Patent Publication Number: US-11649859-B1

Title: Vibration dampening for scanner bearing

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to bearings, and more particularly to rolling element bearings for gantry assemblies. 
     Gantry assemblies for machines such as luggage security scanners, CT scanners, radiotherapy machines, etc. typically include a stationary frame with a central opening for receiving a patient or an object being scanned and a rotatable assembly with diagnostic, scanning or/and treatment devices which scan or radiate the object (e.g., a piece of luggage) or patient within the frame opening. Typically, the rotatable assembly is connected with the frame by a bearing assembly, such as a thin section bearing. Due to the relatively large diameter and minimal radial thickness of the bearing rings of a thin section bearing, such bearings may experience a significant degree of vibration, which can generate undesirable noise levels during the use of the scanner assembly. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention is a thin section bearing assembly for a gantry assembly, the gantry assembly including a static housing with a bore and a rotor rotatable about a central axis. The bearing assembly comprises a bearing outer ring disposable within the housing bore and having an inner circumferential surface with an annular groove, an outer circumferential surface, a radial thickness between the inner circumferential surface and the outer circumferential surface and an inside diameter. The outer ring is sized such that a ratio of the inside diameter to radial thickness is at least thirty (30). A bearing inner ring is disposable about the rotor and has an inner circumferential surface, an outer circumferential surface with an annular groove, a radial thickness between the inner circumferential surface and the outer circumferential surface and an inside diameter. The inner ring is sized such that a ratio of the inside diameter to radial thickness is at least thirty (30). A plurality of rolling elements are disposed between the outer ring groove and the inner ring groove. Further, at least one elastomeric annular damper is disposed between the outer ring and the housing and/or between the inner ring and the rotor. The at least one annular damper has a first section and a second section spaced axially apart from the first section. As such, a compression of the first section is greater than a compression of the second section when a bending moment is applied to the bearing assembly in a first angular direction, and the compression of the second section is greater than the compression of the first section when a bending moment is applied to the bearing assembly in a second, opposing angular direction so as to increase a fatigue life of the at least one annular damper. 
     In another aspect, the present invention is again a thin section bearing assembly for a gantry assembly, the gantry assembly including a static housing with a bore and a rotor rotatable about a central axis. The bearing assembly comprises a bearing outer ring disposable within the housing bore and having an inner circumferential surface with an annular groove, an outer circumferential surface, a radial thickness between the inner circumferential surface and the outer circumferential surface and an inside diameter. The outer ring is sized such that a ratio of the inside diameter to radial thickness is at least thirty (30). A bearing inner ring is disposable about the rotor and has an inner circumferential surface, an outer circumferential surface with an annular groove, a radial thickness between the inner circumferential surface and the outer circumferential surface and an inside diameter. The inner ring is sized such that a ratio of the inside diameter to radial thickness is at least thirty (30). A number (R) of balls are disposed between the outer ring groove and the inner ring groove and traverse a circular path with a pitch diameter (D), each ball having a diameter (D R ) and contacting each of the inner ring groove and outer ring groove at a contact angle (α). Further, at least one elastomeric annular damper disposed between the outer ring and the housing and/or between the inner ring and the rotor, the at least one elastomeric annular damper having a radial thickness (t D ), an axial width (b) and a hardness (S) on the Shore A hardness scale. The bearing assembly is configured to support a radial load (F) and to rotate at an angular speed (N) and the radial thickness (t D ) of the at least one annular damper is calculated as follows: 
     
       
         
           
             
