Abstract:
A bearing assembly ( 20 ) for use in a medical, surgical, or dental handpiece ( 10 ) that enables spindle rotation for a supported rotary tool ( 36 ) at high speeds. The bearing assembly ( 28 ) incorporates annular gap shields ( 50, 54 ) at each axial end to prevent contaminate ingress and to retain lubricating grease within the bearing assembly ( 28 ) to avoid re-lubricated after each use or sterilization cycle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is the U.S. National Stage under 35 U.S.C. §371 of International Application Ser. No. PCT/US2009/040907, having an International Filing Date of Apr. 17, 2009 and is related to, and claims priority from, U.S. Provisional Patent Application Ser. No. 61/045,773 filed on Apr. 17, 2008, and is further related to and claims priority from, from U.S. Provisional Patent Application Ser. No. 61/122,624 filed on Dec. 15, 2008, each of which are herein incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     This invention relates to medical, dental, and surgical handpieces, and in particular to a high-speed ball bearing for use in such handpieces. 
     Medical, dental, and surgical handpieces containing rotary tools supported on high-speed bearings are typically powered by an air turbine or electric motor. In either case, there are some common challenges to long bearing life. During operation, the bearings in such handpieces may be exposed to contamination in the form of both solid and liquid biological matter, as well as in the form of synthetic debris. The handpiece must be cleaned and sterilized between each patient. Typical cleaning and sterilization procedures require flushing of the head of the handpiece with solvents followed by high pressure steam sterilization in an autoclave system. Handpieces can reach temperatures ˜134° C. in autoclaving processes. Both procedures are detrimental to the life of the bearings in that they degrade the properties of many lubricants and retainer materials, as well as remove the lubricant remaining in the bearings. In addition, the cleaning process requires that the head of the handpiece be disassembled so that any foreign matter can be removed from the handpiece head. This is time consuming and can be complicated. For this reason, cleaning of foreign matter (as opposed to sterilization) of medical, dental, and surgical handpieces may not be performed as often as is necessary. 
     In order to replenish the lubricant within the bearings, the user must inject oil into the drive system of the handpiece, operate the handpiece to expel excess oil and clean any oil residue from the exterior of the handpiece. In addition to the time and expense required to relubricate the handpiece, there is the unwanted added risk that excess lubrication can contaminate the working environment. Unwanted lubricant can be expelled within the handpiece body cavity during patient treatment and can contaminate the surface of the tooth or bone being treated, risking infection or resulting in poor adhesion of a amalgam or “filling” material, such as employed in dental procedures. 
     For medical, surgical, or dental handpieces having a rotary tool powered by an air driven turbine, air is discharged from exhaust vents located near the top and bottom of the of the handpiece head during use. This exhaust air may pass through the bearings on the way out of the head and accelerate the expulsion of lubricant from the handpiece. While the handpiece is slowing to a stop, the air flow physics of the handpiece are reversed. As the handpiece shuts down, the low pressure caused by the rotating turbine draws debris, into the head of the handpiece and the bearings within. 
     Accordingly, there is a need for an improved high-speed bearing suitable for application medical, surgical, or dental handpeices to provide support for rotary tools, and which is adapted for easy cleaning, exclusion of external contaminates, and which is configured to facilitate the retention of lubricants within the bearing assembly. 
     BRIEF SUMMARY OF THE INVENTION 
     Briefly stated, the present disclosure provides a bearing assembly for use in a medical, surgical, or dental handpiece that enables spindle rotation for a supported rotary tool at high speeds which may approach 500,000 rpm in dental handpieces. The bearing assembly incorporates annular gap shields at each axial end to prevent contaminate ingress and to retain lubricating grease within the bearing assembly to avoid re-lubrication after each use or sterilization cycle. 
     In one embodiment, the bearing assembly comprises an inner ring, an outer ring, a plurality of stainless steel or ceramic balls, and a retainer that separates the balls. The bearing assembly has an angular contact design, whereby one of the rings is relieved for assembly, and the retainer is a one-piece design which fully encompasses the equator of each ball. The retainer cannot be assembled into the bearing after the rings and balls are assembled, and the retainer is necessarily of a “non-crown” or “non-pronged” design. The bearing assembly further includes two shield closures, with the first shield preferably integral to the inner or outer ring, or non-integral and attached to the bearing in some other way. The first single shield is disposed to protect the interior of the bearing assembly from external debris and contaminates, with the shield directly adjacent to the exterior openings of the housing. The second shield, is mounted to either the inner or outer ring on either the ID or OD of the ring such that the original standard bearing chassis cross-section is not changed. The second shield is positioned between the interior of the bearing assembly (balls, retainer, and grease) and the driven member or rotary tool, without altering the external configuration of the bearing assembly. This second shield substantially encloses the annular gap between the inner and outer bearing rings, and prevents forced air that drives the turbine from driving lubricant out of the bearing assembly. The second shield also prevents ingress of cleaning solutions, foreign material, and contamination into the bearing that can compromise, degrade, dissolve, or otherwise remove the initial lubricant from the bearing. In non-air-turbine powered handpieces, the second shield encloses the interior of the bearing assembly more effectively than a single shield design, thus retaining factory-applied grease lubricant more effectively than prior art designs. The bearing can be sized for use in a variety of medical or dental handpiece applications. 
