Patent Publication Number: US-10767653-B2

Title: Vacuum pump

Description:
CROSS-REFERENCE OF RELATED APPLICATION 
     This application is a Section 371 National Stage Application of International Application No. PCT/GB2016/053926, filed Dec. 13, 2016, which is incorporated by reference in its entirety and published as WO 2017/103580 A1 on Jun. 22, 2017 and which claims priority of British Application No. 1521981.9, filed Dec. 14, 2015. 
     FIELD 
     Embodiments relate to a vacuum pump and a bearing support that is used in a vacuum pump. In particular, but not exclusively, the embodiments are utilized in turbo-molecular vacuum pumps comprising bearings for supporting the pump&#39;s rotor having a first bearing located on a high pressure side of the rotor and a second bearing located in the vacuum side of the rotor. 
     BACKGROUND 
     Turbo-molecular vacuum pumps are well known to the person skilled in the art. Such pumps comprise a mechanism suited to pump gases at high vacuum pressures, including rotor and stator components each having blades extending radially from an axis. At high vacuum pressures the gas molecules behave in a molecular flow regime. 
     It can be advantageous to locate rotor bearings at each end of the rotor shaft, depending on the application that the pump is designed to serve. Thus, the rotor can be supported in a way that minimizes rotor movement during operational conditions. As a result, the bearing located on the rotor shaft nearest to the pump inlet is located in an area of high vacuum pressure. 
     In certain circumstances it may been necessary to use non-contacting bearings in this location in order to minimize contamination that might be caused by bearing lubricant. Such non-contacting bearings include magnetic bearings that can configured as passive magnetic bearings or active magnetic bearings. In all cases, the bearings located at the pump inlet side of the rotor are located on the rotor&#39;s axis and coupled to the pump body by a series of spokes extending from a central bearing support hub. Thus, the bearing is secured in the desired position coaxially with the axis of the rotor and/or drive shaft. The spoke and hub arrangement can be called a “bearing spider”; this arrangement for supporting the bearing resembles a spoked steering wheel used in automotive vehicles, typically having an outer circular rim and a central boss connected to the rim by three or more equally spaced spokes. 
     A typical turbo-molecular pump bearing system is disclosed in DE202013009660U1 where the bearing spider is a separate component that is inserted into the pump. An outer rim of the bearing spider is arranged to engage with the pump body to secure a passive magnetic bearing in the correct central location. A similar arrangement is disclosed in US2003/0170132. In both these documents, the bearing spider is shown as a separate component that is located in position during pump assembly. 
     It is also known to provide the bearing spider as a component that is integral with the pump body. Such arrangements are known from pumps sold by Pfeiffer Vacuum GmbH, such as the Hipace 80 and Hipace 300 pumps. Additionally the EXT555 vacuum pump sold by Edwards Limited has a similar arrangement. In this integral configuration the spokes of the bearing spider extend in a generally radial direction from the inner section of the pump&#39;s inlet flange towards the central hub. The pump body and bearing spider can be machined from the same block of material, thereby reducing the number of component parts. Furthermore, this integral configuration for the bearing spider has other advantages in that the number of assembly steps can be reduced, tolerance stack-up can be reduced, the length of the pump can be reduced and controls on the bearing assembly process can be improved. 
     There is a general desire to improve the performance of turbo-molecular pumps that have a bearing spider on the high vacuum side of the rotor where the bearing spider is an integral part of the pump body. Furthermore, there is a desire to reduce the size and power consumption of such vacuum pumps without reducing the performance parameters of the pump. 
     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 
     In order to try and achieve these desires, there is provided a turbo-molecular vacuum pump comprising: a body having an inlet flange disposed at a high vacuum side for connecting the turbo-molecular vacuum pump with a chamber to be evacuated, and an outlet port disposed at a low vacuum side for exhausting pumped gases from the turbo-molecular vacuum pump; disposed within the body there is a stator and a rotor arranged to move relative to the stator such that, during use, gas molecules are urged from the inlet flange towards the outlet port, the rotor is coupled to a motor by a drive shaft for driving the rotor in a rotary motion; and bearings for supporting the drive shaft and/or the rotor in relation to the body, wherein a first bearing is coupled to the rotor and disposed on supporting mount at the high vacuum side of the rotor, radial struts extend from the supporting mount and form an integral part of the body; characterized in that the radial struts comprise a hinge portion that can flex to allow relative movement of the inlet flange with respect to the supporting mount. In addition, the embodiments provide a body for a turbo-molecular vacuum pump comprising the features described above. 
