Gear reduction units is particularly applicable to mixing apparatus and methods. A motor driven gear reduction unit has two parallel shafts coupled respectively to a driving pinion gear and a driven gear meshing with the pinion gear. Pivoting the cartridge eccentrically carrying one or more pinions moves the meshing pinion clear of the low speed gear such that the cartridge can be lifted out of the drive. In a multiple reduction arrangement, pivoting also can move an intermediate reduction pinion clear of the low speed gear, enabling the cartridge to be lifted clear.