A gear machine, such as a gear pump or gear motor, has a housing in a chamber of which there are journalled two gears which mesh with one another. One of the gears is mounted on a drive shaft the end of which is extended outwardly from the housing. The housing is closed at one side thereof with a cover having a stepped bore for receiving the end portion of the drive shaft. The cover is formed with a circumferential projection concentric with the drive shaft and adapted for centering the cover relative to an external element to be coupled with the gear machine. The stepped bore provided in the cover forms a chamber in which a centering ring with a certain radial play is positioned, which centering ring surrounds the drive shaft. A sealing ring is also positioned in the chamber formed in the stepped bore, which sealing ring sealingly closes the housing. The centering ring provides for reliable aligning of the gear machine to the external element adapted to be coupled with the machine.
A rotary lobe pump has a pair of rotary lobes rotatably mounted in a pump chamber having end walls perpendicular to axes of the lobes and a side wall with which the rotors making sealing contact. At least one of the end walls is provided with a shallow groove in a region between the axes, the groove being of a selected depth and width to permit solid material lodged between the rotors and at least one of the end walls to become disengaged and flushed out while allowing a minimum of fluid to flow through the groove causing pressure loss in the pump.
