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?

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.

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.


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.

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


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.:slight_smile: :slight_smile:

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


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

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.


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.

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.


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.

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.


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)

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.


On our first failed ball control system, we used a free-floating sprocket as a chain tensioner, just as you are describing. It worked great, but did fall off once or twice with a hard hit. I think it was a 24 tooth 25-chain sprocket that was in the middle of two 12 tooth sprockets spaced about 6 inches apart. So it was a small system, but it did work very well.

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.

With short chain runs, the stretching is negligible and is often not an issue. We have used this numerous times over the past few years with no issues. On the longer chain runs, tensioning is much more necessary.

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.