how does one go about making a 6 wheel config drivetrain?

[EricVicenti]

Originally Posted by EricH

Some teams won't touch #25 in the drive with the length of the overpass. They say it's too weak. It isn't, but if not done right, it can be. (The last time 330 used #25 on a robot was the second stage of their shooter wheel drive in 2006 going to the wheel. It was the first time since 2001, when contact was minimal and #25 was used in the drive. Later, we had a lot of problems with drive chains jumping while using 2001 as a practice bot.)

Agreed, in 2007 we didn't do it right, and chain was snapping all the time. I am trying to remember what exactly went wrong, but our tensioning system was terribly designed, if at all. I dont think we had a plan for tensioning at all untill we were mostly built. Alignment might have also been an issue. (IE, if the sprockets were not close enough to coplanar, the chain would pop off.)

[kramarczyk]

I surprised that nobody has mentioned that 6WD can be done without wheel drop.

488 accomplishes this by using omni wheels on the corners. This style is shown in Madison's preso on FIRST Drivetrains... http://www.chiefdelphi.com/media/papers/2037

A variation of this I've seen on a few bots (45-2007) uses high traction wheels in the center and middle and lower traction wheels on the remaining corners. I'm pretty sure they used no wheel drop in this config. http://www.chiefdelphi.com/media/photos/28857

[Mr. Lim]

Here's a link for FIRST Robotics Canada's Robot Design Gallery. There is a section dedicated to drivetrains, many of which are 6-wheel.

http://www.firstroboticscanada.org/site/node/71

[kaszeta]

Originally Posted by kramarczyk

I surprised that nobody has mentioned that 6WD can be done without wheel drop.

Team 95 has used variants on this team for several year (two primary drive wheels coupled with driven omniwheels to avoid scrub) for several years now, this year in 6WD mode mentioned here.

[Brandon Holley]

A few clarifications:

-Center wheel drop isn't *necessarily necessary (See any of 25's drivetrains from the '03 and on)
-#25 chain can be just as effective as #35 chain

On the 2nd note there, if your team feels like taking a little risk go for the #25 chain, and make sure you do it right. Tension obviously is key. I have used #25 on my '04 drivetrain (where we did all reduction through chain and sprocket to pair the CIM with the drill and then finally drive the wheels, I believe it was 11 or 12 chains per side) I did it in '05 and I did it in '07...I HAVE NEVER SEEN A #25 CHAIN BREAK so I may be slightly biased, but honestly do not let the age old "#25 is too weak" stop you from trying it.

This year however, we used kevlar reinforced timing belts with aluminum pulleys. Around 1/6 of the weight of a comparable chain and sprocket setup, with very little noise and a very sleak design. I love them, and I see no reason to stop using them. See here: http://www.chiefdelphi.com/media/photos/30256

[RyanN]

Team Fusion has used #25 chain for many years now, and have only suffered once. This was last year, during the Bayou Regional when we used some Chinese chain that wasn't worth the weight it took on our robot and cost us the regional. We are now using Diamond chain, and are very happy with it. We have never had a chain break before in the 4 year's I've been on the team. Tensioning is the key. You cannot have slack with #25 chain in your drive train, as a whiplash can cause it to snap. In 2007 and before, we went with plastic sprockets for idlers, but this caused some trouble last year when we snapped the axle that the idler was running on. Now we're using nylon blocks and some hardened steel for tensioners (We put these on in Houston last year, and made something very similar this year). We have been very happy with this method. We use nylon block, but some certain type from McMaster Carr that has some lubrication fused into the nylon. I don't remember what it was called, but it works very well.

This year however, we used kevlar reinforced timing belts with aluminum pulleys. Around 1/6 of the weight of a comparable chain and sprocket setup, with very little noise and a very sleak design. I love them, and I see no reason to stop using them. See here: http://www.chiefdelphi.com/media/photos/30256

How are the belts working this year? We used belts in 2003, and I know it slips real bad, but the belts are 6 years old now and the idler is stretched to the max.

[jax1488]

i would just like to ask, whats the pro's and con's of using 6 wheel drive, as against 4-wheel or treads(which are generally 2 wheel)?

[AndyB]

6 wheel gives you the ability to turn on center. It also allows you to in general, turn better than 4WD robots (unless using omni's in some way).

When it comes to ramps and uneven surfaces, the middle wheel on a 6WD prevents the robot from bottoming out as well.

[jax1488]

Originally Posted by AndyB

6 wheel gives you the ability to turn on center. It also allows you to in general, turn better than 4WD robots (unless using omni's in some way).

When it comes to ramps and uneven surfaces, the middle wheel on a 6WD prevents the robot from bottoming out as well.

and im assuming the con factor is the extreme weight added on by all that chain?

also if using the 6 wheels on the same level configuration, wouldnt one have problems with traction, as well as weight if used omni wheels? if its a defensive game i.e. 06, wouldnt u get pushed around a lot?

