Abstract:
A flywheel for kinetic energy storage and its construction using composite materials. The present invention provides a flywheel assembly having a longitudinal axis and comprising an annular rotor and a rotor support for coupling the rotor to an axial shaft, wherein the rotor comprises fibres in a matrix material, and a ring comprising fibres in a matrix material is mounted on the outer circumference of the rotor support and the rotor is mounted on the outer circumference of the ring, the rotor, rotor support and ring each having longitudinal axes which are coincident with the longitudinal axis of the assembly. The presence of the intermediate ring formed of a composite material assists in the fabrication of the assembly and increases its durability by providing a suitable interface between the rotor and a rotor support.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a submission under 35 U.S.C. §371 of International Application No. PCT/GB2014/052204, filed Jul. 18, 2014, which claims priority to Great Britain Application No. 1312927.5, filed Jul. 19, 2013, the disclosures of which are hereby expressly incorporated by reference herein in their entireties. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to flywheels used for kinetic energy storage and more particularly the construction of such flywheels using composite materials. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is well known to use flywheels to store mechanical kinetic energy. The amount of energy stored depends on the mass of the flywheel and its speed of rotation. The kinetic energy of the flywheel is proportional to the square of its angular velocity. However, the amount of energy that may be stored in a given flywheel is dependent upon the robustness of the flywheel assembly and how it responds to the stresses experienced at high rotational speeds. 
         [0004]    It is preferable to form flywheel assemblies from materials which are of low density and high specific strength. For use in, for example, hybrid vehicles or uninterruptable power supplies, energy storage flywheel assemblies need to operate at extremely high speeds, greater than 10,000 rev./min. or even more than 50,000 or 100,000 rev./min. Accordingly, there is considerable demand for flywheel assemblies that are able to operate at speeds of these orders. 
         [0005]    U.S. Pat. No. 4,341,001 discloses a flywheel assembly comprising a metallic hub. The composite fibre rim is wound directly onto the hub. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides a flywheel assembly having a longitudinal axis and comprising an annular rotor and a rotor support for coupling the rotor to an axial shaft, wherein: 
         [0007]    the rotor comprises fibres in a matrix material, and 
         [0008]    a ring comprising fibres in a matrix material is mounted on the outer circumference of the rotor support and the rotor is mounted on the outer circumference of the ring, 
         [0009]    the rotor, rotor support and ring each having longitudinal axes which are coincident with the longitudinal axis of the assembly. 
         [0010]    The presence of the intermediate ring formed of a composite material assists the fabrication of the assembly and increases its durability by providing a suitable interface between the rotor and the rotor support. The fibres of the ring may comprise at least one of glass fibres, basalt rock fibres and low specific modulus carbon fibres. 
         [0011]    Preferably, the ring has a greater elasticity than the rotor in the circumferential direction. This allows it to be press fitted into the inner diameter of the rotor. Furthermore, it may also have a greater elasticity than the rotor support in the circumferential direction, so that it can be press fitted onto the outer circumference of the rotor support. 
         [0012]    In preferred embodiments, the rotor support defines a substantially frusto-conical inner surface, which is rotationally symmetrical about its longitudinal axis. The angle defined by the frusto-conical inner surface with respect to a plane perpendicular to its longitudinal axis is selected so as to substantially strain match the rotor support with the ring. This ensures the integrity of the rotor support and ring assembly when subjected to centrifugal forces. 
         [0013]    The outer circumferential surface of the ring may have a chamfer to assist fitting of the ring into the rotor. The chamfer may define a lead in angle of no greater than around 15° with respect to the longitudinal axis of the ring. 
         [0014]    The rotor support and the ring may be configured such that an imaginary surface defined by outwardly projecting the frusto-conical inner surface of the rotor support extends through the ring from its inner circumferential surface to meet an outer cylindrical surface of the ring. As a result, forces exerted on the ring by the rotor support during rotation of the assembly are primarily directed through the outer cylindrical surface of the ring, avoiding any chamfers at its edges, for example, to optimize the load-bearing capability of the ring. 
         [0015]    The outer circumferential surface of the rotor support and/or the inner circumferential surface of the ring may have a chamfer to assist fitting of the rotor support into the ring. Each chamfer may define a lead in angle of no greater than around 15° with respect to the longitudinal axis of the rotor support. This serves to avoid shearing of the corners of the ring as the end cap is press fitted into it. 
         [0016]    The assembly may also include an inner annulus mounted on an inwardly facing surface of the rotor. The inner annulus may include particles of magnetic material to enable the assembly to be magnetically coupled to a stator. 
         [0017]    The outer circumferential surface of the inner annulus and/or the inner circumferential surface of the rotor may have a chamfer to assist fitting of the annulus into the rotor. The chamfer may define a lead in angle of no greater than around 15° with respect to the longitudinal axis of the rotor. 
         [0018]    A volume defined by the inner circumferential, cylindrical surface of the rotor, the chamfer of the rotor support and the chamfer of the inner annulus may be substantially filled with a bonding material. Otherwise, the presence of a void at this location will reduce the ability of the assembly to withstand high centrifugal forces. The bonding material may be a resin material, for example. 
