GB2617843A

GB2617843A - Vacuum pump damping arrangements

Info

Publication number: GB2617843A
Application number: GB2205736.8A
Authority: GB
Inventors: David Smith Paul
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2022-04-20
Filing date: 2022-04-20
Publication date: 2023-10-25
Anticipated expiration: 2042-04-20
Also published as: GB2617843B; GB202205736D0; WO2023203325A1

Abstract

A vacuum pump 10 comprising a pump housing 12, a rotor shaft 16 and a bearing 18; the vacuum pump further comprising a damping arrangement 20 including a resilient bearing support 22 which substantially surrounds the bearing 18, and first and second opposing resilient damping elements 24, 26. The first resilient damping element 24 extends beyond a first end 28 of the resilient bearing support 22 and the second resilient damping element 26 extends beyond a second end 30 of the resilient bearing support. The vacuum pump further comprising a biasing element 42, preferably a locking nut, arranged to compress the first and second resilient damping elements in an axial direction; each of the first and second resilient damping elements remaining under axial compression during use of the vacuum pump. The resilient bearing support having inner and outer portions 32, 34, and a flexible member 36 therebetween.

Description

VACUUM PUMP DAMPING ARRANGEMENTS

Field

[1] The present invention relates to vacuum pumps, for example turbomolecular vacuum pumps, and to damping arrangements for vacuum pump rotor shaft assemblies.

Background

[2] Turbomolecular vacuum pumps typically comprise a rotor shaft supported by bearings located towards each end thereof. Often, an upper bearing, i.e. that which is typically closer to a pump inlet, is in the form of a magnetic bearing and the lower bearing, i.e. that which is typically further from the pump inlet, is in the form of a rolling bearing.

[3] During use of a vacuum pump, it is important that vibrations generated through rotor shaft rotation are not transmitted to the pump housing, or are at least minimised via damping. This is because both the operation of a vacuum pump and of the systems and instrumentation connected to or in the vicinity of the vacuum pump may be compromised by vibrations which are not effectively isolated. It is well known that high frequency vibrations transmit well through various materials and can therefore interfere with nearby instrumentation.

[4] Another important consideration is that, for the sake of pump performance, the rotor shaft and surrounding stator should have minimal clearance therebetween. The mounting of a rotor shaft must therefore be sufficiently axially stiff so that a minimal clearance can be maintained.

[5] Damping arrangements are thus required around bearings, in particular rolling bearings, to maintain clearance and provide appropriate axial and radial flexibility in order to suppress vibrations otherwise transmitted from the rotating assembly into the static structure and beyond. This is difficult to realise.

[6] Existing apparatus for damping include the use of compact metal spring dampers (CMSD), or the like, which are employed to provide relative axial stiffness and relative radial flexibility. However, CMSDs or similar alone are not sufficient in many cases.

[7] Additional damping arrangements, typically including elastomeric damping members configured to flex in both a radial and axial direction, have been previously employed and sought to improve damping characteristics. For example, existing damping arrangements may provide an elastomeric damping adjacent a CMSD.

[8] However, while such arrangements initially provided encouraging reduction in the transmitted high frequency vibration signatures, investigation by the Applicant into higher frequency vibration has determined that improved isolation between the bearing and fixed structure, i.e. the pump housing, is required due to the increased sensitivity to mechanical vibrations for many vacuum pump uses. Certain types of instrument, mass spectrometer systems for example, are now sensitive to mechanical vibrations of around 80 kHz or higher.

[9] Moreover, further investigation by the Applicant has also found that the preload force in turbomolecular pumps is often great enough to hamper the function of existing damping arrangements such that vibrations are transmitted through the damping arrangements, from the rotor shaft assembly to the pump housing. A preload force is typically applied via the upper, magnetic bearing to bias the rotor shaft assembly axially in one direction.

[10] Finding an optimal balance of high static axial stiffness in order to maintain tight clearance tolerance and low effective dynamic axial stiffness in order to eliminate or dampen vibrations is an ongoing challenge, particularly in view of evermore increasing sensitivity to vibration.

[11] Accordingly, there is a need to address these existing drawbacks. The present invention aims to solve these and other problems with the prior art.

Summary

[12] Accordingly, in a first aspect, the present invention provides a pump housing and a rotor shaft assembly including a rotor shaft and a bearing configured to rotatably support the rotor shaft. The vacuum pump further comprises a damping arrangement including a resilient bearing support which is coupled to and substantially surrounds the bearing. The damping arrangement further includes first and second opposing resilient damping elements.

[13] The first resilient damping element extends beyond a first end of the resilient bearing support and the second resilient damping element extends beyond a second opposing end of the resilient bearing support.

[14] The vacuum pump further comprises a biasing element arranged such that the first and second resilient damping elements are compressed in an axial direction, and each of the first and second resilient damping elements remains under axial compression during use of the vacuum pump.

[15] As used herein, the term "axial direction" refers to the first and second resilient damping elements being compressed in a direction which is substantially parallel to a rotational axis of the rotor shaft.

[16] Because the first and second resilient damping elements oppose one another, i.e. they are arranged substantially in series with one another, they are arranged together to effectively bias the rotor shaft assembly towards an equilibrium position. The first and second resilient damping elements thus act as shock absorbers which absorb vibrations generated by the rotor shaft assembly.

[17] The rotor shaft assembly may have a maximum positive axial displacement limit and a maximum negative axial displacement limit, i.e. substantially opposite the positive axial displacement. The first and second resilient damping elements are configured such that they both remain under axial compression between the maximum positive and maximum negative axial displacement limits.

