WO2019171075A1

WO2019171075A1 - Vacuum pump and vacuum pump rotor

Info

Publication number: WO2019171075A1
Application number: PCT/GB2019/050651
Authority: WO
Inventors: Neil Turner; David Alan Turrell; Alan Ernest Kinnaird Holbrook; Matthew Richard WICKES
Original assignee: Edwards Limited
Priority date: 2018-03-09
Filing date: 2019-03-08
Publication date: 2019-09-12
Also published as: GB201803859D0; EP3762611B1; TW201940815A; CN111801498B; GB2571792B; TW201938916A; WO2019171074A1; TW201938915A; WO2019171076A1; CN111801498A; GB2571792A; EP3762611B8; EP3762611A1

Abstract

A rotor and vacuum pump are disclosed for a vacuum pump. The rotor comprises: an outer profile and a hollow inner portion; at least one body within the hollow inner portion; and body constraining means for constraining the body. The at least one body being is such that movement in a direction parallel to a direction of rotation is constrained by the constraining means; and in response to a deceleration of the rotor that is greater than a predetermined critical value a predetermined force is exerted on the body that is sufficient to overcome the constraint exerted by the constraining means and the body moves within the hollow inner portion.

Description

VACUUM PUMP AND VACUUM PUMP ROTOR

FIELD OF THE INVENTION

The field of the invention relates to vacuum pumps and rotors for vacuum pumps.

BACKGROUND

Vacuum pumps have rotors that rotate at high speeds. If the rotors of such a pump are stopped suddenly due to a fault or a sudden increase in pressure at the inlet, then the forces transmitted through the stub shafts and bearings to the headplates can be sufficiently large to break the screws which hold the

headplates to the stator leading to a permanent loss of vacuum integrity. It is desirable if vacuum integrity is maintained after, and preferably during, such an event.

This problem has been addressed previously with the design of lightweight rotors to reduce the energy stored in the rotating mass. This reduces the force imparted to the other components of the pump on sudden deceleration and- reduces the chance that components will break. Lightweight rotors do however have the disadvantage of larger deceleration during pumpdown (and possibly suboptimal pumpdown performance).

A further technique for addressing this problem is the design of deformable rotor profiles which can be used to extend the deceleration time. However, this may have a side-effect of low inertia, and may provide profiles that distort under centrifugal load making it difficult to get good performance over a range of frequencies. A further technique of allowing the fasteners or headplate components to distort without breaking - may help protect the pump to some degree, however, it is still likely that vacuum integrity will be lost temporarily as they slide over one another, and possibly permanently as well.

The invention seeks to provide an alternative way of addressing at least some of the drawbacks listed above. SUMMARY

A first aspect provides, a rotor for a vacuum pump, said rotor comprising: an outer profile and a hollow inner portion; at least one body within said hollow inner portion; and body constraining means for constraining said body; said at least one body being mounted such that movement in a direction parallel to a direction of rotation is constrained by said constraining means; wherein in response to a deceleration of said rotor that is greater than a predetermined critical value a predetermined force is exerted on said body that is sufficient to overcome the constraint exerted by said constraining means and said body moves within said hollow inner portion.

The inventor of the present invention recognised that an elegant solution to the problem of the sudden deceleration of a rotor transferring forces to other components of the pump and causing damage could be addressed by providing a body within the rotor which is held in position during normal operation by constraining or restraining means but which is configured such that the

constraining or restraining forces are overcome when the amount of deceleration of the rotor exceeds a certain value. This results in movement of the body within the rotor which moves the centre of mass of the rotor and absorbs some of the kinetic energy, allowing the outer profile to be decelerated quickly while the inner body decelerates more slowly, leading to an overall reduction in forces

transmitted. This absorption of some of the kinetic energy within the rotor helps protect both the rotor outer profile and other components of the pump from damage.

