THIS ARTICLE IS FOR THUNDER SPECIFIC ROTOBOSS JR DEVICES WITH ACRYLIC PLATES ONLY
Is you tensioner mounted like this?
If so it is at an extreme angle and under pressure.
There is a modification for this version that needs to be done to your rotary that will need to be done.
If you have aluminum plates there are extra holes to mount the tensioner much more perpendicular to the belt. This relieves the strain on the tensioner If your tensioner is already mounted this way you can disregard this article:
If you have acrylic plates you simply drill a hole and relocate the tensioner so it is parallel The red dot is an approximation of this hole location but verify for yourself to be sure: