We are using a combination for direct drive(well direct after the transmission) and timing belt, but for a team without the equipment to do precision machining then you should go with chain. A tiny bit of error with a direct drive system can put you in a world of trouble. Chains are imo the better choice for your situation.

chains also need some precision in their construction. As in all your holes should be centered. We had a lot of problems with sprockets on bending shafts and poor holes in the sprockets. Slowly got worse and worse. So bad that this year we designed an extremely cool direct drive system before the competition got started. then we decided to make a vector drive system sooo we are back to chains. Even our arm uses chain this year. Long strands of 1/4 chain going up a 9 foot arm. looks sweet and is lighter than a solid rod would have been.

Chain is easy
 

I like chain drive. Keep the chain short, try to align it properly, watch the tension and use #35 if you can afford the weight. Chain is strong and forgiving of slight misalignment. We only have access to a wood shop and a couple of drill presses, but have managed to fabricate a pretty good drivetrain.

Our current 'bot has a 17 1/4 wheelbase with 12-tooth sprockets connecting the front and rear wheels (appx 3 feet of chain). Last year's 2-wheel-drive had a really short chain from an 18 tooth sprocket to a 60-tooth sprocket on the drive wheel - perhaps 18" total. That one never needed tensioning, but I suspect this year's will need just a bit, despite the chain being really tight to begin with.

Ken's point,I think, is that unless you have the skill to hold tight tolerences, don't bother trying to build a gear box. You don't need a readout to hold a .0005 tolerance, they just make the job easier. A properly trained machinist will edge find the part, zero it on the readout, move to the XY coordinate, spot the location, remove the part from the mill, setup a angle plate on a granite surface plate, place a gage ball in the spot, measure to the top of the diameter of the ball using a .0001 indicator and Gage Blocks to determine if the location is on center, and if not, measure how much to move to get on center. There is more to tight tolerance work than just haveing the mill. You also have to know how to measure your locations.... Sorry I went on. Thjere is just a lot to building something to that tight a tolerance...

Our team just this year got a mill...talk about an improvement in part production! We built our own gear box with very simple tecniques using only the mill and it's manual gauges...anything is possible with patience, skill, and a little luck!

Mehhh how hard can it be. All you have to be is tediuos. If the wright brothers assistant could build an airplane engine using a natural gas powered lathe and drill, and hand tools to thousandths of an inch how hard could it be to build it using the tools we have today.

Tried and true method
 

I don't get what all the fuss is about these gearboxes. We've built 'em for two years without a mill, or precision machinery.

We have a tried and true method that some may grimace about. First, we order medium-to-large pitch gears that we know we need. Then, once we have them, we take a plate and mount our motor to it, attaching the driving gear to the motor. We locate the second gear, clamp the gear down, remove the motor, and drill straight through the gear into the plate. The hard part is making sure the large gear doesn't move.

We've used this method for two years and it's worked flawlessly. Some more experienced teams may frown on it, but it's not like we're building a robot meant to last for years and years.

Re: Chain is easy
 

I personally prefer a small, tight gearbox, with a single stage of chain and some kind of tensioning adjustment. This allows you to change the final ratio easily if it turns out you've chosen badly for the dominant strategy that evolves per regional, yet have a strong and reliable main gearbox.

If you use chain, IMO it is VITAL to have either:
A) A simple tensioning adjustment (i.e. using an extrusion system and sliding the gearbox slightly along the rail),
B) Having the motor end of the pair spring tensioned, or
C) Having a PAIR of spring tensioners in the middle to eliminate chain slack in BOTH drive directions.

In addition, if you have a long chain run, consider adding idler wheels and/or delron/HDPE grooved CHAIN GUIDES.

The higher the engagement angle, the lower the possibility of chain hop offs. Shoot for MORE than 180 degrees on all end sprockets, if possible, unless the chain is VERY short.

Well, #35 is forgiving, but not #25!

I WISH we've used #35 this year. I assumed we would, but to my surprise our mech team chose #25 for weight savings. Trust me... It was a NIGHTMARE. Way too stiff, too weak, and IMO the alignment was too critical for drivetrain use.

We broke the chain several times in rounds. It also came off more than once, leaving us spinning like a one armed man in a rowboat... The engagement distance is just too small. Therefore, it comes off way too easily from the slightest alignment or tensioning error.

Talking with other teams, #35 chain is a LOT more forgiving of alignment and tension errors, PLENTY strong enough for this work, AND enough other teams use it so you can find parts like master links at a contest in an emergency. My last team used it in the 2001 CDI contest, and it was GREAT! Not a single problem with it.

