18-09-2013 04:10

cbale2000

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.

18-09-2013 04:42

AllenGregoryIV

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.

18-09-2013 09:14

thefro526

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.

18-09-2013 10:19

IndySam

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.

18-09-2013 12:31

Aren_Hill

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

18-09-2013 12:42

AdamHeard

Re: pic: Spectrum Fall 2013 Drivetrain

If I had to pick? All of them

18-09-2013 15:55

thefro526

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.

18-09-2013 16:08

Madison

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.

18-09-2013 17:08

AllenGregoryIV

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.

18-09-2013 17:55

Aren_Hill

Re: pic: Spectrum Fall 2013 Drivetrain

We'll see what we can do.

18-09-2013 22:56

Ryan Dognaux

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.

18-09-2013 23:19

AllenGregoryIV

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.

18-09-2013 23:27

Chris is me

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.

19-09-2013 01:58

DampRobot

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?

19-09-2013 03:00

AllenGregoryIV

Re: pic: Spectrum Fall 2013 Drivetrain

I can't tell you why you should like it but I can tell you what's attractive to us.

Off The Shelf Parts
Outside of the sheet metal which we get produced by our sponsor, every piece in this is off the shelf and can be assembled with little to no modification. Nothing requires a lathe or a mill. Seeing as we don't have access to either (yet), this is an important feature. It's currently near impossible to meet this requirement with belts.

Narrow
The new robot dimensions (assuming they stick around) make the standard kit bot way of doing chains seem really large, the kit bot or VEXpro drive in a day are good examples. For reference those are the types of drives we have used before. A WCD might be a bit thinner but we don't have milling and welding resources but we have a great laser cutter/sheet metal sponsor.

No tensioning or ever throwing belts/chains
A good drivetrain has a great tensioning system and never throws belts or chains. I am not confident enough that we can build a drivetrain that will meet those requirements. The gear drive solves that problem.

No need for access from the top of the box
We now have the entire width of the robot for our game playing elements. We can cover the entire top of the drive train and not worry about having to ever reach in a fix a chain or belt. We can replace wheels extremely quickly with just one bolt each. This also might be marginally safer as it is harder to get a finger or drop something in the drive.

Can change wheel configurations easily
10, 8, or 6 wheel configurations are all available with this robot without changing much of anything except removing a wheel or two. You could build the same thing into a chain/belt system but that would be a lot of tensioning.

It's a prototype
And the last reason is it's a prototype and it's different. In 3 months I might be completely on the other side of this issue and telling people to never build gear drivetrains ever. If that does end up being the case, we'll have with a nice stock of gears to use for future gearboxes and a story to tell.

19-09-2013 10:44

Ryan Dognaux

Re: pic: Spectrum Fall 2013 Drivetrain

Thanks for pointing that out, I didn't even realize there was a 3rd stage available. That makes things a lot easier.

Also thanks to you and Madison for posting your prototype drive trains in the off-season. It's been fun to read the discussion and mull over some ideas for sheet metal fab drive trains.

19-09-2013 14:46

Paul Copioli

Re: pic: Spectrum Fall 2013 Drivetrain

Yep, that's what we do. In fact, the Verskey pattern in the gears was meant to help with gear alignment for this purpose.

19-09-2013 20:11

Chris is me

Re: pic: Spectrum Fall 2013 Drivetrain

I'm not trying to bash your drive nor am I against the concept of a gear drive. If we had the time and the drive to do something different I'm sure my team would prototype one as well. Just thought I would share some info about belts.

If you can manufacture a drivetrain with enough precision for a functional gear drive, you have more than enough precision to make a dead spaced belt drive, which does not need tensioners.

A properly designed belt drivetrain should never need any maintenance. For the past three seasons we have not touched a drive belt since week 4-5 of build season (unless we had to remove an axle for another reason, but that doesn't really count). So this same advantage can apply to belts as well.