     The first bearing can be a passive magnetic bearing and the radial struts can form an integral part of the inlet flange. In addition, the hinge portion can comprise a notch or pinched point in the strut. Thus, the hinged portion (that is, the notch/pinched point) provides an area designed to flex and decouple any movement in the inlet flange from the hinged portion. The axial depth of the strut at the notch should be sufficient to prevent axial movement of the supporting mount relative to the body during normal operation of the turbo-molecular vacuum pump. Also, the axial depth of the strut at the notch should be sufficient to allow inlet flange distortions that can occur (when a flange clamping force is applied) to be decoupled from the supporting mount, thus allowing the supporting mount to move relative to the body or inlet flange. Furthermore, the axial depth of the notch can be between 80% and 10% of the maximum axial depth of the strut, or wherein the axial depth of the notch can be between 50% and 20% of the maximum axial depth of the strut, or the axial depth of the notch can be 33% of the maximum axial depth of the strut. 
     The notch can be formed in the strut at a position where the strut meets the body. This arrangement can maximize the effect of the location of the hinged portion so as to maximize the decoupling effect of the hinge support. Thus, where the radial struts join the pump body at a location close to the inlet flange, the hinge portion of the radial strut is arranged to decouple the bearing support from force that can be applied to the inlet flange. As a result, when the inlet is secured to a chamber or vacuum conduit using a clamping force, torsional movement of the inlet flange is decoupled from the bearing support via the hinge portion or notch. 
     The Summary is provided to introduce a selection of concepts in a simplified form that are further described in the Detail 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 application are now described by way of example with reference to the following drawings, of which: 
         FIG. 1  is a schematic drawing of a portion of a pump; 
         FIG. 2  is a cross section of the portion of the pump shown in  FIG. 1 ; 
         FIG. 3  is a schematic drawing of a portion of a pump in accordance with one embodiment; and 
         FIG. 4  is a cross section of the portion of the pump shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     Greater control of tolerance stack up is required as a result to maintain clearances between the rotor and stator during pump operation. Therefore, the accurate position of bearings on the drive shaft or rotor becomes more critical. A bearing spider can be used to locate the bearing with a high degree of accuracy when a bearing is provided on the vacuum side of the rotor. It has been understood that a relatively high level of accurate positioning can be achieved by integrating the bearing spider with the pump body such that the bearing spider and pump body are formed from a single piece of material. 
     Commercial considerations are increasing the push to reduce pump size, which has resulted (in some circumstances, depending on the pump&#39;s application) in the rotor being placed as near to the pump inlet as practically possible. As a result, the point at which a bearing spider joins the pump body can be coincidental with an inlet flange. The flange can be formed integrally with the pump body typically comprises a standard arrangement that cooperates with standard clamping tools. A high vacuum seal is often used between the pump flange and another surface that engages the flange face. 
     Application of a clamping force to the inlet flange can cause the inlet flange to distort by relatively very small amounts. These distortions are typically insufficient to compromise the vacuum seal that is formed at the flange. As such, until now, the distortion of the inlet flange has been acceptable and of no undue concern. 
     However, we have noticed that these distortions now have an effect on the bearing spider. This undesirable effect is especially noticeable when the bearing spider and pump body are integral with one another and joined at the inlet flange. The torsional moment applied to the flange by the clamp is transmitted to the bearing spider causing the central portion of the bearing spider to become displaced by an undesirable amount. This in turn causes the bearing itself to become displaced such that accurate placement of the rotor is no longer achievable to the now desired amount. As a result, pump failure may occur if the rotor is not located accurately because the rotor and stator clash during operation. The problem is compounded if the bearing spider comprises magnetic bearings that have bearing components which do not make physical contact with one another, but rather rely on magnetic forces of attraction and repulsion. 
       FIGS. 1 and 2  show a pump body  10  having a bearing spider  12  on the vacuum side of a rotor (not shown for clarity). An inlet flange  14  forms the inlet  16  of the pump. The outlet  18  is located at the bottom end of the pump body  10 . The pump body forms a housing in which the pump components, including a rotor, stator, motor, drive shaft and other components are located. 
     The bearing spider  12  is integrally formed on the inside of the body  10  at a location co-located with the inlet flange, which is also integrally formed with the body. The bearing spider  12  comprises three spokes or struts  22  that extend radially out from a central hub  24  located coaxially with the axis of rotation  25 . A portion of bearing system  26  is located on the hub to cooperate with another portion of the bearing that is located on the rotor (not shown). Thus, during pump operation, the bearing spider  22  is located in a region of high vacuum on the inlet side of the rotor. As a result it is typical to use passive magnetic bearings because they are best suited to operating in high vacuum pressure environments, in certain applications. 
     Referring to  FIG. 2 , a cross-section of the pump body is shown. The same numerical indicators have been used on the elements of the pump body described above. The bearing spider  12  and the inlet flange  14  are approximately in the same plane and the point at which the struts  22  meet the body  10  coincides with the inlet flange. The struts have an approximately constant thickness in the axial direction. Thus, the struts provide axial and radial positioning of the central hub  24  with sufficient support to hold the bearing in position during operational use. 
