Sprockets as Chain Tensioners?
 

Hey everyone
I've noticed that a lot of teams place free-spinning sprockets in the middle of their chaining, and I assume that this is for chain tensioning. If that is the case, then is it actually a good method for tensioning chain?
Thanks,
Michael

Re: Sprockets as Chain Tensioners?
 

That's typically how we do it. Last season we had the idler sproket mounted on a short section of c-channel with two bolts holding it to the frame. When we needed to adjust the tension, we put a washer under the c-channel to lift it up a bit.

Re: Sprockets as Chain Tensioners?
 

I think what you are referring to are sprockets that just 'suspend' between the upper and lower chains, some refer to them as "majic sprockets". The sprocket is sized to be a slightly higher tooth count (diameter) than the sprockets on the shafts.

We have used them and in some places they work well. They float between the upper and lower chains, and will move back and forth a bit while the chain is running. I think they work best when you can use a small diameter sprocket for the tensioning (2 to 2-1/2 inches) and shorter runs of chain (12" shaft to shaft).

We used them with about a 4" diameter and in a 24" span, and due to the flexibility of that length of chain and the diameters, they would occasionally pop loose if we were hit hard.

AndyMark sells some very simple tentioners that also work well. They were actually designed by a student on an FRC team (from California I think). Here is a link to the product page.

http://www.andymark.com/ProductDetai...Code=am%2D0286

Re: Sprockets as Chain Tensioners?
 

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.

Re: Sprockets as Chain Tensioners?
 

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

Re: Sprockets as Chain Tensioners?
 

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.:) :)

Re: Sprockets as Chain Tensioners?
 

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?

Re: Sprockets as Chain Tensioners?
 

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.

Re: Sprockets as Chain Tensioners?
 

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.

Re: Sprockets as Chain Tensioners?
 

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.

Re: Sprockets as Chain Tensioners?
 

Thanks!

Re: Sprockets as Chain Tensioners?
 

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.

Re: Sprockets as Chain Tensioners?
 

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.

Re: Sprockets as Chain Tensioners?
 

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.

Re: Sprockets as Chain Tensioners?
 

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)