EP0480629A1

EP0480629A1 - Improvements in mechanical pumps

Info

Publication number: EP0480629A1
Application number: EP91309065A
Authority: EP
Inventors: Henryk Wycliffe; Nigel Thomas Metcalfe Dennis
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1990-10-06
Filing date: 1991-10-03
Publication date: 1992-04-15
Also published as: GB9021780D0; JPH04259691A

Abstract

A mechanical vacuum pump comprising at least two stages with a pair of intermeshing rotors operating in a pumping chamber associated with each stage, the rotor of each pair being on respective shafts passing through the chamber and arranged for rotation in opposite angular directions, wherein the pump has a first body portion containing or having associated therewith support, control and drive means to drive the shafts synchronously in opposite directions with the shafts protruding from this first body portion into a second body portion in which the pumping chambers are situated. Preferably, the pump is designed for operation with the shafts vertically orientated.

Description

This invention relates to mechanical pumps and more particularly to mechanical vacuum pumps incorporating at least one pair of intermeshing rotors, especially rotors of the type known as "claw" rotors, ie. ones having a "Northey" profile.
Mechanical vacuum pumps of the above kind generally have one rotor of each pair mounted on a first common shaft and the other rotor of each pair mounted on a second common shaft. The shafts are positioned in the pump body with the rotors mounted thereon being held in the correct phase relationship by means of a gear at one end of each shaft and by bearings generally positioned at both ends of each shaft; the shafts are therefore arranged for synchronised rotation in opposite directions to cause intermeshing, normally without contact, between the rotors of each pair.
In a single stage pump, the one pair of rotors is constrained to rotate in the above fashion in a single chamber. In general, however, pumps of this type are multistage with each pair of rotors operating in respective chambers that are linked by means of porting within the walls of adjacent chambers. In accordance with the disclosures of our British Specification No 2 111 126, the pairs of rotors in adjacent chambers may usefully be mounted on their respective shafts in reverse orientation to the pairs in the next chamber, thereby making the porting, and in particular its overall length and positioning in the pump, better in comparison with pumps whose rotor pairs are mounted on their shaft in the same orientation.
Normally, pumps of this type are operated with oil-free and lubricant-free pumping chambers and any oil or lubricant associated with the motor which drives one of the shafts, with the timing gears (or whatever) present for synchronised rotation of the shafts or with the bearings holding the shafts in position within the pump body can be kept clear of the pumping chambers.
In general, the pumps may be operated either with the shafts arranged horizontally or with the shafts arranged vertically. In some cases horizontal operation is preferred, for example in semiconductor vacuum processing use such as CVD and other processes generating particles, as it may enhance reliability during long term operation. In other cases, for example, when evacuating vapours which tend to condense or liquefy during the pumping process, it is often preferred for the shafts to be arranged vertically.
In pumps of this type there are generally bearings at the end of each shaft associated with the motor and timing gears and further bearings at the other end of each shaft such that the rotors are positioned between the respective sets of bearings.
The invention is concerned with an improved vacuum pump for use in particular with pumps designed primarily to operate with vertically arranged shafts.
In addition, however, vacuum pumps of this type are generally costly to manufacture, especially when three or more stages are needed in order to obtain the required vacuum. A considerable proportion of the cost is associated with the rotor assembly in terms of the need for each pair of rotors to be accurately mounted on the respective shafts so that they intermesh closely without touching and to operate within the volume of the relevant chamber within the pump housing.
The present invention is also concerned with the provision of an improved pump whose manufacture and assembly can be simplified, thereby allowing costs to be reduced.
In accordance with the invention there is provided a mechanical vacuum pump comprising at least two stages with a pair of intermeshing rotors operating in a pumping chamber associated with each stage, the rotor of each pair being on respective shafts passing through the chamber and arranged for rotation in opposite angular directions, wherein the pump has a first body portion containing or having associated therewith support, control and drive means to drive the shafts synchronously in opposite directions with the shafts protruding from this first body portion into a second body portion in which the pumping chambers are situated.
Preferably the support, control and drive means comprises bearings, gears and a motor respectively. Advantageously, the support, control and drive means are all contained within the first body portion and sealed therein with respect to the second body portion. Preferably the drive means comprises a motor.
Such a sealed arrangement is beneficial in terms of maintaining a "clean" environment in the vicinity of the pumping chambers whatever type or construction of rotor is employed.
The pumps of the invention preferably possesses two (or more) bearing means associated with each shaft and spaced from each other in that part of the shafts contained in the first body portion. Commonly, one (the lower) of these bearing means will be positioned about one end of the shaft and permit axial movement of the shaft to allow for thermal expansion in particular and the other one(s) (the upper) will be positioned as close as possible to the second body portion (but within the first body portion) to locate accurately the axial position of the shaft in particular.
In such cases, it is preferred for that part of the shafts within the second body portion to be unsupported (thereby avoiding lubricants, etc which would necessarily be associated with the bearings) and the shaft construction can therefore be regarded as being of "cantilevered" or of "overhang" design.
With regard to the ease of manufacture, the pump of the invention preferably has an integral rotor assembly associated with each shaft.
In certain preferred embodiments, the multi-stage rotor assembly and its shaft may be integrally formed throughout as a one piece body, for example from an aluminium alloy body.
In certain other preferred embodiments, the multi-stage rotor assembly may be mounted on a non-integrally formed shaft made from a material such as steel by means of, for example, force fitting, expansion fitting or by bolting.
With preferred embodiments of the invention, assembly of the pumps is made easier by allowing for the pump body in the vicinity of the rotors, ie. the second body portion, to include individual internal wall components to define at least some of the dividing walls between pumping chambers and preferably also to include individual stator components to define at least part of the external walls of the pumping chambers.
In such preferred embodiments of the invention, the internal wall and stator components can conveniently be alternately built around the individual rotor pairs of the respective aligned shafts during assembly of the pump.
Most preferably each internal wall component is divided, for example into two parts, so that it can be mounted in situ about the aligned shafts during assembly of the pump.