[18] Typically, as used herein, the term "axial compression" may refer to the axial squeeze of the first and second resilient damping elements. For example, axial compression, or squeeze, of the first and second resilient damping elements may be expressed as a percentage determined using the initial axial thickness of the resilient damping element and the change in axial thickness during compression.

[19] There is substantially no (or negligible) net change in the total axial compression force applied to the first and second resilient damping elements.

[20] Typically, in use, as the rotor shaft assembly is displaced axially, the compression of the first and second resilient damping elements may correspondingly increase or decrease in magnitude, but both remain under axial compression. In other words, there may be substantially no (or negligible) net change in the total axial compression of the first and second resilient damping elements. The first and second resilient damping elements are arranged such that a positive axial compression on one of the first and second resilient damping elements is simultaneously countered by a negative axial compression on the other of the first and second resilient damping elements. In other words, the sum of the axial compression force acting on the first and second resilient damping elements together remains substantially constant during use of the vacuum pump.

[21] Correspondingly, the combined axial force exerted on the pump housing by the first and second resilient damping elements may remain substantially constant during use of the vacuum pump. In other words, an axial displacement of the rotor shaft assembly causes an increase in the axial force exerted on the pump housing by one of the first and second resilient damping elements, and a simultaneous decrease in axial force exerted on the pump housing by the other of the first and second resilient damping elements.

[22] The damping arrangement of the present invention is configured such that, during use, an axial movement of the rotor shaft assembly simultaneously causes an increase in compression of one of the first and second resilient damping elements (and a corresponding increase in the axial force exerted on the pump housing by that same resilient damping element) and a substantially equivalent decrease in compression of the other of the first and second resilient damping elements (and a corresponding decrease in the axial force exerted on the pump housing by that same resilient damping element). Importantly, both resilient damping elements remain axially squeezed during use of the vacuum pump and act, substantially in series with one another, to absorb or dissipate vibrations transmitted through axial displacement of the rotor shaft assembly.

[23] The configuration of the present invention is particularly advantageous because the damping arrangement provides and maintains a relatively high static axial stiffness in order that a tight clearance tolerance can be maintained between the rotor shaft and the surrounding vacuum pump stator, while it also provides a relatively low dynamic axial stiffness in order to dampen vibrations generated by the rotor shaft assembly in use.

[24] Thus, the present configuration minimises vibration transmission from the rotor shaft assembly to the pump housing, without compromising on performance of the vacuum pump. Advantageously, this configuration allows the described technical effect to be achieved using readily available materials, such as elastomers (Nitrile, for example).

[25] The combination of the resilient bearing support and the first and second resilient damping elements provides improved radial and axial flexibility characteristics. The increased sensitivity of connected or neighbouring systems is therefore addressed. In particular, the Applicant has found that this configuration provides improved reduction in the transmitted high frequency vibration signature typical of standard turbomolecular pumps (TMPs).

[26] In embodiments, the biasing element comprises a displaceable member which is configured to abut the damping arrangement and axially compress the damping arrangement and in particular the first and second resilient damping elements. In embodiments, the biasing element comprises a locking nut which is arranged below the damping element and may be loosened to release and tightened to axially compress the first and second resilient damping elements. The biasing element may comprise one or more compression positions, the compression positions being such that the magnitude of axial compression of the first and second resilient damping elements may be modified.

[27] In embodiments, at least part of the first resilient damping element is located between the first end of the resilient bearing support and a first radially extending surface of the pump housing, and at least part of the second resilient damping element is located between the second end of the resilient bearing support and a second radially extending surface of the pump housing, the second radially extending surface being substantially opposite the first radially extending surface.

[28] The simultaneous axial compression of the first resilient damping element and second resilient damping element between the resilient bearing support and pump housing, and the resilient flexibility of the resilient damping elements, thus creates a forced balance condition therebetween. This may also be referred to as the first and second resilient damping elements providing quasi-zero stiffness in order to isolate or dampen undesirable vibrations which otherwise might be inadvertently measured by or might disturb connected or nearby instrumentation.

[29] Because the resilient damping elements are always under axial compression, even in the absence of axial displacement of the rotor shaft assembly, the vacuum pump has improved static stiffness. However, the resilient damping elements are configured to absorb or dissipate further axial compression upon axial displacement of the rotor shaft assembly. Therefore, the vacuum pump also has improved dynamic axial flexibility.

[30] In embodiments, the first and second resilient damping elements may be formed as separate units. In alternative embodiments, the first and second resilient damping elements may be integrally formed.

[31] In embodiments, the first and second resilient damping elements may be substantially planar and extend longitudinally, i.e. substantially in parallel with the rotational axis of the rotor shaft, beyond the said ends of the resilient bearing support.

[32] In embodiments, the rotor shaft assembly may have an equilibrium position in the absence of axial displacement thereof. In other words, the rotor shaft assembly may have an optimum position during use, wherein the rotor shaft assembly and surrounding stator of the vacuum pump are optimally arranged for minimum clearance therebetween. In embodiments, the damping arrangement, and more specifically the first and second resilient damping elements, may be together configured to bias the rotor shaft assembly towards said equilibrium position during use of the vacuum pump, for example following axial displacement of the rotor shaft assembly.

