pic: Team 192's 2014 Gearbox

[Rauhul Varma]

Originally Posted by asid61

Hi,
I was wondering what the weight of this gearbox was? Did you cheesehole any of the gears?

The gearbox weighs 1.5 lbs without motors and 9.3 lbs including them.
As for cheese holing, the pinions were too small to be cheese holed and most the aluminum on the bull gears was bored out to make cavities for the bearings to press into.

[alb4h]

I'd like to hear more about the design process that you went through. What design tools did you use? How were various iterations evaluated? What recommendations would you give other teams interested in designing their own gearboxes?

Thanks,
Ann

[Rauhul Varma]

I’ll start with the last 3 questions then get into the design process.

What design tools did you use?

The whole gearbox was completely modeled in Inventor and then individual aspects were analysed in Solidworks. The reason for the two programs is because the team models in Inventor, however Inventor struggles with FEA while Solidworks does not. Additionally I used the gates technical manuals a lot as well as their belt theory manuals to get a strong understanding of how the belts preform.

How were various iterations evaluated?

The competition gearbox is actually about the 8th iteration of the gearbox, the 7th iteration was the one that was built in the fall as the prototype. Several of the earlier iterations were improved on so quickly in CAD that they weren’t fully completed before moving onto a updated design. Most of the evaluation of these early iterations was done almost entirely by myself with a little input from Joey Milia. Once the 6th iteration was completed in CAD it was more formally reviewed by Joey and a few other mentors and members of the team. The design was heavily assessed for, manufacturability, ease of assembly, ease of maintenance, reliability, cost, size, and weight.

After the corrections, parts of this 7th iteration were tested in solidworks using FEA and the gearbox was manufactured to do physical testing. The changes between the prototype and final are subtle, mainly changes in motor placement, gearing, and the shifting shaft profile. The shifting shaft continued to be tweaked nearing the end of build as I saw how they performed.

What recommendations would you give other teams interested in designing their own gearboxes?

My main piece of advice would be to have a clear goal of what you want the gearbox to do and know what having this custom gearbox would let you achieve. If the taxing on the team’s resources outweighs the benefits, don’t waste your time; there are a lot of good gearboxes you can just buy and I’d suggest just buying one of those.

If you do decide that a custom gearbox is beneficial I’d recommend you make really a good layout sketch that has all the parts in your gearbox. Having a sketch with every element of the gearbox on it makes transitioning into 3D, and quick adjustments to the entire design, very easy, speeding up the iterative process.

From here you can base all of your parts off of these one or two layout sketches. That way, if you make any changes in the sketch, all the parts update so you don’t have to remake the entire part.

For example, here is the layout sketch of the build gearbox:


I'd like to hear more about the design process that you went through.

The best place to start would be the goals for the Drivetrain. For the past couple years 192’s main goal has been space efficiency, and sometimes that was at the cost of power efficiency. For this year I wanted the gearbox to both, have a smaller footprint than it has had in the past, and not have the inefficient right angle stages we’d used in the past. To achieve this I looked to combine techniques used by others and that we had used previously. I drew on, 971’s gearboxes that place the motors over the wheels, the VEX ball shifter that reduced the size of a two speed gearbox, and the use of belts and placing the motors on top of the gearbox that we used in 2013.

While I was deciding what options to pursue to reach the goals I made the below matrix of the possible ideas. I highlighted the possibilities I liked in red and added +1J to the options that Joey approved of.



I used some of the highlighted/+1J ideas as a goal for the design I wanted to prototype. (Note these were only things I wanted to do, and it was completely acceptable to cut some if they proved to make the design too bulky or raised other problems.)



Once the general aspects of the gearbox had been established, a modified version of JVN’s design calculator was used to determine the final gearing. I was careful to use only gears available in aluminum from WCP or VEX and belts and pulleys available from SDP-SI.

The next step is the actual design of the gearbox. Moving into inventor and laying out all the parts of the gearbox and playing with geometry. This sketch included everything, gears shafts, bearings; everything that would affect geometry. From here the design process moved along like I described in the question about iterations. I found a layout that worked and made lots improvements from there.

[asid61]

Originally Posted by Rauhul Varma

The gearbox weighs 1.5 lbs without motors and 9.3 lbs including them.
As for cheese holing, the pinions were too small to be cheese holed and most the aluminum on the bull gears was bored out to make cavities for the bearings to press into.

1.5lbs? Not 2.5? That's insane (ly awesome)!

[alb4h]

Thank you so much for taking the time and posting these details!!!!
Ann

[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.

