Patent Publication Number: US-2010119188-A1

Title: Bearings

Description:
This invention relates to bearings and particularly, although not exclusively, relates to roller bearings. 
     Roller bearings are widely used on crankshaft/connecting rod mechanisms. By way of example, reference is made to  FIG. 1  which illustrates use of a conventional roller bearing  10  in an internal combustion engine of a motorcycle, lawn mower or the like. The bearing is located at the junction of a hole  21 , at one end of a connecting rod  20 , and a crank pin  40 . The bearing  10  is positioned eccentrically to a crankshaft  30  which consists of two circular disks. The crankshaft  30  has an axle  31  on each of two sides. The roller bearing  10  has rolling elements  12  and cage  11 . The hole  21  has an inner surface serving as an outer raceway  22  for the roller bearing  10 . The crank pin  40  serves as an inner raceway for the roller bearing  10 . 
     Referring to  FIGS. 2 to 5 , the roller bearings  10  include an annular cage  11  made of metal and rolling elements  12 . Commonly used cages  11  may be in the forms of a one piece cage  11  with recesses  14  in outer surface  13  (see  FIG. 2 ); or a one piece type cage  11  with a flat outer surface  13  (see  FIG. 3 ); or a split type cage  11  (consisting of two semi-circular parts) with recesses in outer surface  13  (see  FIG. 4 ); or a split type cage  11  with a flat outer surface  13  (see  FIG. 5 ). 
     As the roller bearing  10  for the crankshaft/connecting rod mechanism is located at the junction of connecting rod  20  and the crank pin  40 , when the crankshaft  30  rotates, the roller bearing  10  is driven to perform two types of motion: first, spinning about the crank pin  40  as the centre; second, moving in association with crank pin  40  to rotate about the axle  31  of the crankshaft  30 . 
     As the roller bearing  10  is eccentric relative to the axle  31  of the crankshaft  30 , the second motion generates a centrifugal force on the roller bearing  10 . As a result, the cage  11  is constantly rubbing against the inner surface of the hole  21  that also serves as the outer raceway  22  of the bearing. The rubbing will cause wear on the outer raceway  22 . To reduce the wear, the conventional method is to electroplate a copper layer or a silver layer on a copper layer on the outer surface  13  of the cage  11 . Such an approach allows the softer copper/silver layer to be consumed to defer the wear on the outer raceway  22 . 
     However, the process of electroplating a copper layer or a silver layer on the copper layer is expensive. Moreover electroplating processes may generate hydrogen embrittlement (HE) problems which may cause fracture of cage  11  while the bearing is running. 
     It is an object of the present invention to address the abovedescribed problems. 
     According to a first aspect of the invention, there is provided a bearing structure for supporting movable elements of a bearing, the structure having an outer surface which is defined at least in part by a composition which comprises a first polymeric material having a Rockwell hardness (M scale) in the range 80 to 130. 
     Said composition may have a Rockwell hardness (M scale) in the range 80 to 130. 
     Said first polymeric material (and optionally said composition) may have a Rockwell hardness (M scale) of less than 120, preferably less than 110, more preferably less than 105. Suitably, said hardness is in the range 90-105. 
     Said first polymeric material may be selected from a polyamide, polyetherimide (PEI), polyacetal, polyester (e.g. polyethylene terephthalate or polybutylene terephthalate), polycarbonate, polyphenylene sulphide or a polymeric material of a type which includes: 
     (a) phenyl moieties; 
     (b) ketone and/or sulphone moieties; and 
     (c) ether moieties. 
     Said first polymeric material may have a Tg of less than 260° C., for example less than 220° C. or less than 200° C. In some cases, the Tg may be less than 190° C., 180° C. or 170° C. 
     Said first polymeric material suitably has a melt viscosity (MV) of at least 0.06 kNsm −2 , preferably has a MV of at least 0.08 kNsm −2 , more preferably at least 0.09 kNsm −2 . 
     MV is suitably measured using capillary rheometry operating at 400° C. at a shear rate of 1000s −1  using a tungsten carbide die, 0.5×3.175 mm. 
     Said first polymeric material may have a MV of less than 1.00 kNsm −2 , preferably less than 0.5 kNsm −2 , more preferably less than 0.38 kNsm −2 , especially less than 0.25 kNsm −2 . 
     Said first polymeric material may have a tensile strength, measured in accordance with ASTM D790 of at least 40 MPa, preferably at least 60 MPa, more preferably at least 80 MPa. The tensile strength is preferably in the range 80-110 MPa, more preferably in the range 80-100 MPa. 
     Said first polymeric material may have a flexural strength, measured in accordance with ASTM D790 of at least 145 MPa. The flexural strength is preferably in the range 145-180 MPa, more preferably in the range 145-165 MPa. 
     Said first polymeric material may have a flexural modulus, measured in accordance with ASTM D790, of at least 2 GPa, preferably at least 3 GPa, more preferably at least 3.5 GPa. The flexural modulus is preferably in the range 3.5-4.5 GPa, more preferably in the range 3.5-4.1 GPa. 
     Said first polymeric material may have a tensile strength, measured in accordance with ASTM D790 of at least 20 MPa, preferably at least 60 MPa, more preferably at least 80 MPa. The tensile strength is preferably in the range 80-110 MPa, more preferably in the range 80-100 MPa. 
     Said first polymeric material may have a flexural strength, measured in accordance with ASTM D790 of at least 50 MPa, preferably at least 100 MPa, more preferably at least 145 MPa. The flexural strength is preferably in the range 145-180 MPa, more preferably in the range 145-164 MPa. 
     Said first polymeric material may have a flexural modulus, measured in accordance with ASTM D790, of at least 1 GPa, suitably at least 2 GPa, preferably at least 3 GPa, more preferably at least 3.5 GPa. The flexural modulus is preferably in the range 3.5-4.5 GPa, more preferably in the range 3.5-4.1 GPa. 
     Said first polymeric material may be a homopolymer having a repeat unit of general formula 
     
