This is a chain tensioner on the kitbot from our 2005 robot. It is adjustable with a wrench and prevents us from having to design a wheel sliding scheme for chain tensioning.
Great idea!
Have you had any problems with the chain skipping off from under your idler?
is that 25 chain or 35 chain?
looks like 25
Brilliant!
Looks familiar 
Ours were a little bulkier though, they probably weigh a pound each :yikes:
Nope, for 2 years the only chain we’ve lost was due to damage sustained to a sprocket from an impact.
It is 25 chain, we’ve been using it the past few years to help save weight without a problem. We use one chain to power all 3 wheels on a side, so we only need one of these tensioners rather than 2 for 2 seperate chain sections.