[33] Thus, each of the first and second resilient damping elements may be subjected to an increase or decrease in axial compression, during use, but they are together configured to bias, or return, the rotor shaft assembly to an equilibrium position. Vibrations generated by the rotor shaft assembly are thereby more effectively dampened. In the equilibrium position, the axial compression force, i.e. squeeze, exerted on each of the resilient damping elements may be substantially equal.

[34] This balance of forces may also be reflected in the axial length of the first and second resilient damping elements. For example, as the first resilient damping element is subjected to a greater compression force, its axial length may correspondingly decrease and, simultaneously, as the second resilient damping element is subjected to a lesser compression force and its axial length may correspondingly increase, and vice versa, depending on the axial displacement of the rotor shaft assembly.

[35] In embodiments, the first and second resilient damping elements may be configured to absorb both axial and radial vibrations transmitted by the rotor shaft assembly in use of the vacuum pump. For example, each resilient damping element may bound an end and an outer circumferential surface of the resilient bearing support and may be arranged between the resilient bearing support and axially extending and radially extending surfaces of the pump housing such that radial and axial vibrations may be substantially prevented from being transmitted to the pump housing from the rotor shaft assembly.

[36] In embodiments, the first and second resilient damping elements may be configured to be axially compressed to a substantially equal degree. In other embodiments, the axial compression of one of the first and second resilient damping elements may be greater than the axial compression of the other resilient damping element. For example, in the absence of an axial displacement of the rotor shaft assembly during use of the vacuum pump, the first resilient damping element may be compressed axially to a greater extent than the second resilient damping element.

[37] In embodiments, the first and second resilient damping elements may have a hardness of between around 50 Shore A hardness to around 90 Shore A hardness, optionally between around 70 Shore A hardness to around 80 Shore A hardness. Typically, the hardness of the first and second resilient damping elements may be determined using ASTM standard D2240.

[38] In embodiments, the resilient damping elements may have a static stiffness of around 700 N/mm to around 1500 N/mm, optionally around 900 N/mm to around 1400 N/mm.

[39] In embodiments the first and second resilient damping elements may have a damping constant of between around 10 Ns/m to around 50 Ns/m. Preferably the first and second resilient damping elements may have a damping constant of around 20 Ns/m. Typically, the damping constant of the first and second resilient damping elements may be determined using ASTM-E756-05. For example, an impact hammer modal testing method may be used, wherein a measured response peak (e.g. a 3dB width of a main response peak) provides a measure of the damping ratio of the first and/or second resilient damping element, from which the damping constant may be derived. Other properties of the first and second resilient damping elements, for example their Young's modulus, may also be determined.

[40] For the avoidance of doubt, all measurements, including hardness, provided herein are measured at 20°C and 1 atmosphere unless stated otherwise. Typically, the vacuum pump may operate at a temperature of between around 20°C and around 70°C.

[41] In embodiments, the vacuum pump may further comprise a preload damping insert arranged between the resilient bearing support and a radially extending surface of the pump housing. In embodiments, the preload damping insert may be configured such that, when the rotor shaft assembly undergoes a preload axial displacement, the preload damping insert is axially compressed.

[42] As used herein, the term "preload axial displacement" refers to displacement of the rotor shaft assembly in an axial direction, typically prior to rotational actuation of the rotor shaft assembly. In other words, preload axial displacement typically takes place prior to the vacuum pump producing a vacuum. In embodiments, preload axial displacement occurs during assembly of the vacuum pump. For example, an upper magnetic bearing of the vacuum pump may be activated, which axially displaces the rotor shaft assembly in a substantially upward direction. Prior to this, the first and second resilient damping elements may be compressed in an axial direction by the biasing element. Biasing the rotor shaft assembly axially in one direction is typically desirable to improve bearing performance. In embodiments, the vacuum pump may include a magnetic bearing separate to the rotor shaft assembly, and the preload axial displacement may be adjusted by adjusting the offset of the magnetic bearing of the vacuum pump. The magnetic bearing may be active or passive.

[43] As stated above, further investigation by the Applicant has found that in some vacuum pumps, for example in split flow turbomolecular vacuum pumps, the preload force of the pump is great enough that the function of the first and second resilient damping elements described above may be jeopardised. In a split flow turbomolecular vacuum pump, for example, a larger, heavier rotor requiring a stronger magnetic bearing (e.g. a 6-magnet upper magnetic bearing) increases the preload axial force applied to the rotor shaft assembly. One of the first and second resilient damping elements would thus be compressed to an extent that it cannot provide dynamic axial flexibility with the other resilient damping element in use. For example, at least one of the first and second resilient damping elements may be subject to compression set, i.e. permanent or semi-permanent deformation, where the preload axial force applied to the rotor shaft assembly (and thereby the displacement of the rotor shaft assembly upon application of the preload axial force) is too great.

[44] In simple terms, if either one of the first and second resilient damping elements is compressed beyond a given threshold, for example more than 15% squeeze (i.e. greater than a 15% change in axial thickness of the resilient damping element), that resilient damping element can no longer provide sufficient damping, i.e. the damping arrangement can no longer work in a quasi-zero stiffness condition, and vibrations may be transmitted from the rotor shaft assembly, through the damping arrangement, and into the pump housing.

[45] Moreover, at the observed axial load levels seen in some vacuum pumps, the required stiffness of the first and second resilient damping elements cannot be straightforwardly obtained with commonly available materials, e.g. elastomers, particularly within the size constraints imposed by typical vacuum pump designs.