       
         
         
             
             
         
       
     
     or a homopolymer having a repeat unit of general formula 
     
       
         
         
             
             
         
       
     
     or a random or block copolymer comprising a unit of IV or V (preferably comprising at least two different units of IV and/or V) 
     whexein B represents 0 or 1, D represents 0 or 1, E′ represents an oxygen atom or a direct link, m represents 0 or 1, w, r, s, z, t and v independently represent 0 or 1 and Ar is selected from one or more of the following moieties which is bonded via one or more of its phenyl moieties to adjacent moieties 
     
       
         
         
             
             
         
       
     
     Unless otherwise stated in this specification, a phenyl moiety suitably has 1,4-, linkages to moieties to which it is bonded. 
     In (i), the middle phenyl may be 1,4- or 1,3-substituted. It is preferably 1,4-substituted. 
     Preferably Ar is selected from one or more of the following moieties which is bonded via one or more of the bonds extending from the phenyl, moieties to adjacent moieties. 
     
       
         
         
             
             
         
       
     
     In (vii), the middle phenyl may be 1,4- or 1,3-substituted. It is preferably 1,4-substituted. 
     Preferred first polymeric materials may be homopolymers or copolymers (preferably homopolymers) of formula IV or V having substituents as detailed in the table below. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                   
               
               
                 Formula 
                 Ar 
                 m 
                 E′ 
                 w 
                 r 
                 B 
                 s 
                 z 
                 t 
                 v 
                 D 
               
               
                   
               
             
            
               
                 IV 
                 (iv) 
                 0 
                 oxygen 
                 1 
                   
                 1 
                 0 
                   
                   
                   
                   
               
               
                 IV 
                 (ii) 
                 0 
                 direct 
                   
                   
                 0 
               
               
                   
                   
                   
                 link 
               
               
                 IV 
                 (ii) 
                 0 
                 oxygen 
                 1 
                 0 
                 1 
                 1 
               
               
                 IV 
                 (i) 
                 0 
                 direct 
                 0 
                 0 
                 0 
                 0 
               
               
                   
                   
                   
                 link 
               
               
                 IV 
                 (iv) 
                 0 
                 oxygen 
                 0 
                 1 
                 1 
                 1 
               
               
                 IV 
                 (iv) 
                 1 
                 oxygen 
                 1 
                 0 
                 1 
                 0 
               
               
                 IV 
                 (iv) 
                 1 
                 oxygen 
                 1 
                 0 
                 1 
                 1 
               
               
                 V 
                 (i)** 
                 0 
                 oxygen 
                   
                   
                   
                   
                 1 
                 0 
                 0 
                 1 
               
               
                 V 
                 (iv) 
                 0 
                 oxygen 
                   
                   
                   
