This is a premium aftermarket tension idler for several Takeuchi mini excavator models. The idler is the larger roller on the opposite end of the undercarriage from the sprocket. The idler attaches to the tensioner and this is the roller that extends as you grease up the tensioner and tighten the track.

Usually when your track starts sagging operators assume the track is stretched out. It is very likely that the sprocket and the idler wear is preventing the track from tensioning properly so it is possible that simply replacing the idler alone will give extended life to the track. Check your sprocket and rubber track and consider replacing all of the worn parts at the same time when possible.