Rotary expansible chamber devices – Interengaging rotating members – Each tapered
Reexamination Certificate
1999-06-16
2001-04-17
Denion, Thomas (Department: 3748)
Rotary expansible chamber devices
Interengaging rotating members
Each tapered
C418S201100, C418S206700
Reexamination Certificate
active
06217305
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to vacuum pumps and more particularly to screw pumps.
Screw pumps usually comprise two spaced parallel shafts each carrying externally threaded rotors, said shafts being mounted in a pump body such that the threads of the rotors intermesh. Close tolerances between the rotor threads at the points of intermeshing and with the internal surface of the pump body, which acts as a stator, causes volumes of gas being pumped between an inlet and an outlet to be trapped between the threads of the rotors and said internal surface and thereby urged through the pump as the rotors rotate.
Such screw pumps are potentially attractive since they can be manufactured with few working components and they have an ability to pump from a high vacuum environment at the inlet down to atmospheric pressure at the outlet.
Conventional screw pumps with shafts which are either mounted cantilever fashion within the pump body or supported at each end with bearings use a common head plate or plates to support the bearing or bearings of both shafts. The head plate or plates are then fixed to the pump body.
This construction has several disadvantages, for example, the head plate(s) has to be or is usually cooled to keep the bearing within its operating temperatures. However, the pump body (stator) is often run much hotter particularly on screw pumps used in semiconductor manufacturing processes. This gives rise to differential thermal expansion such that the stator bores move apart but the head plate and bearings and therefore the rotors do not move as far and become off-centered relative to their respective bores. This requires the screw pump to be made with large running clearances to prevent or minimize the possibility of seizure.
Further, accurate centering of the shafts and rotors within the bores is also difficult due to the tolerance stack-up of the bearing centers and the positioning of the head plate relative to the stator which is typically effected with doweling.
It is an aim of the present invention to provide a screw pump which prevents or mitigates against the problems associated with conventional screw pumps and in particular the problem of thermal distortion.
SUMMARY OF THE INVENTION
According to the present invention a screw pump comprises a first shaft and spaced therefrom and parallel thereto a second shaft mounted in a pump body, both shafts adapted for axial counter-rotation relative to each other, a first rotor mounted on the first shaft and a second rotor mounted on the second shaft, each rotor being formed on an outside surface with at least one helical vane or thread, the helical vanes or threads intermeshing together so that rotary movement of the shafts will cause a fluid to be pumped from an inlet towards an outlet of the pump, and in that a first bearing arrangement is associated with the first shaft and a second bearing arrangement is associated with the second shaft, a bearing carrier being provided for each bearing arrangement, the bearing carriers each being mounted to the pump body independently one from the other.
In a preferred embodiment the rotors are hollow and a bearing carrier extends within each hollow rotor.
The bearing carriers should be mounted in the pump body in a manner such that centering of the bearings, and hence the shafts held in the bearings, is achieved despite thermal expansion of the pump body due to changes in working temperature of the pump body. This can be achieved in particular by attaching at least one end/edge of each bearing carrier to that part of the pump body which acts as the pump stator, normally that part which is parallel (or substantially parallel) to the pump/shaft main axes.
Each rotor may be substantially cylindrical or alternatively may be tapered in a direction from the inlet of the screw pump towards the outlet.
REFERENCES:
patent: 1191423 (1916-07-01), Holdaway
patent: 2079083 (1937-05-01), Montelius
patent: 2511878 (1950-06-01), Rathman
patent: 3116871 (1964-01-01), Mishina et al.
patent: 4078653 (1978-03-01), Suter
patent: 4952125 (1990-08-01), Nagai
patent: 195 22 555 (1997-01-01), None
patent: 1309885 (1961-09-01), None
patent: 384355 (1932-12-01), None
patent: 167597 (1965-01-01), None
North Michael Henry
Schofield Nigel Paul
Denion Thomas
Pace Salvatore P.
The BOC Group plc
Trieu Theresa
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