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     In a further aspect, the present invention is again a thin section bearing assembly for a gantry assembly, the gantry assembly including a static housing with a bore and a rotor rotatable about a central axis. The bearing assembly comprises a bearing outer ring disposable within the housing bore and having an inner circumferential surface with an annular groove, an outer circumferential surface, a radial thickness between the inner circumferential surface and the outer circumferential surface and an inside diameter. The outer ring is sized such that a ratio of the inside diameter to radial thickness is at least thirty (30). A bearing inner ring is disposable about the rotor and has an inner circumferential surface, an outer circumferential surface with an annular groove, a radial thickness between the inner circumferential surface and the outer circumferential surface and an inside diameter. The inner ring is sized such that a ratio of the inside diameter to radial thickness is at least thirty (30). A number of balls are disposed between the outer ring groove and the inner ring groove. Further, at least one elastomeric annular damper is disposed between the outer ring and the housing and/or between the inner ring and the rotor, the at least one elastomeric annular damper being configured to have a first natural frequency that is no greater than about one third of a ball pass frequency of the bearing assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The foregoing summary, as well as the detailed description of the preferred embodiments of the present invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings, which are diagrammatic, embodiments that are presently preferred. It should be understood, however, that the present invention is not limited to the precise arrangements and instrumentalities shown. In the drawings: 
         FIG.  1    is broken-away, axial cross-sectional view of an upper, central portion of a gantry assembly having two bearing assemblies in accordance with the present invention, shown with an outer annular damper and an outer axial damper; 
         FIG.  2    is a top plan view of one bearing assembly separate from the gantry assembly; 
         FIG.  3    is an enlarged, broken-away view of the two bearing assemblies of  FIG.  1   , shown with both inner and outer annular dampers and an outer axial damper; 
         FIG.  4    is more enlarged, broken-away view of the outer portion of a single bearing assembly; 
         FIG.  5    is a broken-away, axial cross-sectional view of a portion of an alternative gantry assembly, shown with the outer rings of two bearing assemblies disposed in contact; 
         FIG.  6    is a broken-away side elevational view of an upper portion of a combined annular damper and integral axial damper disk, shown with an annular damper having two annular damper sections; 
         FIG.  7    is a broken-away, side elevational view of an upper portion of a combined annular damper and integral axial damper disk, shown with an annular damper formed of a single section; 
         FIG.  8    is a broken-away, side elevational view of an upper portion of a combined annular damper and two axial damper disks; and 
         FIG.  9    is a side plan view of one arcuate section of the first construction annular damper and integral axial damper. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Certain terminology is used in the following description for convenience only and is not limiting. The words “lower”, “upper”, “upward”, “down” and “downward” designate directions in the drawings to which reference is made. The words “inner”, “inwardly” and “outer”, “outwardly” refer to directions toward and away from, respectively, a designated centerline or a geometric center of an element being described, the particular meaning being readily apparent from the context of the description. Further, as used herein, the words “connected” and “coupled” are each intended to include direct connections between two members without any other members interposed therebetween and indirect connections between members in which one or more other members are interposed therebetween. The terminology includes the words specifically mentioned above, derivatives thereof, and words of similar import. 
     Referring now to the drawings in detail, wherein like numbers are used to indicate like elements throughout, there is shown in  FIGS.  1 - 9    a thin section bearing assembly  10  for a gantry assembly  1 , the gantry assembly  1  including a static housing  2  with a bore  2   a  and a rotor  3  rotatable about a central axis A C . Preferably, the gantry assembly  1  is part of a luggage security scanner including a rotatable drum  7  ( FIG.  1   ) connected with the rotor  3 , on which are mounted various imaging or scanning equipment, and a support frame (not shown) connected with the housing  2 , the details of which are beyond the scope of the present disclosure. However, the gantry assembly  10  may be incorporated into a “CT” scanner (i.e., computed tomography scanner), a radiation treatment machine (e.g., external beam radiotherapy) or any other machine or device for rotating imaging or treatment equipment, or any other appropriate devices, about a central axis A C . The bearing assembly  10  basically comprises a bearing outer ring  12  disposable within the housing bore  2   a , an inner ring  14  disposable about the rotor  3 , a plurality of rolling elements  16  disposed between the inner and outer rings  12 ,  14  and at least one elastomeric annular damper  18  disposed between the outer ring  12  and the housing  2  or/and the inner ring  14  and the rotor  3 . In use of the gantry assembly  1 , the bearing assembly  10  is intended to support a radial load F between two thousand, two hundred Newtons (2200 N) and twenty two thousand Newtons (22,000 N), or between five hundred pounds force (500 lbf) and five thousand pounds force (5000 lbf), and to rotate at an angular speed N within the range of fifty rotations per minute and about five hundred rotations per minute (500 rpm). 
