An apparatus for tensioning a belt consisting of a tensioner arm having an idler pulley for engagement with a surface of a drive belt, structure for mounting the tensioner arm to a suppport for pivoting movement about a pivot axis in first and second opposite directions, and structure for biasing the tensioner arm in one of the first and second directions. The mounting structure includes first and second members with first and second relatively movable cooperating friction generating surface for damping rotation of the tensioner arm. At least a portion of one of the friction generating surfaces is concave opening towards the tensioner with at least one of the friction generating surfaces having at least a portion defined by other than a radius centered on the pivot axis of the tensioner arm to thereby produce a greater resistance to rotation of the tensioner arm in the other of the first and second directions than in the one of the first and second tensioner arm pivoting directions.