[46] The Applicant has therefore determined that it is important to compensate for this increased preload axial displacement in such a vacuum pump, in order to enable the first and second resilient damping elements of the damping arrangement to function properly so that, in particular, an acceptable level of dynamic flexibility is maintained [47] Thus, the preload damping insert is configured such that, when the rotor shaft assembly undergoes a preload axial displacement as described, the preload damping insert is axially compressed. The preload damping insert therefore substantially absorbs an axial preload force of the vacuum pump in order that the first and second resilient damping elements are not axially compressed by the preload force to an extent that the dynamic stiffness of the damping arrangement is too high. By providing the preload damping insert, the preload force is typically at least partially absorbed prior to use of the vacuum pump, for example in a preloaded state of the vacuum pump before the vacuum pump is producing a vacuum.

[48] The first and second resilient damping elements are thereby prevented, or substantially saved, from having to absorb the axial preload force. The preload damping insert thereby counteracts the preload force to maintain the balance conditions required to achieve a high static axial stiffness whilst also providing low dynamic stiffness to provide effective damping.

[49] In embodiments, the rotor shaft assembly may undergo preload axial displacement prior to the biasing element placing the first and second resilient damping elements under axial compression. Thus, in a method of operation of the vacuum pump, the rotor shaft assembly may subjected to a preload axial displacement, followed by displacement of the biasing element to axially compress the first and second resilient damping elements. In other embodiments, the biasing element may axially compress the first and second damping elements prior to subjecting the rotor shaft assembly to a preload axial displacement.

[50] In embodiments, the preload damping insert may be arranged substantially in parallel with one of the first and second resilient damping elements. In other words, the preload damping insert and the first resilient damping insert may be arranged in parallel axial planes. For example, the resilient bearing support may include a radially extending protrusion or flange from which the preload damping insert extends towards a radially extending surface of the pump housing.

[51] The preload damping insert may be configured so that it contacts and is compressed between the resilient bearing support and the pump housing before the first and second resilient damping elements are compressed in order that an axial preload force of the vacuum pump is absorbed by the preload damping insert in parallel with one of the first and second resilient damping elements. For example, the preload damping insert may have a greater axial length than the first and second resilient damping elements, and/or the preload damping insert may have a lower stiffness than the resilient damping elements.

[52] In embodiments, the preload damping insert may be arranged substantially in series with one of the first and second resilient damping elements. For example, the preload damping insert may be located substantially above the first and second resilient damping elements in the vacuum pump, in order that an axial preload force of the vacuum pump is substantially absorbed by the preload damping insert.

[53] In embodiments, the vacuum pump may comprise more than one preload damping insert.

[54] In embodiments, the pump housing may include a divider arranged between the preload damping insert and at least one of the first and second resilient damping elements. Thus the preload damping insert is substantially prevented from contacting the first resilient damping element or the second resilient damping element.

[55] For example, the preload damping insert may be arranged in parallel with one of the first and second resilient damping elements and the divider may be in the form of an axially extending projection which extends between the first damping element and the preload damping insert. In embodiments, the divider may have a substantially L-shaped axial cross-section.

[56] In embodiments, the preload damping insert may have a stiffness which is substantially equal to or less than the stiffness of the first and second resilient damping elements. Preferably, the preload damping insert may have a stiffness which is less than the stiffness of the first and second resilient damping elements. In embodiments, the preload damping insert may have a stiffness which is around a fifth, i.e. 20%, of the stiffness of the first and/or second resilient damping elements.

[57] In embodiments, the first and second resilient damping elements may be of substantially equal stiffness. In embodiments, the first and second resilient damping elements may be of substantially equal size and shape.

[58] In embodiments, the preload damping insert may have a substantially greater axial thickness than the first and second resilient damping elements.

[59] In embodiments, the vacuum pump may have an unloaded state in which no or little axial force is being applied to the bearing (e.g. by an upper magnetic bearing) and the rotor shaft assembly is thus subjected to little or no axial displacement. In embodiments, the vacuum pump may have a preload state in which a preload axial force is applied to the bearing (e.g. by an upper magnetic bearing) in order that the rotor shaft assembly is subject to preload axial displacement, but the vacuum pump is not producing a vacuum. In embodiments, the vacuum pump may have a use state (i.e. a state in which the rotor shaft assembly rotates about a rotational axis thereof to produce a vacuum). Typically, the rotor shaft assembly may be biased axially in one direction via the application of an axial force in the preload and use states. Typically, in the preload state at least one of the first and second resilient damping elements is axially compressed. In the use state, both the first and second resilient damping elements are axially compressed.

[60] In embodiments, in the preload state of the vacuum pump, the preload damping insert may be substantially axially compressed and the first and second resilient damping elements may be substantially axially uncompressed.

[61] In embodiments, in the use state of the vacuum pump, both the preload damping insert and each of the resilient damping elements may be substantially axially compressed.

[62] In embodiments, the resilient bearing support may include an inner portion coupled to the bearing, an outer portion, and a resiliently flexible member arranged therebetween. In embodiments, the resilient bearing support may be a compact metal spring damper (CMSD) or similar. The first and second resilient damping elements may be arranged substantially in series with the resiliently flexible member of the resilient bearing support.

[63] In embodiments, the first and second resilient damping elements may together substantially surround the outer portion of the resilient bearing support. In other words, the first and second resilient damping elements may wrap around the resilient bearing support.

[64] In embodiments, the outer portion of the resilient bearing support may include a radially extending protrusion which is configured to support the preload damping insert.

[65] In embodiments, the first resilient damping element and/or the second resilient damping element may substantially bound an outer circumferential surface of the resilient bearing support.

