6wd - chain path revisited
 

I am a mentor for a rookie team - hence, I am a rookie as well. :)

We have been researching 6wd systems and am curious what the pro's and con's are for the chain paths that could be used. I've included some very rough drawings (please don't pick on me too much over these!) to illustrate what we have been discussing.

I have my own ideas of some pro's and con's for these designs but would like some unbiased feedback so I will keep those to myself for now.

things to consider are drag created by the tensioners (if any), chain stretch even with the tensioners, dropping a chain, sprocket changes, wheel changes, stuff like that.

we will most likely be using as much of the kit parts as possible and filling in any gaps with similar or identical equipment - unless there are other cheap alternatives presented here.


thanks for the help!!

PS. the blue circles are the tensioners (still trying to figure these out), red are the sprockets, and the light blue are wheels - probably a mix of Omni's and kit wheels.

Re: 6wd - chain path revisited
 

I personally recommend the bottom one, as there are the fewest possible fail points. It's what 114 has used the last few years, and it has never failed us.

Re: 6wd - chain path revisited
 

Absolutely key to keep this as simple as possible. Less things to fail, also - cheaper. Out of the options shown, I would also choose the last drawing, although there are other options as well. Let me make a quick sketch in paint.



Only one tensioner, pushing up away from the wheel. Idealy there would be more chain wrap around the middle wheels sprocket, but this was just to show one other, simpler option. regardless of your setup, chain stretch will occur, just plan on shortening your chain at some point.

Note: Disreguard - this was not thaught out.

Re: 6wd - chain path revisited
 

We drive each wheel of a six wheel drive robot with an individual chain for each wheel. Hence, if a chain (or two) breaks or comes off, you still can drive and control your robot. :)

Re: 6wd - chain path revisited
 

Good point, I definately agree, you should look at running possibly two of the wheels off one chain, then the other wheel off a seperate chain so in an off chance the chain breaks you can still participate in some capacity.

Re: 6wd - chain path revisited
 

We have used a version of the bootom one for years and have never had a failuer. (knock on wood) We have a different version of the tenioners that do not touch the chain. If you send me an e-mail, we can send you the CADs of our drive train.

Shawn

thefamh@frontiernet.net

Re: 6wd - chain path revisited
 

We have used one not mentioned in your design, however similar to the bottom. In ours we we mounted our gearboxes above our center wheels and ran #35 chain from the gearbox to the wheel. We then ran #25 chain from the center wheel to both outside wheels. This means that your center wheel will have 3 sprockets. An advantage is that you should never lose complete control of one side. However, I assume in your bottom design that you have a live axle, so you wouldn't lose control of the center wheel.

We tensioned our chain by moving our wheels, but this was pretty cumbersome, so I would imagine idlers would be more convenient to change. We never threw a chain without getting something (such as a game piece) getting caught in them.

The problem I forsee with your top design is that if you throw that chain, you're done. Even if you use #25 chain correctly its quite possible that a chain will break. It shouldn't, but always plan for the worst case. If you use #35 chain, it will be heavy. Really heavy. And if you plan to play with trackballs, you will most likely have weight problems. Pretty much everyone does.

Good Luck!

Re: 6wd - chain path revisited
 

The bottom one would be my preferred method. Simple, minimalistic, least number of chain tensioners, and they have the option to mount the transmission on any of the three axles for weight distribution purposes.


As for chain tensioners, these can be made really simple; just use round bar stock of HDPE with an off-center hole, with a bolt going through that hole. As you rotate the HDPE bar around the hole, it increases or decreases the diameter.

Team 228 has used these as chain tensioners since 2004, and we've had great success with their simplicity and performance.

Here's a good photo I found on our team website of these chain tensioners on three different applications on our 2007 robot: our elevator (25 pitch chain), arm (35 pitch chain), and drive train (35 pitch chain in the background behind the orange chassis rail).

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Re: 6wd - chain path revisited
 

IMHO... the third design the best with a slight mod.
Directly connect the center wheel to the motor and run chains for each the front and rear wheels from the center (power) wheel.
Even if both chains break... you should still have control of your robot... unless the motor goes south... then you DNF.

Re: 6wd - chain path revisited
 

Okay - to summarize the inputs so far:
1. having 1 chain - you risk loosing that whole side of the drive train if you pop a chain or it breaks.

2. Love the chain tensioner design by Team 228 as a static chain tensioner.

3. Most seem to like the live axle for the center wheel (or one of the wheels depending on weight distribution).


So, a couple specific questions:
1. how about a self adjusting tensioner (spring loaded)? Has anyone had success with these?

2. If you directly connect one wheel to the gearbox output, then your top speed is regulated by the gearbox - correct? we will be using the Kit ToughBox gearboxes - how much of a limitation is this? seems like being able to change the spockets on the wheels would allow for some fine tuning of the speed and torque of the robot. Using a 3 chain design might work best for this?

thx again.

Bryan

Re: 6wd - chain path revisited
 

This doesn't work. The middle wheel is spinning in the opposite direction of those on the ends.

Re: 6wd - chain path revisited
 

The first design would rotate the center wheels opposite the outer wheels.

