Custom Gearboxes
 

So our team has been thinking about designing custom gearboxes to achieve a fast high gear and strong low gear. How do teams create such fast/strong gearboxes? I tried playing around with the JVN calculator to see what I could get but couldn't get what we wanted.

Re: Custom Gearboxes
 

I'm not really sure what you mean by "we couldn't get what we wanted" out of the JVN calculator. Were you using the single speed or two speed tabs? Were you using all the tooth counts of gears that are available from FIRST vendors?

My typical process for drive train gearboxes is to determine what my design parameters are for the gearbox (shifting or no? max speed/min speed? certain time to distance requirements?), then playing around with JVN until I get a set of gear ratios that work.

I guess the main thing about the JVN calulator is that, like all calculators, it's only as good as what you put into it. It's not magic-- one of the often ignored parts of design is determining what exactly you want out of what you're designing. In my experience there hasn't been a single gearbox I've designed that the design wasn't at least in part simplified by the JVN calculator.

Re: Custom Gearboxes
 

When I said "we couldn't get what we wanted", I meant that we couldn't get a fast speed for high gear and a strong low gear. I was using the two speed tab, and yes, I was using the gears that Vex offered. We were hoping to get at least 13f/s for high gear, and a reduction of at least 10:1 gear ratio.

Re: Custom Gearboxes
 

What's driving these numbers? How do you know you want a 10:1 gear ratio? Wouldn't that depend on the wheel size a little bit? And what you're doing?

Re: Custom Gearboxes
 

Well we just want a general gearbox that would have a fast high gear and a strong low gear, and we use 4 inch wheels.

Re: Custom Gearboxes
 

Actually, I've alwasy been confused about something in the JVN calc: it lists the draw at maximum output load, but is that the normal drive load or pushing match load?

Re: Custom Gearboxes
 

So, why build your own? That sounds like a pretty cut and dry use of COTS components? For example http://www.wcproducts.net/wcp-shifter-3cim/ (scroll down to the charts for the speeds) has several options that meet your needs.

I'm not saying don't go custom, just saying that you might want to look at options and see what concepts you can steal.

Re: Custom Gearboxes
 

WCP's drivetrain calculator gives a more in depth analysis than the JVN calculator. http://www.wcproducts.net/how-to-drivetrain/

Re: Custom Gearboxes
 

I think it would be nice to learn making custom gearboxes. If it doesn't work, we could always buy them. And I never knew that site that you gave me existed, so I didn't know if there was something like that.

Re: Custom Gearboxes
 

This calculator only works with Vexpro and WCP gearboxes, unless you want to use the same gears and ratios for a custom box.

Re: Custom Gearboxes
 

With 4 inch traction wheels 1678 Citrus Circuits ran a high gear of 22 fps(theoretical free speed) and a low gear of 8 fps on our 6 CIM drivetrain using VEXPRO ball shifters.

Re: Custom Gearboxes
 

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 might also want to be a bit more flexible with your design-- if you're designing for a certain speed and ratio, you might need to be flexible with your wheel size, for example.

Re: Custom Gearboxes
 

Did you guys have any breaker issues?

Re: Custom Gearboxes
 

Custom gearboxes are nice, but WCP 3-cim gearboxes are hard to beat.
I would go custom only if I am confident that I can get an appreciable reduction in weight (at most a 2.25lb gearbox) or size (because WCP gearboxes are pretty large). Or if you have a gearbox design you want to try in the offseason, such as a bevel box or something similar then it would be good to try custom.

Re: Custom Gearboxes
 

Neither, it's an overly simplified model of total current draw. It doesn't take into account stuff like battery voltage drop.

JVN's spreadsheet is a very good first pass, but you really should look into it deeper. Check out Ether's drivetrain simulation program that takes that stuff into account.

Re: Custom Gearboxes
 

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

Re: Custom Gearboxes
 

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.

Re: Custom Gearboxes
 

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. :D

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

Re: Custom Gearboxes
 

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.

Re: Custom Gearboxes
 

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.

Re: Custom Gearboxes
 

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 ;)

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

Re: Custom Gearboxes
 

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

Re: Custom Gearboxes
 

In both situations, the process is called dimensional analysis.

Re: Custom Gearboxes
 

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.

Re: Custom Gearboxes
 

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.

Re: Custom Gearboxes
 

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

Re: Custom Gearboxes
 

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

Re: Custom Gearboxes
 

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...

Re: Custom Gearboxes
 

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.

Re: Custom Gearboxes
 

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.

Re: Custom Gearboxes
 

Isn't that only true if you construct the system using dimensionless constants? You can specify that you're looking for ft/s from rev/min, and use dimensions of ft/rev (from the circumference equation, so your π should be included there in the unit conversion) to keep everything consistent.

Re: Custom Gearboxes
 

The whole point of dimensional analysis is usually to do calculations without specific knowledge of the equations governing the system, so doing this sort of defeats the purpose. Clearly one can add a sufficient number of non-physical units (such as "revolutions") and reduce any problem to "dimensional analysis" with enough care, but then you're not really doing dimensional analysis anymore.

Unsurprisingly, you usually only do dimensional analysis when the system is sufficiently complicated that you don't want to solve the problem properly (e.g. estimating the temperature of the sun or the size of the mountains on a planet of a given size).

Re: Custom Gearboxes
 

I know its too much reduction because I have seen the application it is being used in and 2 mini cims is just overkill to move an intake up and down.

And for the rest of the comments, it seems like you know the things I said are the things that could be optimized and you should ask yourself can I do better? Why leave your designs unoptimized? When your design is optimized in weight , speed, and resources that's when your getting the most bang for your buck. Chow out