[Rauhul Varma]

Originally Posted by Chris is me

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.

7000 RPM @ 4 max HP is still well within 5mm pitch GT2 belt limits, so they really could be used on any stage, but as you said they are better suited for end stages, mostly due to their size.

Originally Posted by Chris is me

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.

I'm sure many on 971 could have a long discussion with you about the strengths and drawbacks of the HTD vs GT Series belts. IIRC they ran GT3 belts in their drivetrain due to the shock the belts experienced with their high CoG. (Take that with a grain of salt as I did not look at their robot too much this year)

I personally go with GT2 in every application as I have yet to find a place where they don't 'just work.'

(here a link on htd vs gt2)

[Chris is me]

Originally Posted by Rauhul Varma

I'm sure many on 971 could have a long discussion with you about the strengths and drawbacks of the HTD vs GT Series belts. IIRC they ran GT3 belts in their drivetrain due to the shock the belts experienced with their high CoG. (Take that with a grain of salt as I did not look at their robot too much this year)

I personally go with GT2 in every application as I have yet to find a place where they don't 'just work.'

(here a link on htd vs gt2)

This is something my team has gone back and forth on as well. Our first belt drive had several failures and was done with 9mm HTD. We changed some things including the pulley size and went to 15mm* GT2 for a few years. This year we used 15mm HTD in order to get parts faster and we had no problems. The only reason we don't go GT2 is that pulleys and belts tend to be less available with longer lead times than HTD. For something like a gearbox like this I would absolutely go with the stronger tooth profile.

*(At this point I'm pretty sure that for the end of a driveline 15mm is a lot safer than 9mm for 4" wheels and pulleys in the 24T range, regardless of tooth profile. If the difference in tooth profile is your factor of safety between failure and success, maybe try going a bit wider.)

[Rauhul Varma]

Originally Posted by Chris is me

This is something my team has gone back and forth on as well. Our first belt drive had several failures and was done with 9mm HTD. We changed some things including the pulley size and went to 15mm* GT2 for a few years. This year we used 15mm HTD in order to get parts faster and we had no problems. The only reason we don't go GT2 is that pulleys and belts tend to be less available with longer lead times than HTD. For something like a gearbox like this I would absolutely go with the stronger tooth profile.

*(At this point I'm pretty sure that for the end of a driveline 15mm is a lot safer than 9mm for 4" wheels and pulleys in the 24T range, regardless of tooth profile. If the difference in tooth profile is your factor of safety between failure and success, maybe try going a bit wider.)

I absolutely agree. We run 15mm wide 5mm pitch GT2 belts in our WCD with an exact C-C just so we never have to worry about belt failure nor tension.

I have experienced long lead times for GT2 profile components, but since we order them super early in the season its a non issue.

[Adrian Clark]

Rauhul,

First off, thanks for taking the time posting such informative responses. This thread has been a lot of fun to read.

Do you think you could elaborate on how FEA plays a role in your gearbox design process? Like what aspects of the gearbox do you use FEA to optimize?

Thanks,
-Adrian

[Rauhul Varma]

Double Post

[Rauhul Varma]

Originally Posted by Adrian Clark

Do you think you could elaborate on how FEA plays a role in your gearbox design process? Like what aspects of the gearbox do you use FEA to optimize?

I definitely can! I’ll tackle the second question first. Generally I focus on using FEA on the components of my designs that I’m not sure will be able to take the loads I’ve ballparked. For example, basically all FEA I preformed this year was to analyze whether or not the output shaft in this gearbox and the near identical one in our winch would fail/plastically deform/elastically deform to the point of not functioning correctly.

If I feel like something I’ve designed is risky, I definitely take that part (and system) into an FEA simulation. Since build obviously isn’t infinitely long, I make sure that I only take a small portion of the gearbox into FEA and leave everything in an unoptimized state.

For example, I could have taken much more material off the gearbox plates and made the gearbox *much* lighter/smaller through a couple other changes too, however I left the plates thicker and did not makes these changes cause I knew they would just work as is and this reduced the time needed to design our gearbox. (The plates would have looked a lot more 254 esk, more air than plate.)

Another point of note is that I’ve never had anything close to formal training on how to use FEA to analyze designs, so I always take my results with a mound of salt and run many many tests just to verify that my initial results actually make sense. (I do this mostly because I know how easy it is to mess up an FEA sim and if you just use the results without thinking you can fool yourself into thinking a design will work when it will fail, or the complete opposite.)

I hope this answered your question!