     Preferably, the gantry assembly  1  includes two bearing assemblies  10  coupling the housing  2  and the rotor  3 , as shown in  FIGS.  1 ,  3  and  5   . The two bearing assemblies  10  may be formed so as to be disposed on separate sides of a radial rib  4  extending into the housing bore  2   a , as shown in  FIGS.  1  and  3   , or may be disposed within the bore  2   a  so as to abut each other, as depicted in  FIG.  5   . In either case, the two preferred bearing assemblies  10  are preferably identically formed, although not necessarily so, such that only one bearing assembly  10  is described in detail herein. Further, the gantry assembly  1  may include only a single bearing assembly  10  or three or more bearing assemblies (neither structure shown) as shown and described herein. 
     Referring to  FIGS.  1 - 3  and  5   , the outer ring  12  has opposing axial ends  12   a ,  12   b , an inner circumferential surface  13 A with a groove  15  and an outer circumferential surface  13 B. The outer ring  12  is sized having an axial width w O  between the axial ends  12   a ,  12   b , a radial thickness T RO  ( FIG.  2   ) between the inner and outer surfaces  13 A,  13 B and an inside diameter ID O . As the bearing assembly  10  is of the type referred to as “thin section” as discussed above, the outer ring  12  is sized such that a ratio of the inside diameter ID O  to the radial thickness T RO  is at least thirty (30). Further, the bearing inner ring  14  has opposing axial ends  14   a ,  14   b , an inner circumferential surface  17 A and an outer circumferential surface  17 B with an annular groove  19 . The inner ring  14  is sized having an axial width w I , a radial thickness T RI  between the inner and outer surfaces  17 A,  17 B and an inside diameter ID I . As with the outer ring  12 , the inner ring  14  is sized such that a ratio of the inside diameter ID I  to the radial thickness T RI  is at least thirty (30). 
     Further, the plurality of rolling elements  16  are disposed between the outer ring groove  15  and the inner ring groove  19  so as to be rollable simultaneously along the outer and inner rings  12 ,  14 . Preferably, each rolling element  16  is a ball, but may alternatively be a cylindrical roller, a tapered roller, a needle, or any other appropriate type of rolling element. In any case, each of the plurality of rolling elements  16  traverse a circular path about the central axis A C  with a pitch diameter D, which preferably has a value within the range of about fifty centimeters (50 cm) and one hundred eighty centimeters (180 cm). The number “R” of rolling elements/balls  16 , the “ball count”, in each row of rolling elements  16  is preferably between one hundred (100) and five hundred (500) and each rolling element  16  preferably has a diameter D R  within the range of about six millimeters (6 mm) and twenty millimeters (20 mm). Further, in one preferred application, the bearing assembly  10  is an angular contact ball bearing (“ACBB”) in which each rolling element  16  contacts the outer ring groove  15  and the inner ring groove  19  at a contact angle α, as indicated in  FIG.  3   , as opposed to contacting both of the grooves  15 ,  19  within a single vertical plane. The contact angle α preferably has a value in the range of between about fifteen (15°) and forty-five degrees (45°). 
     Due to the relatively substantial flexibility of the rings  12 ,  14 , which is conventional for any thin section bearing, the bearing assembly  10  typically experiences bending moments under loading, which generate vibrations, that exceed the moments and resulting vibration experienced by more conventional bearings. That is, conventional bearings are significantly more rigid than thin section bearings due to the substantially lower ratio between the bearing ID and the ring thickness T R . As such, to reduce vibrations and particularly the noise produced thereby, the bearing assembly  10  includes at least an outer elastomeric annular damper  18  disposed between the outer ring  12  and the housing  2  or/and an inner elastomeric annular damper  18  disposed between the inner ring  14  and the rotor  3 . Depending on the specific application of (e.g., medical scanning, luggage scanning, etc.) and the resultant loading on the gantry assembly  10 , the bearing assembly  10  may include only an outer elastomeric damper  18  ( FIG.  1   ), only an inner elastomeric damper  18  (not shown) or both an outer elastomeric damper  18  and an inner elastomeric damper  18  ( FIG.  3   ). 
     Further, each elastomeric annular damper  18  is preferably formed of an elastomer having a hardness on the Shore A scale of between 20 and 90 and has opposing axial ends  18   a ,  18   b  and inner and outer circumferential surfaces  20 A,  20 B. With an outer annular damper  18 , the damper inner circumferential surface  20 A is disposed about the outer surface  13 B of the bearing outer ring  12  and the damper outer circumferential surface  20 B is disposed against an inner circumferential surface  2   b  of the outer housing  2 . As shown in  FIG.  3   , with an inner annular damper  18 , the damper inner circumferential surface  20 A is disposed about an outer circumferential surface  3   a  of the rotor  3  and the damper outer circumferential surface  20 B is disposed against the inner surface  15 A of the bearing inner ring  14 . 
