Abstract:
This set of roller bearings, which is a grounding bearing for a rotary machine, comprises a first-angular contact rolling bearing having a first outer race, a first inner race and first rolling elements, a second angular-contact rolling bearing having a second outer race, a second inner race and second rolling elements, a mount in which the first angular-contact rolling bearing and the second angular-contact rolling bearing are mounted. The first outer race and the second outer race are arranged in the mount either directly one beside the other or via the interposition of an intermediate member, the axial stiffness of which is greater than that of the mount and than those of the first and second outer races. Application for example to asynchronous motors.

Description:
FIELD OF THE INVENTION 
     Embodiments of the present invention relate to a set of roller bearing unit consisting of a bearing block for a rotating machine. 
     BACKGROUND OF THE INVENTION 
     The following types of bearing sets are known to the art. These sets are, for example, mounted within an asynchronous motor, the rotor of which is housed within the main bearings, which are magnetic. 
     The bearing sets form secondary bearings, also termed gear chassis, which support the shaft in the event the primary bearing malfunctions. 
     The bearing sets known to the state of the art comprise an element that mounts the two external bushings axially within the chassis, which exerts an axial preload on the external bushings, and which is relatively flexible. 
     Moreover, the two external bushings are arranged within the chassis so that there is an end-play between them. 
     A damping element is positioned immediately around the two exterior bushings. 
     The bearings are ceramic bearing elements. 
     The bearing set known to the art have the following drawbacks. 
     When the bearing set undergoes radial stress by the rotor shaft, the coaxial positioning of the two bearings is not ensured, due to the axial end-play between the two external bushings, which in turn is owing to the fact that the external bushings are in direct contact with the damping element, and because of the relatively weak axial preload. 
     The damping element cannot dampen the shaft&#39;s radial displacement without damping the radial displacement of the bearings with respect to the other one within the housing. The angular deviation of the shaft, or the displacement of the shaft around an axis perpendicular to its rotational axis, displaces the bearings with respect to one another. 
     Thus the concentric arrangement of the bushings is not ensured. 
     Since the bearings are angular contact bearings, radial stress is translated into a reactive axial force, which acts against the preload force. The preload, therefore, cannot be defined in this case. 
     Embodiments of the present invention intend to ameliorate these defects and proposes a bearing set that allows dependable functioning when used as a secondary bearing. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention propose a set of the type indicated, characterised in that the primary external bushing and the secondary external bushing are positioned within the housing, one immediately beside the other; with an intermediate element positioned between them, in particular one single intermediate element, the radial rigidity of which is greater than that of the chassis, and that of the primary and secondary bushings. 
     According to some embodiments, the set comprises one or several of the following characteristics: The primary external bushing is in contact with the secondary external bushing; The primary internal bushing is in contact with the secondary internal bushing; The primary and secondary external bushings each contain a bearing track, defined by one partial semi-cylindrical track and one semi-toroidal track; The set comprises means of axial tightening, which tighten the primary and secondary external bushings toward the chassis, with the axial stiffness of the means of tightening under an axial force applied by the secondary external bushing less than or equal to the stiffness of the two external bushings under this axial force; The means of axial tightening comprise means of fastening and a circular plate, and the axial stiffness of the means of fastening is lower than the axial stiffness of the two external bushings and the circular plate by a factor of 10 or more; A damper positioned around the chassis, which damper has a radial stiffness lower than that of the primary and secondary angular contact bearings; The damper is a corrugated sheet, of sheet metal, with the corrugations arranged circumferentially or axially; A housing, with the chassis positioned within the housing, where the chassis is in contact with the housing, and, if necessary, the damper is in contact with the chassis and the housing. 
     Embodiments of the present invention also propose an electric motor consisting of a rotor shaft, a primary bearing, namely a magnetic bearing, and a set of rolling bearings, characterised in that the set of bearings is as described above, where the bearing set forms a secondary bearing, with the secondary bearing supporting the rotor shaft when the primary bearing fails, and in that the rotor is mounted on the primary bearing making no contact with the secondary bearing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood upon reading the following description, provided solely as example, referring to the attached drawings, of which: 
         FIG. 1  is an axial cross-section of a motor according to an embodiment of the invention; 
         FIG. 2  is a larger-scaled view of detail II of  FIG. 1 , and 
         FIG. 3  is an axial cross-section along the line III-III in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an electric motor according to an embodiment of the present invention, designated by the general reference  2 . 
     The electric motor  2  is an example of an asynchronous motor. The motor  2  is an example of a motor with a nominal speed greater than 6,000 RPM. The nominal speed is more particularly greater than 8,000 RPM. 
     The electric motor  2  comprises a housing  4  or shell, a shaft  6 , a rotor  8 , and a stator  10 . 