         [0019]    The edge at each end of the inner circumferential surface of the inner annulus may have a chamfer which defines an angle in the range 20 to 60° (or more preferably in the range 50 to 55° with respect to that surface, to avoid cracking of these edges at high rotational speeds. 
         [0020]    The present invention also provides a method of manufacturing a flywheel assembly, the assembly comprising an annular rotor, a rotor support for coupling the rotor to a support shaft, and a ring, wherein the rotor and the ring comprise fibres in a matrix material, the ring is mounted on the outer circumference of the rotor support and the rotor is mounted on the outer circumference of the ring, the method comprising press fitting the rotor support into the ring. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Embodiments of the present invention will now be described by way of example and with reference to the accompanying schematic drawings, wherein: 
           [0022]      FIG. 1  is a cross-sectional side view of a flywheel assembly according to an embodiment of the present invention; 
           [0023]      FIG. 2  is an exploded view of the flywheel assembly of  FIG. 1 ; 
           [0024]      FIG. 3  is an enlarged cross-sectional view of a portion of the assembly of  FIGS. 1 and 2 , together with a further enlarged portion thereof; and 
           [0025]      FIG. 4  is a cross-sectional view of part of the flywheel assembly of  FIG. 1 , with preferred strength values added. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]      FIG. 1  illustrates a flywheel assembly  70  according to an embodiment of the present invention. It includes a rotor assembly having an annular outer rotor portion  26  and an inner annular rotor portion or annulus  66 . The outer rotor  26  is formed of a composite material comprising unidirectional filaments of carbon fibres in a matrix of a resin material. The inner annulus  66  is formed of unidirectional filaments of glass fibre in a matrix formed of a mixture of resin and magnetic particles. 
         [0027]    The rotor assembly is carried by a rotor support in the form of a hub or end cap  74 . A ring  84  is provided between the end cap and rotor assembly in the radial direction, with the interfaces between the ring and the end cap and rotor assembly, respectively, overlapping in the axial direction. Preferably, the interface between the ring and the rotor entirely overlaps the interface between the ring and the end cap, in the axial direction. The end cap is generally conical in shape. Its outer circumference carries the ring  84  which is in turn attached to the outer rotor  26 , on its inner cylindrical surface. The end cap extends outwardly in an axial direction from the rotor assembly and has a central circular opening  68 . 
         [0028]    A central shaft  76  extends through the opening  68  in the end cap. The shaft includes a circumferentially extending flange  88 . The end cap is retained against the flange  88  by a clamping nut  86  which is threaded onto the shaft  76 . An anti-fretting shim  64  is provided between the nut  86  and the end cap  74 . The shaft is supported for rotation about a longitudinal axis  81  of the flywheel assembly by a pair of bearings  90  and  92 . The bearings are in turn supported by a containment (not shown) for the flywheel assembly. The rotor assembly is rotatable about an electrical motor generator stator  94  carried by the containment. 
         [0029]    A flywheel to be used to store energy in a vehicle may have an outer diameter of around 350 mm, with the inner diameter of the outer rotor being around 290 mm and the inner diameter of the inner annulus around 250 mm, for example. 
         [0030]    The fabrication of the flywheel assembly shown in  FIG. 1  will now be described with reference to the exploded view of  FIG. 2 . According to the present invention, the end cap  74 , the ring  84 , the outer rotor  26  and the inner annulus  66  are press fitted together. The components to be press fitted together are heated to around 60-70° C. prior to the press fitting process. Each of these press fitted components is formed of a composite material. The overlaps in the radial direction between press fitted surfaces to achieve the desired interference fits maybe somewhat larger than those typically used with metallic components. For example, the difference in the diameters of the inner and outer surfaces may be of the order of 0.25 mm to 0.85 mm. In addition the components are bonded together to further increase the strength of the assembly. 
         [0031]    Preferably a resin coating is applied to the components to act as a lubricant during assembly which is then cured to produce a bond between the components. 
         [0032]    A resin, for example, Hysol® EA 9394 as manufactured by Henkel Corporation, may be used as the lubricant and bonding agent. 
         [0033]    During the fabrication process, the shaft  76  is inserted through the central opening  68  of the end cap. The end cap is secured in place on the shaft by the clamping nut  86 . The outer circumference of the end cap is then machined as it is rotated about the longitudinal axis  81  of the assembly and shaft. The end cap  74  is precisely machined to the desired size. The glass fibre hoop wound ring  84  is then press fitted over the outer cylindrical surface of the end cap  74 . This process is assisted by the bonding agent acting as a lubricant and provision of  15  degree lead in angles in the form of chamfers on the outer circumferential surface of the end cap and the inner circumferential surface of the ring. In this way, the two components can be fitted together without damage. 
