pump outlet 22 is located downstream from the holweck mechanism 34 the cartridge 14 has three inlets 16 18 and 20 the first low fluid pressure inlet 16 is located upstream of all of the pumping sections in this example the first inlet 16 is substantially orthogonal to the longitudinal axis of the drive shaft 26 as indicated at 36 the second middle fluid pressure inlet 18 is located interstage the first pumping section 30 and the second pumping section 32 in this example the second inlet 18 extends about the longitudinal axis of the drive shaft 26 the third low fluid pressure inlet 20 may be located as illustrated upstream of or alternatively between the stages of the holweck mechanism 34 such that all of the stages of the holweck mechanism are in fluid communication with the each of the inlets 16 18 20 returning now to the housing 12 the bore has an inlet formed in the rear surface 38 of the housing 12 and through which the cartridge 14 is inserted into the housing 12 the inner surfaces 40 42 44 46 of the bore guide the cartridge 14 towards the fully inserted position shown in fig1 to 6 as it is inserted into the bore the end of the bore is profiled as indicated at 48 in fig1 to define abutment surfaces for engaging the front end of the inserted cartridge 14 and which with the rear surface 38 of the housing 12 limit the extent to which the cartridge 14 can be inserted into the housing 12 as shown in fig1 to 6 the housing 12 is shaped so as to expose the bore at a number of locations to allow fluid to enter the fluid inlets 16 18 20 when the.