Custom Gearboxes

[PingPongPerson]

This year CV Robotics did our first custom gearboxes since 2008, because we wanted a more durable and flexible to our design option. When I calculate the speed for gearboxes I design, I usually just do a rough calculation (eg. stoichiometry to get from CIM RPM [5000ish] to feet per second). Using this I can compare it to previously used gearboxes to see about how fast we would be going. This year's gearbox was calculated at around 18ft/s:
https://www.youtube.com/watch?v=feS_KxnsZ8s

You can see our robot on the far side in red here:
https://www.youtube.com/watch?v=3UlV...DIBcQ&index=61

The robot may not be going 18ft/s, but next year if we want a faster robot, we can change the gearing so that our calculated value is a little larger, maybe 21ft/s.

Hope this helps,
Michael

[rnewendyke]

We did custom gearboxes on our 2013 west coast drive and were very happy with the results of it. We went about finding our gear ratios by looking at what other teams had done for speeds in the past as well as analyze how our own drive trains had preformed in the past. It looks like a "fast speed" is around 18 feet per second theoretical (free speed * gear ratio * wheel dia in inches * π / ( 12 * 60 ) and low gears usually run ~ 2 - 2.5 times lower that a gearbox's high gear. The highest high gear that I have ever seen was 24 fps theoretical on 3061's machine this year and in talking to them they said that they felt that their high gear was too high because they couldn't accelerate to top speed on the field and they found it difficult to control.

[VioletElizabeth]

Originally Posted by marandola316

Did you guys have any breaker issues?

In finals 2 at SVR they blew the main breaker: http://www.youtube.com/watch?v=fi68AQWPtCw

I believe they said this was pushing in high gear by accident or something along those lines. They can give better clarification than I, though.

Edit: Found it! Talks about blowing the breaker: http://www.chiefdelphi.com/forums/sh...d.php?t=128530

[TheKeeg]

Truck Town Thunder has built their own transmissions for 16 years and 2014 was the first year we bought them. I guess you can say we were experts at designing and building the strongest transmissions out there but it turns out WCP does exactly the same thing. Buying was difficult for us to do because we had never liked what reductions and speed they gave us, but those WCP 2 speed trannys are pretty sweet. We had a low speed of 5 (strictly for pushing) and a high of 15 (which I think could have been higher but we wanted a lower low speed). Buying them saves a ton of time (like 3 weeks for us) and a lot of resources. If you are set in your ways, then good luck to you, but I strongly suggest a store-bought transmission.

[Arpan]

Originally Posted by rnewendyke

We did an entirely custom gearbox on our 2013 west coast drive and were very happy with the results of it. We went about finding our gear ratios by looking at what other teams had done for speeds in the past as well as analyze how our own drive trains had preformed in the past. It looks like a "fast speed" is around 18 feet per second theoretical (free speed * gear ratio * wheel dia in inches * π / ( 12 * 60 ) and low gears usually run ~ 2 - 2.5 times lower that a gearbox's high gear. The highest high gear that I have ever seen was 24 fps theoretical on 3061's machine this year and in talking to them they said that they felt that their high gear was too high because they couldn't accelerate to top speed on the field and they found it difficult to control.

Yeah, it was far too fast. Our driver couldn't react in time and drove like he was drunk when in high gear.

That said, it was fun to be able to get across the field in just over two seconds when there was nobody in the way.

I wouldn't recommend going higher than 18 theoretical.

[Oblarg]

Originally Posted by cadandcookies

As they say, steal from the best, invent the rest. I'd agree that looking at COTS options and borrowing their ratios is probably the best option for you.

You could even take it a step further and borrow the entire COTS design, and simply make whatever modifications to the plates and gear setups you deem necessary to get what you want out of it. There's no shame in copy-pasting stuff from a published CAD in FIRST

Originally Posted by PingPongPerson

stoichiometry to get from CIM RPM [5000ish] to feet per second

I don't think that's the word you wanted...

