Re: Chain Tensioning
 

Back in 2008 we had problems with the drive chain slipping for our elevator under load. It was in such a weird spot that any conventional tensioner would've either hit our frame or rubbed on the ground. So, I figured, what the heck, let's try a floating idler. And it worked, it make it the entire season without any issues. (Except once - we almost tipped the robot over pushing the limit in some pre-ship practice and the force must've popped the sprocket out...)

Also, we used these guys on our '05 robot: http://www.mcmaster.com/#6233k33/=6iqpvw

I really don't like them because I don't feel that they tensioned the chain all that well, but it also wasn't the best chain routing, so these may work too.

Something else we use on 816 is adjustable transmission mounts. Every robot we've built since 2006 has had one in some form or another. This year, the transmissions are mounted on 1/4-20 threaded rod which allows them to move up and down to tension if needed.

Re: Chain Tensioning
 

The best method of chain tensioning is to properly design your chain run so that there is no need for a tensioner. However, to be more realistic for FIRST, the most robust system of chain tensioning is to be able to vary the distance between two sprockets. Anything else adds unnecessary complexity and strain on what should be a rather simple system.

Once 114 implemented a proper chain tensioning system (Aim High, 06), we did not break a single chain, or have any chain related problems. We have since had zero problems with chain.

Re: Chain Tensioning
 

In 2007, 2008, and 2010 We've used small cylinders of HDPE to tension our chain. We simply bolt it on a piece of angle with the hole off-center, rotate to proper tension, then tighten it down .

(2009 We didn't use chain we did direct gears instead)

Here are pictures of it's implementation on those 3 robots mentioned above:
2007:
http://www.vandenrobotics.com/plogge...n/dsc07244.JPG
2008:
http://www.vandenrobotics.com/plogge...n/dsc07275.JPG
2010:
http://www.vandenrobotics.com/plogge...n/sn850302.JPG

Re: Chain Tensioning
 

We used the same thing, but with delrin.

Re: Chain Tensioning
 

It should also be noted that we had an issue with these tensioners rotating on our 2010 robot no matter how tight the bolts were cranked down.

Re: Chain Tensioning
 

Thanks for the advice guys, we really appreciate all the help.
I was wondering though, if spring tensioners only work in one direction, why not use two?
One oriented one way, and another oriented the opposite way.
Maybe something like this?

Re: Chain Tensioning
 

While we only had this problem about once if you can figure out exactly how much tension you need, then you can drill the hole in the center of the cylinder and mount it like that so even if it rotates you maintain your tension. Works like a charm.

Re: Chain Tensioning
 

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

Re: Chain Tensioning
 

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.

Re: Chain Tensioning
 

Art,

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

Thanks,

-RC

Re: Chain Tensioning
 

We used metric 3mm polycarbonate (.118") sheet for these sprockets, as it reduced the amount of necessary chamfer. The chamfers were done manually with files, since cutting them out on a CNC machine takes long enough as it is.

Re: Chain Tensioning
 

I suspect that the best way to tension a chain will be a long-standing debate in FRC, with no clear winner. Every system has its own pros and cons.

Re: Chain Tensioning
 

Garrett, there is a patent for a dual tensioner very much like the one you illustrated.

Re: Chain Tensioning
 

Would you mind posting a link to the patent so I could check it out?

Re: Chain Tensioning
 

Here's a link to patent 4069719. There are additional illustrations in the patent doc.

http://www.google.com/patents?id=8LAwAAAAEBAJ

I often use the Google patent search for ideas for mechanisms. If you find one relevant patent, there are often others cited and you can check them out.

Then there are endless hours of entertainment reading patents such as the MIG welder attachment for a drill that converts it into a spool gun. Or the golf clubs that are powered by a shotgun shell.