GB2421054A

GB2421054A - Vacuum pump with cooled shaft seal surface and bearing

Info

Publication number: GB2421054A
Application number: GB0426919A
Authority: GB
Inventors: Mark Edwards Tomkins
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 2004-12-08
Filing date: 2004-12-08
Publication date: 2006-06-14
Also published as: GB0426919D0

Abstract

A screw, roots or Northey vacuum pump comprises a stator 12 and shaft 14 located in the stator and supported for rotation relative to the stator. A shaft seal 18 comprising a carrier 26 with PTFE rings 20, 24 contacting a sealing surface 22 of a sleeve 44 mounted on the shaft separates a lubricated bearing pump portion and an oil free portion. A channel 38 is provided for conveying a coolant e.g. oil between the shaft and at least one of the sealing surface and a bearing 16 for supporting the shaft. The channel may be helical and coolant is supplied through an axial non-rotating tube 30 supported on a shrink fit bearing 35 co-axially with the shaft and through radial ports 30. Cools the seal sleeve and bearing and reduces risk of carbonisation.

Description

VACUUM PUMP

This invention relates to vacuum pumps, and is directed to the cooling of one or more components of a vacuum pump.

Dry pumps are widely used in industrial processes to provide a clean and/or low - pressure environment for the manufacture of products. Applications include the chemical, pharmaceutical, semiconductor and flat panel manufacturing industries.

Such pumps include an essentially dry (or oil free) pumping mechanism, but generally also include some components, such as bearings and transmission gears, for driving the pumping mechanism that require lubrication in order to be effective.

Examples of dry pumps include Roots, Northey (or "claw") and screw pumps. Dry pumps incorporating Roots and/or Northey mechanisms are commonly multi-stage positive displacement pumps employing intermeshing rotors in each pumping chamber defined by the stator of the pump. The rotors are located on contra- rotating shafts, and may have the same type of profile in each chamber or the profile may change from chamber to chamber.

In vacuum pumps operating near ultimate pressure, the majority of the pressure differential occurs near the exhaust end of the pump. This is due to the high volumetric efficiency necessary to achieve good ultimate pressures. As a result of this high volumetric efficiency, a significant proportion of the displaced gas is continually conveyed back and forth in the exhaust line, generating heat.

Surfaces normal to the direction of gas flow see the maximum impact velocity, and so the impacting of gas molecules on these surfaces gives excellent heat transfer.

As the pumped gas generally moves parallel to the surfaces of the stator, the gas does not impact on the surfaces of the stator as much as it does on the surfaces of the rotor, and so the stator has a lower heat transfer rate than the rotor.

As the rotors of the pump heat up, heat is conducted from the rotors to the shafts.

Unless there is shaft cooling or other means of dissipating the heat then the temperature of the shafts will increase until it approaches the pumped gas temperature. This can cause the heating of other components of the pump that are in contact with the shaft, such as static and dynamic seals, bearings, gears and lubricating oils. This can lead to carbonisation of a thin film of mineral or hydrocarbon lubricating oil disposed between the shaft and the contact surface of an elastomeric or PTFE lip seal typically used as a seal for isolating the pumping chambers from the gearbox of the pump. This can, in turn, lead to contamination of the lubricating oil and failure of the lip seal.

It is an aim of at least the preferred embodiment of the present invention to reduce seal temperatures by conducting away heat generated by the seal, and by reducing the amount of heat that is transferred from a shaft to the seal.

The present invention provides a vacuum pump comprising a stator, a shaft located in the stator and supported for rotation relative to the stator, a shaft seal for contacting a sealing surface extending about the shaft, and a channel for conveying a coolant between the shaft and at least one of the sealing surface and a bearing for supporting the shaft.

By conveying a coolant between the shaft and the sealing surface, the coolant can take heat away from the sealing surface, thereby cooling the surface of the shaft seal contacting the sealing surface. Due to the reduced temperature at the sealing surface, the risk of carbonisation occurring at the seal is reduced. This can extend the lifetime of the seal and any lubricant located between the seal and the sealing surface, and thus ultimately the lifetime of the pump.

Furthermore, by conveying a coolant between the shaft and a bearing, the coolant can take heat away from the inner race of the bearing. As well as also preventing carbonisation of lubricant located in the bearing, the reduced temperature of the bearing (and the shaft seal) can allow the shaft to be rotated at a greater speed, thereby improving pump performance. Thus, the present invention also provides a vacuum pump comprising a stator, a shaft located in the stator and supported for rotation relative to the stator, and a channel for conveying a coolant between the shaft and a bearing for supporting the shaft.