     Referring particularly to  FIG.  9   , each annular damper  18  is preferably formed of a plurality of arcuate sections  22  (only one shown), most preferably three or four sections  22 , spaced circumferentially about the central axis A C  and each having two opposing circumferential ends  22   a ,  22   b . Each circumferential end  22   a ,  22   b  of each one of the arcuate sections  22  is disposed at least adjacent to one of the circumferential ends  22   b ,  22   a  of another one of the arcuate sections  22 , and preferably the adjacent ends  22   a ,  22   b  are abutting. By forming each annular damper  18  as a plurality of arcuate sections  22 , the damper  18  is installable by placing each section  22  about the outer surface  13 B of the outer ring  12  or about the inner surface  3   a  of the rotor  3 . However, each damper  18  may alternatively be formed as a one-piece annular member which is stretched over the appropriate surface  13 B,  3   a.    
     Furthermore, to increase the vibration dampening capability, the bearing assembly  10  preferably further comprises at least one elastomeric axial damper disk  24  disposed either between one axial end  12   a  or  12   b  of the outer ring  12  and a radial surface  5  of the housing  2  and/or between one axial end  14   a ,  14   b  of the inner ring  14  and a radial surface  6  of the rotor  3 . In addition to damping vibration within the bearing assembly  10 , and thus within the gantry assembly  1 , the elastomeric annular damper(s)  18  and the axial damper disks  24  also function to electrically and thermally insulate the bearing rings  12 ,  14 . That is, an outer annular damper  18  will prevent or at least reduce thermal heat transfer and electric current flow between the bearing outer ring  12  and the housing  2  and an inner annular damper  18  will prevent/reduce heat transfer and current flow between the inner ring  14  and the rotor  3 , with any axial damper disks  24  also preventing thermal and electric flow between the rings  12 ,  14  and the housing  2  or the rotor  3 . Preferably, each axial damper disk  24  is integrally connected with an adjacent annular damper  18  by a web section  26 , but may alternatively be formed as a completely separate disk  24  (structure not shown). Further, as shown in  FIG.  8   , two axial damper disks  24  may be integrally formed with a single annular damper  18 . 
     To enhance the vibration reduction capability of each elastomeric annular damper  18 , the one or more dampers  18  are each configured, i.e., by appropriate selection of material and dimensions, to have a first natural frequency ω that is no greater than about one third (⅓) of a ball pass frequency BPF of the bearing assembly  10 . As is known, the ball pass frequency BPF of a bearing assembly can be calculated as follows: BPF=½ NR*(1±((D R /D)*cos α))); in which N is the bearing rotational speed, R is the number of balls per row of the bearing assembly, D R  is the diameter of each rolling element  16 , D is the pitch diameter of the bearing assembly  10  and α is the contact angle of the bearing assembly  10 , as each is described above. The natural frequency ω of each one of the annular dampers  18  may be calculated as follows: ω=√(K/m); in which K is the stiffness of the elastomeric annular damper  18  and m is the mass of each annular damper  18 . Both the stiffness K and the mass m are each dependent upon the dimensions of each elastomeric annular damper  18 , such as the damper radial thickness t D , the axial width b and the diameter D D . 
     The stiffness K is also dependent upon the material properties of the specific elastomer and is preferably calculated as follows: K=E*A/L; in which E is the modulus of elasticity of the particular elastomeric material, A is the cross-sectional area of the damper  18  and L is the circumferential length of the annular damper  18  (i.e., Π*Diameter D D  of the damper  18 ). 
     In calculating the stiffness K and thereafter the natural frequency ω of each annular damper  18 , the diameter D D  of the annular damper  18  is determined by size of the particular bearing ring  12  or  14  to which the damper  18  is applied, and thus is established by the size of the bearing assembly  10 . Also, each annular damper  18  preferably extends across substantially the entire width w O  or w I  of the particular ring  12 ,  14 , respectively. Specifically, each outer annular damper  18  is preferably sized to extend substantially across the entire axial width w O  of the outer ring  12  and each inner annular damper  18  is sized to extend substantially across the entire axial width w I  of the inner ring  14 . Most preferably, each annular damper  18  has an axial width b that is sized such that a ratio of the axial width b of the damper  18  to the axial width w O  or w I  of each one of the outer and inner rings  12 ,  14  is within the range of about 0.75 and 0.90. That is, b/w O =0.75-0.90 and b/w I =0.75-0.90. 