     The electric motor  2  defines a main axis X-X, which is the rotational axis of shaft  6  and rotor  8 . In what follows, the expressions “axially”, “radially”, and “circumferentially” shall be used with respect to the main axis. The axes of the elements described below are deemed coaxial with the main axis, unless otherwise indicated. The motor  2  also defines a median plane M-M, extending perpendicularly to the main axis X-X. The median plane M-M is positioned at the centre of the axis of rotor  8 . 
     The shaft  6  and rotor  8 , taken together, have a mass that is greater than 500 kg, in particular greater than 750 kg. 
     The electric motor  2  also comprises two primary bearings  12  adapted to support shaft  6  whilst rotating, and thus also rotor  8 . In this case the primary bearings  12  are magnetic bearings, more particularly with an actively supported drive shaft with two guide pins. The two primary bearings  12  are positioned axially on either side of rotor  8 . 
     The electric motor  2  also comprises two sets of roller bearings  14 . 
     Each roller bearing set  14  is a secondary or bearing block adapted to support the shaft  6  when the associated primary bearing  12  fails. 
     In the case represented, the two roller bearing sets  14  are identical and are arranged within the housing  4 , symmetrically with respect to the medial plane M-M. 
     In a variant not represented here, the two roller bearing sets  14  are arranged symmetrically with respect to the medial plane M-M. Moreover, the roller bearings in a roller bearing set  14  may differ in size to the bearings in other roller bearing sets  14 . 
     In what follows, only one of the roller bearing sets  14  shall be described, making reference to  FIG. 2 . 
     The roller bearing set  14  consists of a primary angular contact bearing  16 , a secondary angular contact bearing  18 , a chassis  20 , a damper  22 , a portion  24  of housing  4 , and means of axial tightening  26 . 
     The two angular contact bearings  16  and  18  are mounted in an X-form. 
     The primary angular contact bearing  16  comprises a primary external bushing  30 , a primary internal bushing  32 , and primary rolling elements  34 . In this instance the primary rolling elements  34  are balls. The primary rolling elements  34  are made of metal, notably steel, for example. As a variant, the primary rolling elements  34  are made of ceramic. 
     The secondary angular contact bearing  18  comprises a second external bushing  36 , a second internal bushing  38 , and secondary rolling elements  40 . In this instance the primary rolling elements  40  are balls. The secondary rolling elements  40  are made of metal, notably steel, for example. As a variant, the secondary rolling elements  40  are made of ceramic. 
     In the present case, the primary external bushing  30  and the second external bushing  36  each comprise an external track  42  formed by a semi-cylindrical track  44  and a semi-toroidal track  46 . In profile, the semi-cylindrical track  44  is continuous with the profile of the semi-toroidal track  46 . Moreover, the two semi-cylindrical tracks  44  of the two external bushings  30  and  36  are adjacent. The two tracks  42  of the primary external bushing  30  and the secondary external bushing  36  are thus open axially on one side, so that lacking any external check, the external bushings  30  and  36  can be axially removed from the associated rolling elements. 
     The primary internal bushing  32  and the secondary internal bushing  38  each comprise an internal track which is exclusively in the form of a semi-toroid and which is axially closed on both sides. 
     The primary external bushing  30  and the secondary external bushing  36  are arranged immediately side by side in the chassis. In the present case, the primary external bushing  30  and the secondary external bushing  36  are in contact with one another. 
     More particularly, the primary angular contact bearing  16  and the secondary angular contact bearing  18  are identical. The only difference is their symmetrical positioning with respect to one another. 
     The chassis  20  comprises an opening  50  for receiving rolling elements. The chassis  20  is formed by a cylindrical wall  52  and a circular base wall  54 . Thus, the opening  50  is formed by the interior surface of the cylindrical wall  52  and by the interior surface of the base wall  54 , 
     The primary external bushing  30  is in contact with the base wall  54  and with the cylindrical wall  52 , whilst the secondary external bushing  36  is in contact with the cylindrical wall  52 , but not with the base wall  54 . 
     The primary internal bushing  32  is in direct contact with the secondary internal bushing  38 . 
     The primary angular contact bearing  16  and the secondary angular contact bearing  18  are mounted within the chassis  20 . The means of axial tightening  26  are adapted to tighten the primary and secondary external bushing  30  and  36  axially against the chassis  20 , more precisely toward the base wall  54 . 
     The axial stiffness of the means of tightening  26 , under an axial force applied by the secondary external bushing  36 , is equal to or less than the stiffness of the two external bushings  30  and  36  under this same force. In other words the means of axial tightening  26  have more axial deformation than the two external bushings  30  and  36 . 
     The axial stiffness of the means of tightening  26  is significantly lower than the stiffness of the two external bushings (by a factor of 10 or more). 