                   
                 1 
                 — 
                 0 
                 1 
               
               
                 V 
                 (ii) 
                 0 
                 Direct 
                   
                   
                   
                   
                   
                   
                   
                 0 
               
               
                   
                   
                   
                 link 
               
               
                 V 
                 (iv) 
                 1 
                 oxygen 
                   
                   
                   
                   
                 1 
                 0 
                 0 
                 1 
               
               
                 V 
                 (iv) 
                 1 
                 oxygen 
                   
                   
                   
                   
                 1 
                 1 
                 1 
                 1 
               
               
                   
               
            
           
         
       
     
     Said first polymeric material may be amorphous or semi-crystalline. 
     Said first polymeric material is preferably semi-crystalline. The level and extent of crystallinity in a polymer is preferably measured by wide angle X-ray diffraction (also referred to as Wide Angle X-ray Scattering or WAXS), for example as described by Blundell and Osborn (Polymer 24, 953, 1983). Alternatively, crystallinity may be assessed by Differential Scanning Calerimetry (DSC). 
     The level of crystallinity in said first polymeric material may be at least 1%, suitably at least 3%, preferably at least 5% and more preferably at least 10%. 
     In especially preferred embodiments, the crystallinity may be greater than 30%, more preferably greater than 40%, especially greater than 45%. 
     The main peak of the melting endotherm (Tm) for said first polymeric material (if crystalline) may be at least 300° C. 
     Said first polymeric material preferably comprises, more preferably consists essentially of, a repeat unit of formula (XX) 
     
       
         
         
             
             
         
       
     
     where t1, and w1 independently represent 0 or 1 and v1 represents 0, 1 or 2. Preferred polymeric materials have a said repeat unit wherein t1=1, v1=0 and w1=0; t1=0, v1=0 and w1=0; t1=0, w1=1, v1=2; or t1=0, v1=1 and w1=0. More preferred have t1=1, v1=0 and w1=0; or t1=0, v1=0 and w1=0. The most preferred has t1=1, v1=0 and w1=0. 
     In preferred embodiments, said first polymeric material is selected from polyetheretherketone, polyetherketone, polyetherketoneetherketoneketone, polyetheretherketoneketone and polyetherketoneketone. In a more preferred embodiment, said polymeric material is selected from polyetherketone and polyetheretherketone. In an especially preferred embodiment, said polymeric material is polyetheretherketone. 
     Said first polymeric material may make up at least 60 wt %, suitably at least 70 wt %, preferably at least 80 wt %, more preferably at least 90 wt %, especially at least 95 wt %, of the total amount of thermoplastic polymer(s) in said composition. Preferably, substantially the only thermoplastic polymer in said composition is said first polymeric material. 
     Suitably, said composition includes at least 80 wt %, preferably at least 90 wt %, more preferably at least 95 wt %, especially at least 99 wt % of said first polymeric material, especially preferred being a polymer of formula (XX) referred to above. 
     Said first polymeric material is preferably of a type which includes: 
     (a) phenyl moieties; 
     (b) ketone and/or sulphone moieties; and 
     (c) ether moieties. 
     Said bearing structure preferably includes a substrate provided with a layer defined by said composition. Said substrate preferably includes a material which is different to said composition of said layer. Said substrate may comprise (e.g. at least 70 wt %, 80 wt %, or 90 wt % comprises) for example consists essentially of, a metal, for example steel. 
     For example, said layer may comprise a coating layer on the substrate, wherein said coating layer suitably consists essentially of said composition described. 
     A layer defined by said composition may have a coat weight of at least 1.0 mg/cm 2 , preferably at least 1.2 mg/cm 2 . The coat weight may be less than 20 mg/cm 2 . 
     The layer may include at least  1 . 0  mg, preferably at least 1.2 mg of said first polymeric material per cm 2  of said layer. The layer may include less than 20 mg of said first polymeric material per cm 2  of said layer. 
     Said layer may have a minimum thickness (measured across its whole extent) of 1 μm, suitably of at least 5 μm, preferably of at least 8 μm. 
     Said layer may have a maximum thickness (measured across its whole extent) of less than 150 μm, preferably less than 100 μm, more preferably less than 50 μm, especially less than 25 μm. 
     Said layer may have an average thickness measured over its whole extent in the range 10 to 150 μm. 
     Said minimum/maximum thicknesses may be measured using a Sheen Eco Test Plus BFN Coating Thickness Gauge Type 121-17-00 from Sheen Instruments Ltd. 
     Said layer may have a total weight of at least 1 mg, preferably at least 5 mg, more preferably at least 10 mg. 
     Said layer may include at least 1 mg, preferably at least 10 mg, especially at least 1 g of said first polymeric material. 
     The bearing structure may include at least 10 mg, preferably at least 500 mg, especially at least 1 g of said first polymeric material. 
     Said composition which defines said outer surface may include greater than 50 wt %, greater than 75 wt %, greater than 90 wt %, greater than 95 wt %, greater than 98 wt % of said first polymeric material. 
     Properties of said outer surface may be adjusted by including materials in said composition in addition to said first polymeric material. 
     Said composition may include a second polymeric material. A said second polymeric material may be selected from a fluorocarbon resin or a polymeric material as described above comprising: 
     (a) phenyl moieties; 
     (b) ketone and/or sulphone moieties; and 
     (c) ether moieties. 
     When said second polymeric material comprises a fluorocarbon resin, said resin may comprise perfluoroalkoxy tetrafluoroethylene (PFA), polytetrafluoroethylene (PTFE) or tetrafluoroethylene-hexafluoropropylene copolymer resin (FEP). 
     A perfluoroalkoxy polymer (herein referred to as PFA) may be represented by the following general formula: 
     