The constraining means and the mass of the body can be selected to have suitable values such that only when a deceleration that is deemed to be critical for the mechanical strength of the pump is reached will the force of the

constraining means be overcome and the body allowed to move. The constraining means may be configured in a number of ways but in some embodiments said constraining means comprises distortable material, said distortable material being configured to distort under said predetermined force.

In addition to the movement of the body changing the centre of mass of the rotor the distortion of the distortable material will also absorb some of the energy of the rotor.

In some embodiments said distortable material is configured to distort by one of bending, compressing and stretching.

In some embodiments the constraining means comprises a post extending in a radial direction said body being mounted on said post and said post being configured to distort in response to said deceleration being above said

predetermined critical value.

One way of constraining the body may be to mount it on a post, the post being configured such that when a certain force is exerted on the post it will distort and the body will move within the hollow portion of the rotor, movement of the body extending the deceleration time and bending of the post absorbing some of the energy.

In some embodiments, said post is attached to said rotor at a point towards a centre of said rotor and extends towards said outer profile.

One way of allowing the guide to bend is to attach the guide which may be a post to the centre of the rotor and to leave the outer end free. If the guide or post is suitably configured then a force on the body due to a sudden deceleration will cause the post to bend and the deceleration of the mass inside the rotor will be slower than the deceleration of the outer profile.

In some embodiments said body is mounted at a position on said post towards said outer profile. It may be advantageous to mount the body towards the outer edge of the rotor as in such a position movement of the body has a greater effect on the moment of inertia of the rotor and thus, can absorb more of the energy. Furthermore, many rotors are shaped such that their cross section may be larger towards the outer portion allowing the body to move further within the rotor.

In some embodiments, said post is configured to have at least one predetermined portion that is weaker than other portions of said post, such that in response to said critical rate of change in velocity said guide preferentially distorts at said at least one predetermined portion.

It may be desirable to provide further control of the deceleration of the body and this may be provided by configuring the guide to have a portion that is weaker than other portions of the guide. This enables the distortion or bending of the guide in response to the deceleration to be controlled and the movement of the mass within the rotor to follow a predetermined path. This may protect the outer profile of the rotor from being unduly damaged in response to at least some decelerations.

In some embodiments, said body is mounted to slide on said post between an inner and outer position in response to changes in a rotational velocity of said rotor.

Where the body is mounted on a post then in addition to providing some protection from sudden deceleration by allowing the post to distort, the post may also serve as a guide along which the body can slide and thereby change the moment of inertia of the rotor. In this regard, the properties of a rotor change with the operation of the pump. Thus, during startup for example, where the pump is to be accelerated a low inertia rotor will accelerate more quickly. However, when operating at full speed a rotor with a high moment of inertia will be more resistant to disruptions due to changes in pressure and will decelerate more slowly in response to such changes. A body mounted such that it slides within the rotor in response to rotational velocity can be used to provide a rotor with a lower moment of inertia at lower rotational velocities where the body is towards the centre of the rotor and a rotor with a higher moment of inertia at higher velocities where the body is towards the outer portion of the rotor. In this regard, a biasing means may be used to bias the body towards the inner position allowing it to move to the outer position when speeds above a certain level are reached.

Where the body is in the outer position then the force due to deceleration will be higher on the post and it is more likely to distort. It is when the rotor is rotating at or towards full speed that the protection of the rotor due to sudden deceleration events is particularly important.

Alternatively and/or additionally, said constraining means comprises a

compressible material mounted within said hollow inner portion.

The body may be constrained in a number of ways provided that the constraining means holds the body in place until a predetermined force which is triggered by sudden deceleration is exerted on the body, this force being sufficient to overcome the constraining force. In some cases the constraining means may be a compressible material such that the body is mounted within a hollow rotor, in some cases towards the trailing edge of the outer profile with the compressible material lying between the body and the leading edge of the outer profile. The compressible material is configured such that it compresses in response to a predetermined force and the body moves from the trailing edge towards the leading edge of the outer profile of the rotor.