19-09-2013 20:26

AllenGregoryIV

Re: pic: Spectrum Fall 2013 Drivetrain

Yes but where do I get pulleys with the 6 hole pattern already machined (and preferably with versakeys)? Aren or Paul want to chime in here?

Also I did a quick cost break down of the Gear Drive vs. an 8 wheel #25 chain drive.
The gear system is about $170 more.

Here are the spreadsheets.

19-09-2013 21:20

donkehote

Re: pic: Spectrum Fall 2013 Drivetrain

andymark?

http://www.andymark.com/product-p/am-2234.htm

we used these in our shooter this year. worked great! I would love to see more sizes available in plastic. Buying from gates in canada takes a long time, and is far too expensive. Still did it this year though, and it was worth it.

19-09-2013 21:34

AllenGregoryIV

Re: pic: Spectrum Fall 2013 Drivetrain

Thanks Kevin, I should have clarified, that I was looking for more sizes that just the 42t AM pulleys.

20-09-2013 00:25

AdamHeard

Re: pic: Spectrum Fall 2013 Drivetrain

OH yeah! GT2 or HTD pulleys with a versakey pattern! Aren make it happen!

20-09-2013 00:45

DampRobot

Re: pic: Spectrum Fall 2013 Drivetrain

I'd be surprised if this wasn't already in the works at IFI or at other suppliers.

20-09-2013 08:52

Ajennings8896

Re: pic: Spectrum Fall 2013 Drivetrain

Looks like a really good drivetrain, I'll have to have my team look at this and maybe try something new this year. We usually just use the standard drivetrain but this looks like something we could do this year.

20-09-2013 19:26

donkehote

Re: pic: Spectrum Fall 2013 Drivetrain

I wish there were more sizes too. Ive been thinking of cadding up a drivetrain using only gears, but I could never justify the weight when compared to the same thing with the plastic 42 tooth pulleys and 15mm belts. Always liked the idea though.
How are you planning on adjusting the backlash on all those gears, or are you not worried. I cant see it being a major issue, but i thought id ask.

20-09-2013 20:40

cadandcookies

Re: pic: Spectrum Fall 2013 Drivetrain

This. Please. Please. Please. (If we keep asking does the chances of it actually happening go up).

The only current problem with belts (at lease in my opinion) is that pulleys require decent amount of post machining to be utilized the most effectively. COTS FIRST-standard pulleys would fix that (as I understand it there's something about licensing the tooth profile that stands in the way, but mon dieu if this could happen it would make my 2014 build season).

20-09-2013 20:54

AllenGregoryIV

Re: pic: Spectrum Fall 2013 Drivetrain

I agree that belts would be lighter, though we really haven't had a weight problem the last two years. We actually have been struggling to get up to 120lbs, to get more traction. Right now I would take the smaller footprint over the lesser weight option.

I haven't thought about the backlash, several teams have done gear drives and haven't mentioned it being a problem.

21-09-2013 03:22

IndySam

Re: pic: Spectrum Fall 2013 Drivetrain

Backlash, what backlash? Not a problem. Also we were under 80lbs this year so weight wasn't a problem for us.

The new small drive size was one of the factors that made a gear drive practical this past year in both weight and cost.

21-09-2013 23:25

Seth Mallory

Re: pic: Spectrum Fall 2013 Drivetrain

I like your effort for making a smaller footprint. For the last couple of years 192 has been working to reduce the footprint in a different way. For those that know 192 you will be shocked to hear that the students have a new gear box that should be more space saving then the last 2 years.

22-09-2013 14:50

Re: pic: Spectrum Fall 2013 Drivetrain

More than the last two years?! I can't wait to see this. Any idea when this super-compact transmission will debut?

22-09-2013 16:31

Seth Mallory

Re: pic: Spectrum Fall 2013 Drivetrain

As a mentor I cannot release that information. That is up to the students. Announcing that we are building a new gear box is not news as in last 12 years the students have designed and built new gear box's all but 1 year. Gear box's are a great engineering project for students.