In the preferred embodiment, the outer surface of the shaft is profiled to define, at least in part, the channel. For example, the channel may be defined, at least in part, by one or more grooves formed on the outer surface of the shaft. One or more helical grooves may be formed on the outer surface of the shaft to convey coolant along and about the outer surface of the shaft. Alternatively, a plurality of axially extending grooves may be formed on the outer surface of the shaft, which grooves may be arranged substantially parallel to the longitudinal axis of the shaft, or may be curved.

The sealing surface is preferably located on a sleeve mounted on the shaft, the channel extending between the sleeve and the shaft. When the outer surface of the shaft is profiled to define part of the channel, the channel may be fully defined by the outer, profiled surface of the shaft and the inner, cylindrical surface of the sleeve. As an alternative to profiling the outer surface of the shaft to define the channel, the inner surface of the sleeve may be profiled to define the channel, the channel now being fully defined by the outer, cylindrical surface of the shaft and the inner, profiled surface of the sleeve. As another alternative, both the inner surface of the sleeve and the outer surface of the shaft may be profiled to define the channel.

To supply the coolant to the channel, the pump preferably comprises a cavity located within the shaft, means for conveying a coolant to the cavity, and means for conveying coolant from the cavity to the channel. In the preferred embodiment, the means for conveying a coolant to the cavity comprises a supply tube located at least partially within the shaft for supplying coolant to the cavity.

The supply tube is preferably substantially co-axial with the shaft, and is preferably supported by a bearing located between the supply tube and the shaft to inhibit rotation of the supply tube with the shaft.

The means for conveying coolant from the cavity to the channel preferably comprises means for conveying the coolant radially outwards from the cavity towards the shaft. For example, the coolant may be conveyed through at least one bore located within the shaft and extending between the cavity and the channel. In the preferred embodiment, a plurality of bores are spaced about the longitudinal axis of the shaft to supply coolant from the cavity to the channel.

The pump may be a dry pump, in which the pumping mechanism comprises first and second intermeshing rotors adapted for counter-rotation within the stator. The rotors may have a Roots, Northey or screw profile as required. The pump may be in the form of a multi-stage pump in which the stator defines a plurality of interconnected pumping chambers arranged in series and each housing respective rotors.

Preferred features of the present invention will now be described, by way of example only, with reference to the following drawing, which is a cross-section through a part of a vacuum pump.

The pump 10 comprises a stator 12 housing a drive shaft 14 for rotating a rotor (not shown) within a pumping chamber defined by the stator 12. The shaft 14 is supported for rotation relative to the stator 12 by bearings 16, which may be in the form of magnetic bearings or, as illustrated rolling bearings. Rolling bearings typically comprise two rings, an inner ring and an outer ring, between which rolling elements run in raceways. To prevent mutual contacts between the rolling elements, they are often guided and evenly spaced by a cage. A lubricant establishes a loadcarrying film separating the bearing components in rolling and sliding contact in order to minimize friction and wear. Other purposes include the prevention of oxidation or corrosion of the bearing components, the formation of a barrier to contaminants, and the transfer of heat away from the bearing components.

A shaft seal assembly 18 is provided for sealing against lubricant flow along the shaft 14 from a lubricated region (on the left of the drawing as illustrated) and an oil free region (on the right of the drawing as illustrated) during use of the pump 10. In this embodiment, the seal assembly 18 comprises a first sealing ring 20 formed from polytetrafluoroethylene (PTFE) or other elastomeric material having a peripheral sealing lip which extends towards the lubricated region and bears io radially against a sealing surface 22 of the shaft 14, and a second, similar sealing ring 24 having a peripheral sealing lip which extends towards the oil free region and bears radially against the sealing surface 22 of the shaft 14. The sealing rings 20, 24 are mounted in a carrier 26 by any suitable method, for example by bonding or crimping.

The shaft 14 includes a longitudinal bore 28 that passes partially along the length of the shaft 14 and is co-axial therewith. A coolant supply tube 30 is located within the longitudinal bore 28. The coolant supply tube 30 extends through the longitudinal bore 28 such that a first end 32 is located within the bore 28 and a second end thereof (not shown) extends from the other end (not shown) of the shaft 14. The second end of the coolant supply tube 30 may be retained by any convenient means. To inhibit rotation of the coolant supply tube 30 within the longitudinal bore 28 with rotation of the shaft 14, a bearing 34 is provided between the outer surface of the coolant supply tube 30 and the inner surface of a ring 35 located within the longitudinal bore 28 to support the coolant supply tube 30. The ring 35 may be inserted in the longitudinal bore 28 using any convenient technique, such as shrink fitting in which the ring 35 is initially shrunk using liquid nitrogen, for example, and inserted into the longitudinal bore 28 so that subsequent thermal expansion causes the ring 35 to be rigidly located within the longitudinal bore 28.

