Abstract: The improved hourglass worm gear which has been produced by grinding a blank for the worm gear by rotating a grinding wheel about a grinding wheel shaft A while turning the grinding wheel shaft A about a tool shaft A3 which extends in parallel with a wheel shaft A2 perpendicular to a worm shaft A1 is characterized in that a cross-sectional shape of the grinding wheel taken in a plane inclusive of the grinding wheel shaft A exhibits a circular arc.Since the cross-sectional shape of the grinding wheel for successively grinding a series of gear tooth surfaces on the hourglass worm gear is contoured in the form of a circular arc, the hourglass worm gear assures that a radius r of the circular arc has an increased degree of freedom and an ideal contact pattern can be obtained by properly adjusting the radius r.

Abstract: The improved hourglass worm gear which has been produced by grinding a blank for the worm gear by rotating a grinding wheel about a grinding wheel shaft A while turning the grinding wheel shaft A about a tool shaft A3 which extends in parallel with a wheel shaft A2 perpendicular to a worm shaft A1 is characterized in that a cross-sectional shape of the grinding wheel taken in a plane inclusive of the grinding wheel shaft A exhibits a circular arc.Since the cross-sectional shape of the grinding wheel for successively grinding a series of gear tooth surfaces on the hourglass worm gear is contoured in the form of a circular arc, the hourglass worm gear assures that a radius r of the circular arc has an increased degree of freedom and an ideal contact pattern can be obtained by properly adjusting the radius r.

Abstract: A housing of reduction gears adapted to be directly coupled to a motor is improved in order to make it possible for a drive shaft of the reduction gears to assume various positions and attitudes. The improvements reside in that the housing has a hexahedral shape, and setting holes are provided on the respective planar surfaces of the housing except for that surface through which the housing of the reduction gears is directly coupled to the motor. Preferably, bolt pockets are provided on two opposed supporting planar surfaces, and bolt holes extend to the respective bolt pockets from a supporting planar surface adjacent to the two opposed supporting planar surfaces.