# Team 2102 Presents Our Offseason Robot

#21

do you mean two arms and one central gear?

awesome bot btw :]

#22

You are right. That was the option.
I am extremely proud of the team for building an awesome and effective rendition of the robot they intended to build. And, training our new designers in the process.

#23

I have a hypothetical question. If you have an overall 1:7 ratio when driving a rotating arm at the point of rotation, will direct driving the gear track with a ratio of 1:7 have the same speed and torque?

#24

Yes. We did a simmilar setup with chain this year for our wrist. It worked well with 0 issues. https://imgur.com/a/UOLA370#U5imHJN

#25

Then where is the benefit if the result is the same?

#26

Pardon the MS paint.

Let’s say that with a dynamic load, we’re putting 20lb on the arm.

If we were powering the arm in the usual way, with the weight out 19.75" from the pivot, we need 20lb * 19.75in = 395 in*lb at the pivot. Illustrated here. That’s a lot of torque. Enough that you have to start worrying if your parts are strong enough.

Doing it this way though, you end up with less torque at the driving axle. The radius of the large gear is 8 in, and the upwards force at the gear needs to balance out the downwards force at the gripper. So:
8 * F = 19.75 * 20
F = 19.75 * 20 / 8 = 49.4 lb
Illustrated here

Because the little gear has a radius of 1 in, the torque on that axle is:
49.4 lb * 1 in = 49.4 in*lb
Illustrated here

So, by driving the arm this way, the largest force any axle sees is 1/8 of the conventional way. (Which makes sense, since those gears are an 8:1 reduction).

#27

So it reduces the stress on the system, but does it make it faster? Btw, I realize my questions might sound criticizing, but I’m just genuinely curious about your design.

#28

Nope. The speed of the system is totally dependent on the gearing and motors. We could have sped it up by having less reduction in the gearbox.

This rack and pinion gearing setup is essentially the final reduction stage in the gearing. It’s an 8:1 reduction. The gearbox we have on this is 53.6:1. Once you run it through the rack and pinion gears, it ends up being 428.6:1.

So, if we’d just made this a traditional arm with a 428:1 gearbox, it would move the same speed with the same torque. However, it would have way higher stress on it.

I’d like to reiterate though, we did this like 85% because it’s cool, 15% because it’s smart.

#29

The torque benefits aren’t from the unique gear arrangement, but rather that the arm pivot doesn’t transfer torque.

A traditional FRC arm on a dead axle has the same thing going for it.

The novel arrangement can also be packaged into a lower pivot as the bottom half the larger gear isn’t necessary (as it’s non-rotating).

#30

Yup! The arm doesn’t have more torque, but the axles don’t have as much stress on them.

#31

Which to be clear is not unique to this configuration at all. Any arm with a final dead axle will not have torque in it.

The shaft for the pinion for whatever the final stage is will have the same torque in both setups assuming same ratios.

#32

What size teeth did you end up using for these gears? I’m curious to try something like it on our router.

#34

They were 8DP, with a pressure angle of (I think) 14.5 degrees.

Edit: Pressure angle is 20°, and we filleted the roots of the gear for manufacturability.

#35

You can find the CAD model for this robot here: