Patent Publication Number: US-2022235778-A1

Title: Rotor support and vacuum pump with such a rotor support

Description:
CROSS-REFERENCE OF RELATED APPLICATION 
     This application is a Section 371 National Stage Application of International Application No. PCT/EP2020/065222, filed Jun. 2, 2020, and published as WO 2020/249431 A1 on Dec. 17, 2020, the content of which is hereby incorporated by reference in its entirety and which claims priority of British Application No. 1908277.5, filed Jun. 10, 2019. 
    
    
     FIELD 
     The field of the invention relates to vacuum pumps, and to rotor supports or mounting means for supporting a rotating shaft of a vacuum pump. 
     BACKGROUND 
     Vacuum pumps such as turbomolecular pumps typically comprise a body and a rotor supported for rotation relative to the body. The rotor when rotating draws gas from a tool connected to the inlet of the pump. The rotor is supported by a bearing arrangement. In some cases there are two bearings, the upper bearing being in the form of a magnetic bearing, and the lower bearing in the form of a rolling bearing. 
     It is important in such pumps that the rotor is mounted in an axially stiff manner, such that the clearances between rotor and stator components are maintained and the magnetic bearings operate effectively. However, it is also important in some applications that vibrations that arise due to rotation of the rotor are isolated from the pump body to impede these vibrations being transmitted to the tool connected to the inlet. This is particularly the case in scientific instruments such as electron microscopes. 
     A further issue with mounting the shaft arises when the pump is serviced and the bearings need to be replaced. Using new bearings within such a pump conventionally requires the pump to be rebalanced, however, were a bearing with sufficient radial flexibility to be used that also had axial stiffness, this rebalancing step might be dispensed with allowing the pump to be serviced in the field. 
     Some of these issues have been addressed using an insert or compact metal spring damper as disclosed in EP2126365 which is used to mount the bearings to the body of the pump and provide axial stiffness and radial flexibility. 
     However the amount of vibrations transmitted to the pump body is still too high for some applications. This is particularly, the case where the rolling bearings become worn and high frequency vibrations may be transmitted between the rotor and the pump body. 
     It would be desirable to provide a pump with improved isolation from vibrations due to the rotating rotor. 
     The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background. 
     SUMMARY 
     A first aspect provides a rotor support configured to rotatably mount a rotor shaft in a vacuum pump, said rotor support comprising: a rolling bearing for rotatably supporting said shaft; an insert and at least one resilient damping member, said insert and said at least one resilient damping member surrounding said rolling bearing; wherein said insert comprises inner and outer annular portions connected by a plurality of flexible members, said plurality of flexible members being configured to flex in a radial plane and resist movement in an axial plane, thereby absorbing radial movement of said shaft; and said at least one resilient damping member is formed of an elastomeric material configured to flex in both a radial and axial direction; wherein said at least one resilient damping member is configured to support said insert and is arranged in series with said insert. 
     The inventor of the present invention recognised that although axial stiffness and some radial flexibility are desirable when mounting a rotor in a vacuum pump where both accurate axial positioning of the rotor and low vibrations are important, the use of an insert where the axial stiffness is too high may provide a path for vibrations to travel between the rotor, bearings and pump body. 
     Providing some further isolation between the bearings and pump body may help alleviate this problem, while still providing acceptable axial positioning. In particular, providing an elastomeric material that has both axial and radial flexibility to support the insert and that is arranged in series with it provides isolation and damping of vibrations. 
     The resilient damping member is located in series with the insert such that it may be located between the ball bearing and the insert and/or it may be located around the outer surface of the insert such that it is located between the pump housing and the outer annular portion of the insert when the rotor support is supporting a rotor in a pump. In effect providing a resilient damping member in series with the insert when mounting the rotor allows the rotor support to provide the functionality of axial stiffness of the insert while providing improved isolation from vibrations due to the additional flexibility of the damping member between the ball bearing and pump housing. 
     It should be noted that although an elastomeric material provides flexibility in both axial and radial directions and good isolation from vibrations, the more flexible the elastomer the greater both the radial and axial flexibility are, such that independent control of each is to some extent lost. However, the use of a resilient member formed of elastomeric material in conjunction with an insert which is configured to provide radial flexibility with axial stiffness, is a combination that allows both a degree of independent control of the amount of axial and radial stiffness through design of the insert, while providing additional isolation to reduce vibration transmission with the damping member. 
     In some embodiments, said resilient damping member is arranged in series with said insert in at least one of the following configurations: at least one of said at least one resilient damping member is located between said insert and said rolling bearing; and said outer annular portion of said insert is located between at least one of said at least one resilient damping member and said rolling bearing. 
     In some embodiments, said insert is formed of a stiff material and said flexible members are longer axially than they are wide, thereby providing axial stiffness and radial flexibility. 
     The axial stiffness and radial flexibility may be provided by making the insert of a stiff material such as a metallic or plastic material and making the flexible members axially long so that they resist bending in an axial direction, while being relatively thin so that they can flex radially. In this way the degree of flexibility in each direction is built into the design with the material and dimensions of the flexible members. 
     Where the insert is made of a metallic material, then the metal to metal junctions between the insert and the pump body and the insert and the rolling bearing provide a path for vibrations which can be effectively impeded where an elastomeric material is used to interrupt the path. A plastic material may of itself reduce some of the vibration transmission, however, a further resilient member will impede the vibrations still further. 
     Although the flexible members can be formed in a number of ways, in some embodiments, each of the flexible members comprises an elongate, arcuate member substantially concentric with the inner and outer annular portions. 
     In some embodiments, the flexible members provide a plurality of integral leaf springs providing radial flexibility. 
     In some embodiments, said rolling bearing comprises an inner race, an outer race and a plurality of rolling elements located between the races, said outer race being bonded to said insert. 
     It should be noted that the resilient damping member may be located between the insert and pump body when mounting the rotor to the pump, or between the insert and the rolling bearing. Where the insert is bonded to the rolling bearing it may be directly bonded to them, or where the resilient damping member is between the rolling bearing and the insert, the resilient damping member may be bonded to the rolling bearing and the insert to the resilient damping member. Bonding the insert to the rolling bearing, allows them to be accurately and consistently located axially with respect to each other. 
     In some embodiments, said at least one resilient damping member is located between said insert and said rolling bearing. 
     Alternatively or additionally, said insert is located between said rolling bearing and said resilient damping member. 
     Where the insert is located adjacent to the rolling bearing then the bonding of the insert to the rolling bearing may be more easily and effectively performed. The bonding of the elastomeric materials may be more difficult and in some embodiments they will not be bonded. 
     In some embodiments, the rotor support further comprises at least one resilient axial damping member formed of an elastomeric material and mounted such that axial movement of said rolling bearing changes a compression on said resilient axial damping movement. 
     As noted previously using an insert formed of a stiff material with little axial flexibility may allow some vibrational noise to be transmitted to the pump body. This is reduced with a resilient damping means providing some axial and radial flexibility. However, the axial flexibility should be limited and it may be further limited with a resilient axial damping member which is configured such that axial movement compresses or squashes the material, providing an effective resistance to axial movement. The resilient damping member that provides radial support is sheared by axial forces and may provide less resistance to axial displacement. 
     In some embodiments, said at least one resilient axial damping member is mounted on an outer surface in a radial plane of said rotor support. 
     In some embodiments, in order to provide additional axial stability the resilient axial damping member may be mounted on a surface that lies in a radial plane, that is perpendicular to the axis of the shaft and is configured such that when mounted in the pump it cooperates with a similarly radially extending surface on the pump, such that the resilient axial damping member is mounted between the two surfaces, relative axial movement of the surfaces towards each other compressing the damping member by different amounts. 
     In some embodiments, said insert comprises an extension extending inwardly from an end of said inner annular member, said surface extending over said rolling bearing, said resilient axial member being mounted between a surface of said rolling bearing facing said extension and a surface of said extension facing said rolling bearing. 
     The surface in the radial plane may be on an extension of the inner annular member of the insert which extends radially across and axially displaced from an end surface the rolling bearing so that the resilient axial member is mounted between the extending surface and the rolling bearing surface. 
     In some embodiments, said at least one resilient damping member is axially more flexible than said resilient axial damping means. 
     The resilient axial damping member is provided to impart a higher axial stiffness than the at least one resilient damping member which may be more than 1.5 times as axially flexible as said resilient axial damping member. This may be due to its shape, and/or the fact that it is mounted between two surfaces in the radial plane, so that axial movement compresses it, and/or it may be due to the material it is formed of. 
     In some embodiments, said resilient axial damping member comprises a member with a substantially rectangular cross section when not compressed. 
     The resilient axial damping member may be a gasket with a flat type shape and a rectangular cross section. 
     In some embodiments, said resilient axial damping member and said at least one resilient damping member comprise a single member. 
     In some embodiments they comprise a single L-shaped member. 
     Although the two or more damping members may be formed separately, in some embodiments, they may be formed as a single piece, perhaps as an L-shaped member, operable to provide an elastomeric element both in a radial path between components and in an axial path between components. 
     In some embodiments, said at least one resilient damping member comprises at least one O-ring. 
     The at least one resilient damping member may be one or more O-rings which allow the roller bearing and insert to be slid into position within the pump. 
     A second aspect provides a vacuum pump comprising: a rotor comprising a shaft rotatably mounted within a pump body on a rotor support, said rotor support comprising a rotor support according to a first aspect. 
     In some embodiments, an outer one of said at least one resilient damping member or said insert are bonded to said pump body. 
     The rotor support may be bonded to the pump, which will provide additional axial stability, in such a case a resilient axial damping member may not be used. 
     In some embodiments, said O-ring is mounted in a recess on said pump body. 
     The O-ring(s) may be mounted within recess(es) in the pump body, allowing the rotor support to be slid into position. 
     In some embodiments, said vacuum pump comprises a turbomolecular pump. 
     Turbomolecular pumps are often used to provide the high vacuums necessary for some scientific analysis and in such cases a low vibrational environment may be required. This is for example the case with electron microscopes. Thus, providing these pumps with means for isolating the rotors from the pump body can improve their performance. 
     In some embodiments, said vacuum pump comprising further bearings, said further bearings comprising magnetic bearings, said rolling bearings being mounted towards one end of said shaft and said magnetic bearings towards an opposing end. 
     Many pumps such as turbomolecular pumps have magnetic bearings to reduce frictional forces and provide clean bearings that can be used close to the pump inlet. These magnetic bearings require the axial positioning of the shaft to be accurately controlled for them to be effective, they also provide axial loading on the shaft biasing the shaft towards the bearings. This biasing can hold the shaft in position in particular when used in conjunction with the resilient axial damping members located between two radially extending surfaces, one associated with the rotor support and one with the pump body. 
     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. 
     The Summary is provided to introduce a selection of concepts in a simplified form that are further described in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which: 
         FIG. 1  shows a cross section through a rotor support means according to a first embodiment; 
         FIG. 2  shows the insert of the rotor support means of  FIG. 1 ; and 
         FIG. 3  shows a cross section through one side of a rotor support means according to a further embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Before discussing the embodiments in any more detail, first an overview will be provided. 
     To improve the vibration levels of a vacuum pump and in particular, a turbo pump with an insert in the form of a compact metal or plastic spring damper, an elastomeric element is added in series with the compact metal spring damper CMSD or insert. 
     In applications such as pumping electron microscopes the level of vibration caused by the turbo pump is very critical to the microscope resolution. To reduce the level of vibration caused by the pump a compact metal spring damper may be used to mount the ball bearings. This uses circumferential spring elements to give a low radial stiffness, but high axial stiffness, thus absorbing radial movement of the shaft due to imbalance, but maintaining a good axial location of the rotor. This achieves a high level of isolation and a good vibration performance, however there is still room for improvement in the most demanding applications. 
     Embodiments seek to enhance the performance of such an insert by using an elastomeric element to support the insert, giving a degree of axial and radial compliance and further isolating the bearing noise and vibration from the main pump housing. 
     The compliant element may be made up of one or more components, such as one or more O-rings for radial location and a flat elastomeric ‘washer’ for axial location. Alternatively an ‘L’ shaped elastomeric component could provide both axial and radial location. 
     In one embodiment, the elastomeric element(s) is/are mounted between the insert and pump housing, alternatively, or additionally, they are mounted between the bearing and insert. 
     This arrangement is used in pumps with a single ball bearing and a passive magnetic bearing or in pumps with two ball bearings supporting the shaft. 
       FIG. 1  shows a cross section through a rotor shaft  20  that is supported by a rotor shaft support  5  according to an embodiment. The rotor shaft support  5  comprises rolling bearings  10  which are held in position by a compact metal spring damper or insert  70 . Insert  70  is in this embodiment bonded to the bearing  10  with adhesive. Insert  70  provides axial stiffness and some radial flexibility for the bearing  10 , helping to maintain them in position while allowing some radial movement to absorb vibrations. The rotor shaft  20  is the shaft of a turbomolecular pump. 
     Insert  70  is mounted to pump body  30  via a resilient damping member  40  formed of elastomeric material mounted between the insert  70  and the pump body  30 . This resilient damping member  40  has some axial and radial flexibility and helps reduce vibrations still further. In some embodiments, the shaft is held in position by adhering the rotor support to the pump body  30 , however, in this embodiment, the rotor support is held in position by a further resilient damping member in this case an axial resilient damping member  42 . 
     Axial resilient damping member  42  is mounted between a projecting portion of the pump housing  30  and the upper surface of the insert  70 . In this embodiment axial resilient damping member  42  is a ring shaped gasket. The resilient damping member  40  has an annular form and is thicker than the flat gasket  42  allowing more axial movement. Gasket  42  is arranged such that axial movement of the bearing compresses the gasket and thus, axial movement is resisted. 
     Shaft  20  is mounted on a rolling bearing  10  located towards the outlet end of the pump and on magnetic bearings (not shown) located towards the inlet end of the pump. These magnetic bearing provide the biasing force to hold the insert  70  against gasket  42 . In this way gasket  42  helps provide axial alignment of the rotor shaft  20 , while its elasticity helps reduce vibration transmission from the rolling bearing  10  to the pump body  30  along the axial path. The elasticity of gasket  42  is selected to be relatively low such that axial movement is restricted and axial alignment is maintained within acceptable limits. The elasticity of resilient member  40  is selected to be higher providing more flexibility and reducing vibration transmission particularly in the radial path between the insert  70  and pump body  30 . 
       FIG. 2  shows the insert  70  in more detail. The insert  70  is provided to mount the shaft within the rolling bearing in a way that is axially stiff and radially flexible. In this embodiment insert  70  is formed of a metallic material, although in other embodiments it may be formed of a plastic. Insert  70  has integral inner and outer annular portions  54 ,  56  connected together by a plurality of flexible members  58 . In this embodiment they are formed by machining slots  60  in the insert  70 . Each flexible member  58  is connected by a first resilient hinge  62  to the inner portion  54 , and by a second resilient hinge  64  to the outer portion  56 . Each flexible member  58  is in the form of an elongate, arcuate member substantially concentric with the inner and outer annular portions  54 ,  56 , and, as illustrated in  FIG. 2 ., the flexible members  58  are preferably circumferentially aligned. The flexible members  58  of the resilient support  52  thus provide integral leaf springs of the resilient support or insert  70 . As can be appreciated the elasticity of the insert  70  in each of the radial and axial directions is dependent upon the material of the insert and the thickness and shape of the flexible members  58  and on their hinged portions  64 ,  62 . Thus, the insert can be designed according to requirements, with desired axial and radial properties. 