       
         
         
             
             
         
       
     
     wherein Rf is a fluoroalkyl group (so —O-Rf is a perfluoroalkoxy group). 
     A tetrafluoroethylene/hexafluoropropylene copolymer (herein referred to as FEP) may be represented by the following general formula: 
     
       
         
         
             
             
         
       
     
     Among commercially available FEPs, those copolymers having a hexafluoropropylene content of from 18 to 25% by weight are preferably used. 
     Said composition may include a filler means. Said filler means may be particulate and/or may comprise nano fibres, nano tubes or other nano fillers. Examples of fillers include glass particles, ceramic particles, metallic particles, mineral particles and pigments. Specific examples include glass beads or microspheres, silica particles, carbon black, titanium dioxide particles, barium titanate particles, molybdenum disulphide and mica. 
     When said composition does not consist essentially of a said first polymeric material, it preferably includes a said fluorocarbon resin (especially PTFE), graphite and/or molybdenum disulphide. Said composition may also include a pigment for example carbon black. 
     The sum of the amount of said second polymeric material and filler means in said composition may be in the range 0 to 50 wt %, preferably 0 to 25 wt %, more preferably 0 to 10 wt %, especially 0 to 5%. 
     Said composition preferably includes 60-100 wt % of said first polymeric material (especially polyetheretheketone), 0-40 wt % of a second polymeric material as described and 0-10 wt % of filler means as described. Said composition preferably includes at least 70 wt % of said first polymeric material (especially polyetheretheketone) and the sum of the amount of second polymer material and filler means is in the range 0-30 wt %. Preferably, said second polymeric material and filler means are selected from fluoropolymers (especially PTFE), graphite and molybdenum disulphide. 
     In an especially preferably embodiment, said composition includes at least 90 wt % polyetheretherketone. The composition may consist essentially of polyetheretheketone and pigment (e.g. carbon black) at a level to provide the composition in a desired colour. 
     Said bearing structure is preferably arranged to engage a first part for example to fit within an opening (e.g. a circular opening) in said first part, (such as within an inner raceway such as of a connection rod). The first part is preferably arranged to be mounted for rotational movement relative to the structure. Said structure is preferably arranged to engage a second part for example by the second part (which may have a circular cross-section) fitting within an opening within said bearing structure. The second part may be arranged to be mounted for rotational movement relative to the structure and may comprise a pin such as a crank pin. Suitably, said first and second parts are arranged to move independently of one another in use when mounted as described. 
     Said outer surface of said structure preferably faces outwardly. It preferably comprises a surface which may contact (and therefore cause and/or be susceptible to wear as a result thereof) another part for example said first part described above (when provided). 
     Said structure is preferably substantially annular. Said outer surface preferably faces radially outwardly. Said outer surface preferably extends around (preferably substantially 360° around) a rotational axis of the structure. 
     Said outer surface may be arranged to accommodate movable elements of a bearing. For example, it may be arranged to accommodate movable elements which are rotatable. Such movable elements may have axes of rotation which extend substantially parallel to a rotational axis of the structure around which said outer surface extends. Said outer surface may include elongate slots for accommodating moveable elements as aforesaid. 
     The outer surface may include regions (suitably which extend around the rotational axis of the structure) at respective ends of said elongate slots. The regions may define lands at the ends of said slots. Said regions may contact (and therefore cause and/or be susceptible to wear as a result thereof) another part for example said first part as described above (when provided). The outer surface of said regions is preferably defined by (and more preferably comprises a coating of) said composition. 
     In one embodiment (represented in  FIGS. 3 and 4  hereinafter) said regions at respective ends may be part of a smooth outer surface which is only interrupted by slots arranged to accommodate movable elements. In another embodiment (represented in  FIGS. 2 and 4  hereinafter) said regions at respective ends may be spaced apart from depressions in the outer surface. 
     In a preferred embodiment substantially the entirety of the outer surface of the bearing structure is defined by (for example provided with a coating of) said composition. 