         [0034]    The end cap and glass ring combination is then pressed into the outer carbon hoop wound rotor  26 . Again, this is assisted by the lubrication of a bonding agent and 15 degree lead in angles on the outer circumferential surface of the ring  84  and the inner cylindrical surface of the rotor  26 . Then, the inner hoop wound magnetic composite annulus  66  is pressed into the carbon outer rotor  26 , once again employing the bonding agent as a lubricant and assisted by 15 degree lead in angles on the outer circumferential surface of the annulus and the inner cylindrical surface of the rotor. 
         [0035]    After press fitting, the components are heated to cure the bonding agent. 
         [0036]    Chamfers are defined at each end of the inner cylindrical surface of the inner annulus  66 . The chamfers preferably define angles with respect to the longitudinal axis  81  of at least 20 degrees, more preferably 20-60 degrees, and more preferably still 50-55 degrees, to avoid cracking of the edge at high rotational speeds. 
         [0037]    An enlarged cross-sectional view of the rotor assembly shown in  FIGS. 1 and 2  is illustrated in  FIG. 3 . Dashed line  160  is an imaginary line projecting the inner conical surface of the end cap  74  in an outward direction through the ring  84  and rotor  26 .  FIG. 3  also includes a further enlarged view of the location where this line intersects with the interface between the ring and the rotor. As discussed above, chamfers (labeled  100  and  102 , respectively in  FIG. 3 ) are formed on the outer circumferential edges of the ring  84  and annulus  66  to assist the assembly process. As a result, a volume  98  is defined between the outer rotor  26 , the ring  84  and the annulus  66 . To ensure that the outer edge of the end cap is fully supported by the ring  84 , the assembly should be configured such that the projected line  160  intersects with the interface between the ring  84  and the outer rotor  26  at a location outside the volume  98 . Furthermore, it is desirable to space this intersection from the volume  98 , and the greater the spacing the better, subject to other configuration requirements. Preferably, the volume is filled with a bonding agent during the assembly process to ensure that the full widths of the ring  84  and annulus  66  are supported by the outer rotor  26 . 
         [0038]    The rotor support or end cap  74  has a conical configuration such that it is strain matched with the surrounding ring  84 . Its elastic deformation during rotation therefore substantially matches that of the ring to avoid separation during rotation at high speeds. The angle defined by the body of the rotor support relative to a plane perpendicular to its axis of rotation is preferably in the range 17-26 degrees, and more preferably around 20 degrees. 
         [0039]    The ring  84  acts as a load-spreading intermediate member. It is tough enough to support the outer edge of the end cap. This avoids the end cap digging into or significantly fretting the surface of the outer rotor  26  and forming stress initiation points. A glass fibre-based composite may be used for the ring, for example. 
         [0040]    During rotation, the end cap grows in diameter due to centrifugal force. In the absence of ring  84 , the tough outer edge of the end cap would be forced into the relatively softer outer rotor, causing areas of high stress. This could lead to break up of the rotor at lower rotational speeds than would otherwise be the case. 
         [0041]    The ring is preferably formed of a material having a greater circumferential (hoop) elasticity than the material of both the end cap and the outer rotor. This allows the ring to be press fitted (expanded) onto the end cap and then press fitted into the carbon rotor  26 . The ring preferably has a higher radial compressive strength than the outer rotor and a lower radial compressive strength than the end cap. 
         [0042]      FIG. 4  includes indication of strength values for the rotor  26 , annulus  66 , ring  84  and end cap  76  according to one embodiment of the present invention. 
         [0043]    As shown in  FIG. 4 , the hoop tensile strength of the outer rotor  26  in this embodiment is relatively high, around 180 to 195 MPa so that it can withstand the loads experienced during rotation at high speed. In contrast, the hoop tensile strength of the ring  84  is relatively low (around 50 to 60 MPa), and is similar to the hoop strength of the end cap (around 45 to 55 MPa). 
         [0044]    The elasticity of the ring  84  in the circumferential direction is greater than that of both the outer rotor and the end cap. This facilitates the press fitting of the ring onto the end cap, and in turn the press fitting of the ring and end cap combination into the rotor. 
         [0045]    The radial (and axial) transverse compressive strength of the ring (14 to 16 MPa) is greater than that of the rotor (6.5 to 7.5 MPa), but less than the tensile strength of the end cap (60 to 70 MPa) in the radial direction. The ring is therefore tougher than the rotor in the radial direction, and prevents the much harder end cap perimeter digging into and damaging the relatively soft rotor during assembly. 
         [0046]    The transverse compressive strengths of the rotor and inner annulus in the axial direction are relatively low (6.5 to 7.5 MPa and 13 to 15 MPa, respectively). The hoop tensile strength of the inner annulus is lower than that of the outer rotor, at around 33 to 35 MPa. 
         [0047]    While the present invention has been illustrated by description of various embodiments and while those embodiments have been described in considerable detail, it is not the intention of Applicants to restrict or in any way limit the scope of the appended claims to such details. Additional advantages and modifications will readily appear to those skilled in the art. The present invention in its broader aspects is therefore not limited to the specific details and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of Applicants&#39; invention.