A shredder or mixer apparatus for the shredding or mixing of bulk material and which is mounted on a mobile chassis which may be driven by a power source on the chassis or pulled by a tractor or truck therefor. A specially designed power assist transmission receives power from the power source and is capable of being manually shifted in order to change a transmission power ratio between the power source and a shredding or mixing member, such as an auger, without any interruption of power to the auger. Thus, changes in loads on the power source can be compensated by reducing or increasing the transmission ratio without power interruption. A specially designed hydraulically operated transmission is effective for this purpose. A manually actuable shift means on the transmission is capable of being engaged by an operator and moved to any of a plurality of selected positions for shifting the transmission to different power output ratios.