     Said bearing structure may include a radially outwardly facing outer surface as described, an inner surface which suitably faces in an opposite direction to said outer surface; and respective side surfaces which suitably face in a direction which is transverse to said outer surface for example in the direction of a rotational axis of the structure. Preferably, said radially outwardly facing outer surface is provided with a coating of said composition, preferably so that substantially the entirety of the outer surface is defined by said composition. Preferably, in addition, said inner surface is provided with a coating of said composition, preferably so that substantially the entirety of the inner surface is defined by said composition. Preferably, said respective side surfaces are provided with a coating of said composition, preferably so that substantially the entirety of the side surfaces are defined by said composition. Preferably, substantially the entirety of said bearing structure is defined, for example provided with a coating of, said composition. 
     In one embodiment, said bearing structure may comprise an endless annular structure (as in  FIGS. 3 and 5  described). In another embodiment, said structure may comprise an interrupted (e.g. split ring) annular structure (as in  FIGS. 2 and 4 ) 
     Said bearing structure is preferably a cage for roller bearings. The cage may be arranged to accommodate rolling elements. 
     The invention extends to a cage for roller bearings, comprising: 
     a body of the cage; and 
     a coating layer containing polyetheretherketone to cover 
     an outer surface of the body. 
     The bearing structure (for example cage) may be for a roller bearing having movable elements (for example rollers) having a diameter of 6 mm or less (referred to as “needle rollers” which are used in needle roller bearings). 
     According to a second aspect of the invention, there is provided a bearing structure for supporting movable elements of a bearing, the structure having an outer surface which is defined at least in part by a composition which comprises a first polymeric material which includes: 
     (a) phenyl moieties; 
     (b) ketone and/or sulphone moieties; and 
     (c) ether moieties. 
     The composition and said first polymeric material of the second aspect may be as described according to the first aspect. 
     Any feature of the bearing structure of the first aspect may be applied to the structure of the second aspect mutatis mutandis. 
     According to a third aspect of the invention, there is provided a method of making a bearing structure as described in the first or second aspects, the method comprising: 
     (a) selecting a bearing structure for supporting movable elements having an outer surface which is not defined partially or completely by a said composition of said first and/or second aspect; 
     (b) contacting said bearing structure with a composition of said first and/or second aspects so that an outer surface of said bearing structure is defined at least in part by said composition. 
     Features of the composition, outer surface and bearing structure may be as described according to said first and/or second aspects. 
     Said composition may include a said first polymeric material having a D 50  of less than 100 μm, suitably less than 60 μm, preferably less than 40 μm, more preferably less than 20 μm, especially less than 10 μm. The D 50  may be greater than 1 μm. Said first polymeric material may have a D 90  of less than 50 μm, preferably less than 40 μm, more preferably less than 30 μm, especially less than 20 μm. Said first polymeric material may have a D 99  of less than 60 μm, preferably less than 50 μm, more preferably less than 40 μm, especially less than 30 μm. 
     D 50 , D 90  and D 99  referred to herein may be measured by laser diffraction, for example using a Sympatece Helos (HO476) RODOS Analyser with Windocs Software from Sympatece GmbH, Germany. 
     Preferably, at least 1.0 g, more preferably at least 1.2 g, per cm 2  of said first polymeric material is applied in said method to form said outer surface. 
     Preferably, the method involves contacting said bearing structure with a spray which comprises said composition. 
     The method may involve contacting the bearing structure with a powder which includes said first polymeric material or contacting the substrate with a liquid which includes said first polymeric material. When a powder is used, it is preferred that the bearing structure is at a temperature above ambient temperature at the time of contact with said composition. Suitably, the substrate may be at a temperature of greater than 100° C., preferably greater than 200° C., more preferably greater than 300° C., especially greater than 350° C., at the time of contact with said composition. 
     When a liquid is contacted with said bearing structure, said liquid and/or said bearing structure may be at a temperature in the range 10° C. to 50° C. at the time the substrate and first polymeric material are contacted. 
     Preferably, the method involves subjecting the bearing structure, after it has been contacted with said composition, to an environment wherein the temperature is at least 100° C., preferably at least 200° C., more preferably at least 300° C., and especially at least 350° C. The temperature is suitably selected to cause said first polymeric material to fuse. The temperature is suitably selected so that said first polymeric material is not degraded or otherwise detrimentally affected. Suitably, the temperature is less than 500° C., more preferably less than 450° C. 
     The method may be used to prepare an outer surface having a total weight of at least 1 mg, preferably at least 5 mg, more preferably at least 10 mg, after said first polymeric material has been fused as described. 
     Preferably, in the method, a layer is formed which has a minimum thickness of 5 μm, preferably 8 μm, after said first polymeric material has been fused has described. 
     The method may be used to apply multiple layers of said composition. 