In other embodiments, said constraining means comprises a biasing means attaching said body to said outer profile and configured to move by tensile deformation in response to a force greater than or equal to said predetermined force. An alternative and/or additional means of constraining the body may be to use a biasing means such as a spring which holds the body close to the trailing edge of the outer profile and in response to a predetermined force is configured to stretch or compress such that the body moves towards the leading edge of the outer profile. The body may in some embodiments be mounted within or on some guide means to guide its path within the rotor from the trailing to the leading edge.

In some embodiments, said outer profile of said rotor is substantially lighter than said at least one body.

Where embodiments may be particularly effective is if the body has a substantial mass relative to that of the outer profile. Where this is the case then the slower deceleration of the internal body due to it moving within the outer profile from a position closer to the trailing edge of the rotor towards a position closer to the leading edge has a greater effect on the absorption of the deceleration or kinetic energy and thus results in a reduction in forces being transmitted to other components of the pump.

In some embodiments, said outer profile is more resistant to deformation than said constraining means.

Although, it may be advantageous for the outer profile to be relatively light compared to the body it may be desirable for it to be mechanically robust and more able to resist deformations due to the deceleration force than the

constraining means within the profile such that in response to sudden

deceleration it is the constraining means that distorts and not the outer profile. Where the outer profile is mechanically robust it may be that it is not damaged by the deceleration and can be reused although in some embodiments the whole rotor will need to be replaced following such an event. ln some embodiments, said rotor comprising a fluid flow path between a leading and trailing side of said rotor, said body being mounted to obstruct said fluid flow path in a closed position and to move to an open position and not to obstruct said fluid flow path in response to a deceleration of said rotor exceeding a first value.

In some embodiments, the moveable body within the rotor may also serve as a valve means for obstructing or opening a flow path through the rotor. In this regard, the body forms part of a pressure relief valve within the rotor itself to allow flow of fluid from the leading to the trailing edge of the rotor in response to sudden high pressure events. Having a moveable body within the rotor configured to move in response to sudden deceleration provides an opportunity for this body to be used not only to reduce the speed at which the overall rotor slows but also to open a valve within the rotor and relieve some of the pressure increase. Sudden deceleration is often caused by an increase in pressure and it may therefore be advantageous to provide some pressure relief.

In some embodiments the constraining means is a resilient constraining means and the body may open the valve in response to a first predetermined

deceleration and return to its closed position when the deceleration is over. In this way, protection of the rotor may be improved. In some embodiments in response to a higher deceleration the resilient means may be distorted further, in some cases beyond its elastic limit and the body will move further thereby providing the absorption of deceleration energy that provides at least some protection from damage to the pump and rotor. In some embodiments, the first value and critical value are the same value.

In some embodiments, the rotor comprises at least two constraining means, a first constraining means being configured to allow said body to move a small distance in response to said deceleration exceeding said first value and a second constraining means being configured to allow said body to move a further distance in response to said deceleration exceeding said critical predetermined value. Where the rotor has a movable body which acts both as a valve body and as a protective means against sudden deceleration, then it may be advantageous to use two different constraining means. Such an arrangement may be configured to allow the body to move a small distance to open the valve under a first lower force and a further distance absorbing more of the deceleration energy under a larger critical force.

The two different constraining means can be a number of things provided that their constraint is overcome by different sized forces, allowing control of a first movement in response to a lower force and a further movement in response to a higher force. In some embodiments, said first constraining means comprises a shape of an internal space of said rotor in which said body is mounted, said shape being such that on rotation of said rotor said body is constrained by centrifugal force exerted on said body to obstruct said fluid flow passage.

