FIELD OF THE INVENTION

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

BACKGROUND

When designing a rotor for a vacuum pump, the properties of the rotor which affect the performance of the pump need to be considered. In this regard some of the properties of the rotor which are desirable change depending on the stage of operation of the pump. It is advantageous, for example, if a rotor of a vacuum pump can accelerate quickly during start up of the pump as this allows it to reach operational speed in a shorter time and start the effective pumping of a chamber. However, it is also desirable for a rotor to be able to provide some resistance to disruptions due perhaps to increases in pressure. Lightweight rotors do have the disadvantage of larger deceleration during pumpdown and also possibly suboptimal pumpdown performance.

In effect, a rotor with a low moment of inertia is desirable during the acceleration phase as it allows increased acceleration for the same force. However, when the pump is operating at full speed a rotor with a lower moment of inertia is more prone to disruptions and in particular, an increase in pressure at the input will cause a lower inertia rotor to decelerate more than it would one with a higher moment of inertia.

It would be desirable to be able to change the properties of a rotor depending on the stage of operation of a pump to provide improved performance.

SUMMARY

A first aspect provides a rotor for a vacuum pump, said rotor comprising: an outer profile; at least one body within said outer profile mounted such that it is movable between an inner position towards a centre of rotation of said rotor and an outer position away from said centre of rotation, a moment of inertia of said rotor changing with a change in position of said at least one body; biasing means for biasing said at least one body towards said inner positon; wherein said body is configured to move between said inner and outer position in response to changes in forces acting on said body triggered by changes in a rotational velocity of said rotor.

The inventors of the present invention recognised that some preferred properties of a rotor, such as inertia, change with the stage of operation of the pump. They also recognised that the speed of the rotor was also characteristic of the stage of operation and thus, an elegant solution might be to vary the moment of inertia of the rotor in dependence upon the rotational speed. Such a rotor would have preferred properties both when accelerating and when operating at full speed.

In some embodiments, said biasing means and mass of the body are configured such that said mass moves between said inner and outer position in response to said rotor passing a predetermined angular velocity.

Embodiments provide a rotor whose properties change at a predetermined rotational velocity. This can be achieved with a movable body mounted within the rotor and configured such that at the predetermined rotational velocity the body moves from an inner to an outer position and the moment of inertia of the rotor is correspondingly changed. The body is held in the inner position by a biasing means, properties of the biasing means and body being selected to provide movement between the two positions at a preferred rotational speed.

In this regard, the properties of the biasing means and the size of the mass will affect the force required to move the body between the inner and outer positions. Thus, the rotor may be designed by suitable selection of these properties so that the mass moves at a predetermined angular velocity. In some cases this may be selected to be the angular velocity close to full speed of the pump such that when not operating at or close to full speed the rotor inertia is lower and once full operational speed is reached the moment of inertia of the rotor rises to a higher value.

In some embodiments, said rotor comprises at least one guide, said guide forming a path between said inner and said outer position, said at least one body being mounted to move along a corresponding guide.

The one or more bodies may be mounted on or within a guide such that the movement of the body is constrained by the guide, the guide can be configured to provide the required, generally radial path, from the inner to the outer position.

In some embodiments, the guide may be a post and the body may be mounted to slide on the post between the inner and outer position. In other embodiments, the guide may be a channel along which the body my slide or roll depending on its formation.

In some embodiments, said guide is configured to provide mechanical stiffening of said rotor.

In order to allow a body within a rotor to move some portion of the internal part of the rotor will be hollow. This may render the rotor less mechanically robust than a completely solid rota. Where the guide is a post or some other solid feature then it may be advantageous if the guide is configured to provide mechanical stiffening of the rotor and thereby increase its robustness.

In other embodiments, said guide is configured to bend in response to a rate of change in velocity above a predetermined critical value.

In some cases, rather than being used to provide additional mechanical stiffening of the rotor, the guide for mounting the body may be configured to bend in response to a rate of change in velocity above a predetermined critical value. In this regard, sudden deceleration of a rotor can cause damage to the rotor. Making the rotor more robust to reduce the possibilities of damage can lead to other portions of the pump being damaged by the sudden deceleration of the rotor. The inventors of the present invention recognised that having a movable mass within a rotor not only allows the moment of inertia to vary but also provides a possibility of controlling the forces exerted on the rotor due to the deceleration to some extent. In particular, the outer profile of the rotor which may not have a substantial mass could decelerate quickly in response to an external event while with a suitably configured rotor, the mass inside could continue to move and decelerate more slowly, by for example bending the guide and thereby absorb some of the energy of the deceleration. Such a configuration improves the robustness of the pump to such events.

In some embodiments, said guide is attached to said rotor at a point towards said inner position and is not attached to said outer profile towards said outer position.

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 guide is configured to have at least one

predetermined portion that is weaker than other portions of said guide, such that in response to said critical rate of change in velocity said guide preferentially distorts at said a 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 outer profile of said rotor is substantially lighter than said at least one body.