[AndyB]

Well, MOST 6WD drive configurations lower the center wheel at least 1/8".

I don't understand your question about being pushed.

There is significant weight added. However, the bonus to that is that it is also lowering your CG. Omni wheels weigh just as much as regular wheels in most cases. You lose no traction. In fact, if using a grippy tread, like the KOP wheels or IFI nitrile tread, you gain a little traction.

[RyanN]

For two more wheels on our robot, it probably adds at most 3lbs. We only have 4 #25 chains on our drive train, which is probably only a pound once you add them all up. The only way someone can push you with 6WD is if they get in front of you and push you back against your motors, or if they get on a corner and push, but then all they do is turn you.

[jagman2882]

on the chain subject...i have seen a #25 chain break and it wasnt crappy chain either....IMO its worth it to spend a little more weight to get the #35 chain which you know will not break with the amount of force that is put on it....again thats just my personal opinion but 1126 used #25 from '03-'06 and then we had a severe failure in 06 that caused us to switch to #35

[jax1488]

Originally Posted by AndyB

Well, MOST 6WD drive configurations lower the center wheel at least 1/8".

I don't understand your question about being pushed.

There is significant weight added. However, the bonus to that is that it is also lowering your CG. Omni wheels weigh just as much as regular wheels in most cases. You lose no traction. In fact, if using a grippy tread, like the KOP wheels or IFI nitrile tread, you gain a little traction.

say a robot is to come and push against a corner, wouldnt you spin and not be able to push back, if u have omni wheels for front and back?

and for the 1/8th lowered, why would u only want 4 wheels on the floor at the same time? i wouldnt think that the weight added would be worth the extra 2 wheels if u dont use them. you lose traction for the wheels that arent on the ground, say someone bumps into u around that area, you would spin

im not bashing 6wd, im trying to find out pros and cons

[Billfred]

Originally Posted by jax1488

and im assuming the con factor is the extreme weight added on by all that chain?

also if using the 6 wheels on the same level configuration, wouldnt one have problems with traction, as well as weight if used omni wheels? if its a defensive game i.e. 06, wouldnt u get pushed around a lot?

Weight's never been an issue for us--neither of 1618's 6WDs have really broken 100 pounds. This year's robot weighed in around 78 pounds at Chesapeake, minus the forks we'd removed. That's all with #35 chain, and last year's robot was bolted together. A judicious application of #25 chain (or, if you're 125 this year, belts) and rivets could drive that down further.

Omniwheels don't have to weigh much either--compare the without-bearings weight of the 2007 kit wheel (0.75 pounds) and the AndyMark 6" plastic omniwheel (0.75 pounds).

I know there have been teams that have had defensive-oriented robots with omnis on the corners--488 last year comes to mind--but I'll yield to them since we've always run traction wheels in our 6WDs.

Originally Posted by jax1488

say a robot is to come and push against a corner, wouldnt you spin and not be able to push back, if u have omni wheels for front and back?

and for the 1/8th lowered, why would u only want 4 wheels on the floor at the same time? i wouldnt think that the weight added would be worth the extra 2 wheels if u dont use them. you lose traction for the wheels that arent on the ground, say someone bumps into u around that area, you would spin

im not bashing 6wd, im trying to find out pros and cons

I'm still yielding on the omnis-on-corner discussion, but I'll bite on the other.

We actually use all six wheels on our 6WD, just not at the same time. Consider a robot that didn't have the front two wheels, with the other two left where they are. If you stop, the parts of the robot still doing 14 fps are going to want the robot to tip forward. Without those wheels, we're digging into the ground or rolling on some type of caster that affects how we turn (read: easily). The added weight is a non-zero amount--probably about a pound and a quarter or a pound and a half per side for us--but it's worth it in our determination.

As for traction, you're not really losing any. Granted, that particular wheel has zero traction with the floor if it's raised, but your four wheels in contact with the ground make up for it well enough to make up for it in most situations. Also consider that their front probably takes enough room to hit the middle wheel as well--unless they clip a corner and clip it well, you're still probably not turning.

And, since 6WD can enable easier turning, you can readjust quickly.

[Brandon Holley]

Originally Posted by RyanN

How are the belts working this year? We used belts in 2003, and I know it slips real bad, but the belts are 6 years old now and the idler is stretched to the max.

As with any good drive system that uses some form of drive like chain and sprocket/belt or whatever else, tension is key.

The fact that chain "stretches" was enough to make us consider using something else. Kevlar does not stretch, or at least, the stretch is minimal to the point that we dont need to adjust tension. This allowed for a very simple solution to tensioning, which was putting a piece of delrin, turned down to the proper diameter, placed over one of the standoffs already in place on the drivetrain. The results so far?? No skipping, slipping or sliding. I couldn't be happier with belts.

A LOT OF THOUGHT needs to be put into a drive system. You have to try and be adjustable for different situations, but you need to really commit to something from the start. I love how our drive train came out this year. If you have any more specific questions let me know.