The first constraining means may simply be the shape of the space in which the body is mounted, the shape being such that when the rotor is rotating the body is thrown by centrifugal force into a position at which it obstructs the fluid flow path. Deceleration of the rotor moves the body away from this position and opens the path. Further movement beyond this initial open position is constrained by the second constraining means which requires a further force to allow the body to move further,

In other embodiments, said first and said second constraining means comprise distortable material, said distortable material of said first constraining means being configured to distort under a first force and said distortable material of said second constraining means being configured to distort under a second higher force. ln some embodiments, said distortable materials of said first and second constraining means are each configured to distort by one of bending,

compressing and stretching.

A second aspect provides a vacuum pump comprising a rotor according to a first aspect.

In some embodiments, said rotor comprises a two lobe rotor, each of said lobes comprising one of said at least one body.

In some embodiments, said vacuum pump comprises a mechanical booster pump.

In some embodiments, said vacuum pump comprises a two rotor Roots pump.

Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims.

Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

Figure 1 shows a rotor according to a first embodiment;

Figure 2 shows a rotor according to a further embodiment;

Figure 3 shows a rotor according to a still further embodiment; and

Figure 4 shows a rotor according to a yet further embodiment. DESCRIPTION OF THE EMBODIMENTS

Before discussing the embodiments in any more detail, first an overview will be provided.

A rotor for a mechanical booster with an energy absorbing core to reduce the loads transmitted to shafts and headplates during a sudden deceleration (timing slip) without unduly reducing the inertia of the rotor is disclosed.

The rotor profile is configured to be relatively light, but still strong enough to maintain its shape under the centrifugal load encountered in normal operation. In some embodiments, the inertia is maintained with the addition of a separate “lollipop” built into the cavity that would normally be called the lightening hole. In a sudden deceleration event the outer profile stops quickly, but, thanks to its low inertia, does not transmit very much load to the stubs, bearings or headplates. The lollipop which comprises a mass mounted at the end of a post extending from a central portion of the rotor, initially continues to rotate, and decelerates over a longer period than the outer profile by bending its stick. Energy is absorbed in the stick, and the extended deceleration reduces the force

transmitted to the headplate.

In some embodiments, the outer profile is configured to be mechanically robust such that it does not distort under forces under which the lollipop post distorts and the damage is localised to the lollipop, making the profile reusable after replacing the damaged parts.

Almost all existing solutions to this problem involve lightening the rotor, and do not take into consideration the associated loss in pumpdown performance that this would entail.

The lollipop stick or post can be weakened in a favourable location, so that the head of the lollipop has a large range of movement without colliding with the inside edge of the outer profile. Figure 1 shows a rotor 5 comprising an outer profile 10 and a body 20 mounted on posts 30 according to an embodiment. The posts 30 and body 20 are configured such that in response to a deceleration that exceeds a predetermined critical value the posts 30 will bend and body 20 will move from its central position towards the leading edge of the rotor. In this way, the time during which deceleration occurs is extended and the forces transmitted to other components of the pump will be reduced compared to the case were there not to be some internal movement of masses within the rotor.

In some embodiments, body 20 is also configured to slide along post 30 allowing the inertia of the rotor to change in dependence upon the rotational velocity of the rotor. Thus, depending on the rotational velocity a centrifugal force exerted on the body will change and at a certain point it will be sufficient for the body 20 to move from an inner position to the outer position shown. In this regard, the body may be retained in the inner position by biasing means such as springs not shown. The biasing means are configured to stretch such that the body moves to the outer position in response to forces that are exerted when the rotor is operating at or close to full speed.

Thus, when operating at full speed the body will be in the position shown in Figure 1. It is at full speed that the pump is at most risk of damage from sudden deceleration due to pressure increases or faults within the pump. Thus, where the bodies are able to slide along post 30 they will be in the position shown in Figure 1 at higher rotational speeds. In other embodiments the bodies 20 will not be mounted to move in a radial direction. In either case sudden deceleration above a critical value from full speed will cause post 30 to bend and will cause posts 30 to bend and bodies 20 to move in response to the deceleration force.

