustable idler's shaft slid in a slot milled into the two lexan plates, relying on the tension in the chain to keep it pressed up againt the bolt. The fixed idler's shaft just fit into two holes.

Another thing to note is that if you don't need your motors spinning in the same direction (which we didn't this year because we could pneumatically lock the two sides of the driveline when we needed to go straight), it is possible to design the system without the stationary idler (thus having your chain make a 'z' shape).

[Avarik]

Cheesy Poofs had a little rubber chain tensioner last year, though I have no clue what it was actually called (I believe they ordered it from McMaster?)

Basically you would squeeze it in and stick it in the chain. As the chain loosened, it expanded and tensioned it. I am not quite sure how well this worked, but I did hear them say that it never fell off their robot. Try asking them at nats what it was exactly.

[Mr. Ivey]

I'm sure the others have given out the knowledge that I'm about to post, but I would like to just sum up my knowledge. It never hurts to have more info out there.

Chains are fun, till they pop off, then you have a problem, because you are dead in the water.

Tensioner
A tensioner will help you avoid the problem of a chain popping off. Reason, it adds tension to the line so there is no play in the line allowing the chain to stay securely on the sprocket. But you must have the tensioned in really really really good alignment with the other sprockets. If it's 1/2 an inch wide from the drive sprocket you have a big problem. Time for example lesson: Little Johnny V-Neun is riding his Schwin Apple Crate, and he shifts to 5 gear, and his chain pops off. Why, his derauler pushed the chain too far past the 5th sprocket. End of lesson. Moral if the tensioner is too far wide it will pull the chain right off your drive sprockets. That's the reasoning behind that point of alignment. Next is free spin of the tensioner. Remember this little guy has to spin with the chain, so you don't want to tack weld it to a shaft, which is obvious. This little guy needs to have something holding him in place to the shaft that it's on. You can use many "holding devices" like washer but steel ones don't allow much spin. You can use ball bearings on the sprocket and have something light weight holding them in which would be really cool but costly in weight and price. I recommend using something to the extent making a designed fault into this system, sorta. You really don't want to mess up your sprocket. So the intentional weak in my opinion would need to be the things holding the sprocket in line, but don't make them out of foam or Krispy Kreme Doughnuts. Use something like nylon washers with lots of lube between them, with a nylon spacer between the shaft and the sprocket. This will allow free spin and good strength holding the tensioner in space. No where do you want them, how many do you want? The best way I think to do a tensioner is using 2, one on each side of the chain, having them on some sort of screw so both sprockets are evenly set on the chain. Orentation of the tensioners yet again in my opinion is in the middle, having them on the outside of the chain, pinching it creating tension. Spreading the chain out is inviting something to get stuck in it. The biggest fault about a good tensioner is the torque that is put on it. If you over stress the shaft that the tensioner is mounted to you can cause trouble. By bending the tensioner too far, you can cause the chain to jump or other issues, like loosing tension and not going anywhere.

No Tensioner
This is the way I would honestly go, nothing to get in the way, or add weight, or break in my opinion. Just cut out a few links in the chain, and you have nothing to worry about, except if you don't leave enough play in the chain. If you don't leave enough play in the chain you can cause the system to bind, or you can torque the shaft that the sprockets are connected to and then you have even bigger problems. But you can reduce these problems by using mounts, so you don't couter leaver the drive shafts.

Just wanted to get what I know out there for those that asked for information. Hope it helps.
ivey

[Brandon Holley]

Originally Posted by kacz100

Just some friendly advice (for next time) go find the formula that will tell you the center to center distance of the sprockets so that the chain fits and no tensioner is needed

Unfortunately the world isnt perfect and that doesnt always work. Sometimes u may just be between a half and whole link