[66] In embodiments, the first resilient damping element and/or the second resilient damping element may be arranged such that the outer portion of the resilient bearing support is not in contact with the pump housing.

[67] In embodiments, the first resilient damping element and/or the second resilient damping element may include a radial lip arranged to overlay a said end of the resilient bearing support.

[68] In embodiments, the first resilient damping element and/or the second resilient damping element may have a substantially L-shaped axial cross section.

[69] As stated, the first and second resilient damping elements may be integrally formed, or the first and second resilient damping elements may be formed of separate units which are contiguously arranged in the damping arrangement.

[70] In embodiments, the first and second resilient damping elements may be substantially formed of an elastomer. For example, the first and second resilient damping elements may be substantially formed of Nitrile, a fluoroelastomer such as VitonTM, silicone, or ethylene-propylene-diene-monomer (EPDM). In embodiments, the first and second resilient damping elements may be substantially formed of a woven wire mesh.

[71] In embodiments, the first and second resilient damping elements may be substantially formed of different materials from one another.

[72] In embodiments, the preload damping insert may be substantially formed of an elastomer. For example, the preload damping insert may be substantially formed of Nitrile, a fluoroelastomer such as VitonTM, silicone, or ethylene-propylene-dienemonomer (EPDM). In embodiments, the preload damping insert may be substantially formed of a woven wire mesh.

[73] In embodiments, the preloading damping insert may be substantially formed of a different material to the first and second resilient damping elements.

[74] In embodiments, the first resilient damping element and/or the second resilient damping element and/or the preload damping insert may be substantially annular.

[75] In embodiments, the first resilient damping element and/or the second resilient damping element and/or the preload damping insert may comprise a gasket, or 0-ring.

[76] In a further aspect, the present invention provides a method of assembling a vacuum pump in accordance with any preceding aspect, the method comprising the steps of: adjusting the biasing element of the vacuum pump to compress the first and second resilient damping elements of the vacuum pump in an axial direction such that the first and second resilient damping elements remain under axial compression during use of the vacuum pump; and applying a preload force to the rotor shaft assembly of the vacuum pump.

[77] In embodiments, the step of adjusting the biasing element is carried out prior to the step of applying the preload force to the rotor shaft assembly.

[78] In embodiments, the method may comprise a calibration step wherein the axial compression of the first and second resilient damping elements may be calibrated. Preferably the calibration step is carried out via displacement of the biasing element.

[79] In embodiments, the vacuum pump may include a preload damping insert, and the step of applying the preload force to the rotor shaft assembly may be such that the preload damping insert is axially compressed.

[80] In a further aspect, the present invention provides a method of operating a vacuum pump in accordance with any preceding aspect, the method comprising the steps of compressing the first and second resilient damping elements in an axial direction; and actuating the vacuum pump to produce a vacuum; wherein each of the first and second resilient damping elements remain under axial compression during use of the vacuum pump.

[81] In embodiments, the method may comprise the step of adjusting the biasing element to axially compress the first and second resilient damping elements.

[82] In embodiments, the method may comprise a calibration step wherein the axial compression of the first and second resilient damping elements may be calibrated.

[83] In embodiments, the vacuum pump may include a preload damping insert, and the method may comprise the step of subjecting the rotor shaft assembly to a preload axial displacement prior to actuation of the vacuum pump to produce a vacuum, such that the preload damping insert is axially compressed.

[84] In embodiments, the step of subjecting the rotor shaft assembly to a preload axial displacement may occur prior to the step of adjusting the biasing element to axially compress the first and second resilient damping elements.

[85] In embodiments, the method may comprise the steps of ceasing production of a vacuum; ceasing the preload axial displacement of the rotor shaft assembly and adjusting the biasing element such that the first and second resilient damping elements are no longer axially compressed by the biasing element.

[86] In embodiments, the step of adjusting the biasing element such that the first and second resilient elements are no longer axially compressed by the biasing element occurs prior to ceasing the preload axial displacement of the rotor shaft assembly.

[87] In a further aspect, the present invention provides a damping arrangement for a rotor shaft assembly of a vacuum pump. the rotor shaft assembly including a rotor shaft and a bearing which rotatably supports the rotor shaft.

[88] The damping arrangement comprises a resilient bearing support configured to couple to and surround the bearing of the rotor shaft assembly; a first resilient damping element configured to extend beyond a first end of the resilient bearing support, and a second resilient damping element configured to extend beyond a second end of the resilient bearing support [89] The first and second resilient damping elements are configured to be axially compressed by a biasing element of the vacuum pump, and each is configured to remain under axial compression during use of the vacuum pump.

[90] In embodiments, the resilient bearing support may include an inner portion, and outer portion, and a resiliently flexible member arranged therebetween; and a bearing may be coupled to the inner portion and may be configured to rotatably support the rotor shaft of the vacuum pump.

[91] In embodiments, the damping arrangement may further comprise a preload damping insert configured to locate between the resilient bearing support and a radially extending surface of the pump housing of the vacuum pump. In embodiments, the preload damping insert may be configured such that, when the rotor shaft assembly of the vacuum pump undergoes a preload axial displacement, the preload damping insert is axially compressed.

[92] In embodiments, at least part of the first resilient damping element may be configured to locate between the resilient bearing support and a first radially extending surface of the pump housing of the vacuum pump and at least part of the second damping insert may be configured to locate between the resilient bearing support and a second radially extending surface of the pump housing of the vacuum pump.

