Chain Tension-er?

[Massako]

Originally Posted by Mr. Lim

If you are REALLY tight on resources, one of the simplest tensioning techniques is to stick a big sprocket into the middle of the loose chain run - without actually attaching the sprocket to anything - it just floats in the middle of the chain run. The sprocket has to be big enough to take up all the slack, and is held in place by the chain run itself, with chain running above and below it holding it "securely" in place.

Don't know if this makes sense, but it does work decently well. Surprisingly the sprocket won't fall out unless directly disturbed...

This is a hack we used on 188 pretty effectively back in the early days.

lol thanks. i dont know how well this would work on a drive train but we can defiantly try it.

[Joe Johnson]

Originally Posted by Massako

lol thanks. i dont know how well this would work on a drive train but we can defiantly try it.

MCMASTER sells a dohicky that works like that. Only for #35 chain and bigger but it at least confirms the concept.

Dr. Joe J

[eneubec1]

Our team was in the same situation last year. We use 16' of chain to drive our lifting mechanism to lift totes/bin. We spent 2 days trying to figure out how to tension the chain, and in the end decided we didn't need one. Those 2 days would have been nice to have back and have some drive practice.
We went the entire season and never had a problem with our chain.

[Breadbocks]

Originally Posted by Joe Johnson

MCMASTER sells a dohicky that works like that. Only for #35 chain and bigger bit it at least confirms the concept.

Dr. Joe J

So that's where what we had came from. Why on earth does that cost $70?

[Joe Johnson]

Originally Posted by Breadbocks

So that's where what we had came from. Why on earth does that cost $70?

I saw that, yeah, $70 is crazy for a bit of plastic (or rubber or silicone or whatever). One thing to keep in mind is that while it may keep your chain from derailing it is not going to do anything for the backlash as you go from fwd to rev and vice versa. Not a huge deal on wheels (probably) but feedback loops hate backlash so... ...keep that in mind (and do the right thing, put in a real chain tensioner ;-)

Dr. Joe J.

[MisterG]

We found that flexible sprocket device too and I was wondering if something like that could be 3D printed out of some material that would give a similar effect. Anyone have thoughts about this?

Also from my reading it seems like tensioning addresses at least two separate factors. The first is making the chain the correct length for the application. This can also be solved with careful design and manufacturing. The second is compensating for chain stretch. Which of these sounds like the bigger factor for an application like a drive train?

[Breadbocks]

I suspect if you could get it to print right with the proper spacing, ABS would have enough flex without being too squishy or too rigid. The trick is getting it right.

As far as spacing, the only actual problem is length, which chain stretch contributes to. The three options are 1. ignore it, 2. tension it, 3. have some way to move a sprocket around so you can move it back when chain stretch.

[Joe Johnson]

I would laser cut this thing before I tried to 3D print them. But even that seems a bit sketchy to me. As I recall (I have only held one in my hand once and it was years ago), the reason the device is a hoop and not a disk is that the hoop structure allows it to become an ellipse and thereby add some springiness to the system. As slack becomes available the ellipse gets more round and takes up that slack.

Bottom line, this thing flexes. I think a 3D printed version is doomed to destroy itself over time.

Dr. Joe J.

[elil]

I highly recommend an idler pulley. My team has used a bolt with a piece of pvc around it. To attach the bolt, we tighten it with a lock nut through a slot on a piece of sheet metal. The one thing about this is that the nut has to be really tightened down. Otherwise it's about as light and simple as you can get.

We've also used spring loaded tensioners. These had their advantages and disadvantages. The big advantage was never needing to worry that the chain was tensioned after the tensioner was attached. The disadvantages are of course cost, but also weight. Additionally, support like a wood block must be added to the bolts used to attach the spring tensioners if they cannot be mounted directly onto a frame element.

We have also used 3d printed pieces on a belt drive. They are a half moon shape sized to tension the belt without needing to be adjusted.

[Nirnaeth]

We've used this product from McMaster every year.

Expensive, but totally worth it.

http://www.mcmaster.com/#chain-tensioners/=10yrv27

[Joe Johnson]

Originally Posted by Nirnaeth

We've used this product from McMaster every year.

Expensive, but totally worth it.

http://www.mcmaster.com/#chain-tensioners/=10yrv27

Counter point.

Build your own. It's not that hard and it is totally worth it. ;-)

Dr. Joe J.

P.S. Serious. Designing chain tensioning into your robot is just a skill that FIRST teams should acquire and then nurture and grow. It is a core skill that makes your robot SO MUCH more reliable if you do it right. Learn it. Love it. Pass it on before you graduate. JJ

[artdutra04]

Originally Posted by Massako

lol thanks. i dont know how well this would work on a drive train but we can defiantly try it.

When I was on 228 in 2011, we used CNC milled polycarbonate floating idlers to tension the 25-pitch roller chain in the drive train. They never fell out, but had to be adjusted a few times early on as the chain 'stretched'.

I don't have any photos of the 2011 drive train usage, but I do have one available from when I first used them in 2009. We used the same style polycarbonate floating idlers in 2011.

[Taylor]

For a nice history lesson, research how WCP got its start. linky