The shaft 14 further includes a plurality of radial bores 36, each extending between the longitudinal bore 28 and a helical groove 38 formed on the outer surface 40 of the shaft 14. The outer surface 40 of the shaft 14 thus defines both with the inner surface 42 of a seal sleeve 44 mounted on the shaft 14 to provide the sealing surface 22 of the shaft 14, and with the inner surface 46 of the bearing 16, a helical channel 47 which extends along and about the outer surface 40 of the shaft 14 between the oil free region and the lubricated region of the pump.

In use, a stream of coolant, for example a coolant oil, is supplied from a source thereof to the second end of the coolant supply tube 30. The source may be conveniently provided by an oil reservoir located external to the stator of the pump in which the rotor is housed, for example in the gearbox of the pump 10. The coolant flows through the bore 48 of the coolant supply tube 30 and into a cavity defined between the end of the longitudinal bore and the ring 35. From the Is cavity 50, the coolant flows radially outwards through the radial bores 36 and enters the helical channel 47 defined between the shaft 14 and the seal sleeve 44 and the bearing 16 in turn, within which it flows back along the outer surface 40 of the shaft 14, that is, in a direction opposite to the direction of the coolant flow through the bore 48. From the helical channel 47 the coolant is exhaust back into the oil reservoir, from which the coolant may be pumped back to the second end of the coolant supply tube 30 via a suitable heat exchange mechanism. The arrows in the drawing indicate the direction of the coolant flow through the illustrated part of the pump 10. As the coolant enters the reservoir, it can cause a turbulent oil splash or mist that lubricates the gears within the gearbox.

The flow of coolant through the channel 47 thus extracts heat from the seal sleeve 44 and the bearing 16. Cooling of the seal sleeve 44 and bearing 16 reduces the risk of carbonisation at the bearing 16 and the sealing surface 22, and also advantageously increases the viscosity of lubricant provided between the lips of the seals 20, 24 and the sealing surface 22. -.7-

Claims

1. A vacuum pump comprising a stator, a shaft located in the stator and supported for rotation relative to the stator, a shaft seal for contacting a sealing surface extending about the shaft, and a channel for conveying a coolant between the shaft and at least one of the sealing surface and a bearing for supporting the shaft.

2. A vacuum pump according to Claim 1, wherein the outer surface of the shaft is profiled to define, at least in part, the channel.

3. A vacuum pump according to Claim 1 or Claim 2, wherein the channel is defined, at least in part, by a groove formed on the outer surface of the shaft.

4. A vacuum pump according to Claim 3, wherein the groove is a helical groove extending about and along the outer surface of the shaft.

5. A vacuum pump according to any preceding claim, wherein the sealing surface is located on a sleeve mounted on the shaft, the channel extending between the sleeve and the shaft.

6. A vacuum pump according to any preceding claim, comprising a cavity located within the shaft, means for conveying a coolant to the cavity, and means for conveying coolant from the cavity to the channel.

7. A vacuum pump according to Claim 6, wherein the means for conveying a coolant to the cavity comprises a supply tube located at least partially within the shaft.

8. A vacuum pump according to Claim 7, wherein the supply tube is substantially co-axial with the shaft.

9. A vacuum pump according to Claim 7 or Claim 8, wherein a bearing is located between the supply tube and the shaft to inhibit rotation of the supply tube with the shaft.

10. A vacuum pump according to any of Claims 6 to 9, wherein the means for conveying coolant from the cavity to the channel comprises means for conveying the coolant radially outwards from the cavity towards the shaft.

11. A vacuum pump according to Claim 10, wherein the means for conveying coolant from the cavity to the channel comprises at least Is one bore located within the shaft and extending between the cavity and the channel.

12. A vacuum pump according to Claim 11, comprising a plurality of said bores spaced about the longitudinal axis of the shaft.

13. A vacuum pump according to any preceding claim, wherein the shaft seal comprises at least one resilient sealing ring surrounding the shaft and having a lip bearing radially against the sealing surface.

14. A vacuum pump comprising a stator, a shaft located in the stator and supported for rotation relative to the stator, and a channel for conveying a coolant between the shaft and a bearing for supporting the shaft.