Position idlers to get better chain wrap -- a question about good practices.
 

So, I guess I'm looking for words of advice and consolation as I deal with trying to elegantly tension chain and achieve good wrap around my drive sprockets.


Assume that tensioning is achieved by moving the outer drive wheel and sprocket away from all of the others -- toward the end of the robot. The idler is attached to the frame rail and is fixed in place.

How close can I reasonably put the idler to the gearbox output sprocket (shown floating in space) to achieve increased chain wrap without it being so close that the sprockets are acting as spur gears, too. :)

Are there any situations that may arise from creating serpentine chain wrap, speaking generally? I've never designed such a complex chain run before; and I realize it's not even that complex.

Re: Position idlers to get better chain wrap -- a question about good practices.
 

Maybe I don't understand what you are saying, but why not try this? I made it in Paint in about 10 seconds, so don't kill me...

Re: Position idlers to get better chain wrap -- a question about good practices.
 

In 2007 we had an two 10 tooth sprockets stacked vertically with about 3/8" clearance between them. We had no problems with it at all. The chain was coming in about 30 degrees off horizontal from each side (30 degrees towards the side that gave it more wrap).

Hope that helps. The mentor who designed it with us was from 687 and he had build a lot chain driven robots in the past and I believe he said as long as there is room for the chain to go trough, there won't be problems.

Re: Position idlers to get better chain wrap -- a question about good practices.
 

Well, I guess I'm trying to find out what sort of advice or experience folks have with such an arrangement -- which is really just an extreme case for what I have on my model. It seems to me that, as you increase chain wrap with such a setup, you correspondingly increase the chain's desire to tear the idler off its mount entirely. :) I'm just curious to learn what folks have found to be a happy medium or that I'm concerned for nothing.

Re: Position idlers to get better chain wrap -- a question about good practices.
 

Bicycle idler sprockets routinely have a serpentine chain wrap--more than you seem to want. You should have no problem.

Re: Position idlers to get better chain wrap -- a question about good practices.
 

Well, this is true. We broke the shaft holding on our idler last year, but it was really a bad design flaw because the metal was so thin where it broke. The amount of force on the idler is a lot (I think our engineer was saying that we had over 400lbs of force on our idler) We ended up replacing our lathe built idler bolt assembly with just a plain 'ole bolt that worked for the rest of the regional.

Re: Position idlers to get better chain wrap -- a question about good practices.
 

We used a serpentine chain-wrap system in 2006 and it was a disaster. The chains fell off, they even broke! We had a six motor drive train and so much of the power was lost to friction that we barely had any pushing power at all!

I'll never use tensioners of any kind again. Not in the drive train, on the arm, or anywhere else. Moving the axels is a far more elegant and efficient solution.

And yes, I realize hundreds of teams use tensioners just fine. Thats just my opinion.

Re: Position idlers to get better chain wrap -- a question about good practices.
 

That's weird.... we had a serpentine wrap in a six motor drive and there we no problems. Never had a chain derail, ever. We still had great pushing power as well, no more power loss than a regular setup.

Re: Position idlers to get better chain wrap -- a question about good practices.
 

true, but they cheat and make sure there is very little chain tension on the part of the chain with the derailleur (the part with all that wrap). The loaded (top) part of the chain is a straight shot from crank sprocket to wheel hub sprocket.

One thing we learned in 2006 is that if you use the "elegant" solution of making the axle moveable to adjust chain tension, that you really do need to make it elegant, not just a bolt thru a slot.

One thing we learned in 2007 is that the idlers/tensioners need to be part of the initial design concept, it's not easy to just add them in later and have it all work out.

In other words, I don't have an easy answer for Madison!

Re: Position idlers to get better chain wrap -- a question about good practices.
 

You won't have trouble with chain if you have the correct setup. Little sprockets save weight, but they put more tension on the chains, allowing them to snap. The constant forward and reverse of our robots makes the chains slap around, often derailing them, so use guides (we make our own nylon guides, and haven't had a chain derail since 2005. Just my advice...

Re: Position idlers to get better chain wrap -- a question about good practices.
 

There's a nice picture of the setup on page 45 of FIRST Robots: Behind the Designs.

Re: Position idlers to get better chain wrap -- a question about good practices.
 

From the pictures in FIRST Robots: Behind the Design, it appears that the idlers that 100 had in 2006 consisted of cylinders of plastic (looks to be polyethylene or maybe nylon) so it isn't really a big surprise to me that it had high frictional losses. On the other hand, we have used ball bearing idler sprockets for the last two years, and they spin really easily even with a significant amount of force applied to the chain.

Re: Position idlers to get better chain wrap -- a question about good practices.
 

oh man.... dont have my book with me, but that much wrap on plastic rod seems like a lot of friction. No wonder.

Re: Position idlers to get better chain wrap -- a question about good practices.
 

Actually if they did it right there might not have been much friction at all. The trick is to install the rod in such a way as to make sure the chain is tight and then let it "wear in" a slot on the plastic rod. You migh want to pre-wear a groove in the rod to make sure the chain stays put, but it probably is not necessary. Once the tight chain wears into the rod a little bit it will go slightly slack. It will then cease to wear and the friction will be negligible.

Another approach is to not use tensioners at all. We have done this the past few years. In this case you need to be able to locate the sprocket axles precisely. You need to design the chain loop to have an integer number of links and preferably an even integer.

If there's interest I can do a white paper on how to do this.

Chris

Re: Position idlers to get better chain wrap -- a question about good practices.
 

If you can mount the drive motor/trans assembly midway between the wheels AND have it mount such that it can be moved vertically slightly, you could use a double-row drive sprocket and two independent chains. Each chain run is shorter, which seems to reduce tensioning problems. To tension it, just loosen the drive assembly and shim it up slightly. Team 975 has done this a couple of years. Tensioning with #35 chain was rarely required, and slipping a thin washer or two between the drive mounting plate and the frame seemed to take up more slack than you'd imagine.

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