If we ever use chain again in a FIRST contest (which we probably will), I'll DEFINITELY argue for using #35!

- Keith

chain
 

Actually, I've only used #35. Considering all the info, I'll stick with it. Our 4-wheel-drive bot used 2 pounds of it this year. Plus, you can get #35 links at places like Home Depot and Loew's.

We've had a fair amount of stretch or break-in wear with 35, and I would imaging 25 is just that much worse.

Re: chain
 

OOC, One item that was recommended by one team at GLR was using "standard bicycle chain".

I'm not so sure about that... Does anyone have any experience with this? How does that compare to the "numbered" roller chains size and strength wise? Is it strong enough, and RELIABLE enough for our use? Heck, you can get THOSE sprockets, chains, and links at numerous stores in virtually any mid-sized town! (Personally, I'm still leaning toward #35 for next year...)

- Keith

--
"How did your team break your chain?" "Why, with a 'chain saw', of course..."

Re: Re: chain
 

I don't know how bike chain is sized but it looks to me like #35. I couldn't imagine why it would be on a different standard. Anyway, yes bike sprockets and chain could be a very cheap effective way to get the job done. It just depends on what you are trying to do. We use all hubbed keyway bore sprockets that are not found on bikes.

#25 Chain is plenty strong for drivetrain usage ~900lbs break strength, we used in on this years bot and trust me, we are putting out some serious power in this years drivetrain (CIM&Drills Already have ripped 4 Brecoflex belts in half after two reigonals) As for alignment on the #25 it is much more difficult and critical to keep the sprockets aligned. At Buckeye we were throwing the chains on high speed impacts so we made some delrin circular plates bolted to the sides of the sprocket that make a v for the chain to rest in, worked great never broke or derailed at GLR and we were in the winning alliance. :)

Would I do it again? No Way. All gear drive next year and you never have to worry about it again plus you get even better efficiency.

I'm All for Shimano!!
 

I worked in a bicycle shop for 10 years. There are 2 types of bicycle chain-Standard Coaster Brake Chain and Derailluer Chain. They are both 1/2 inch pitch. The Standard Chain is 1/8 inch wide and uses a master link. The Derailluer Chain is 3/32 inch wide and uses no master link. You need a Chain tool to push the pins out and back in to connect the chain. Bicycle chain is very different in pitch and width and is not compatible with #35 pitch gears.

The past 3 years we have been using #25 chain in our 6 wheel drive set-up. This year's 'Bot has 2 chains per side, 1 short and 1 long. I modified my Park Chain Tool to work with the #25 Chain pins, so Iwe are not running any master links in the drive chains.(This dramatically increases the chains working load) I posted a few photos in the Gallery of how the output shaft is coupled together. The trans can be removed in 1 minute from the chasis without disturbing the chain Adjustments. Just after the last match with 302/67/226 in Ypsi, I pulled the trans' apart, inspected, and reinstalled in the chasis in less than 15 minutes. We have not had any trouble with the chains breaking or jumping off. We played very hard all weekend, and had very little trouble with our drivetrain. The only damage we sustained was qualifying match #102 against 302. They drove us into the wall, which we then drove over them, continued about our way to the rest of the match, until the end when we drove on top of them to claim our territory on the ramp. We did bend a 60 tooth sprocket, which was easily swapped out.

I have considered running Bicycle chain for next year because so many ratios are available in very lightweight sprockets. The tensile strength should be about the same as #35 chain. In fact, I don't think 130lb robot can exert as much force on a chain as a 220lb rider cranking up hill would. :cool:

I have noticed that gearboxes can be a little... touchy. This year, we went with the FIRST supplied gear boxes, with slight modifications. Never again. Something was seriously wrong with the right hand side of our drivetrain, and we ended up dissassembling it 6 times. Just before our last match, we blew the side out of the plastic gearbox. We believe that it was a combination of overtightened bolts, slightly oversized bushings, and excessive force from the motor. Unfortunately, our expierence with chain drives hasn't been much better, as we had a fair amount of trouble with the chain jumping last year that was never really resolved. One of the major lessons was that the whole gearbox needs to be aligned correctly, and everything test fitted and measured several times. I'm sure we would have caught the problem had we had more time to practice, but I believe it is finally resolved. I will definitely be making a tour of the pits this year looking at the drivetrains!

S cubed

Yeah I'm sure they built the plane in six weeks too.