pic: Spectrum Fall 2013 Drivetrain
 

Re: pic: Spectrum Fall 2013 Drivetrain
 

A few suggestions...

1. Consider direct driving the center wheel from the gearbox. There's really no reason not to and it virtually guarantees that regardless of what might break elsewhere on the drive system (short of the gearbox itself) you'll always have one working wheel. It's also a good way to help balance the robots weight. Team 703 has done this for several years and its worked great.

2. Consider vertically offsetting the outer wheels from the center one. Doing this greatly improved the robots ability to turn. In the past when we used large numbers of wheels in our drive, we would position the wheels on an arc of something like ~30ft, it worked well because it could turn on a dime, but in a pushing match you'd always have at least 4 wheels on the floor.

3. Without knowing your teams experience with them, I might advise against using the VersaWheels, I've talked to several teams that have used them and complained that they wear down quite quickly, and unlike Traction Wheels, must be completely replaced, instead of replacing just the tread. That said, this is just what I've heard, your experience (if any) may very well be different.

Re: pic: Spectrum Fall 2013 Drivetrain
 

Thanks for the suggestions.

This setup allows us to get all the reduction we need without adding another gear before the drive shaft. If we direct drove we would have to do something similar to the VEX 3 stage ball shifter and it would push our gearboxes further into the chassis. This system should be nearly as reliable as a direct drive. This also requires less machining, since we don't ever need a bearing on the far plate.

The two outer wheels in each set of 5 are raised 3/16" (which is a lot but it's on purpose). The middle six shouldn't have much of a problem turning, since it's like a normal 8 wheel with the added benefit of part of the load on the two center wheels that don't scrub.

We ran an 8 wheel VersaWheel setup this year and loved them when they were new. (Ask the Hawaiian Kids about trying to push us at IRI on Saturday afternoon.) However they do wear down. We have swapped them three times this season. They cost about a 1/4 of a comparable wheel, so that's still a bargain but it does take up time. That is why we have them only in the corners and raised more than normal. Our 4 outer wheels didn't wear nearly as badly as the others since they are only engaged in pushing matches. We also learned that you basically can use them twice if you just swap them front to back when the edge that is pushing starts to go away.

Re: pic: Spectrum Fall 2013 Drivetrain
 

This drive train is pretty awesome, we used some similar concepts on our Drive in 2013 and it ran really well.

If I remember correctly (It's been a while) you don't need to drill out any of Vex Pro's 1/2" Hex Gears to clear the tube axles. One of our wheels per side was driven the same way that you're proposing, and all we had to do was bolt the gear onto the side of the wheel and go.

We also noticed the same wear characteristics on our Versa wheels, we ran an 8WD with the outer corners raised 3/32", and got some fairly even wear on 6 of the 8 wheels, but after a while the wheels would wear enough to make the drive sit on all 8 wheels which was solved by swapping the front and rear pairs. (sounds like you did the same thing)

It might be worth looking into the viability of using a quick change gear pair in between the Transmission and wheels. There are a handful of available ratios that have a pretty nice spread for tuning the drivetrain's final ratio.

Re: pic: Spectrum Fall 2013 Drivetrain
 

I love more and more teams doing all gear driven drives.

I would suggest that you be sure and make a cover for those trans servicing holes. The small fittings on those cylinders are very fragile and are easily broken by stuff that may end up on the field.

Re: pic: Spectrum Fall 2013 Drivetrain
 

If you guys had to pick, which gear sizes with the VersaKeys would you want with a 1.125" bearing bore?

-Aren

Re: pic: Spectrum Fall 2013 Drivetrain
 

If I had to pick? All of them ;)

Re: pic: Spectrum Fall 2013 Drivetrain
 

Seems like 40T*, 44T*, 50T, 60T and possibly 64T would be the 'most useful' to have with an R8-Sized Bore - Although it would be amazing if every gear were available with some sort of bearing (bushing**) bore.

*The 40T and 44T don't have the VersaKey, but would still be nice to have with a bearing bore.

**I think the 30T would be the smallest gear that could reasonably fit an R8 Bore, so at some point it may need to step to an R6 bore, or even a bushing bore, I'm not sure.

Re: pic: Spectrum Fall 2013 Drivetrain
 

As things currently stand, you have to be careful when bolting gears onto Versawheels with bearings pressed in, as the boss around the bore on gears prevents the face with the versa pattern from being flush with the wheel hub. We laser cut an .063" spacer with the versa pattern to take up the space and make sure overzealous students don't crush the bearing while bolting the gear to the wheel.

Having the option to select gears with bearing bores would prevent this; it'd also give us more options for clustering idler gears on common shafts. So, yeah -- make 'em all with bearing bores. :)

Re: pic: Spectrum Fall 2013 Drivetrain
 

Very good idea, I'll look into this. Thanks

That's one of the reasons I was looking at drilling out the gears, it will make it possible to use the same length spacers for idler gears and the drive gears on the inner sides of this assembly.

Re: pic: Spectrum Fall 2013 Drivetrain
 

We'll see what we can do.

Re: pic: Spectrum Fall 2013 Drivetrain
 

Any suggestions for easily doing this with the VexPro ball shifter? We've been looking at multiple gearbox options and the long output hex shaft that's sold only gives us 1.375 inches, which isn't a lot if you're trying to directly drive a wheel and a sprocket, or a gear.

Re: pic: Spectrum Fall 2013 Drivetrain
 

The "long" output shaft is used in the 2 stage transmission to allow for a sprocket(s) or another gear to drive another one that actually powers the wheel. Direct driving off of only the two stages internal to the ball shifter would normally make for a robot that is geared for too high a top speed. If you download the CAD for the 3 stage shifter you can see that the output shaft is noticeably longer for use with direct driving wheels.

Re: pic: Spectrum Fall 2013 Drivetrain
 

I really don't understand the functional difference between direct driving a wheel using a 3 stage ball shifter and using a two stage ball shifter + gear reduction to drive the wheel. I'm pretty sure there isn't one, really. You're already filling the drive line with gears, might as well put a reduction in that drive line too.

Re: pic: Spectrum Fall 2013 Drivetrain
 

I'm a skeptic. Why should I like geared drivetrains?

At least in my mind, a timing belt drivetrain is both lighter and more efficient, and a chain drive is much easier to build and quicker to fix. I do see that chains and belts becoming de-tensioned is a disadvantage, but the solution to this is a cam or exact c-c system. Is there some specific advantage that gears offer over belts or chain that can't be attained in either system?