Embodiments are particularly effective where the outer profile of the rotor has a substantially lower mass than the body. A higher mass body which is moving makes the relative change in moment of inertia of the rotor larger than would be the case were the body to have a lower mass compared to the outer profile which does not move.

In some embodiments, said rotor comprises a fluid flow path between a leading and trailing side of said rotor, said body being mounted to obstruct said fluid flow path when in said outer position and not to obstruct said fluid flow path when in said inner position.

One alternative and/or additional way of dealing with the pressure at the input of the pump suddenly rising and causing deceleration of the rotor is to provide a pressure relief valve which is conventionally mounted in the stator and opens when the pressure at the pump outlet exceeds the inlet pressure by more than a certain amount. This helps reduce the pressure difference between the inlet and outlet and protects the pump from sudden deceleration. The inventors recognise that having a moving body within the rotor of a pump could be used in

conjunction with fluid flow paths inserted into a rotor between a leading and trailing side of the rotor to provide a pressure relief valve that is activated by movement of the body between the inner and outer position triggered by changes in rotational velocity of the rotor. In this regard, in some embodiments the body moves from the outer to the inner position when the rotor slows below a predetermined angular velocity. Such a deceleration may well be caused by an increase in pressure at the inlet and it may be advantageous if such deceleration of the moving body were also to cause a pressure relief valve to open. In this regard, the moving body could itself be the valve member which obstructs a valve seat when in the outer position as well as a moving mass changing the inertia of the rotor.

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

Although, the biasing means may be a number of things, in some embodiments it comprises a spring.

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

Although the vacuum pump may be a number of things, in some embodiments the vacuum pump comprises a mechanical booster pump. Mechanical booster pumps are used in situations where the pressure at the inlet may vary as chambers are pumped down or as valves are opened and thus, such pumps are particularly prone to deceleration events and the associated requirement for reacceleration. Thus, it is advantageous in such pumps if the inertia of the rotor can change such that reacceleration can be done quickly and efficiently and deceleration can be resisted to some extent by a higher inertia rotor.

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

In some embodiments, said predetermined angular velocity comprises a velocity between 2 and 20% lower than a full speed of said vacuum pump.

The biasing means and mass of the body may be configured such that the body moves from the inner to the outer position at a predetermined angular velocity. It may be advantageous if that is close to the full speed of the pump such that re- acceleration is efficiently performed with a low inertia rotor and only when it is close to full speed is the inertia of the rotor suddenly increased. In some embodiments, said vacuum pump comprises a drive controller configured to control the driving of the rotor, said drive controller being configured with a range of skip frequencies around a frequency of rotation at which said body is configured to move positions.

Such an arrangement allows the vacuum pump to reduce the time that the pump runs when the balance may be upset by the body moving or being in an intermediate position.

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 with the body in the outer position according to one embodiment;

Figure 2 shows a rotor with the body in the inner position according to one embodiment;

Figure 3 shows a graph indicating how the radial position of the moveable body varies with the frequency of rotation of the rotor; and

Figure 4 shows an alternative embodiment of a rotor. DESCRIPTION OF THE EMBODIMENTS

Before discussing the embodiments in any more detail, first an overview will be provided. When performing a chamber pumpdown with a vacuum pump such as a booster combination, if there is any significant volume ratio between the Mechanical Booster & Dry Pump (always) and there is no by-pass valve capable of handling the full throughput from the Mechanical Booster (almost always), then the load on the MB will be initially tremendous and the rotor will slow down. In the later part of the pumpdown, the MB will reaccelerate to its full speed. If a booster rotor has a high inertia then it will accelerate only slowly, and the pumpdown is delayed. If the inertia is reduced to aid reacceleration, then the initial load on the rotor causes a much deeper deceleration than it otherwise would, and the pumpdown is delayed. For a rigid rotor there is an optimum inertia that produces the quickest pumpdown, but the optimum depends on chamber size, base-pressure required, DP size, drive torque, etc.

E.g. for an example pump operating on a 500 litre chamber the following characteristics are observed:

I.e. a high inertia rotor provides the fastest pumpdown (although the frequency doesn’t recover in the time of the pumpdown, which could restrict cycle times). However, for a 1000 litre chamber, the opposite is the case and a low inertia rotor gives the quickest pumpdown times. The application proposes a rotor with variable inertia, which“snaps” between a high value when the rotor is running at full speed, and a lower value when the rotor is already slowed down using a system of moving and in some cases sliding masses attached to the rotor profile. This inhibits the rotor from slowing down too fast or much when suddenly loaded, but enables it to be easily reaccelerated afterwards.

Embodiments provide a rotor that has the advantages of low inertia at lower speeds when the rotor may be accelerating and higher inertia at higher speeds, this facilitates the initial rapid pumpdown from a high inertia rotor running at or close to full speed and the rapid acceleration and later pumpdown performance from a lower inertia rotor

The body is retained in the inner position using a biasing means, such as a spring. A mass on a spring in a rotating frame sits at an equilibrium radius r, where k(r-ro )=mr ² . The equilibrium radius diverges r=(kroH(k-mcu ² )) and becomes infinitely large, as the frequency approaches the critical frequency (jUc=V(l*fm). By making the critical frequency just slightly lower than the normal running speed of the machine, and restraining the mass when it reaches the outside, it can be arranged for the mass to (effectively) rapidly switch from an equilibrium close to the axis to an equilibrium position at the periphery of the rotor as the rotor approaches maximum frequency. The natural length of the spring holding the mass in place will determine the range of frequency used in the transition.

