Belt tensioners for a power transmission belt system are disclosed. The belt tensioners have a support member, an arm, a torsion spring operatively coupled therebetween, and an improved damping assembly. The arm includes a housing section pivotally mounted to the support member for rotation about the arm. This housing section has a rub surface against which the damper assembly is seated. The damper assembly has a body having a friction surface frictionally contacting the rub surface of the arm and a damper spring integrated with the body. The improvement of the damper assembly is that the rub surface of the arm includes a trough concentric about the axis of rotation and the friction surface of the body includes an annular protrusion seated in the trough.