     According to a fourth aspect of the invention, there is provided a bearing comprising a structure as described in accordance with the first, second and/or third aspects in combination with movable elements. 
     Said moveable elements are preferably arranged to rotate relative to, for example within, the structure. Said movable elements may be arranged around substantially the entire extent of the outer surface of the structure. Said movable elements may be arranged to project from the outer surface. They may also be arranged to project from an inner surface of the structure which inner surface faces in an opposite direction to said outer surface. 
     Said movable elements suitably include outer surfaces arranged to contact a surface and roll over the surface in use. Outer surfaces of said movable elements are preferably defined by a material which is different to the composition which defines the outer surface of the bearing structure. Outer surfaces of said movable elements preferably do not include a coating which comprises a polymeric material of a type which includes: 
     (a) phenyl moieties; 
     (b) ketone and/or sulphone moieties; and 
     (c) ether moieties 
     as described in the first aspect. 
     Said movable elements may comprise a metal. 
     Said movable elements may comprise balls or rollers. For example balls may be mounted and retained within the structure. Preferably, said movable elements comprise rollers each of which is suitably mounted for rotation about a respective axis. The rollers may have a circular cross-section. They may be tapered. Preferably, the cross-section is substantially constant along the extent of the rollers. Said rollers are preferably substantially cylindrical. 
     Said movable elements, for example rollers may have a length (l) and an outer diameter (d) wherein the ratio l/d is greater than 2, suitably greater than 3, preferably greater than 4, especially greater than 5. The outer diameter suitably refers to the maximum diameter of the movable elements. 
     Said bearing is preferably a roller bearing, wherein said structure comprises a cage and said movable elements comprise rollers. The bearing may be a needle roller bearing comprising needle rollers having diameters of 6 mm or less or may comprise a bearing with rollers having diameters of greater than 6 mm. 
     According to a fifth aspect of the invention, there is provided a method of assembling a bearing according to the fourth aspect, the method comprising 
     (i) selecting a bearing structure as described in accordance with the first, second and/or third aspects; 
     (ii) selecting a multiplicity of movable elements; 
     (iii) mounting the movable elements in the structure, for example within slots which interrupt the outer surface of the structure. 
     According to a sixth aspect of the invention, there is provided an assembly comprising: 
     (a) a bearing of the fourth aspect; 
     (b) a first part which is mounted for rotational movement relative to the bearing structure of the bearing. 
     The first part may comprise an opening within which the bearing is arranged, suitably with movable elements of the bearing abutting, and suitably being arranged to roll over, a surface which defines the opening. The assembly may be such that, in use, the outer surface of the bearing structure may be urged towards and/or against said surface which defines the opening, for example under a centrifugal force. This may risk the outer surface of the bearing structure and the surface which defines the opening becoming worn. However, advantageously, a bearing structure comprising a composition as described herein may have a lower tendency to wear and cause wear than hitherto and may therefore extend the useful life of the bearing and said first part. 
     The assembly of the sixth aspect may additionally include a second part which is mounted for rotational movement relative to the bearing structure and is suitably independent of said first part. 
     The second part may be arranged to extend with an opening in the bearing, suitably with movable elements of the bearing abutting and suitably being arranged to roll over a surface of said second part. 
     Said first part may comprise a circular cross-section opening. It may define an inner raceway and may be associated with a connecting member for example a connecting rod. 
     Said second part may comprise a circular cross-section and may be cylindrical. It may comprise a crank pin. 
     The assembly of the sixth aspect may comprise a crank shaft and connection rod assembly. 
     The assembly may include said first and second parts which are suitably associated with said bearing and which suitable rotate about the same axis of rotation. The assembly may include an axle, for example of a crank shaft, which is suitably arranged to rotate about an axis which is not coincident with the axis of rotation of said first and second parts. The axle may be arranged to rotate about an axis which is offset relative to (preferably which extends substantially parallel to) the axis of rotation of said first and second parts. 
     Said assembly may be for an internal combustion engine. Said assembly may include a plurality of needle roller bearings. 
     The invention extends to an internal combustion engine which includes an assembly as described herein. 
     Any feature of any aspect of any invention or embodiment described herein may be combined with any aspect of any other invention or embodiment described herein mutatis mutandis. 
    