[93] In embodiments, the preload damping insert may have a stiffness which is less than the stiffness of the first and second resilient damping elements. In embodiments, said first and second resilient damping elements may be of substantially equal stiffness.

[94] In a further aspect, the present invention provides a resilient bearing support for a rotor shaft assembly of a vacuum pump; the resilient bearing support comprising an inner portion configured to couple to a bearing of the rotor shaft assembly; an outer portion, and a resiliently flexible member arranged therebetween; wherein the outer portion of the resilient bearing support includes a radially extending protrusion.

[95] In embodiments, the radially extending protrusion of the outer portion of the resilient bearing support may be configured to support a preload damping insert.

[96] For the avoidance of doubt, features of aspects and embodiments described herein may be combined, and still fall within the scope of the present invention.

Brief Description of Figures

[97] Preferred features of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: [98] Figure 1 illustrates an axial cross-section of part of a vacuum pump according to the present invention, with part of the cross-section enlarged.

[99] Figures 2A to 2C illustrate a series of schematic illustrations of a vacuum pump according to the present invention in an unloaded state, a preloaded state, and a use state.

Detailed Description

[100] Figure 1 shows part of a vacuum pump of the present invention, referenced generally as 10. The vacuum pump 10 comprises a pump housing 12 and a rotor shaft assembly 14. The rotor shaft assembly 14 includes a rotor shaft 16 and a bearing 18 configured to rotatably support the rotor shaft 16.

[101] The bearing 18 is a rolling bearing, and is one of two bearings of the vacuum pump 10. The other bearing (not shown) is a passive magnetic bearing configured to apply an axial preload force to the rotor shaft assembly 14 prior to use and during use in order that the rotor shaft assembly 14 is subjected to preload axial displacement. Typically, the passive magnetic bearing is preloaded during assembly of the vacuum pump 10.

[102] The vacuum pump 10 further comprises a damping arrangement 20. The damping arrangement 20 includes a resilient bearing support 22 in the form of a compact metal spring damper (CMSD). The resilient bearing support includes an inner portion 32 coupled to the bearing 18, an outer portion 34 and a flexible member 36 arranged therebetween. The resilient bearing support 22 is coupled to the bearing 18 and substantially surrounds the bearing 18 about the outer circumference thereof.

[103] The damping arrangement 20 further comprises first and second resilient damping elements 24, 26 in the form of substantially annular gaskets. The first and second resilient damping elements 24, 26 are formed as separate units but are contiguous with one another.

[104] The first resilient damping element 24 extends beyond a first end 28 of the resilient bearing support 22 and the second resilient damping element 26 extends beyond a second end 30 of the resilient bearing support 22. More specifically, each of the first and second resilient damping elements 24, 26 substantially surround the outer portion 34 of the resilient bearing support 22 such that the outer circumferential surface of the resilient bearing support and each end of the outer portion thereof are bounded by the first and second resilient damping elements 24, 26. Part of each of the first and second resilient damping elements 24, 26 is arranged between respective ends of the resilient bearing support 22 and substantially radially extending surfaces of the pump housing 12.

[105] The vacuum pump further comprises a biasing element in the form of a locking nut 42. The biasing element is arranged such that the first and second resilient damping elements 24, 26 are compressed in an axial direction substantially parallel to the rotational axis of the rotor shaft 16. The first and second resilient damping elements 24, 26 are thus under axial compression and remain under axial compression during use of the vacuum pump. Typically, the locking nut 42 is displaced to axially compress the first and second resilient damping elements 24, 26 prior to use, and preferably prior to application of a preload force on the rotor shaft assembly 14 to displace the rotor shaft assembly. In this way, correct axial compression of the first and second resilient damping elements 24, 26 can be ensured prior to use. This also allows a specific axial compression level of the first and second resilient damping elements 24, 26 to be met prior to use. It may be that the rotor shaft assembly 14 is axially displaced in a direction substantially opposite to the preload axial displacement thereof, in order to test that the magnetic bearing is correctly displacing the rotor shaft assembly 14.

[106] Referring briefly to Figures 2A to 2C, the vacuum pump 10 has an unloaded, or non-use state (Fig. 2A) in which the biasing element 42 is may or may not be axially compressing the first and second resilient damping elements 24, 26 (the biasing element is not providing any axial compression in Figure 2A) and little or no axial force is being applied to the rotor shaft assembly 14 and the rotor shaft assembly 14 is thus not axially displaced. The vacuum pump 10 also has a preload state (Fig. 2B) in which the biasing element 42 has been tightened in order to axially compress the first and second resilient damping elements 24, 26 and an axial preload force is being applied to the rotor shaft assembly 14 to displace it in an axial direction, but wherein the vacuum pump is not producing a vacuum. The vacuum pump 10 also has a use state (Fig. 2C) in which the rotor shaft assembly 14 rotates about a rotational axis thereof to produce a vacuum, and wherein the first and second resilient damping elements 24, 26 remain axially compressed.

[107] In the unloaded state, when the biasing element 42 is adjusted to axially compress the first and second resilient damping elements 24, 26, the first and second resilient damping elements 24, 26 are both subjected to around 10% squeeze.

[108] In the preload state, i.e. once the rotor shaft assembly 14 has been axially displaced towards the passive magnetic bearing, the first (upper) resilient damping element 24 is typically subjected to around a 15% squeeze while the second (lower) resilient damping element 26 is typically subjected to around a 10% squeeze. Beyond a squeeze of around 20%, the resilient damping elements 24, 26 are at greater risk of compression set, i.e. being permanently or semi-permanently deformed.

