Abstract: A rotary vacuum pump having a low vacuum stage and a high vacuum stage. Each stage includes a stator body having a bore and a rotor mounted eccentrically in the bore to form a cavity between the stator body and the rotor. The rotor of each stage has two vanes slidably positioned in diametrically opposed slots in the rotor which in use of the pump are substantially in contact with an inner wall of the stator body during rotation of the rotor. The stator body of each stage has an inlet and an outlet to allow, in use of the pump, fluid being pumped to enter into and be expelled from the cavity by means of the rotating vanes. Oil to be injected into the low vacuum stage and, when required, into the high vacuum stage also. Differential flow rates (including zero) of ballast gas are introduced to the low vacuum stage, for example in an amount in excess of ten percent of the overall vacuum pump capacity (which equates to the capacity of the high vacuum stage).

Abstract: A vacuum pump which includes an oil pump for raising the pressure of oil in the vacuum pump during operation of the vacuum pump and an inlet valve for isolating the vacuum pump inlet during non-operation of the vacuum pump. The vacuum pump has an operating valve acting in an oil line between the oil pump and the inlet valve and in an oil line between the inlet valve and an oil reservoir. The operating valve is movable by means of oil pressure between a second position associated with vacuum pump operation in which the oil line between the inlet valve and the oil reservoir is closed and a first position associated with vacuum pump non-operation in which the oil line between the inlet valve and the oil reservoir is open. A means is provided to allow high pressure oil from the oil pump to communicate with the inlet valve in at least the second position.