Rotary expansible chamber devices – With mechanical sealing
Reexamination Certificate
1998-04-24
2001-06-12
Nguyen, Hoang (Department: 3748)
Rotary expansible chamber devices
With mechanical sealing
C418S206600
Reexamination Certificate
active
06244841
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to vacuum pumps and, more particularly to oil-free or dry mechanical vacuum pumps.
Oil free or dry vacuum pumps, ie those having an oil free swept volume are well known and are extensively used to evacuate enclosures in clean environments such as those found in the semiconductor industry.
Typical pumps of this type can comprise a chamber having one or, more commonly, a plurality of, for example four, pumping stages, each containing inter-meshing pairs of rotors to effect a pumping action and urge gas being evacuated from a pump inlet, through the pumping stages in turn, and subsequently urge the gas out of the pump through a pump outlet.
One rotor of each pair is attached to a first shaft passing through the stage(s) with the second rotor of each pair being attached to a second shaft also passing through the stage(s). One of the shafts is driven by a motor and the other is usually driven synchronously in the opposite direction by means of timing gears attached to the respective shafts.
The rotors of each pair are commonly of the “Roots” type or of the “Northey” (or “Claw”) type, both well known per se in the vacuum pump industry. In multi-stage pumps, each stage may possess the same type of rotor pairs or there may be different types of rotor pair in different stages. For example, one commercial vacuum pump sold by the Applicants comprises a first stage possessing a “Roots” type rotor pair and the second, third and fourth stages all possessing “Claw” type rotor pairs.
Alternative pumps of this type are regenerative pumps in which a disc shaped rotor attached to a shaft is driven at high speed by an electric motor usually positioned about the shaft. The rotor has a plurality of teeth on its edge or arrayed on one or both of its faces and, in use, the teeth rotate within passageways in a pump stator and urge molecules of gas being pumped through the passageways. At one place in the passageway, a stripper deflects the molecules in to the next passageway or to a pump exhaust.
A gearbox is usually positioned at the driven end of the shaft(s) containing the shaft end(s), bearings within which the shaft(s) rotate, any timing gears and commonly the motor positioned about the driven shaft. Further bearings may optionally be present at the opposite (non-driven) ends of the shaft(s).
For reasons of cleanliness and non-contamination of the gases being pumped to avoid in particular the possibility of transfer of such contamination back in to the enclosure being evacuated, the oils and/or greases necessarily associated with the gearbox need to be contained and isolated within the gearbox.
For the same reasons, the enclosed gearbox is normally positioned adjacent the pump stage associated with the pump outlet. However, for practical engineering reasons, the gearbox cannot be fully isolated, in particular because of the slight leakage always associated with shaft seals which need to be present about the shafts and attached to a head plate between the gearbox body and the pump stages. This is particularly true for seals of the non-contacting type which are often used to minimise power consumption or because the speed of shaft rotation is too high for contact seals such as lip seals.
As such, a common problem with some designs of vacuum pump is that the gearbox shaft seals at the exhaust end of the pump tend to be exposed to different pressures depending on the pump duty. The seal pressure is typically some intermediate pressure between that of the inlet and outlet of the final pump chamber. During pump “roughing” condition this will tend to be greater than atmospheric pressure, whereas at ultimate vacuum condition this will tend to be lower than atmospheric pressure. This can lead to a certain flow of gas past the shaft seals, ie in to or out of the gearbox respectively. This can be particularly the case with non-contacting shaft seals where this may have the undesirable effect of carrying contamination in to the gearbox and oil/lubricant mist out of the gearbox.
SUMMARY OF THE INVENTION
The invention is concerned with the provision of a vacuum pump in which such a problem is minimised and generally overcome.
In accordance with the invention, there is provided a vacuum pump comprising a pump chamber having an inlet and an outlet and through which gas from an enclosure connectable to the inlet can be pumped to a pump exhaust, the chamber possessing at least one rotor adapted for high velocity rotation within the chamber and mounted on a shaft extending from the chamber and in to a pump gearbox substantially isolated from the chamber by means of a shaft seal associated with the shaft, wherein the shaft seal is of a close tolerance but non-contact design and means are provided to buffer, in use of the pump, the gas pressure in the vicinity of the shaft seals from the changeable gas pressures associated with operation of the pump rotor.
The shaft seal must be of a close tolerance, non-contact design. The use of one or more metal rings held within grooves in the pump (stator) body which allow for limited movement—radial and axial—in the grooves and with an internal diameter only very slightly larger than the external diameter of the shaft are useful in this respect. In use of such seals, the metal ring(s) is centred about the shaft in a non-contacting manner.
Alternatively, the shaft seal may be one having a lip held in the pump (stator) body with its lip in close tolerance proximity (but non contacting) to a rotating part or vice-versa.
The invention is based on the surprising discovery that the use of such a close tolerance but non-contact seal coupled with the means to buffer the gas pressure in the vicinity of the seal allows for an effective seal without the need to establish a specific gas flow (or gas circulation) in to the seal area to ensure that the oil is retained in the gear box.
Pumps of the invention can comprise a single stage within the pump chamber or, more commonly, a plurality of individual stages within the chamber in a manner known per se with the first stage being adjacent the chamber inlet and the final stage being adjacent the chamber outlet which is itself connected to the pump exhaust. Each stage may posses a single rotor but more usually possesses a pair of rotors as described above and is separated from its adjacent stage or stages with ports and passageways linking the stages so that gas can be pumped from the chamber inlet to the chamber outlet in sequential order through the stages.
The gas pressure buffering means can comprise a pressure equilibrium volume or “plenum” volume. The plenum volume is generally connected to the gas exhaust line of the pump, thereby ensuring a substantially damped and therefore constant pressure at the shaft seals in order to prevent a significant flow of gas or vapour in to or out of the gearbox. Advantageously, the plenum volume is generally of annular shape and positioned about the shaft(s).
In preferred embodiments of the invention, the gas pressure buffering means is linked to the pump exhaust, preferably directly and not via a pumping chamber.
This advantageously takes the form of a pressure buffering equalisation line linking the pressure buffering means, for example a plenum chamber, and the exhaust. Preferably such a pressure equalisation line can be formed in a head plate member between the pump chamber(s) and the gearbox.
It is generally advantageous for the pressure buffering means, preferably a plenum volume, to be present between two seals of the close tolerance, non-contacting type, both seals generally being positioned on either side of the pressure buffering means and within the head plate area of the pump. In such embodiments, a pressure equalisation line can link the pressure buffering means with the pump exhaust, thereby substantially isolating the equalisation line from the pump chamber(s).
Such embodiments are particularly relevant to vacuum pumps having a single “Claw” stage or to ones having a “Claw” stage at the chamber outlet as the outlet from a “Claw” stage ten
Birch Peter Hugh
Morris Michael
Schofield Nigel Paul
Nguyen Hoang
Pace Salvatore P.
The BOC-Group, plc
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