In most preferred embodiments, the pump is designed for operation with the shafts vertically orientated, ie. with a lower (first) body portion containing or having associated therewith support and control means in the form of bearings, timing gears and means to drive the shafts synchronously in opposite directions with the shafts protruding from this lower body portion into an upper (second) body portion. As such, assembly of the upper pump body comprising the internal wall components and stator external wall components can be achieved in situ, together with relevant sealing components and then a top portion attached.
The pump inlet can usefully be contained in such a top portion and the internal wall components will generally have slots or other porting for the flow of evacuated gases through the individual chambers of the pumps and hence to an outlet, preferably in the lower area of the pump body.
Overall, pumps of the invention are designed to have optimum operational characteristics in conjunction, in preferred embodiments, with rotor pairs having a reverse orientation of adjacent pairs which allows for a simplified, and generally shorter, ducting arrangement between adjacent chambers of the pump. They are also designed primarily, although not exclusively, with a multi-stage pump whose rotor pairs consist entirely of "claw" type profiles.
To illustrate the invention, reference will now be made to the accompanying drawings, for the purpose of exemplification of the invention only, in which:
Figure 1 shows (schematically) a vertical section through a pump of the invention.
Figure 2 shows a section (not to scale) through the line II-II of Figure 1 showing a pair of rotors in particular.
Figure 3 shows a section (in the scale of Figure 2) through the line III-III of Figure 1 showing a two-piece internal wall component in particular.
Figure 4 shows in isolation a perspective view of a single shaft with integrally formed rotors of the pump of Figure 1.
Figure 5 shows in isolation two synchronously positioned shafts with corresponding integrally formed rotors of the pump of Figure 1.
Figure 6 shows a vertical section through a second pump of the invention.
Figure 7 shows a vertical section through a third pump of the invention.
With reference to the drawings, the pump of the invention comprises a lower body portion 1 within which are positioned shafts 2 and 3 by means of respective end bearings 4 and 5 and respective location bearings 6 and 7.
An upper end portion of the shaft 2 has formed integrally therewith four individual "claw" shaped rotors 8, 9, 10, 11 and an upper end portion of the shaft 3 has formed integrally therewith four further individual "claw" shaped rotors 8′, 9′; 10′; 11′. Each pair of rotors 8, 8′; 9, 9′, 10, 10′ and 11, 11′ are arranged for rotation about their respective shafts in the manner known for "Northey" type pairs. Rotation of the shafts 2, 3 within bearings 4; 5 and 6; 7 respectively is controlled by a pair of timing gears 12 and is actuated via the shaft 2 by drive means 13 operated by belt means (not shown) engaging the annular groove 14 of the drive means.
An annular central body portion 15 is positioned on the lower body portion 1 enclosing the bearing means 6; 7 and sealed to the lower body portion 1 by means of an 'O' ring.
An upper body portion is formed from individual circular stator components 16 alternating with individual internal wall components 17 secured to the central body portion 15 and to each other by a series of further 'O' rings.
The shape of the individual stator components 16 is shown most clearly in Figure 2 with the pair of rotors 11; 11′ located for rotation about their respective shafts 2, 3 in intersecting bores 18 with a flange being present for "sandwiching" the internal wall components 17 between adjacent starter components.
The shape of the individual internal wall components 17 is shown most clearly in Figure 3. Each component is in two parts so that they can, during assembly, be inserted about the shafts 2, 3 and thereafter "sandwiched" between the adjacent stator components 16. In addition to the split circular hole 19 for receiving shaft 2, the internal wall component has a further hole 20 for receiving the other shaft 3 but which is enlarged to act as a transfer port between the chambers on either side of the component.
The position of this enlarged hole 20 is reversed with the smaller hole 19 on adjacent internal wall components so that, in use, the porting provided by enlarged hole 20 allows flow of evacuated fluid in the direction of the arrows in Figure 1.
In an alternative stator arrangement, all individual stators 16 and wall components 17 could be integral but split axially (like item 17) to form a two-part stator.
A top body portion 21 is positioned on top of the uppermost stator component 16, and is sealed thereto by means of a further 'O' ring, and has defined therein a pump inlet 22.
This construction therefore provides four pumping chambers separated by the internal wall components 17 in which the rotor pairs operate. Evacuated gas pumped from the inlet and through each of the four chambers exits via an outlet 23 found in the central body portion 15. In particular, there are no bearings or other components requiring lubrication (or whatever), associated with the upper body portion, ie. within the pumping chamber.
Turning to Figure 6, this shows a further pump of the invention of substantially similar construction to that shown in Figure 1 to 5 inclusive. However, in this pump, the rotor assembly 30 associated with a first shaft 31 is again integrally formed but is not integral with the shaft 31. Instead, it is mounted on the shaft 31 by means of a bolt 32 and washer 33 which engages a circular cavity 34 in the top surface of the rotor assembly 30 and is screwed into a threaded bore of the shaft 31. Again, there are no bearings associated with the top end of the shafts.
The further rotor assembly 35 associated with a second shaft 36 is similarly constructed and similarly attached to the second shaft 36 by means of a further bolt 37 and washer 38.
With regard to Figure 7, this shows a further pump of the invention which is very similar to that of Figure 6 but with its shaft bearings, timing gears and drive motor all contained within the lower body portion 1 and sealed therein with respect to the upper body portion containing the pump chambers.
Figure 7 shows in particular that the interior of the lower (first) body portion 1 is sealed with respect to the second body portion by means of shaft sealing means in the form of piston rings 39 about the shaft 2 and by means of shaft sealing means in the form of piston rings 40 about the shaft 3.
Within the lower body portion 1 are situated the pair of timing gears 12 attached to the respective shafts and a motor 41 arranged to drive the shaft 2 comprising annular stationary windings 42 and, in use, rotating part 43.
The use of such a sealed lower body portion thereby allows the upper body portion containing the four pump chambers to be devoid of any oil or lubricants associated with the bearings 4,5,6,7, the timings gears 12 and the electric motor 41.
Overall, pumps of the invention provide an effective design for easy construction and maintenance of dry vacuum pumps in particular. The material used for the various components may vary depending on pump size and end use. Generally, however, iron, steels and aluminium or aluminium alloys are suitable for the rotor assembly and, in relevant cases, for the shaft as well; in the case of aluminium or aluminium alloys, however, especially in the case of heavier duty pumps, it may be necessary to manufacture the lower ends of the shafts, ie remote from the integrally formed rotor assembly, from a stronger material such as a steel. The steel (or whatever) part and the aluminium part can be keyed together in a variety of known ways.
In all cases, the invention is particularly suitable for use with multi-stage pumps employing "claw" type rotors in each stage. It would, however, be possible to use in the invention a mixture of rotors and include, for example, a mixed "Roots" and "Claw" chamber system (especially one with a roots chamber adjacent the pump inlet) particularly in pumps not being manufactured with an integral rotor assembly for each shaft.