    
     
       Specific embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, which: 
         FIG. 1  is an exploded view of a roller bearing in association with a crankshaft/connecting rod mechanism as described above; 
         FIGS. 2 to 5  are respective perspective views of roller bearings of different constructions, as described above; 
         FIG. 6  is a cross-section through a roller bearing in accordance with a preferred embodiment of the present invention. 
     
    
    
     Referring to  FIG. 6 , a cage  100  includes a body  110  to hold the rolling elements  200 . The rolling elements  200  may be needle rollers having a diameter smaller than 6 mm or rollers having a diameter greater than 6 mm. In addition, the body  110  has the outer surface covered by a polyetheretherketone (PEEK) containing coating layer  120  which is formed at a thickness between 1 μm and 50 μm. Moreover, additive(s) may be added to the coating layer, such as PTFE, graphite, MoS 2  or a compound thereof. A pigment such as carbon black may also be added for aesthetic purposes. 
     The body  110  of the cage  100  is annular and can be a one piece type with recesses in its outer surface, a one piece type with a flat outer surface, a split type, consisting of two semi-circular rings, with recesses in its outer surface or a split type with a flat outer surface, as described with reference to  FIGS. 2 to 5 . 
     Compared with the known cages which have an electroplated silver layer on a plated copper layer on an outer surface of the cage, the polyetheretherketone (PEEK) containing coating layer  120  can remain on cage  100  for a longer period of time and can more effectively defer the wear on the outer raceway. This is illustrated below. 
     For comparison purposes a Test piece A was selected wherein the outer surface of the cage was electroplated with copper at a thickness of 10 μm and further electroplated with silver at a thickness of 10 μm, with the total plating layer at a thickness of 20 μm. 
     A Test piece B having a polyetheretherketone (PEEK) coating layer  120  at a thickness of 10 μm was prepared as follows: 
     A dispersion comprising polyetheretherketone, Aerosol OT75E, carbon black and water was used to coat the outer surface of the cage to define the 10 μm coating layer  120  using the procedure described in PCT/GB2004/003710. 
     Test pieces A and B were formed into bearings by incorporation of rollers and were assembled in a raceway and tested under the following test conditions: 
     The bearing was rotated at 9000 RPM, the centrifugal force of the cage exerted on the outer raceway was 95 Newtons, the relative speed between the cage and the outer raceway was 7.2 m/sec, lubrication oil temperature was 80° C., and the hardness of the bearing outer raceway was HV720-750 
     Test results after 100 hours testing were as follows: 
     1. Test piece A: The plated silver layer on the outer surface of the cage had been totally worn out, and a portion of the plated copper layer had been worn out also. The maximum wear on the bearing outer raceway was about 2 μm. 
     2. Test piece B: The Polyetheretherketones (PEEK) containing coating layer on the outer surface of the cage remained intact, and the maximum wear on the bearing outer raceway was less than 1 μm. 
     The test results indicate that the polyetheretherketones (PEEK) coating layer  120  can remain on the outer surface of the cage longer than the conventional copper plus silver plating. Also, the polyetheretherketones (PEEK) containing coating layer can more effectively defer wear on the outer raceway. 
     The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.