Abstract: The belt of a pump jack connects an output pulley of a motor and an input drive pulley of the transmission and is tensioned by an idler roller with end flanges which is spring tensioned into engagement with the belt. The idler roller is carried on a base frame adjustable longitudinally and transversely to the belt on a concrete support base. An upstanding support is carried on the base frame including a platform carrying bearings for a pivotally mounted arm extending to the roller for supporting the roller axis transverse to the belt and a spring connects to the arm for applying a force to the arm to press the roller against the belt. The upstanding support and the pivotally mounted arm are all arranged one side of the idler roller so as to present the idler roller to the belt from one side of the belt.