[EricH]

Sometimes, a little bit of slack in the belt won't hurt (mainly conveyors). Not the case on an arm, though. Go with a tensioner.

Or...precision-drill the hole locations for the belt's shafts and hope it holds tension. I've done that--330 (2006) had a very high RPM motor attached to a belt, and we never tensioned, other than location of the other end. The same went for the other belt system on the same structure. (We did have one other belt, but one of the two units was tensioned and one was not. I forget which one was on the robot. It was also a very different type of belt.)