Chain Tensioning

[Rion Atkinson]

Okay, I was talking with someone about ways to tension a chain. And one of the things that was brought up was a spring. (Or some kind of potential energy device) Basically you would have a rod mounted on a pivot. you would have a spring attached to one side, and a free floating sprocket on the other. (Or you could use AM chain tensioner if you modified it a little.)

We loved the idea. But that's just it; it is still an idea. We do not know of anything like this every being used on a FIRST robot. We know it has been used before, but it's unknown if it's been used on FIRST robots. Is there a specific reason? Or is it just because no one has tried it?

Thanks
-Rion

[angelawence11]

I don't think that there is any reason why this wouldnt work. We were actually discussing doing something like this, but we found it much simpler to do it another way. But in truth, its a great idea and somebody should do it!!! Let us know how it goes!

[Rion Atkinson]

Originally Posted by angelawence11

I don't think that there is any reason why this wouldnt work. We were actually discussing doing something like this, but we found it much simpler to do it another way. But in truth, its a great idea and somebody should do it!!! Let us know how it goes!

A better way? Care to elaborate? I'm okay dieing after you tell me if I must.

[GarrettF2395]

Well a big question we (me and Rion) had about using this kind of tensioner, was why don't teams use spring tensioners on their drive chains (chains that run drive wheels)? Ive seen one or two used on random chains that drove mechanisms on robots, but never a spring tensioner on a drivetrain. Could anyone explain why this is?

[Boommaster713]

Last year 932 used this method for tensioning our gates belts for our ball gathering system
I'll try and get a picture but we had to scavenge some parts from the ball gatherer for this years design

As for why, in my opinion (my own experience with robot drive chains) it requires a structure around the chains to mount the bar and a 2nd structure for the spring, versus the andymark tensioners which do not require said structure

[Aren_Hill]

Bike's usually have the spring loaded tensioning setup, this is because the chain is only ever going one direction with force, meaning the spring takes up tension on the side that is always slack.

Robot drivetrains dont do this for the reason they run both directions and placing a spring loaded tensioner would yield situations in which the motor would be pulling in chain directly against the spring and when this happened slack would develop in the other areas of the chain run giving increased likelyhood of chain popping off.

Using a spring loaded idler to reduce harmonic bouncing of the chain over long spans though is a good use though.

[Leav]

Also I'll suggest that my first intuition regarding this is that you would need a very stiff spring.

if the spring is... anti-stiff (what's the word for that? )... then under load it could allow the chain to lose tension somewhere along it's length, which could cause the chain to slip off.

without having much experience with chain tensioners, my intuition tends to lean towards an idler gear that can be moved in order to tension the chain, and then can be locked into place. this could be designed to be fairly simple.

currently my team has one axle on each chain loop mounted on slots so it can be tensioned pulled away and tension the chain. this is somewhat problematic since the axle is moving and thus is less than perfectly secure (even when locked into place).

I'd continue working on this idea and developing ways to tension chains..

maybe try making a list of possible solutions, and then make a pro/con list for each solution...

-Leav

[moojoe]

Page 1022 on McMaster's website.
devices similar to what you explained are used quite commonly in industry. they're great ways to keep tension, especially if the mechanism is subject to shock loads.

I personally haven't seen these used on robots, probably due to weight, but other then that I dont really see why they wouldnt be since they are great for keeping constant tension on the mechanism.

[GarrettF2395]

This is similar to the type of tensioner we are talking about.
http://www.cheapbikeparts360.com/images/b/13958.jpg

[artdutra04]

Even simpler than spring-loaded tensioners: floating idler sprockets.

[RMS11]

Originally Posted by artdutra04

Even simpler than spring-loaded tensioners: floating idler sprockets.

Did you all lasercut that yourselves? How did you decide how big it needed to be and other design decisions? We use 25 chain so we cant use mcmasters. Thanks!

[WJF2011]

Originally Posted by RMS11

Did you all lasercut that yourselves? How did you decide how big it needed to be and other design decisions? We use 25 chain so we cant use mcmasters. Thanks!

Its true rick but wouldnt we need a plastic cutting bit?

[artdutra04]

Originally Posted by RMS11

Did you all lasercut that yourselves? How did you decide how big it needed to be and other design decisions? We use 25 chain so we cant use mcmasters. Thanks!

They were milled on a CNC machine; you can't laser cut polycarbonate.

As for design decisions, we just made a bunch of different sizes. To tension the chain, simply pop the floating idler sprocket out and move it closer toward one of the driving/driven sprockets. You may have to readjust the idler position somewhat frequently as the chain breaks in, but once the chain finishes stretching you can leave the floating sprockets in the same position for the rest of the season.

If you don't have [access to] a CNC machine, you can always just get plastic (nylon) sprockets and use them as floating idlers.

[R.C.]

Originally Posted by artdutra04

They were milled on a CNC machine; you can't laser cut polycarbonate.

As for design decisions, we just made a bunch of different sizes. To tension the chain, simply pop the floating idler sprocket out and move it closer toward one of the driving/driven sprockets. You may have to readjust the idler position somewhat frequently as the chain breaks in, but once the chain finishes stretching you can leave the floating sprockets in the same position for the rest of the season.

If you don't have [access to] a CNC machine, you can always just get plastic (nylon) sprockets and use them as floating idlers.

Art,

Did you guys put a chamfer on the sprockets and how thick are they?

Thanks,

-RC

[Evert Timberg]

Our team loves to use simple aluminum spaces as a temporary tensioning method. We use a lathe to make a hole down the axis of a cylindrical piece of aluminum and bolt it in place. The chain runs over the aluminum spacer and the size and diameter of the spacer can be easily changed. This works fairly well as a temporary measure and is lightweight.

That being said, the spacers are damaged by the chain and eventually break. I'd use them only if you really need to, otherwise the idler gear would be a better bet as there is no risk of damaging the chain.

Another solution we found was half links. Until this year we'd never heard of them but when used with a chain puller, its easier to make chains that are of the proper length.