Of course, as the mass slides outwards, it experiences a Coriolis force, which represents an additional torque demand on the motor, which will momentarily slow the acceleration. However, if it is configured to occur just as the rotor is reaching full speed, there should be only a light gas load at the time, and plenty of motor torque to spare. Balance is maintained by knowing that the moving masses will almost always be either completely pushed in, or completely flung out, and almost never anywhere inbetween. The drive can be programmed with a very narrow range of skip frequencies to avoid running for any length of time when the balance might be upset.

Embodiments, provide increases in inertia of around 45% in transitioning between the two configurations illustrated, and the theoretical range of frequency over which the transition occurs is from 94.3 to 95.5 Hz.

Figure 1 shows a rotor 5 according to an embodiment. Rotor 5 comprises an outer profile 10 extending from a central portion 15 which is mounted to a drive shaft. The outer profile 10 surrounds a at least partially hollow inner portion. Within the inner portion of each lobe is a body 20 which is mounted to slide on guide 12.

In this embodiment, the outer profile 10 is configured to have a relatively low mass while the inner body 20 has a relatively high mass. Thus, movement of the mass 20 between the inner and outer positions changes the moment of inertia of the rotor significantly.

In this embodiment the guide that controls the motion of the body 20 is in the form of two posts extending from the central portion 15. The body is mounted to slide along these posts in response to a predetermined rotational velocity. In this regard, a biasing means, not shown which may be in the form of a coiled spring for example mounted around the posts 12 biases the body 20 towards the inner position. The mass of the body 20 and the characteristics of the spring determine the rotational velocity at which the centrifugal force on the body overcomes the spring force of the spring and the body moves from the inner to outer position.

In embodiments, the body and spring force are configured such that this angular velocity is close to the full speed of the pump such that during normal operation the body 20 is in the position shown in Figure 1 and the moment of inertia of the rotor is high. This makes the pump resistant to deceleration due to events such as a rise in pressure within the chamber being pumped and improves the operation of the pump. However, when the speed falls below the critical velocity which may be between 2 and 20% below the full speed of the pump depending on the embodiment, then the body will move from the outer position as shown in Figure 1 to the inner position as shown in Figure 2. At this point the moment of inertia of the rotor is significantly reduced. This can be advantageous when the speed of rotation of the rotor has slowed as it allows the rotor to accelerate more quickly and easily in response to an accelerating force rotor. At start-up for example when the rotor needs to be accelerated or following an event where the pressure in the chamber has increased, accelerating the rotor back to full speed may be achieved more quickly and easily. It should be noted that the preferred properties for any rotor will depend on the capacity of the pump and the chamber and thus, the rotor can be configured appropriately.

In some embodiments, the posts on which the body 20 is mounted may act as a structurally strengthening means to reinforce the rotor and make it robust.

Alternatively, in some embodiments the one or more posts may be configured to distort in response to high forces perhaps caused by sudden deceleration of the rotor. This can be advantageous as it can provide protection of the rotor against sudden deceleration allowing the outer low inertia profile to decelerate quickly and the inner body to decelerate more slowly thereby absorbing some of the energy and providing at least some protection to the rotor and other components of the pump form the high forces triggered by such an event.

Figure 2 shows the rotor of Figure 1 with the body 20 in the inner position.

Figure 3 shows a graph indicating how the radial position of the moveable body varies with the frequency of rotation of the rotor. The varying line shows how, the body moves from the inner to the outer position within a small rotational frequency range, this design allows potential imbalance due to movement of the body to be reduced to a small time frame and improves the performance of the pump. The varying line showing the body’s movement, shows how the body would move were it not constrained within the rotor. The horizontal lines represent the restraints imposed on the motion by the inner part of the rotor (lower horizontal line) and by the outer profile of the rotor (upper line). In some embodiments, the pump drive may be configured such that it skips frequencies of rotation that correspond to the movement of the body between the two positions.

Figure 4 shows an alternative embodiment of a rotor where guide 12 in this embodiment is a hollow channel within an otherwise solid rotor. The body being spherical and rolling within the channel in response to changes in angular velocity.

In this embodiment there is a flow path between an inlet 1 1 a in the leading edge of the outer profile 10 of the rotor and an outlet 1 1 b in the trailing edge of the rotor that the body 20 obscures when in the outer position and which is open when the body moves from the outer position towards the inner position. In this way in addition to acting to changing the moment of inertia of the rotor the body 20 acts to open and close a flow path which acts as a pressure relief valve. In this way in addition to having the increased moment of inertia at full speed the rotor has a pressure relief valve that opens in response to increases in pressure at the pump inlet making the rotor more resistant to deceleration due to these increases in pressure.

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 11a inlet

11b outlet

12 guide

15 central portion 20 body