The transmission assembly incorporates a mixer gearbox assembly, a system for centering and retaining a mixer shaft and drive and a seal. The mixer gearbox assembly is operable to drive a mixer shaft capable of receiving either a complete electric drive motor on a first face of the gearbox housing for operatively coupling with a first end of a long cross-shaft in the housing, or alternatively a partial electric drive motor on a second face of the gearbox housing (4) operating coupling with an opposite end of the cross-shaft. The system for automatically centering and rigidly retaining a mixer shaft in a mixer drive includes a hollow quill in a mixer drive which is provided with an axial bore having a first diameter along a central portion thereof and tapering linearly and axially in opposite directions away from the central portion with increasing diameter to define a pair of opposed conical portions of the axial bore on either side of the central bore. A smooth-surface cylindrical mixer shaft is inserted into the bore and is retained by the tightening of upper and lower tapered cylindrical locks which are slidable along the mixer shaft and which have conical outer surfaces which are matable with the upper and lower conical portions of the quill bore. The labyrinth shaft seal prevents the migration of fluids through a rotary joint, especially the migration of gear lubricant from a mixer drive assembly into a batch of process materials being mixed.
