01-09-2008 14:51

R.C.

Re: pic: FRC 1771 Drivetrain

What ratios of gears are you using on the gearbox and it looks very promising. If you have access to a CNC, might want to lose some weight.

01-09-2008 15:08

=Martin=Taylor=

Re: pic: FRC 1771 Drivetrain

You might want to support the bumpers at the ends. Otherwise they're likely to rub on the wheels, causing problems.

This was a problem for us this last season, and is one of the reasons why we probably won't do cantilevered wheels again.

01-09-2008 22:58

sdcantrell56

Re: pic: FRC 1771 Drivetrain

Thanks for the advice. This will be done on a CNC so lightening patterns may show up on this if we decide to do it. Although ~35lbs for a drivetrain may prove that reducing weight is unnecessary.

I will take into consideration the issue with bumpers flexing.

On a seperate note does anyone have any experience with UHMW wheels, specifically in a cantilevered scenario like this.

01-09-2008 23:45

R.C.

Re: pic: FRC 1771 Drivetrain

Quote:

Originally Posted by sdcantrell56 Thanks for the advice. This will be done on a CNC so lightening patterns may show up on this if we decide to do it. Although ~35lbs for a drivetrain may prove that reducing weight is unnecessary. I will take into consideration the issue with bumpers flexing. On a seperate note does anyone have any experience with UHMW wheels, specifically in a cantilevered scenario like this.

01-09-2008 23:56

sdcantrell56

Re: pic: FRC 1771 Drivetrain

Yeah I will look into cutting weight out of the frame. As for the wheels, I really want to go custom for a lot of reasons. Primarily we can get the exact size we want (5"), they are also keyed for hex shaft without a hub, and finally they are way cheaper as the material to make 12 wheels only costs ~$70. The wheels will of course be treaded with roughtop or wedgetop. I guess I am primarily asking for the durability of the actual uhmw portion of the wheel and if we go this route we will probably have a full set of spares with us at all times

01-09-2008 23:58

Jon Jack

Re: pic: FRC 1771 Drivetrain

I would be concerned about losing the bumpers with you're bumper mounts. I looks like if you got into a good pushing battle that you're bumpers could end up coming off.

Also, one of the big benefits for the west coast drive is it's maintainability. When designing something always ask yourself what do I do if X breaks, how do I repair or replace X? What happens if you break a belt? Right now it looks like you'd have to remove and disassemble part of the transmission in order to do that. That's probably going to be a lengthy repair. I would recommend using 25 chain instead.

Even if the numbers are in your favor, it's best to have the ability to repair something rather than leave it to chance. You never know what could happen.

02-09-2008 00:01

R.C.

Re: pic: FRC 1771 Drivetrain

Quote:

Originally Posted by Jon Jack I would be concerned about losing the bumpers with you're bumper mounts. I looks like if you got into a good pushing battle that you're bumpers could end up coming off. Also, one of the big benefits for the west coast drive is it's maintainability. When designing something always ask yourself what do I do if X breaks, how do I repair or replace X? What happens if you break a belt? Right now it looks like you'd have to remove and disassemble part of the transmission in order to do that. That's probably going to be a lengthy repair. I would recommend using 25 chain instead. Even if the numbers are in your favor, it's best to have the ability to repair something rather than leave it to chance. You never know what could happen.

02-09-2008 00:07

sdcantrell56

Re: pic: FRC 1771 Drivetrain

Yes in order to change a belt the transmission would have to be removed. I figure the entire operation would take slightly longer than putting on a chain however the weight savings for a belt versus even #25 chain is pretty significant. This would be something that we would really have to weigh the tradeoffs on before deciding. Plus the belts really have a cool factor and Ive been wanting to try them for a while.

02-09-2008 00:10

Jon Jack

Re: pic: FRC 1771 Drivetrain

I don't think 25 chain is lighter than belts, I would just use 25 chain to make the drivetrain easier to maintain. Another thing to consider too is that belts will stretch more than chain. This means that the slots that the front/read bearing houses sit it will have to be longer to pick up any slack created from the stretching.

02-09-2008 00:19

R.C.

Re: pic: FRC 1771 Drivetrain

Belts will break, a team posted that and it happened at the San Diego Regional. Chain is the way to go unless you have an expert helping you. Also, belts need to be tensioned before and after each match, this may become bothersome after a few matches. FIRST gets interesting and people tend to forget minor things, I know my team did. Ahh the good times.

02-09-2008 00:26

sdcantrell56

Re: pic: FRC 1771 Drivetrain

Actually belts stretch much less than chain. In multiple posts by team 125 they talk about there belts not stretching at all after initially tensioning them. they rely on kevlar belt which is very strong stuff. The only positive I see to the chain is that it is easier to change if it breaks. In all other aspects, belt is the superior solution.

02-09-2008 00:29

sdcantrell56

Re: pic: FRC 1771 Drivetrain

I forgot to post the gear ratios so here goes. The first stage is 12 to 48 reduction, and then the second stage is either a 15 to 60 or 30 to 45 reduction.

02-09-2008 00:32

Dancin103

Re: pic: FRC 1771 Drivetrain

Other than needing to loose a few pounds, i would say it looks good. Keep up the good work.

