WCD questions

[Fusion_Clint]

My search skills must be weak, I could not find an answer to my questions, please help us out.

Fusion is contemplating switching to WCD for this upcoming season, mainly to reduce machining time and increase drivetrain serviceability. We are starting to look at designs and plan to make a couple of chassis's this summer/fall.

My question: Why do teams put their chains on the inside of the rail instead of the outside with the wheels? It looks to really complicate the build (requires: smaller sprockets, all live axles, transmission standoffs, tensioning issues etc).

FRC teams are smart, so their must be a reason other than appearance. Show me the light. Thanks

[billbo911]

One really good reason to put the chains and sprockets on the inside of the rails is to get the rails as wide as possible. This gives you a larger frame which allows you more room to put in other components. If the sprockets are on the outside of the frame rails, it forces the frame rails closer together reducing internal space.

[Fusion_Clint]

Thanks, but would not the space savings issue be a moot point?

You still can't use the extra space because it is occupied by the chain and sprockets? If you build on top of the chain and sprockets you reduce serviceability.

[Mr V]

One reason to put the chains on the inside is to allow for some uprights attached to the outside of the frame rail to create a frame perimeter outside of the wheels and a place to mount the bumper. Depending on your wheel width you can get away with 1x2 or 1x3 for those uprights.

Another benefit is that it reduces the cantilever of the axle by putting the wheels as close to the bearing as possible rather than spacing it out for two sprockets and any spacer required to prevent the chain from rubbing on the frame or wheels.

[GeeTwo]

In the interest of disclosure, this is all from my browsing - 3946 has not built a chain-in-tube drive, even as an off-season project.

Putting the chain inside the tube provides a built-in chain guard. This protects the environment in case a chain breaks, but probably more importantly excludes stray carpet and bumper threads and other foreign materials from getting caught in the chain.

Placing the sprockets between the two supports reduces stress all over, especially given that there is already plenty of stress on those cantilevered axles. (already noted by Mr. V)

Finally, many of the chain-in-tube designs appear to go for the largest sprocket such that the link plates will just barely fit inside the tube. This offers "insurance" against chain stretch - even if the chain stretches, it cannot come off the sprockets, because there is not enough room for the plates and rollers to fit in the space between the sprocket teeth and the tube.

[Fusion_Clint]

GeeTwo,

We have zero interest in placing the chain in the tube.

We are thinking of switching from having 2 plates with the wheels, chain, and sprockets sandwiched between them; to using the West Cost Drive system of a rectangular tube with internal transmission and exposed wheels.

[Rafi Ahmed]

Putting the chain inside of the chassis give you a safeguard for the chain as GeeTwo said, even though it's not inside the tube. WCD was developed before bumpers and other robots could get tangled/break the chain. Also, there wasn't OTS FRC gearboxes then so just about everything in the gearbox was custom. Teams that developed WCD also were fairly strong in their machining capabilities, so live shafts weren't as big of an issue.

Early WCDs (with six wheels), didn't have as small sprockets. There were cutouts in the electrical boards for the chain runs. You can make it out here. Tensioning was also solved with eye shaped bearing blocks and cams.

[Chak]

Having the wheel on one side and the chain on the other also makes maintenance a bit easier, since you can remove either without bothering the other.

[asid61]

Putting the chain on one side of the tube and the wheels on the other has always had two reasons for me: Firstly, to balance out the loads between wheel and sprocket by making the bearings like a "pivot" for them to cancel each others' forces out, and secondly to reduce the cantilever and the torque arm length on those bearings. I would not run sprockets on the same side as the wheels mainly for the second reason.
It also increases serviceability by letting you access the chains without removing wheels or bumpers, although in practice bumper removal at least has to happen anyway.

[ASD20]

Chain-in-Tube is also safer, especially if the bot is going to be used for demos.

[Sperkowsky]

It's also much easier to swap wheels. By having chain/belts on the other side not connected to the wheels it's usually just a matter of getting rid of a shaft collar or loosening a screw to take off a wheel. This isn't a problem if your using colons but some of the softer grippier wheels require swapping everyone once in a while.

[Chris is me]

There is zero reason to use 2 tubes at all. It will actually hurt your efficiency (more misalignment) and you are just throwing weight away. If you want chain on the outside of the frame just put it there, between the frame and the wheels. If you use live axles this won't interfere at all with your ability to change wheels, and it's a perfectly valid solution for a WCD to do this. Teams have run WCDs like this many times before with no problems.

[RoboChair]

Originally Posted by Mr V

Another benefit is that it reduces the cantilever of the axle by putting the wheels as close to the bearing as possible rather than spacing it out for two sprockets and any spacer required to prevent the chain from rubbing on the frame or wheels.

This, seconded.

[Michael Hill]

Originally Posted by Mr V

Another benefit is that it reduces the cantilever of the axle by putting the wheels as close to the bearing as possible rather than spacing it out for two sprockets and any spacer required to prevent the chain from rubbing on the frame or wheels.

Thirded. Saint-Venant's Principle is useful. Additionally, using 2x2 box tubing rather than 1x2 would help things out. In normal cantilevered bearing applications, it's preferable to keep the bearings 3-5 characteristic distances (read diameters) apart, so with a 0.5" shaft, having the bearings 2" apart (4 characteristic distances) would really be ideal for long bearing life. But...space is at a premium and R8 bearings can be found rather cheaply.