[109] In the use state, any change in axial position due to displacement resulting from vibrations will cause an increase in axial force exerted on the pump housing 12 by one of the elements 24, 26 and a decrease in axial force of a similar magnitude exerted on the pump housing 12 from the opposing resilient damping element 24, 26.

[110] Referring back to Figure 1, the first and second resilient damping elements 24, 26 are under axial compression, at least in the use state of the vacuum pump 10, but may also be under axial compression in the preload state.

[111] During use of the vacuum pump 10, the first and second resilient damping elements 24, 26 are under sufficient axial compression to provide acceptable static axial stiffness, but allow acceptable dynamic axial flexibility. In other words, the damping arrangement allows the static axial stiffness to be such that minimal clearance between the rotor shaft assembly 14 and surround stator (not shown) of the vacuum pump can be maintained, but that vibrations generated by the rotor shaft assembly 14 can be effectively dampened during use.

[112] During use, when the vacuum pump is producing a vacuum, the rotor shaft assembly 14 may be displaced axially in a first, positive direction and/or a second, negative direction as the rotor shaft 16 rotates. The rotor shaft assembly 14 has a maximum positive displacement limit and a maximum negative displacement limit. At both the maximum positive and maximum negative displacement limits both the first and the second resilient damping elements 24, 26 are under axial compression.

[113] Thus, during use, an axial movement of the rotor shaft assembly 14 simultaneously causes an increase in compression of one of the first and second resilient damping elements 24, 26 and a substantially equivalent decrease in compression of the other of the first and second resilient damping elements 24, 26. Corresponding changes in the axial force exerted on the pump housing 12 by the resilient damping elements 24, 26 also occur.

[114] The first and second resilient damping elements 24, 26 are in series with one another such that their compression is linked. An upward axial movement of the rotor shaft assembly 14 typically causes an increase in axial compression of the first resilient damping element 24 and an equivalent decrease in axial compression of the second resilient damping element 26.

[115] Because the resilient damping elements 24, 26 are located and 'sandwiched' between the resilient bearing support 22 and the pump housing 12 in this way, and are continuously axially compressed in the use state, the rotor shaft assembly 14 is urged towards an equilibrium position following axial movement thereof. Typically, in the use state and in the absence of axial movement of the rotor shaft assembly 14, i.e. when the rotor shaft assembly 14 is in its equilibrium position, the axial compression force exerted on the resilient damping elements is substantially equal.

[116] In the embodiment of Figure 1, the first and second resilient damping elements 24, 26 are of a substantially equal size and shape, and are both formed of an elastomer having substantially similar hardness.

[117] As can be seen most clearly in the enlarged section of Figure 1, the first and second resilient damping elements 24, 26 are arranged such that the resilient bearing support 22 does not contact the pump housing 12 in use.

[118] Referring back to the schematic diagrams of Figures 2A, 2B and 2C, the vacuum pump 10 further comprises a preload damping insert 38. The preload damping insert 38 is in the form of an annular gasket. The preload damping insert 38 is arranged between the resilient bearing support 22 and a substantially radially extending surface of the pump housing 12. The outer portion of the resilient bearing support 22 includes a radially extending protrusion 44 on which the preload damping insert 38 is supported. The preload damping insert 38 is configured such that, when the rotor shaft assembly undergoes a preload axial displacement, e.g. when displaced by the upper passive magnetic bearing of the vacuum pump 10, the preload damping insert 38 is axially compressed.

[119] The preload damping insert 38 thus substantially absorbs the axial preload force produced when the vacuum pump is in the preload state or the use state. The preload damping insert 38 is configured to substantially absorb the axial preload force such that the first and second resilient damping elements 24, 26 do not become over-compressed. Were the first and second resilient damping elements 24, 26 to become over-compressed, they would not provide sufficient axial or radial flexibility for vibrations to be dampened, and may be subject to compression set.

[120] In Figures 2A to 2C, the preload damping insert 38 is arranged in parallel with the first resilient damping element 24.

[121] In Figures 2A to 2B, the preload damping insert 38 is configured such that it contacts and is compressed between the resilient bearing support 22 and the pump housing 12 before the resilient damping elements 24, 26 are compressed therebetween (see Figures 2A and 2B). In this way, the preload force of the vacuum pump 10 is absorbed by the preload damping insert 38 before the resilient damping elements 24, 26 are engaged between the pump housing 12 and the resilient bearing support 22 during a transition from the unloaded state to the use state.

[122] The pump housing 12 includes a divider 40 in the form of an axial projection which separates the preload damping insert 38 from the first resilient damping element 24. A displaceable locking nut 42 is arranged to lock the rotor shaft assembly 14 in place during use. As stated, the locking nut 42 also acts as a biasing element to axially compress the first and second resilient damping elements 24, 26 such that the first and second resilient damping elements 24, 26 remain axially compressed during use of the vacuum pump 10.

[123] The preload damping insert 38 has a stiffness which is less than the stiffness of the first and second resilient damping elements 24, 26. In the embodiment of Figures 2A to 2C, the stiffness of the first and second resilient damping elements 24, 26 is around at least 5 times greater than the stiffness of the preload damping insert 38. Thus, following application of the preload force on the rotor shaft assembly 14, the preload damping insert 38 is substantially axially compressed. It may be that further axial compression of the preload damping insert 38 is possible during use of the vacuum pump 10, or the preload damping insert may be completely axially compressed and act to transmit force between the resilient bearing support 22 and the pump housing 12. The first and second resilient damping elements 24, 26 subsequently provide sufficient axial and radial flexibility (together with the resilient bearing support 22), to dampen vibrations generated via the rotor shaft assembly 14.

