The grinder mixer assembly includes a drum assembly with end plates rotatably supported on stub shafts. These stub shafts are secured to arms of a yoke assembly. The spindle of a first hydraulic motor is secured to one of the stub shafts. The housing of the first motor is connected to a center portion of the drum by a key. The spindle of a second hydraulic motor is secured to the other stub shaft. The housing of the second motor is to a center portion of the drum by a key. The keys transmit torque from drive rings attached to each motor to driven rings attached to the drum mid way between the end plates of the drum.