In some embodiments, the posts 30 are configured to deform preferentially at a certain point. This allows movement of the body in response to sudden deceleration to be more predictable and controlled. It may be advantageous to control it such that the body 20 does not contact the outer profile 10 in response to a deceleration at or close to the critical post bending value, and thus, the rotor outer profile may be reusable.

Figure 2 shows an alternative embodiment where the constraining means is in the form of a compressible material 32. In this case, forces due to deceleration of the rotor acting on body 20 will cause the body 20 to push against the

compressible material 32. The compressible material may be configured such that a certain force on body 20 is sufficient to compress the material 32 and allow body 20 to move.

Figure 3 shows an alternative embodiment where the constraining means is in the form of a spring 34. In this embodiment, on deceleration the spring will extend and the body 20 will move within the internal space of the rotor. In this embodiment there is a guide 36 for constraining the movement of body 20 in response to the deceleration force. In some cases there may be openings within the outer profile 37a and 37b the opening 37b being obstructed by body 20 when in the constrained position. When not in the constrained position opening 37a is no longer obstructed and fluid can flow from an opening 37b in the leading edge of the rotor to an opening 37a in the trailing edge. The flow path is in this embodiment through the guide 36 which may be is larger than the moving body 20, in other embodiments there may be an adjacent channel running between the openings. The flow path and openings acts as a pressure relief path and help reduce the deceleration forces further. In some embodiments, the spring is configured to extend within its elastic limit to open opening 37a in response to a first deceleration force. If the deceleration force is larger than a second critical value, then the spring may extend beyond its elastic limit and the body will move further and this movement will absorb some of the deceleration energy.

In some embodiments, the constraining means may be a combination of one or more of the constraining means disclosed in Figures 1 to 3. For example in the embodiment of Figure 3, there may be a compressible material within the guide towards the leading edge, which acts to cushion the body and slow its movement when the spring has moved a certain distance, in some cases beyond its elastic limit. Similarly there may be compressible material within the hollow inner space of the rotor of Figure 1 or the body in the embodiment of Figure 2 may be attached to the trailing side of the outer profile of the rotor by a spring means.

The use of different resilient means to impede the movement of the body 20 may allow different critical rotational velocities to trigger movements by different amounts. This can be used to provide limited movement of the body sufficient to open the pressure relief valve at a first deceleration and to provide further movement where deceleration is higher. The further movement may result in permanent distortion of the resilient means and in some embodiments damage to the rotor.

Figure 4 shows such an embodiment where a rotor has two constraining means for body 20, a first constraining means 34, comprising a spring configured to distort a small distance in response to a first force, thereby opening port 37a and providing a passage through the rotor 5 to inlet 37b. In response to a larger deceleration force the constraining means 34 distorts beyond its elastic limit and the body 20 moves to contact compressible material 32 which acts as a crash mat to receive body 20 and compresses allowing body 20 to move towards the leading edge of the rotor and absorb some of the energy of the deceleration.

In an alternative embodiment to that shown by Figure 4, springs 34 may be absent and the body 20 may be constrained simply by the internal shape of the rotor to obstruct port 37a when the rotor is rotating and centrifugal force is exerted on the body. In this case deceleration will cause the body 20 to move away from port 37a and the fluid flow path through the rotor will open. The body will be constrained from moving further by compressible material 32. A higher deceleration force may be sufficient to compress this material and the body will move further into this material absorbing some of the kinetic energy. In summary Figure 1 shows a device configured to absorb energy by bending, Figure 2 shows a device which absorbs energy by compression. Figure 3 shows a device which absorbs by a tensile deformation. Figure 4 shows an embodiment with two constraining means where in one example one deforms by tensile deformation and the other by compression.

