swept back shape of the blades 47 on impellers 46 and 48 has been found to cause a more aggressive threshing of tailings 60 and forces the tailings 60 to the radially outer portion of the blades 47 faster which has been found to increase conveying capacity threshing plate surfaces 64 a and 68 a may each have a rough surface texture or smooth and or can include elements such as raised protuberances and the like for imparting a desired turbulence to the tailings flow for performing a desired threshing function as discussed in more detail below impellers 46 and 48 each include a mounting portion 82 which is preferably a hub mountable to a rotatable member such as shaft 58 of conveyor 40 in the instance of impeller 46 for rotation with the rotatable member in a predetermined rotational direction such as direction a about a rotational axis such as axis c as best shown in fig1 each impeller 46 and 48 includes a plurality of blades 47 preferably four in number which extend generally radially outwardly from mounting portion 82 at equally spaced locations around the rotational axis as noted before each blade 47 includes a surface 47 a facing in a direction opposite the rotational direction and a surface 47 b facing in the rotational direction referring back now to prior art fig2 and 3 the prior art cleaning systems required a conventional bearing 102 and locking collar.