     As a result of the diameter D D  (and thus the circumferential length L) being established by the size of the bearing assembly  10  and the desired coverage of substantially the entire width w O  or w I  of the bearing rings  12  or  14 , the capability of adjusting or establishing the desired natural frequency ω of each elastomeric damper  18  is primarily achieved by careful selection of the maximum thickness t D  of the particular damper  18 . As a rough estimate of the damper thickness t D  with intended gantry assemblies  1  requiring the outer and inner rings  12 ,  14  to be diametrically sized within the range of between about three and one-half feet (3.5 feet or 1.07 meters) and about five feet (5 feet or 1.52 meters), the radial thickness t D  of each of the one more elastomeric annular dampers  18  is preferably between about 0.020″ inches (0.5 mm) and about 0.32″ (6 mm). 
     As the damper thickness t D  is particularly critical to optimize vibration damping as described above, it has been determined that a desired radial thickness t D  of each annular damper  18  may be calculated for a desired bearing rotational speed N and applied loading F on the bearing as follows: 
     
       
         
           
             
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     Referring now to  FIGS.  4 ,  6 ,  8  and  9   , to optimize the bearing assembly  10  to handle the expected bending moments on the gantry assembly  1  as discussed above, the at least one elastomeric annular damper  18  preferably has a first section  30  and a second section  32  spaced axially apart from the first section  30  so as to form a double-section damper  18 . Specifically, the first section  30  is disposed adjacent to the first axial end  12   a  or  14   a  of the respective outer ring  12  or inner ring  14  and the second elastomeric damper section  32  is disposed adjacent to the second axial end  12   b  or  14   b  of the respective outer ring  12  or inner ring  14 . With this structure, the compression of the first elastomeric damper section  30  is greater than the compression of the second elastomeric damper section  32  when a bending moment M1 is applied to the bearing assembly in a first angular direction D1, as indicated in  FIG.  3   . Conversely, the compression of the second elastomeric damper section  32  is greater than the compression of the first elastomeric damper section  30  when a bending moment M2 is applied to the bearing assembly  10  in a second, opposing angular direction D2. Thus, the bending loads M1, M2 are primarily absorbed or supported by only one damper section  30  or  32 , respectively, as opposed to the entire damper  18  experiencing the full amount of compression, such that the fatigue life of each annular damper  18  formed with two sections  30 ,  32  is increased. 
     Preferably, the damper first section  30  and the damper second section  32  each have a radial thickness t D , preferably calculated as described above, and are connected by an intermediate damper section  34 . The intermediate damper section  34  has a radial thickness ti with a value that is less than the value of the radial thickness t D  of each one of the annular first and second sections  30 ,  32 . As such, the intermediate section  34  primarily functions to connect the first and second damper sections  30 ,  32  and does not support loading between the outer ring  12  and the housing  2  or between the inner ring  14  and the rotor  3 . Further, by providing the intermediate section  34  connecting the damper sections  30 ,  32 , the damper first and second sections  30 ,  32  are installable as a single unit while establishing a desired axial spacing between the two damper sections  30 ,  32 . Although the one or more elastomeric dampers  18  are preferably arranged in two sections  30 ,  32 , most preferably connected by an intermediate section  34 , each damper  18  may be formed as a single-section damper of constant radial thickness t D  as depicted in  FIGS.  3 ,  5  and  7   . 
     Representative, non-limiting examples of the present invention were described above in detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. 
     Moreover, combinations of features and steps disclosed in the above detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Furthermore, various features of the above-described representative examples, as well as the various independent and dependent claims below, may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings. 
     All features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter, independent of the compositions of the features in the embodiments and/or the claims. In addition, all value ranges or indications of groups of entities are intended to disclose every possible intermediate value or intermediate entity for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter. The invention is not restricted to the above-described embodiments, and may be varied within the scope of the following claims.