02-09-2008 00:35

R.C.

Re: pic: FRC 1771 Drivetrain

Quote:

Originally Posted by Brandon Holley Thanks for the shout out akash. While I am no expert, I did spend a considerable amount of time this past season working with belts. We used .200" pitch, .375" wide kevlar reinforced timing belts from www.sdp-si.com. We finally had a belt go down after the beantown blitz this year (2 regionals and 2 off-season events). The thing about a belt is even though ours were kevlar reinforced, they are just plastic teeth. Keeping them tight will help a considerable amount, and I am pretty sure you guys did to have the belt(s) last as long as they have. http://en.wikipedia.org/wiki/Fatigue_(material) <---- FATIGUE. Fatigue is basically repeated stress over a relatively long period of time. Even though your belts may not have seen any greatly large shock loads, seeing small loads over an entire FIRST season probably fatigued the material in the belt enough that it finally broke. Congrats on making it this far into a season before having to change them! Hope it helps. Brando

02-09-2008 00:37

sdcantrell56

Re: pic: FRC 1771 Drivetrain

Im hoping I can get this drivetrain under 30 lbs. I havent made one yet that could actually function that was that light. Im working on a single speed version in case there is no need for a shifter like this past year.

02-09-2008 01:21

=Martin=Taylor=

Re: pic: FRC 1771 Drivetrain

Quote:

Originally Posted by sdcantrell56 Yeah I will look into cutting weight out of the frame. As for the wheels, I really want to go custom for a lot of reasons. Primarily we can get the exact size we want (5"), they are also keyed for hex shaft without a hub, and finally they are way cheaper as the material to make 12 wheels only costs ~$70. The wheels will of course be treaded with roughtop or wedgetop. I guess I am primarily asking for the durability of the actual uhmw portion of the wheel and if we go this route we will probably have a full set of spares with us at all times

02-09-2008 01:35

R.C.

Re: pic: FRC 1771 Drivetrain

Quote:

Originally Posted by Hachiban VIII I'm worried about hex-broaching plastic. Doesn't seem like a good idea to me. Might want to go with some of those Al AM hubs.... or wheels.... For the record you CAN buy 5" plastic wheels for less than $70.

02-09-2008 03:59

Cory

Re: pic: FRC 1771 Drivetrain

Quote:

Originally Posted by Dancin103 Other than needing to loose a few pounds, i would say it looks good. Keep up the good work.

02-09-2008 08:45

sdcantrell56

Re: pic: FRC 1771 Drivetrain

I don't know of any place to purchase 5" plastic wheels from other than colson wheels which are not hey keyed and which do not allow for tread attachment. Also the price of $70 is not per wheel that was the approximate price that itwould cost us to make 12 wheels, enough for 2 robots.

Has anyone actually tried broaching UHMW or any plastic for that matter? I would think it wouldn't be a problem but if that is not the case then I have to change my plan.

I agree and was under the assumption that anything under 35lbs or even 40lbs is a light drivetrain. I now have 2 drivetrains designed under 35lbs, one cantilevered and the other using plates and standoffs.

02-09-2008 15:17

Madison

Re: pic: FRC 1771 Drivetrain

Quote:

Originally Posted by Cory 35 pounds is a very admirable weight for a rolling drivebase. Very few teams have made theirs lighter.

02-09-2008 15:51

=Martin=Taylor=

Re: pic: FRC 1771 Drivetrain

Quote:

Originally Posted by M. Krass Our '08 drivetrain was 35 lbs. with electronics. Making it lighter would require so much additional work that it's probably not worth it, but that doesn't mean we're not trying. Of course, it's quite a bit easier to hit these low weights when you drop a CIM motor or two.

02-09-2008 15:54

sdcantrell56

Re: pic: FRC 1771 Drivetrain

Exactly. If we dropped a cim per transmission this would automatically be 5.5lbs less. Also the spur gear style transmission is heavier than a dewalt. Additionally this frame has more metal than yours. It is true we could possibly shave ~1-2lbs out of the frame, but the amount of machining might not be worth it if we know well ahead of time that we will make weight. A ~30lb drivetrain is imo very light particularly with a 2 speed transmission and 2 cims.

I am working on an updated version that uses custom hubs in the wheels and that has some more details added and Ill work on getting it up soon.

02-09-2008 16:43

s_forbes

Re: pic: FRC 1771 Drivetrain

The frame looks great, nice and simple, and the bumper mounts are a very clever approach. I'd avoid cheese-holing it if I were you... It's already very light! Just put as much thought into your mechanisms as you did into this drivetrain and you won't have any weight problem at all.

And belts are awesome!

Quote:

Originally Posted by rc_cola1323 Yeah, I agree with Hachiban. Hexing UHMW is harsh, our capitan put ball bearings in them and it worked fine, but hexing is a different story. Where can you get 5 inch plastic wheels???

02-09-2008 17:05

sdcantrell56

Re: pic: FRC 1771 Drivetrain

Those wheels are definitely worth a look however I am going for something with a bit more traction (roughtop). I am also debating doing a finesse version of this one using custom omni wheels in the corners and a center traction wheel plus single speed. I just finished my single speed and with it I have gotten this drivetrain under 30lbs (~29.5)