[PingPongPerson]

You are not calculating reactant amounts, but the process is the same

[Tristan Lall]

Originally Posted by PingPongPerson

You are not calculating reactant amounts, but the process is the same

In both situations, the process is called dimensional analysis.

[asid61]

Originally Posted by Tristan Lall

In both situations, the process is called dimensional analysis.

We should probably calm down. Ask yourself: does it matter? Why am I arguing?

Generally, when doing actual speed calculations you should use a good calculator like the JVN one. I can't comment on it's accuracy, but at least it provides a general estimate of what to expect with the tread you are using.

[Oblarg]

Originally Posted by Tristan Lall

In both situations, the process is called dimensional analysis.

Well, I hope you're not calculating your robot speed strictly by dimensional analysis, because on these scales a factor of, say, 2pi is important.

[PingPongPerson]

All I am saying is that using dimensional analysis can be a quick and easy way to compare speeds of gearboxes to previous designs assuming there is not a drastic change in speed. For example, our robot last year ran at a dimensionally calculated speed of 20 fps so by gearing a gearbox for 18 fps we get a slightly slower gearbox.

For me at least, 18 fps means nothing because I cannot actually visualize how fast that is just from the number. Using a previous reference point has been useful for me at least. If there is another way to visualize this I would really appreciate the information.

JVN's design calculator is great and I still use it occasionally, but I find that a quick calculation proves just as useful in many cases.

Sorry for the confusion,
Michael

[Mk.32]

Minus drive gearboxes (since WCP WCD Gbs are so nice and basically the same thing I would build), I usually do custom gearboxes everywhere.

I just started with this team this year (2485) and apparently this is the team's first custom gb.

http://www.chiefdelphi.com/forums/sh...17&postcount=5

[Chowmaster4695]

Originally Posted by Mk.32

Minus drive gearboxes (since WCP WCD Gbs are so nice and basically the same thing I would build), I usually do custom gearboxes everywhere.

I just started with this team this year (2485) and apparently this is the team's first custom gb.

http://www.chiefdelphi.com/forums/sh...17&postcount=5

This looks like a good first attempt at custom gearbox. Couple of things I am confused about here. It looks like the cim mounting holes are not easily accessible and theres just way too much reduction for your application. Also confused on why the plate does not contour the spacers and why there are such big bolts holding the whole thing together. Seems like your shafts will locate the gearbox well enough without gigantic bolts and the pocket seems a little off... and could be more aggressive...

[Mk.32]

Originally Posted by Chowmaster4695

This looks like a good first attempt at custom gearbox. Couple of things I am confused about here. It looks like the cim mounting holes are not easily accessible and theres just way too much reduction for your application. Also confused on why the plate does not contour the spacers and why there are such big bolts holding the whole thing together. Seems like your shafts will locate the gearbox well enough without gigantic bolts and the pocket seems a little off... and could be more aggressive...

The CIM mounting holes are decently accessible, hard to see in this photo.

How do you know there is to much reduction? If you read the thread in entirely I believe I commented on it was on the slow side, but worked beautifully.

We are using 1/4-20s to hold the gb together (pretty standard in FRC AM uses it a lot), since that's what we had and they were alum bolts so they weighed nothing compared to the steel ones. I am sure 10-32s would have worked fine but what we had is what we had.

Why does the pocketing have to contour the spacers? It got a little weird since we needed to throw in all the slots which is what bolts it to the robot. And honestly I didn't really care much in the terms of aesthetics; it could have been pocketed more heavily but the mentors on the team wanted to be on the safe side. And the weight difference would have been in the grams.

[Oblarg]

Originally Posted by PingPongPerson

All I am saying is that using dimensional analysis can be a quick and easy way to compare speeds of gearboxes to previous designs assuming there is not a drastic change in speed. For example, our robot last year ran at a dimensionally calculated speed of 20 fps so by gearing a gearbox for 18 fps we get a slightly slower gearbox.

Strictly speaking, what you're doing is not simply dimensional analysis. Dimensional analysis would be multiplying the characteristic size of the wheel by the characteristic speed of the motor; it thus usually results in loss of dimensionless constants.