A speed reducer comprising a worm gear of the type suitable for driving proportioning pumps. The worm is rotated by an input shaft which is suitably journalled within a speed reducer housing wherein the input shaft bearings are respectively carried within aligned housing openings. Either one of the input shaft bearings is adjustable by means of a threaded hole in the housing where the input shaft is inserted. A threaded adjusting piece is screwed against the race of one of the bearings that hold the input shaft to precisely adjust the longitudinal alignment of the worm relative to the gear and to precisely load the two bearings that hold the input shaft to substantially eliminate excessive wear, vibration and chatter.