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.