Offered only during 1966 and 1967 the South Bend variable-speed unit was similar to those adapted for use on Rockwell-Delta and Sheldon lathes with expanding and contracting pulleys side-by-side on a common, overhung shaft. Of a type known in the UK as an "Ainsworth Drive", after its Victorian inventor, similar devices have been made by a number of manufactures including Browning, Gerbing, Lewellen, Lovejoy, Maurey, Reeves, Speed Selector and T.B. Woods. The assembly was a self-contained unit, very heavily built and carried on a cast-iron frame bolted into a sheet-steel cabinet. A handwheel on the front of the stand controlled the mechanism through a universally-joined shaft and screwed rod that moved the vari-speed pulley assembly up and down a pair of vertical bars positioned between a fixed motor and fixed final-drive shaft. Forcing the unit in either direction caused the tension of the two belts to simultaneously open and close their pulleys and so vary the drive ratio. From the expanding and contracting pulleys the drive passed up to a shaft, mounted in bearings on a swinging plate, that carried a single pulley from which the headstock spindle was driven by an enormously wide flat belt. Tension of the final drive belt could be adjusted independently by a right-and-left-hand-threaded turnbuckle.
To save the expensive of a electric revolution counter the long handwheel boss was provided with a sliding pointer to indicate spindle r.p.m and is possible that, like those used on Delta-Rockwell lathes, a mechanism could have been incorporated to limit maximum speed when the lathe was used for training apprentices - and especially in schools.
The drive unit used on the 10-K was made by Hi-Lo Manufacturing of Minneapolis,  Minnesota, a company still in business and providing both traditional, simple drives as well as more fully-developed and complex versions for special purposes..