this driving wheel is provided with holes 13 for the passage of lubricants i e drilling mud and it will be provided with a pair of cheeks which are parallel planes one of which is indicated at 14 secured to the rotor head 10 on its lathe axis will be a cylinder 15 and a sleeve bearing 16 will be held in position on the cylinder 15 by means of a cap 17 fastened into the member 15 for example by means of the screw 18 since the stator member of the helical gear pair within which the rotor terminating in the head 10 rotates and orbits is generally of rubber or similar material misalignment can occur and for this reason and to allow for freedom of movement the spacing between the cheeks 14 is preferably very slightly larger than the outside diameter of the sleeve 16 this along with the radial freedom provided by the cheek arrangement takes care of any possible misalignment from the foregoing description it will be clear that as the inner member or rotor of the helical gear pair rotates and gyrates as is common in moineau type gear pairs the cylinder 15 will describe theoretically a circular path within the drill pipe 11 and as it describes this path it will bear against one or the other of the cheeks 14 depending upon the direction of rotation and will transmit torque from the member 10 to the driving wheel 12 the sleeve 16 is free to rotate on the cylinder 15 and a cap 19 will provide a thrust bearing against the bottom 20 of the slot between the cheeks 14 in fig2 there is shown a slightly modified embodiment of the.