Re: 6wd - chain path revisited
 

MKRass, I noticed my mistake right after I posted it, and was somewhat embarrased. You are, correct - the middle wheel would be spinning the opposite direction. I don't function well on no coffee, and I know, poor excuse. - can't believe I slipped like that.

Re: 6wd - chain path revisited
 

Our team has been using the second arrangement as shown in your original drawing. The only difference might be with the orientation of the chain tensioner. We tried out a live axle last year and it did what it was supposed to but the extra work didn't seem to buy us much. This year we are going to try a 1/2 inch diameter bolt as our axle and mount the wheels on bearings. In the end it is really just figuring out what works best for your team.

Re: 6wd - chain path revisited
 

Yeah - I noticed that too - meant to draw it with the chain under the sprocket rather than over the sprocket - then they all rotate in the same direction.

Re: 6wd - chain path revisited
 

i too would choose the last one. I have designed this one for the past 3 years and just continue to improve pond it. Though im running outa things to improve:D :D knock on wood*

Re: 6wd - chain path revisited
 

i updated the image to correctly draw the chainpath for the first image.

Re: 6wd - chain path revisited
 

I like the second method best. This way if you break a chain or two you are in no way out of commission. I would use #35 chain for the motor-center wheel link, then #25 for the other 2 chains. This saves weight while still having some dependable #35 chain on there.

Re: 6wd - chain path revisited
 

My team has used the three-chain design for two years now, to great success. Though we have never used the kitbot transmissions.

I can't recall the exact shaft size we used for our axles, but I do remember that, two years ago, by the time we were half way through the qualifying rounds, the bolts we used were toast. We upgraded to some grade 8 bolts of the same size and haven't had a problem since. Though, getting those bent axles out of our wheels was a bit of a challenge I must admit. You can see a picture of the design we used two years ago here. I couldn't find much of a shot of the drive train from last year.

Other than the bent axles, the only trouble we ever had was getting those poof-balls stuck in the chains. We actually cut one in half during a match once, that was pretty cool... well, until it finally popped one of the chains off anyways. *Hopefully* we won't have a problem with that this year though...

Re: 6wd - chain path revisited
 

One more possibility would be to omit the tensioners at all.

Use a live center axle

If you use sprockets with an even tooth count and space them an even multiple of the chain pitch apart in a single drive/driven configuration you would be able to keep it really simple.

If the chain stretches enough to need tensioning throw it out.

It's really not worth losing the chance to go to Atlana for a couple of bucks worth of streched chain.

You also don't need the extra hardware needed for tensioning to complicate things as well.

(in the dark, on carpet ... with a ref asking you if you can continue. been there ... unfortunately done that... GTR finals 05)

Re: 6wd - chain path revisited
 

Unfortunately, we built a 2wd robot for an earlier competition and the chains stretched enough during the game that they tended to jump off. Using #25 chain just seems to require it if you have any distance to cover with it. Anyone else concure or disagree? all inputs are welcome. thx.

Re: 6wd - chain path revisited
 

I would recomend the last setup (your third picture). That is the same as the drivetrain we had last year and we had zero problems.

Our first year we tensioned our chain by moving the wheel axles, and it didn't work well at all. Chain tensioners are not as much of a hassle as they are made out to be, and if they are done right they work very well. Here's a picture of some of our tensioners from last year.

What kind of transmissions are you using?

Re: 6wd - chain path revisited
 

Here's a YouTube video of 1583's 6wd. http://www.youtube.com/watch?v=px-QCB9UeCsa It uses a single drive chain. We ran a similar design last year with no problems.

Re: 6wd - chain path revisited
 

Here's a working youtube video link. http://www.youtube.com/watch?v=px-QCB9UeCs

Re: 6wd - chain path revisited
 

If your team attends multiple competitions, it's a lot cheaper and more robust to just use a chain tensioner than to constantly keep replacing roller chain because it stretched.

Personally, I love robot drive trains that once they are designed, built, and assembled, you can more or less forget about them. (As opposed to constantly tweaking them between matches.) Nothing is better than going to a competition, getting through inspection Thursday morning, and relaxing in the pits between matches, because nothing broke or needs to be replaced.

Re: 6wd - chain path revisited
 

That is what we did last year as well. One thown chain does not take out the entire drive on one side of your bot.

Re: 6wd - chain path revisited
 

Team 190 used the bottom design with no failures through two regionals, championships, and a good half dozen offseasons and a few dozen demos with no failures or chains dropped. For tensioners we simply had rollers of varying sizes we could swap on and off instead of an adjusting mechanism.

Re: 6wd - chain path revisited
 

For the last 2-3 years, 330 has used the middle design. We don't use tensioners very much; last year the potentiometer mount served as a tensioner on the gearbox-to-center chain. We used #35 and figured out exactly where to place the wheels to eliminate tensioners. The big advantage is that a single thrown chain can't do a whole lot to you, unless it's the gearbox-to-center one, and even then you still have one wheel. We've only had one problem, and that was when a thrown/broken chain jammed between sprockets at the same time that we got stuck on an inner tube at the end of a match. It was fixed within one timeout.