# pic: Automatic Tensioner

Just an idea that was bouncing around my head. The idler sprocket is mounted on slots and can slide vertically. A spring pulls a wedge shape underneath the sprocket plate, forcing it upwards. So as the chain stretches, the wedge would move further, and the sprocket would take up the slack. The slope on the wedge is gradual enough that the idler can’t be pushed back down. In theory, you would never have to adjust your chain. I don’t know if we would build this, but I thought it might be worth sharing.

Great idea, how would you interface this with your drive train, and where would you get the springs?

A spring as chain tensioner work great, when the chain is only being driven in one direction. Ask yourself these two things:

What happens when the chain is driven the other way?
How stiff would that spring have to be to maintain tension when the chain is driven in both directions?

Edit: If the ramp truly won’t allow the sprocket to be pushed down, it’d work in theory. Assuming a coefficient of friction of 0.3 that would be around 16.7deg.

Idea: What if you put the idler sprocket on spring-loaded swing arm and prevented back-driving with a one-way roller bearing?

In answer to the first questions, this type of tensioner would probably be best suited to a drive system that is built with two side plates. A non cantilevered design. Then you could bolt this assembly on one of the plates under the chain run. Any extension spring should work, or you could even use surgical tubing.

I realize spring loaded tensioners don’t work well on systems that drive in both directions. The purpose of this exercise was to come up with a mechanism to take up the slack in the chain as it stretches. I believe timing chain tensioners on motorcycles work the same way.

If you were to actually build one of these, calculating the optimal angle based on the coefficient of friction would be wise.

Using a one way roller bearing on a pivot arm is a neat idea! I was thinking of wrapping a torsion spring around a pivot to accomplish the same thing.

Mentioning motorcycle timing chain tensioners reminded me of another automatic tensioning system! The engines that I have taken apart all used a timing chain system like this. I’m sure there are many other ways.

The slack chain guide side is tensioned by a hydraulic piston, as seen below.

You can see a linear ratchet and pall system on the piston’s shaft to prevent the piston from back-driving and losing tension on the chain. Perhaps a spring or pneumatic piston with a similar ratchet and pall system would work well too… just more food for thought!