Chainless drive system?

[sithmonkey13]

We have been working on several possible drive systems for our robots (both FIRST and on our own) and we have hit a question. We would like to have the possibility of mounting the gearbox and motor directly to the axle (avoiding using a drive chain, which we seem to always have a problem with.) Our lead mechanical guy asked, "How do you mount the bearings? Normally they are the wheel to allow it to spin around the axle, so would you mount them to the axle or what?" This question has been bugging us, and we view it as the step to maybe going to more advanced drive systems (such as swerve, mecanum or holonomic instead of 2 or 4 motor drive (with one gearbox per side of the robot) and chains going to the wheels.))

[kgzak]

You can attach bearings to the shaft and have to the gearbox turn that shaft. Use bearing blocks or something to that effect and hold the bearings with the shaft and then attach the output to the shaft on the bearing blocks. We tried this our second year and it would have worked, if we had the right gear ratio.

Or

Use Hubs and attach the wheel directly to the output shaft of the gearbox. Assuming your shaft is long enough to get the appropriate hub and spacing on it. This is the method we now use. It's very reliable and not as many fail points.

[EricH]

There are some comments I want to make here.

1) Many, many mecanum, swerve, and holonomic drives use chain. Don't ask how many chains a typical 118 swerve robot has; they rarely if ever throw one. I believe they use floating tensioners.

2) By eliminating the chains, you eliminate an easy place to adjust the gear ratio if you need to. This can be a factor on some "random" Thursday when you realize that the robot can't go over a field obstacle and needs to, or loses every pushing match, or isn't fast enough...

3) If you're always having trouble with chains, it probably means that you have them improperly tensioned or possibly there's a misalignment between sprockets. #25 chain is unforgiving but light; #35 is heavier but more forgiving, though even it has a limit. It's also possible that you may need to check your masterlink practices...

4) One big potential problem is that if the drive wheel on one side comes off the ground, that side is stuck. The easiest solution? Chains/belts/gears to another wheel on that side. (Mecanum and swerve can avoid this easily; omni may be able to if designed right.) Guess where that leaves you?

5) Other options: Gears (yes, gears--ask 25 and/or 703), belting (Gates is giving it out to FRC teams), and proper use of chain.

I'm not saying that direct drive is a problem--it isn't--but there are other options out there to play around with.

[Chris is me]

All of that being said, chains are annoying and loud and you have to tension them, so I don't see why you would use them when unnecessary. How often do you gear the drivetrain wrong enough that you want to change a sprocket anyway?

Anyway, you can drive a wheel off an axle by either attaching a hub to an existing wheel, or using a live axle wheel. Most wheels you're familiar with are dead axle, and mount bearings in the wheel; live axle wheels receive power through the axle and thus shouldn't have any bearings in them.

Live axle / direct drive from gearbox drivetrains can be implemented on a lot of robots to save a bit of weight and to decrease the amount of sprockets and chain on a robot. Many robots direct drive the center wheel and then chain sprockets out from the center.

[IndySam]

We had 4 AndyMark nano's directly driving our wheels last year and it worked great.

No chain hassle to deal with and no heavy chains.

This year AM has their line of nano tubes, chain, belt or gear driven.

[JamesCH95]

Just FYI: you're going from a 'dead axle' (where the wheel axle doesn't spin) to a 'live axle' drive system.

An OTS mounted bearing might work well for a prototype, check out McMaster PN 5913K61 as an example. You could also machine your own bearing brackets. I would look at machining U-channel to mount the transmission to one leg, a bearing in the other leg, and machine out the webbing to let the wheel pass through. Good luck!

[Alan Anderson]

If you put the wheel directly on the output shaft, there are already bearings in place as part of the gearbox. There's a rule of thumb saying not to have more than two bearings on a shaft. Leaving it unsupported at the "far" end, using a cantilevered wheel, usually works fine as you take care not to put "too much" side load on the gearbox shaft.

Non-scientific anecdote: four AndyMark nanoboxes hold a robot's weight this way quite nicely.