     The inlet flange  14  comprises an engagement portion  28  arranged to engage with and locate a mechanical clamp. The mechanical clamp is used to secure the flange in place with an opposing flange member of a vessel to which the pump is attached (not shown). A suitable sealing O ring is used to provide a gas tight seal that can seal the inlet of the pump (at high vacuum pressures of the order of 10 −8  mbar) from atmospheric pressure outside the pump body. To achieve this, the clamps are tightened with a torque force of 6-10 Nm or more. This clamping force causes the inlet flange to deform and twist slightly, as indicated by arrow  30 . As a result of this twisting moment applied to the inlet flange, the struts  22  are also displaced because the inlet flange, pump body and bearing spider struts are co-located and integrally formed with one another. Thus, the twisting moment is transmitted to the bearing spider and causes the spider to move towards the rotor, thereby displacing the bearing as indicated by arrow  32 . 
     Passive magnetic bearings are sensitive to axial movement because the magnets on each side of the bearing (the stator bearing component and the rotor bearing components) have to be accurately located in relation to each other so that the correct force is applied to the bearing components. In the circumstances described above, a relatively small displacement of one bearing components caused by the clamping force applied to the inlet flange can result in problems with the bearing that may ultimately lead to pump failure. 
     For instance, our tests have shown that the central bearing spider hub of a pump as shown in  FIG. 2 , can become displaced by up to 25 μm when a 10 Nm clamping force is applied to the inlet flange by claw clamps. When using a rotatable flange collar applying a torque force of 10 Nm the bearing spider hub can become deflected by 200 μm. 
     An embodiment is shown in  FIGS. 3 and 4 . The same reference numerals are used to indicate features common with the pump described above. The embodiments aim to overcome the issues that we have found and described above. Referring to  FIG. 3 , a portion of a vacuum pump body according to the embodiments is shown. A cross-section of a portion of the same pump body shown in  FIG. 3  is provided in  FIG. 4 . In general terms, the central bearing spider is provided in a location that is coaxial with the pump axis  25 . The bearing spider has three struts or legs that extend radially from the central hub towards the inlet flange  14 . 
     A pinched section or notch  40  is formed at the point where the struts are integrally formed with and meet the pump body. The pinched section has an axial thickness that is substantially reduced with respect to the maximum axial thickness of the strut, as indicated by arrow  42 . Typically, a reduction in thickness of the order of between 10% and 80%, or more preferably between 20% and 50%. In the device shown in  FIG. 4 , the pinched section is 33% the maximum axial thickness, thus the axial thickness at the narrowest portion of the pinched section  40  is ⅔ the maximum axial thickness. 
     This arrangement provides a flex point in the strut that can hinge or bend when a force is applied to the strut. As a result, when a torsional clamping force (as indicated by arrow  30 ) is applied to the inlet flange clamp engagement portion  28 , the resulting movement in the flange is significantly decoupled from the bearing spider  12 . The pinched section provides a hinged portion of the strut. By providing the pinched point  40  close to the inlet flange, any rotational movement of the flange caused by the clamping force results in the struts bending, hinging or flexing at the pinched point. This flexing reduces axial movement in the bearing spider that may otherwise have occurred should the pinched point not be utilized. 
     The pinched point  40  should be strong enough to provide structural strength to the bearing spider support during the operational cycle of the pump. The thickness of the struts in an axial direction should be greater than the pinched point to provide additional rigidity and stiffness such that the flexing of the strut occurs in the desired location. Thus, one design option is to provide an increased thickness of the struts between the bearing spider and the pinched point. The pinched point can be arranged to have an axial thickness that is sufficient to withstand operational loads and provide the desired strength for the structure. In other words, the strut of the embodiment is generally thicker in an axial direction than the known struts used in current devices, apart from at one location where the strut has a similar thickness to known devices. This location is the pinched point, located at the point where the strut and inlet flange are co-located. 
     We have found that using this arrangement has significant benefits in reducing the movement of the bearing spider when a clamping force is applied to the inlet flange. For instance, our tests have shown that the central bearing spider hub of an embodiment (as shown in  FIG. 4 ), becomes displaced by 5 μm when a 10 Nm clamping force is applied to the inlet flange by claw clamps—this is a reduction of 20 μm from the device shown in  FIG. 2 , as described above. When using a rotatable flange collar applying a torque force of 10 Nm the bearing spider hub deflected by 45 μm, compared to 200 μm on known systems. As a result, the potentially detrimental displacement of bearing spider caused by the application of a clamping force can be greatly reduced by utilizing the embodiments. 
     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.