pic: Team 192's 2014 Gearbox

[s_forbes]

Beautiful. I suspect that the multiple mounting points for each CIM are to vary the belt tension? Also curious on what you did to manufacture the plate with a gradual and accurate bend. The details on this gearbox are amazing!

[MetalJacket]

Wow, almost thought that was a render at first. Neat idea using pulleys for your first stage of reduction - I'm guessing that was to allow you to locate your output shaft off to the side like it is (motors over the wheel well?).

[Rauhul Varma]

Originally Posted by MetalJacket

Wow, almost thought that was a render at first. Neat idea using pulleys for your first stage of reduction - I'm guessing that was to allow you to locate your output shaft off to the side like it is (motors over the wheel well?).

I did use belts on the initial reduction so I could put the motors wherever I wanted and the motors do hang over the center wheel.

[Steven Donow]

I've yet to see a gearbox from you guys that isn't a work of art. Such s nice gearbox

[Jonathan Norris]

Just incredible, so compact and light. Never thought of using belts on the motors, the weight savings must be huge.

[roystur44]

Great job,

Would love to see the other side with the gear shifting layout. Is there a encoder on the transmission? Neutral?

[Rauhul Varma]

Originally Posted by roystur44

Great job,

Would love to see the other side with the gear shifting layout. Is there a encoder on the transmission? Neutral?

Unfortunately I'm not sure when I will be able to post a side view as, but I can give you a some details and will post a picture once the others are assembled.

The gearbox features a custom ball shifting mechanism running at 19.5 ft/s and 8.5 ft/s with VexPro (originally 1/2" Hex) Gears on R10 Open bearings from USA Bearings and Belts (unlike the bushing in the VexPro gearbox). There is an encoder from US Digital encoder (S4-120-250-N-S-B) on a 3D printed mount connected to the shaft with the two black plastic pulleys via a section of 1/4" ID surgical tubing.

[BBray_T1296]

I am surprised you opted to have the 2 CIMs with the pulleys at the shaft tips and the one with the pulley at the bushing.

Is there any specific reason for this? I would have thought the other way around would be better.

[Rauhul Varma]

Originally Posted by BBray_T1296

I am surprised you opted to have the 2 CIMs with the pulleys at the shaft tips and the one with the pulley at the bushing.

Is there any specific reason for this? I would have thought the other way around would be better.

The main reason reason was so that the various mounting holes for the motors were not covered up by a belt, also I prefer having the longer lengths of belts closer into the plate.

Another thing I forgot to mention, the gearbox actually is 2 CIMs and 1 MiniCIM, with a CIM and MiniCIM sharing a belt.

[Answer42]

You guys have certainly elevated drive train gearboxes to an art form. I think the thing that strikes me about them is the level of detail and thought put into them is not often seen in a six week design window, even from the great teams. Good luck this season

[DampRobot]

Originally Posted by Answer42

... not often seen in a six week design window...

I believe they at least partially develop thse gearboxes in the fall.

Originally Posted by DampRobot

I believe they at least partially develop thse gearboxes in the fall.

Champions are made in the offseason.

[nathannfm]

Originally Posted by Rauhul Varma

the gearbox actually is 2 CIMs and 1 MiniCIM, with a CIM and MiniCIM sharing a belt.

Originally Posted by Joey Milia

The belts are 9mm wide, 3mm pitch Gates GT2 belts. The pulleys on this gearbox are 20T aluminum and 48T nylon filled ABS.

I am thinking about designing a gearbox that would have a CIM, a Mini CIM, and a larger pulley belted together and I had a few questions.

Are there any problems with belting the 3 together considering the ~15% difference in free speed between the CIM and the Mini CIM?

Is 20T the smallest you can get away with with this type of belt, and why was it chosen over 9mm wide, 5mm pitch HTD belts?

Did you have any problems with this gearbox over the season and did you end up needing to use the tension adjustment CIM mount holes?

Thanks.

[Rauhul Varma]

Originally Posted by nathannfm

I am thinking about designing a gearbox that would have a CIM, a Mini CIM, and a larger pulley belted together and I had a few questions.

Are there any problems with belting the 3 together considering the ~15% difference in free speed between the CIM and the Mini CIM?

We did not have any problems with the mismatched free speeds. CIMs and MiniCIMs were designed to be interchangeable; Additionally the free speeds of both motors have a huge tolerance range so you really have no idea how mismatched they are.

CIM Free Speed: 5,310 rpm (+/- 10%)
MiniCIM Free Speed: 6,200 rpm (+/- 10%)

Originally Posted by nathannfm

Is 20T the smallest you can get away with this type of belt, and why was it chosen over 9mm wide, 5mm pitch HTD belts?

Firstly, no; you can put a smaller pulley on the CIMs. GRT has used 16 tooth pulleys in the past and you might be able to go smaller, we haven’t tried.

However, there are a lot of considerations that you have to be aware of when designing belt/pulley system. (here are just a few)
1) To get the full strength from a Gates Powergrip series belt you need at least 6 teeth in mesh and at least 60 degrees of wrap. When you use a smaller pulley this becomes harder to do.
2) When you use a smaller pulley you put the belt through a tighter radius, which can lead to a decreased belt life.
3) The width of a belt (IIRC) linearly correlates with it’s maximum power transfer

HTD vs GT2: HTD belts have substantially lower max power ratings so switching to HTD would have put the belt far out of spec.

5mm vs 3mm Pitch: using 5mm pitch components would have been safer due to their increased load ratings, however the pulleys required to get the reduction (with correct wrap, ect.) I wanted would have been a lot bigger and did not fit with the rest of the design.

Originally Posted by nathannfm

Did you have any problems with this gearbox over the season and did you end up needing to use the tension adjustment CIM mount holes?

We never removed the gearboxes from the robot over the course of the season and they never need any repairs. The holes are to account for the manufacturing tolerance in the belt length; once installed, the belts should not need to be retensioned.

[Chris is me]

Originally Posted by nathannfm

Is 20T the smallest you can get away with with this type of belt, and why was it chosen over 9mm wide, 5mm pitch HTD belts?

9mm wide, 5mm pitch is more suited for the higher torque, lower speed belt spans of drivetrains. For an initial reduction, smaller belts of smaller pitches can perform just as well.

Whether HTD or GT2 is better is subject to some debate, and it also depends on the application. GT2 is rated for higher loads, but some say HTD's deeper tooth allows it to handle reversing loads better. HTD is probably a bit closer to optimal at the end of a driveline compared to the beginning.