单项选择题
A.injector pumps
B.positive displacement pumps
C.centrifugal pump
D.centripetal pumps
单项选择题 Shaft sealing in a centrifugal pump can be effected by （）.
单项选择题 As for a hydraulic vane pump, which of the following is in contact with the vanes during working: （） (1)the rotor (2)the pump shaft (3)the external surface of the housing
单项选择题 As for a hydraulic single action vane pump, the inner wall of the stator is usually of （） shape.