be used to prevent the pawl 46 from moving beyond the point at which the cam surface 45 on the pawl will properly engage the ratchet it will be apparent that as the rotor turns a full revolution the pawl 46 will be carried to a place where the surface of the pawl 46 will slide onto the ratchet 45 and then to where the ratchet teeth 50 will be engaged by the step 51 on the pawl 46 as the rotor continues the ratchet wheel 45 will be turned a short distance to move the seal 21 rotatably through a few degrees of rotation that turning will be sufficient to prevent the seal 21 from presenting the same path across its face to the disk thus any incipient groove in the face of the seal 21 will be rotated out of the path of whatever irregularity on the plate 35 might be causing the groove and the wear on the seal will be equalized although it is not necessary for the successful operation of the device provision may be made for varying the flow rate of liquid through the machine one simple way is by using orifice members 26 having openings of varying sizes this means is effective and is readily possible because of the use of a separate orifice member another means of accomplishing the same purpose is also.