Abstract: An idler mechanism for maintaining tension on a drive belt in a drive belt system comprising an idler arm, a pulley mounted on the arm for rotation about a rotational axis, a pivotally adjustable mounting plate, means pivotally mounting the arm to the mounting plate eccentrically with respect to the pivot axis of the plate, and resilient means operatively associated with the pivotal mounting means for resiliently biasing the pulley into tensioning engagement with the drive belt. Application of a predetermined input torque to the pivot axis of the adjustable plate imparts a substantially constant tension to the drive belt substantially independent of belt length within predetermined tolerances.