Claims

A mechanical vacuum pump comprising at least two stages with a pair of intermeshing rotors operating in a pumping chamber associated with each stage, the rotor of each pair being on respective shafts passing through the chamber and arranged for rotation in opposite angular directions, wherein the pump has a first body portion containing or having associated therewith support, control and drive means to drive the shafts synchronously in opposite directions with the shafts protruding from this first body portion into a second body portion in which the pumping chambers are situated.
A mechanical pump according to Claim 1 in which the support, control and drive means comprises bearings, gears and a motor.
A mechanical pump according to Claim 1 and Claim 2 in which the support, control and drive means are all contained within the first body portion and sealed therein with respect to the second body portion.
A mechanical pump according to any preceding claim in which the pump has an integral rotor assembly associated with each shaft.
A mechanical pump according to any preceding claim in which the rotor assembly and its shaft is integrally formed throughout as a one piece body.
A mechanical pump according to any preceding claim in which the rotor assembly is mounted on a non-integrally formed shaft.
A mechanical pump according to any preceding claim in which internal wall and stator components of the second body portion are alternately built around the individual rotor pairs of the respective aligned shafts during assembly of the pump.
A mechanical pump according to any preceding claim in which each internal wall component is divided so that it can be mounted in situ about the aligned shafts during assembly of the pump.
A pump according to any preceding claim in which the pump is designed for operation with the shafts vertically orientated.