[FTC]: Drive Platform- Design Exercise

[wgardner]

Great! I look forward to seeing pix of the final result, and learning how much torque it can withstand.

Cheers.

[DavisDad]

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[GeeTwo]

Originally Posted by wgardner

FYI, if you're considering this CAD model for use on a real robot, you might want to read some of the reviews on the Dremel angle drive complaining of lack of durability. It's probably made for high speed but not necessarily high torque. It would be a shame to buy 4 of them at $20 each, build a robot based on them, and then find that they break the first time your robot runs into a wall or another robot, stalling your motor.

Even if you solve the durability problem, the roller-on-roller transfer of energy from one axle to the other is not likely to transfer much torque (though it would be good for high-speed, low torque applications, like attaching to a dremel). With that device in your drivetrain on the wheel side of the gearbox, I doubt you would be able to get anywhere near stalling the motor; I'd be more worried about not getting enough traction to get the robot rolling at all. I don't know the contact force or coefficient of friction, so I can't even give a rough numerical estimate of the torque, but it's something else you should check before including it in your design.

[DavisDad]

Originally Posted by GeeTwo

...the roller-on-roller transfer of energy from one axle to the other is not likely to transfer much torque...

The gearbox has straight bevel gears; my model's simplified representation doesn't show the the teeth.

[mhaeberli]

Nice work!

On a very distantly related note, are there any suggestions anywhere as to how to set up an Android Virtual Device Emulator to closely track the ZTE Speed? Obviously, one wouldn't be able to do more than test out stuff not directly related to motor control, etc, except maybe with some input and output test files...

Thanks,

Martin

(one of the mentors for FTC #7593 TigerBots).

[DavisDad]

Hi Martin,

Have you seen the threads at the FTC forum. There's a lot of activity there. Here's an example:

Thread: End to end response times

[mhaeberli]

Thanks!

[DavisDad]

Here's a photo and CAD of angle gear. The square shaft is pretty light, but the design is cool as square drive shaft is routed through the hollow shaft and press fitted at the gear end. There's a lot of flex which make alignment with motor easy. I'll test for ability to handle about 1500 in-oz.

[DavisDad]

Looked at stress in square shaft. Assumed round shaft 2.5mm diam and 500 in-oz (3.53 Nm) and plugged into this on-line calculator: TORSION OF SOLID AND HOLLOW SHAFTS

It returns 160 kPSI (1100 MPa). That about 5x the the stress to break a steel rod. It's not looking good for the square shaft. Now I need to look at the gears; they probably aren't designed for the torque either...

[wgardner]

If the dremel drive doesn't work out, you could probably make your own version using the bevel gears at ServoCity and some Actobotics parts, like what is shown at the bottom of this link. So far, we're quite happy with the 1/4" axles of the Actobotics set: they're much stronger than the Tetrix axles and the bearings for the Actobotics axles seem much nicer than the bronze bushings used in the Tetrix set.

[DavisDad]

Originally Posted by wgardner

If the dremel drive doesn't work out, you could probably make your own version using the bevel gears at ServoCity and some Actobotics parts, like what is shown at the bottom of this link. So far, we're quite happy with the 1/4" axles of the Actobotics set: they're much stronger than the Tetrix axles and the bearings for the Actobotics axles seem much nicer than the bronze bushings used in the Tetrix set.

Thanks for the link, and the price is right. The same steel gears from mfgr are about $25 ea.

[DavisDad]

I tested the angle drive for torque strength. I jigged it up and applied force with a torque wrench set at 20, 25 & 30 in-lb (480 in-oz). At 30 in-lb, the shaft started "unraveling", twisted and broke. It's a bundle of wires formed into a square shaft.

I'll drill out the wire shaft and make a coupling to press fit into the 4mm ID/6mm OD bearing shaft.

[wgardner]

Originally Posted by DavisDad

I've been spending some time working with OnShape.

I'm looking for CAD models of the Modern Robotics components and your nice picture is all that I can find so far.

Did you make that model yourself or get it from somewhere? Are there models for the other components? If you made them yourself, would you be willing to export them and share them?

Thanks!

[DavisDad]

I've modeled the new motor, servo and main power modules. They are "public" at OnShape. I can export to most file types; what do you prefer?

[wgardner]

Originally Posted by DavisDad

I've modeled the new motor, servo and main power modules. They are "public" at OnShape. I can export to most file types; what do you prefer?

Great! I'll just find them myself on OnShape! Thanks!