     The axial stiffness of the means of tightening is very significant. For example, the axial stiffness is greater than the axial stiffness of current systems that use a flexible strip. 
     Tightening serves to put the external bushing  30  and  36  in contact, which before tightening have an initial gap of at least 1/100 mm, and more particularly several hundredths of a mm. The initial gap is an item in the construction specifications of the bearing manufacturer. 
     The means of axial tightening  26  are, for example, made up of a circular plate  56  and means of fastening  58  that fasten the circular plate  56  to the chassis  20 . 
     The means of fastening  58  are, for example, by bolts. 
     The axial stiffness of the means of fastening  58  is lower than the axial stiffness of the set of external bushings  30  and  36 , and of the circular plate  56 . Thus, under axial strain applied by the external bushing  36  on the circular plate  56 , it is the means of fastening  58  that absorb the forces. Under such axial strain, the means of fastening  58  deform axially more than the circular plate  56 . 
     During thermal expansion of the rolling elements  16  and  18 , in particular, the means of fastening  58  grant a certain degree of axial play to bushings  30  and  36 . 
     In a variant not represented here, the means of fastening  58  are a weld, such that plate  56  is welded to chassis  20 . 
     In another variant not represented here, the circular plate  56  has an external diameter that corresponds to the interior diameter of cylindrical wall  52 , where circular plate  56  has a threading that aligns with a bore set in cylindrical wall  52 . 
     Moreover, one or more axial springs, such as a conical spring washer or helical springs, may be positioned between circular plate  56  and external bushing  36 . 
     In this case, each axial spring may be housed within an axial cavity that is set in the circular plate  56 , and which is open to the external bushing  36 . 
     When the means of fastening  58 , in its tightened state, pushes the circular plate  56  toward or against the external bushing  56 , an axial clearance j 1  exists between the circular plate  56  and the chassis  20 . This axial clearance j 1  is not a null value. 
     The damper  22  is positioned around chassis  20 . The damper  22  has a radial stiffness that is less than the radial stiffness of the angular contact bearings  16  and  18 . In this instance the damper  22  is a corrugated sheet. The corrugations extend circumferentially, as is visible in  FIG. 3 . 
     In a variant not represented, the corrugations extend axially. 
     The corrugated sheet is manufactured from sheet metal, for example. 
     The damper  22  is in direct contact with the chassis  20  and with the housing  4 . 
     Alternatively, the damper  22 , is omitted and the chassis  20  is in direct contact with housing  4 . 
     In a variant not represented here, the primary external bushing  30  and the secondary external bushing  36  are arranged immediately side by side in the chassis  20 . In this case, the primary external bushing  30  and the secondary external bushing  36  are positioned within the chassis  20  by interposition of an intermediate element, for example an intermediate ring. The intermediate element is in contact with the external bushings  30  and  36 . The intermediate element has an axial stiffness that is greater than that of the means of fastening  58  and greater than that of the chassis, particularly that of the base wall  54 . The axial rigidity of this intermediate element is also greater than that of the primary and secondary external bushings  30  and  36 . 
     The electric motor  2  works in the following manner. 
     When shaft  6  is mounted on the main bearings, and when the main bearings  12  support shaft  6 , shaft  6  and the primary and secondary internal bushings  32  and  38  define an end-play j 2 . Shaft  6  does not contact the bearing sets  14 , and thus does not contact the internal bushings  32  and  38 . 
     When one or both main bearings  12  fail, particularly whilst the rotor  8  is turning, the shaft  6  comes into contact with the internal bushings  32  and  38  of the associated bearing set. The rotor  8  and shaft  6  are thenceforth borne by the bearing elements  16  and  18 . 
     Thanks to the side-by-side positioning of the primary external bushing  30  and secondary external bushing  36 , the distribution of axial and radial forces applied to the rolling elements is lessened when they come into contact with shaft  6 . Moreover, the fact that the angular contact bearings  16  and  18  are mounted directly on the chassis ensures that axial alignment of the two rolling elements  16  and  18  is maintained. The angular deviation of chassis  20  around an axis perpendicular to the X-X axis produces a defined angular deviation of the two rolling elements  16  and  18 . 
     Moreover, positioning the damper  22  around the chassis  20  allows for easy assembly of the two rolling elements on the chassis. 
     Direct application of the primary external bushing  30  and of the secondary bushing  36  on the wall  52  greatly facilitates assembly. 
     Positioning of the damper  22  around the chassis  20  also contributes to easy assembly, and guarantees that the primary angular contact bearing  16  and the secondary angular contact bearing  18  remain coaxial when contact is made with shaft  6 . 
     Embodiments of the present invention can also generally apply to rotating machines, for example rotating electrical machines such as generators or motors. One variant of a rotating machine could be a compressor. 
     This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.