chain wrap
 

how much is the minumum that 25 pitch chain needs to wrap on a sprocket, with average tension. is there some kind of rule?

Re: chain wrap
 

180 is ideal. I've done 120 with some success but it's not as smooth and will slip under directional changes depending on tensioning.

-Vivek

Re: chain wrap
 

+1

:)

Re: chain wrap
 

From Diamond Chain: http://www.diamondchain.com/support/techfaqs.php

What is the recommended minimum chain wrap in a roller chain drive?

ANSWER: The recommended minimum wrap angle of the smallest sprocket in the drive is 120°. The wrap angle can be reduced to 90°, if good chain tension adjustment is maintained. If chain tension is not closely maintained with less than 120° wrap, the chain can jump teeth, resulting in damage to the chain and/or the sprocket.

Note: For a sprocket ratio of 3:1 or less there will always be 120°or more wrap on the small sprocket, regardless of the center distance.

Re: chain wrap
 

That is true, assuming only two sprockets....but if you have more than two sprockets in the system, things get more complicated.

Re: chain wrap
 

Yeah, don't forget idler sprockets.

Which reminds me, you can often use idlers or tensioners to ensure more chain wrap.

-Vivek

Re: chain wrap
 

I believe maximum recommended tension for #25 chain is about 300 pounds. Not sure about minimum, but generally good tension is where you can pull up the chain just slightly (about 1/2" to 3/4" from horizontal depending on length of the stretch of chain) in the middle of the chain stretch. If you're tensioning the chain and you see the links begin to deform at all, it's too tight. Under proper tension, the chain should eventually stretch out and require retensioning.

Re: chain wrap
 

It is my considered impression that chain doesn't "stretch", except under extreme load. Elongation is due to wear, though it could happen due to exceeding the elastic limit. Chains are not guitar strings, so don't "tension" them. If a chain wears and elongates more than a couple of percent, then you'll have problems with tooth spacing and are in a world of hurt.

Have a look around here: http://chain-guide.com/toc.html

Re: chain wrap
 

I agree, chains wear in, they don't stretch. Once they get some wear at the contact points, they don't elongate much more.

Re: chain wrap
 

Short runs of quality #35 chain are your friend. :D

Re: chain wrap
 

This may just be from working on bikes, but chains seem to stretch to a certain point after their break-in period. Just use a system like we did last year. We had a skate-board wheel with springs pulling it back to put tension on the chain. We had no problems and didn't have to mess with calculations and angles. If you can't put it on the inside of the chain, push against the outside. You need some kind of tensioning. Good luck.