The skilled person would recognise that the constraining means could be formed in different ways. Where the body in the rotor also acts as a valve body then the constraining means may be configured such that an initial movement occurs at lower forces allowing the valve to open, while further movement may occur at higher forces and be energy absorbing. The constraining means used in embodiments may be formed of a distortable material which may distort and absorb energy under any one of bending, compression and tensile deformation. Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

REFERENCE SIGNS

5 rotor

10 outer profile

20 body

30 posts

32 compressible material 34 spring

36 guide

37a, 37b openings

Claims

1. A rotor for a vacuum pump, said rotor comprising:

an outer profile and a hollow inner portion;

at least one body within said hollow inner portion; and

constraining means for constraining said body;

said at least one body being mounted such that movement in a direction parallel to a direction of rotation is constrained by said constraining means;

wherein

in response to a deceleration of said rotor that is greater than a

predetermined critical value a predetermined force is exerted on said body that is sufficient to overcome the constraint exerted by said constraining means and said body moves within said hollow inner portion.

2. A rotor according to claim 1 , wherein said constraining means comprises distortable material, said distortable material being configured to distort under said predetermined force.

3. A rotor according to claim 2, wherein said distortable material is configured to distort by one of bending, compressing and stretching.

4. A rotor according to any preceding claim, wherein said constraining means comprises a post extending in a radial direction said body being mounted on said post and said post being configured to distort in response to said deceleration being above said predetermined critical value.

5. A rotor according to claim 4, wherein said post is attached to said rotor at a point towards a centre of said rotor and extends towards said outer profile.

6. A rotor according to claim 4 or 5, wherein said body is mounted at a position on said post towards said outer profile.

7. A rotor according to any one of claims 4 to 6, wherein said post is configured to have at least one predetermined portion that is weaker than other portions of said post, such that in response to said critical rate of change in velocity said guide preferentially distorts at said a least one predetermined portion.

8. A rotor according to any one of claims 4 to 7, wherein said body is mounted to slide on said post between an inner and outer position in response to changes in a rotational velocity of said rotor.

9. A rotor according to any preceding claim, wherein said constraining means comprises a compressible material mounted within said hollow inner portion and configured to be compressed in response to a force greater than or equal to said predetermined force.

10. A rotor according to any preceding claim, wherein said constraining means comprises a biasing means attaching said body to said outer profile and configured to distort by tensile deformation in response to a force greater than or equal to said predetermined force.

11. A rotor according to any preceding claim, wherein said outer profile of said rotor is substantially lighter than said at least one body.

12. A rotor according to any preceding claim, wherein said outer profile is more resistant to deformation than said constraining means.

13. A rotor according to any preceding claim, said rotor comprising a fluid flow path between a leading and trailing side of said rotor, said body being mounted to obstruct said fluid flow path in a closed position and to move to an open position and not to obstruct said fluid flow path in response to a deceleration of said rotor exceeding a first value.

14. A rotor according to claim 13, said rotor comprising at least two

constraining means, a first constraining means being configured to allow said body to move a small distance in response to said deceleration exceeding said first value and a second constraining means being configured to allow said body to move a further distance in response to said deceleration exceeding said critical predetermined value.

15. A rotor according to claim 14, wherein said first constraining means comprises a shape of an internal space of said rotor in which said body is mounted, said shape being such that on rotation of said rotor said body is constrained by centrifugal force exerted on said body to obstruct said fluid flow passage.

16. A rotor according to claim 14, wherein said first and said second

constraining means comprise distortable material, said distortable material of said first constraining means being configured to distort under a first force and said distortable material of said second constraining means being configured to distort under a second higher force.

17. A rotor according to claim 16, wherein said distortable materials of said first and second constraining means are each configured to distort by one of bending, compressing and stretching.

18. A rotor according to any preceding claim, wherein said rotor comprises a two lobe rotor, each of said lobes comprising one of said at least one body.

19. A vacuum pump comprising a rotor according to any preceding claim.

20. A vacuum pump according to claim 19, wherein said vacuum pump comprises a mechanical booster pump.

21. A vacuum pump according to claim 19, wherein said vacuum pump comprises a two rotor Roots pump.