A rotary spindle assembly for a machine tool including a spindle supported at opposite ends by oppositely directed roller bearings. The bearing at that end of the spindle opposite to the nose end is an outer race which is axially slidable in a housing, and a plurality of compression springs are arranged between the outer race and an annular thrust block to bias the outer race away from the axial center of the spindle. Elements are provided for varying the total biasing force on the outer race and the apparatus includes a regulator for controlling the force-varying elements. The force varying elements may be apparatus for applying hydraulic pressure to supplement the biasing force or apparatus for modifying the initial compression state of the springs. The axial force on the outer race reduces or eliminates play in the bearings.