[124] While, in the embodiments of Figures 1 and 2A to 2C, the vacuum pump 10 comprises two resilient damping elements 24, 26 and the preload damping insert 38, it is envisaged that the vacuum pump 10 may comprise only a single resilient damping element 24 and the preload damping insert 38, or that the vacuum pump 10 may comprise the first and second resilient damping elements 24, 26 and no preload damping insert 38 (as is the case in Figure 1).

[125] It will be appreciated that various modifications may be made to the embodiments shown without departing from the spirit and scope of the invention as defined by the accompanying claims as interpreted under patent law.

Reference Key Vacuum Pump 12 Pump Housing 14 Rotor Shaft Assembly 16 Rotor Shaft 18 Bearing Damping Arrangement 22 Resilient Bearing Support 24 First Resilient Damping Element 26 Second Resilient Damping Element 28 First End of Resilient Bearing Support Second End of Resilient Bearing Support 32 Inner Portion of Resilient Bearing Support 34 Outer Portion of Resilient Bearing Support 36 Flexible Member of Resilient Bearing Support 38 Preload Damping Insert Divider 42 Biasing Element 44 Radially Extending Protrusion

Claims

Claims 1 A vacuum pump comprising a pump housing and a rotor shaft assembly including a rotor shaft and a bearing configured to rotatably support the rotor shaft; the vacuum pump further comprising a damping arrangement including a resilient bearing support which is coupled to and substantially surrounds the bearing, and first and second opposing resilient damping elements; wherein the first resilient damping element extends beyond a first end of the resilient bearing support and the second resilient damping element extends beyond a second opposing end of the resilient bearing support; the vacuum pump further comprising a biasing element arranged such that the first and second resilient damping elements are compressed in an axial direction; each of the first and second resilient damping elements remaining under axial compression during use of the vacuum pump.
2 The vacuum pump according to claim 1, wherein at least part of the first and second resilient damping elements are located between said ends of the resilient bearing support and opposing radially extending surfaces of the pump housing.
3 The vacuum pump according to claim 1 or claim 2, wherein the rotor shaft assembly has an equilibrium position in the absence of axial displacement thereof; and wherein the first and second resilient damping elements are together configured to bias the rotor shaft assembly towards said equilibrium position during use of the vacuum pump.
4 The vacuum pump according to any preceding claim, further comprising a preload damping insert arranged between the resilient bearing support and a radially extending surface of the pump housing; and the preload damping insert is configured such that, when the rotor shaft assembly undergoes a preload axial displacement, the preload damping insert is axially compressed.
The vacuum pump according to claim 4, wherein the preload damping insert is arranged substantially in parallel with one of the first and second resilient damping elements.
6 The vacuum pump according to claim 4 or claim 5, wherein the pump housing includes a divider arranged between the preload damping insert and at least one of the first resilient damping element and the second resilient damping element.
7 The vacuum pump according to any of claims 4 to 6, wherein the preload damping insert has a stiffness which is less than the stiffness of at least one of the first and second resilient damping elements.
8 The vacuum pump according to any of claims 4 to 7, wherein the preload damping insert has a substantially greater axial thickness than the first and second resilient damping elements.
9. The vacuum pump according to any preceding claim, wherein the first and second resilient damping elements are of substantially equal stiffness.
10. The vacuum pump according to any preceding claim, wherein the resilient bearing support includes an inner portion coupled to the bearing, an outer portion, and a resiliently flexible member arranged therebetween; preferably wherein the first and second resilient damping elements together substantially surround the outer portion of the resilient bearing support.
11. The vacuum pump according to any preceding claim, wherein the first and second resilient damping elements are substantially formed of an elastomer; preferably wherein the preload damping insert is also substantially formed of an elastomer.
12. A damping arrangement for a rotor shaft assembly of a vacuum pump, the rotor shaft assembly including a rotor shaft; the damping arrangement comprising a resilient bearing support configured to support the rotor shaft assembly; a first resilient damping element configured to extend beyond a first end of the resilient bearing support; and a second resilient damping element configured to extend beyond second end of the resilient bearing support; wherein the first and second resilient damping elements are configured to be axially compressed by a biasing element of the vacuum pump; and wherein each of the first and second resilient damping elements is configured to remain under axial compression during use of the vacuum pump.
13. The damping arrangement of claim 12, wherein the resilient bearing support includes an inner portion, and outer portion, and a resiliently flexible member arranged therebetween; and a bearing coupled to the inner portion which is configured to rotatably support the rotor shaft of the vacuum pump.
14. The damping arrangement of claim 12 or claim 13, further comprising a preload damping insert configured to locate between the resilient bearing support and a radially extending surface of the pump housing of the vacuum pump; wherein the preload damping insert is configured such that, when the rotor shaft assembly of the vacuum pump undergoes a preload axial displacement, the preload damping insert is axially compressed.
15. A resilient bearing support for a rotor shaft assembly of a vacuum pump; the resilient bearing support comprising an inner portion configured to couple to a bearing of the rotor shaft assembly; an outer portion, and a resiliently flexible member arranged therebetween; wherein the outer portion of the resilient bearing support includes a radially extending protrusion.