Sprockets as Chain Tensioners?

[ajlapp]

We call them Chinese stars......they work great in a pinch, but I typically shy away from designing them in. They can be tricky.

Mcmaster sells a flexible style, way over-priced.....McMaster 5896k1. We usually just stick an AM sprocket in there and let it go.

[Ether]

Originally Posted by ajlapp

They can be tricky.

Could you elaborate on that a little bit? Tricky how?

[Teched3]

We've used them on many drive systems as tensioners with #25 chain. However, we also use them to drive encoders to keep track of the robot during autonomous mode on a live axle (dual purpose device) with shims to adjust tension.

[Chris Fultz]

Look at post #18 in this thread. Is this what you are referring to?

http://www.chiefdelphi.com/forums/sh...t=43750&page=2

Or, are you talking about placing a sprocket on some type of bracket and using that as a tensioner?

[artdutra04]

We've used them before with 25p roller chain, and they work great. After you first install them you need to keep constantly adjusting them every match or so during your first competition or so. Once the roller chain finished stretching, we left it in the same spot and never really needed to adjust it.

[Retired Starman]

Sprockets tend to be heavy. While a sprocket on a tensioned arm or inserted between the chains can make a good tensioner, you pay the weight price. There are other less weighty ways to tension your chain, though the sprocket mounted on an arm makes a great one.

[ajlapp]

Could you elaborate on that a little bit? Tricky how?

Chris captured some of the problems above....

If you deploy a floating star tensioner on a long run of chain, the system tends to "slap" back and forth under load. Hard to explain without a diagram. This action can eventually through the tensioner.....especially if the chain tries to twist at all.

The point is, if you can design to the proper center distance and plan for a tensioning device, say a bolt or cam, I believe it will be more robust for competition.

We have used these in the past many times, but always as a fix, never as the planned way.

[PingPongPerson]

Thanks!

[ajlapp]

You could try a cam....see the picture below. Each cam has a 1/4" shoulder bolt that acts as an axle. The bolt is drilled off center. You twist the cam into the desired position then tighten a set screw....this drives the aluminum cam up into the bolt and locks everything down.

As the delrin wears you can adjust the tension or replace the bushing altogether.

[NickE]

Keep in mind that anything you add into the chain run will add friction and decrease the efficiency of the system.
The only way to avoid this is to either size the center-to-center distances perfectly so that no tensioner is needed or to have a system where the axles slide to tension the chain.

[Ether]

Originally Posted by NickE

Keep in mind that anything you add into the chain run will add friction and decrease the efficiency of the system.
The only way to avoid this is to either size the center-to-center distances perfectly so that no tensioner is needed or to have a system where the axles slide to tension the chain.

Does anyone have any data quantifying this effect ? At least some ballpark numbers. It would be useful to know just how "floating sprocket" tensioners, rotating tensioners, sliding tensioners, and adjustable axles compare.

[Chris is me]

Originally Posted by Ether

Does anyone have any data quantifying this effect ? At least some ballpark numbers. It would be useful to know just how "floating sprocket" tensioners, rotating tensioners, sliding tensioners, and adjustable axles compare.

I've been interested in making a simple test bed for something like this but I have no idea what I should be measuring - I assume current draw?

Personally I'm going to wager a guess that friction in tensioners is essentially negligible. Shaker just uses blocks of Delrin or nylon pushing against the chain (not spinning) and we have no problems at all. We even had a multi-purpose Delrin rod we used as both a tensioner and as a structural member (standoff)

[Ether]

Originally Posted by Chris is me

I've been interested in making a simple test bed for something like this but I have no idea what I should be measuring - I assume current draw?

You need current and speed, or current and voltage, or voltage and speed.

Ideally one of the two parameters is held fixed to make it easier to compare.

If you can measure all three then the extra data can be used as a cross-check of the test's integrity.

[Cory]

Originally Posted by Chris is me

I've been interested in making a simple test bed for something like this but I have no idea what I should be measuring - I assume current draw?

Personally I'm going to wager a guess that friction in tensioners is essentially negligible. Shaker just uses blocks of Delrin or nylon pushing against the chain (not spinning) and we have no problems at all. We even had a multi-purpose Delrin rod we used as both a tensioner and as a structural member (standoff)

I doubt that a static tensioner has a negligible effect on efficiency.

If you've got a two stage spur gear reduction and a single chain run from the gearbox you're already down to 83% efficiency or so. If you then shove a block of plastic into the chain to tension it, it would seem you would be liable to lower efficiency quite a bit.

We have no data, but empirical evidence has suggested to us that our overall drivetrain design is noticeably more efficient than the average drive similar to ours in terms of same number and type of motors, similar gear reductions, top speeds, and wheel type/friction.

[ahecht]

Originally Posted by NickE

Keep in mind that anything you add into the chain run will add friction and decrease the efficiency of the system.
The only way to avoid this is to either size the center-to-center distances perfectly so that no tensioner is needed or to have a system where the axles slide to tension the chain.

Chain "stretches" (actually, it is wearing, not stretching, but the end result is the same) and gets longer with use. A system that is initially designed with perfect spacing will end up with loose chain as the system is used.