       FIG. 3  shows an alternative embodiment of a cross section through one side of a rotor support  5  configured to support shaft  20  of a vacuum pump rotor within pump body  30 . Rotor support  5  comprises insert  70 , rolling bearing  10  and resilient members  40 ,  42  and  43 . 
     In this embodiment, shaft  20  is mounted via the rotor support  5  towards the outlet end of the pump and on magnetic bearings (not shown) towards the inlet end of the pump. The magnetic bearings provide a biasing force  80  on shaft  20  which biases it against axial resilient member  42  and a portion of resilient member  43 . 
     It should be appreciated that the flexible resilient members may be located either between the rolling bearing  10  and insert  70  and/or between the insert  70  and pump body  30 . In this embodiment they are mounted between both, but in some embodiments there are only flexible resilient members  40  and  42  or flexible resilient member  43 . 
     The resilient members may be bonded to the rolling bearing  10  and insert  70  and to the pump body wall  30 , however, generally they are held in position by their elasticity. 
     In the embodiment of  FIG. 3  rolling bearing  10  is adhered to shaft  20 . Insert  70  lies between pump body  30  and rolling bearing  10  and there is an L-shaped resilient member  43  between the rolling bearing  10  and insert  70 . The L-shaped resilient member is formed of an elastomeric material and has a narrower cross sectional width along the radially extending portion than along the axially extending portion. There are further resilient members between the insert  70  and the pump body  30 . These comprise O-ring  40  located in recess  32  in the pump body wall and axial resilient member  42 . In other embodiments L-shaped resilient member  43  may be replaced by two separate resilient members, a ring gasket arranged to lie on the upper surface of rolling bearing  10  and an annular elastomeric damping member arranged to lie around the outer surface of bearing  10  and adjacent to the inner surface of insert  70 . 
     Insert  70  has an inner annular member  54  with a radially inwardly extending portion at one axial end that extends over the rolling bearing  10  and provides a surface for a portion of the L-shaped resilient member  43  that extends over the axial end surface of rolling bearing  10  to abut with and provide additional axial stability. The insert further has a central flexible member  58  extending from the outer annular member  56  to the inner annular member  54  and providing radial flexibility while also biasing the rolling bearings towards a central position. The outer annular member  56  has a radially outwardly extending portion at the other axial end to the radially inwardly extending portion that provides a surface parallel to a protruding portion  34  of the pump wall body  30 . Axial resilient member  42  which is in the form of a gasket is located between the two parallel radially extending surfaces and provides additional axial stability. In this embodiment biasing force  80  biases the bearing against L-shaped member  43  and the insert  70  against resilient member  42  holding the support axially in place, the biasing force  80  resisting axial movement in the downward direction and the two resilient members  42 ,  43  resisting it in the upwards direction and acting to damp any axial vibrations. 
     O-ring  40  and the axially extending portion of L-shaped resilient member  43  provide damping of vibrations travelling radially from the shaft and bearings towards the pump body  30 . The O-ring  40  is mounted in a recess  32  of pump body wall  30  and this allows it to be held in place while the insert, shaft and bearing are slid into position. 
     Were the rolling bearing  10  to need to be replaced, then the arrangement of the rotor support  5  will help to accurately position the shaft both radially and axially allowing the bearings to be changed in the field without the need for rebalancing. 
     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. 
     Although elements have been shown or described as separate embodiments above, portions of each embodiment may be combined with all or part of other embodiments described above. 
     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are described as example forms of implementing the claims.