Custom Two Speed Gearbox

[Jeff Waegelin]

Originally Posted by R.C.

Def missed that guy

You guys still using the 11T pinions?

-RC

Yep. We used them last year on our drive modules, and again this year in our transmissions - we made custom plates and shafts for the AM 2-speed transmission and used the 11T press-on pinions. I love them because you don't have to deal with keys or clip rings or anything.

*The option Art mentioned using 11T gears would be nice and light, but you'd have to come up with 11T gears somehow. They are not available off the shelf, so you'd have to find some 11T spur gear stock or get a company to Wire EDM them for you or something. Incidentally, does anybody know a place that sells 11 tooth stock? That would be a pretty sweet find...

We get ours made via EDM - I've not seen any 11T gear stock, although that's not to say it doesn't exist - I just haven't looked for it that hard.

[Tristan Lall]

One thing to think about: if you want a bit more versatility in keeping your high ratio high, add more motors. That way, you can still manoeuvre hard in high gear, without worrying about current drawn due to acceleration. They gave us a lot of motors in the KOP, so you might as well use them.

I tend to design 2-speed gearboxes with the ability to spin the wheels when pushing against an immovable object in high gear, to account for the fact that shifters don't always work right. Failing to do this means that if you can't shift, you'll stall the motors pushing on things, and possibly damage the motors. (Depending on your confidence in the mechanism, this may be an unnecessary tradeoff.)

[Adam.garcia]

Originally Posted by Tristan Lall

I tend to design 2-speed gearboxes with the ability to spin the wheels when pushing against an immovable object in high gear, to account for the fact that shifters don't always work right. Failing to do this means that if you can't shift, you'll stall the motors pushing on things, and possibly damage the motors. (Depending on your confidence in the mechanism, this may be an unnecessary tradeoff.)

Can you please elaborate on how I should go about ensuring that I design the high gear to slip at stall?

Are there formulas/a calculator/white paper that somebody has written describing this phenomenon?

I'm using Blue Nitrile Tread from McMaster. However, I do not know the coefficient of friction.

Thanks

[Tristan Lall]

Originally Posted by Adam.garcia

Can you please elaborate on how I should go about ensuring that I design the high gear to slip at stall?

If you're designing the wheel to slip instead of stall, the force the gearbox can exert on the ground through the wheel should exceed the maximum frictional force that the ground exerts on the wheel. The maximum gearbox force is a function of the motors' torque at stall. The maximum frictional force is a function of normal force (itself a function of weight) and frictional coefficient.

If you're designing the gearbox to always shift into low in case of a failure, you should characterize the possible failures and try to deal with them. This isn't easy for shifting mechanisms that don't return to a home position (e.g. servos) during a malfunction, and is also difficult if you can't walk up to the robot to override a malfunctioning valve (which you can't, at least not during a match). Since redundant actuators and control channels aren't really feasible under most circumstances, taking this design approach will invariably lead you to compromise—but that might be good enough.

Originally Posted by Adam.garcia

Are there formulas/a calculator/white paper that somebody has written describing this phenomenon?

John V-Neun wrote one, and has updated it over the years. It's very useful. Others have written similar tools (like this and this), but I haven't tried them.

Originally Posted by Adam.garcia

I'm using Blue Nitrile Tread from McMaster. However, I do not know the coefficient of friction.

Probably about 1.0 on carpet (an educated guess). See here for a bit more information.

[sdcantrell56]

Originally Posted by Tristan Lall

Probably about 1.0 on carpet (an educated guess). See here for a bit more information.

Any type of roughtop seems to be more like 1.2 to 1.3. I would try to ere on the side of caution rather than pushing things so definitely use the higher coefficient of friction.