pic: GEAR & CHAIN IN TUBE 8WD

[Chris is me]

Without the top side of the tube being able to constrain the chain, won't the eventual chain stretch and sprocket wear cause it to derail? It was my understanding that the physical constraints of the chain inside the tube were the main factor preventing derailment.

[Cothron Theiss]

Any chance we could get some more detailed renders of the gearbox/chain runs? What I can see looks great, but it's hard to ascertain the specifics from this.

[InFlight]

I understand the attraction of belt/chain in tube drivetrains as a design exercise.

The trade made is very involved and long replacement times if a belt/chain/axle/bearing failed during a competition. You could very easily miss a match for the amount of time it would take to replace any part.

Considering maintenance, and tensioning should be prime considerations in determining the best way to implement a belt/chain drive train.

[Cothron Theiss]

Originally Posted by InFlight

I understand the attraction of belt/chain in tube drivetrains as a design exercise.

The trade made is very involved and long replacement times if a belt/chain/axle/bearing failed during a competition. You could very easily miss a match for the amount of time it would take to replace any part.

Considering maintenance, and tensioning should be prime considerations in determining the best way to implement a belt/chain drive train.

+1

And using 3" tubes means your chain has room to fail. If you can use larger sprockets without increasing the weight too much, the closer you can get to the insides of the tube without interference, the better. also, using sprockets that large means it would take a monumental amount of force to cause a chain failure at the sprockets, because the load would be distributed over so many teeth. This also means less wear, or "stretch" over time.

I would try and tweak the design so that the ends of the drive rails are open, so that if you do need to get to your chain or gears, you don't have to disassemble your chassis frame.

[Ty Tremblay]

Originally Posted by Chris is me

Without the top side of the tube being able to constrain the chain, won't the eventual chain stretch and sprocket wear cause it to derail? It was my understanding that the physical constraints of the chain inside the tube were the main factor preventing derailment.

Using the tube to keep chain on is sub-optimal. In my opinion, you shouldn't use anything other than proper tension, power transmission, and tooth engagement to keep a chain on the sprocket. If you design the c-c spacing properly, the chain shouldn't get loose enough to jump off sprockets as it wears in.

In theory, there's no where for the chain to go if it gets loose and the sprocket teeth are close to the tube wall, but its one of those things where you shouldn't have to take advantage of it if you do it right.

[Chris is me]

Originally Posted by Ty Tremblay

Using the tube to keep chain on is sub-optimal. In my opinion, you shouldn't use anything other than proper tension, power transmission, and tooth engagement to keep a chain on the sprocket. If you design the c-c spacing properly, the chain shouldn't get loose enough to jump off sprockets as it wears in.

It's just that I've built a bunch of chain drives at exact center distance, using sprockets of a variety of different sizes and under different loads, that have stretched to the point of jumping off sprockets. Everything is properly aligned, it's just from wear. I've always been told the "magic" of chain-in-tube is the tight fit prevents a loose chain from jumping, since it has nowhere to go, but maybe I just have bad chain runs or something.

[Cothron Theiss]

I agree that exact c-c is a viable option, and having the chain stay on because of proper tensioning and spacing is preferable, but I think having a tight fit inside the tube adds an extra measure of safety in preventing failures. Hopefully this would be redundant, and the tube would never need to keep the chain on the sprocket, but taking advantage of this peace of mind is one of the benefits of running chain in tube.

[Kevin Ainsworth]

This year for our climber we ran dual #25 chains in-tube, one within each chassis side rail. Since the chain was worked in only one direction, pulling the robot up the wall, there was only one slack side. We did contain the chain with the chassis rail on the slack side. The tensioned side of the chain is in open air, no containment. 200lbs of lifting force with four CIM's in low gear and no failures. It utilized a Vex 22 tooth sprocket on each end to transfer the drivetrain gearbox power via a PTO.

Image of in-tube climber chain run.
https://drive.google.com/open?id=0B6...jRVTzYxNGFjOWM

For a drive train where the chain is worked in both directions I would agree containing the chain on both sides could be beneficial and is good insurance. I wonder if anyone with in-tube chains has had issues with broken chain or chain that gets severely wedged between the sprocket and tube.

On the other side of the coin, we ran our drivetrain sprocket spacing at .015" under theoretical spacing this year (7.985" center to center sprocket spacing for 8" of chain length). There was a very large amount of slack so that the defenses didn't hit tensioned chain and damage it or knock if off the sprockets. Our run of chain was allowed to get pushed up past the chassis rails and the defenses would impact the chassis rails instead so therefore we never lost a chain this year.

[346CADmen]

For a drive train where the chain is worked in both directions I would agree containing the chain on both sides could be beneficial and is good insurance. I wonder if anyone with in-tube chains has had issues with broken chain or chain that gets severely wedged between the sprocket and tube.

346 ran chain in tube for our drive this year. Yes we did experience a broken chain and yes it is "difficult" to fix. Darn the ramparts.

While I was not able to view your design in detail, I would wonder if you considered assembly? It is difficult to assemble chain in tube with fixed centers. Guess I'll take a closer look to see how you overcome that issue.

[waialua359]

Originally Posted by Ty Tremblay

In theory, there's no where for the chain to go if it gets loose and the sprocket teeth are close to the tube wall, but its one of those things where you shouldn't have to take advantage of it if you do it right.

In the past, we used sliding bearing blocks instead of tensioners to ensure that our bike chain would never jump a sprocket, even if the chain were to get looser due to wear and tear.
For the past 4 seasons, our main machinist ensured the 3 things you mentioned and we never had an issue with chain jumping off our drive ever. We basically set it and forget it, thus eliminating the need for adjustable bearing blocks and saving weight/time to make them.