     In an embodiment of the present disclosure, the second shield component of the bearing assembly is a molded or machined component comprised of an engineering plastic (or polymer), composite or other material, such as an amorphous polyetherimide, with or without a glass filler. The second shield is capable of withstanding repeated sterilization, autoclaving, and exposure to associated high temperatures without degradation of mechanical properties, while maintaining a rigid barrier, and while preserving shield integrity. 
     In an embodiment of the present disclosure, the second shield component of the bearing assembly is configured as a “snap” shield, such that it will mechanically interlock with the inner bearing ring in an operational position, such as by snapping over a grove on the bearing ring outer diameter, or mechanically interlock with the outer bearing ring, such as by snapping into a groove on the bearing ring inner diameter. The “snap” mechanical interlocking approach eliminates the need for other fastening methods such as welding or affixing the shield with wire and provides for easy assembly. The shield is preferrably made of an engineering plastic material that will elastically deflect sufficiently during installation to permit a “snap” mechanical interlocking action which return the shield to an undeflected configuration when the shield is disposed at the operational position. 
     In an embodiment of the present disclosure, the retainer (cage) component of the bearing assembly is composed of an engineering plastic with a composition selected to withstand repeated sterilization and autoclaving procedures and temperatures without degradation of mechanical or tribological properties, similar to the second shield component. 
     In one embodiment, the retainer (cage) component of the bearing assembly is composed of a base material such as polyamide-imide (PAI), containing fillers including carbon (graphite or carbon fiber) present in amounts equal to, or greater than 10% of the component weight, and fluoropolymer particles in amounts equal to, or greater than, 1% of the component weight. The fluoropolymer particles may be polytetrafluoroethylene (PTFE, such as Teflon™). The presence of fillers enhance the tribological performance (low friction and wear) of the retainer. 
     In one embodiment, the retainer (cage) component of the bearing assembly is composed of a base material such as polyetheretherketone (PEEK), containing fillers including carbon (graphite or carbon fiber) present in amounts equal to, or greater than 10% of the component weight, and fluoropolymer particles in amounts equal to, or greater than, 5% of the component weight. The fluoropolymer particles may be polytetrafluoroethylene (PTFE, such as Teflon™). 
     The foregoing features, and advantages set forth in the present disclosure as well as presently preferred embodiments will become more apparent from the reading of the following description in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       In the accompanying drawings which form part of the specification: 
         FIG. 1  is a perspective view of a medical, surgical, or dental handpiece; 
         FIG. 2  is an enlarged cross-sectional view of a head of the handpiece shown in  FIG. 1 , incorporating a bearing assembly of the present disclosure; 
         FIG. 3  is an enlarged cross-sectional view of the bearing assembly of the present disclosure; 
         FIG. 4  is an enlarged cross-sectional view of an alternative embodiment of the bearing assembly of the present disclosure; 
         FIG. 5  is an exemplary graph illustrating a normalized life improvement of the bearing assembly of  FIG. 3 , as compared to a prior art bearing in the same handpiece under identical test conditions; and 
         FIG. 6  is an exemplary graph comparing relative life improvement of the bearing assembly of  FIG. 4 , as compared to a prior art bearing in the same handpiece under identical test conditions. 
     
    
    
     Corresponding reference numerals indicate corresponding parts throughout the several figures of the drawings. It is to be understood that the drawings are for illustrating the concepts set forth in the present disclosure and are not to scale. Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. 
     DETAILED DESCRIPTION 
     The following detailed description illustrates the invention by way of example and not by way of limitation. The description enables one skilled in the art to make and use the present disclosure, and describes several embodiments, adaptations, variations, alternatives, and uses of the present disclosure, including what is presently believed to be the best mode of carrying out the present disclosure. 
     Turning to the Figures, and to  FIG. 1  in particular, powered hand-held instruments  10  (handpieces) for supporting rotary driven tools, that are used frequently in medical, surgical, or dental typically have a sleeve  12  and an end portion (or head)  14 . As shown in  FIG. 2 , the head  14  comprises a body  16  and a cap  18  which, in combination, define an interior cavity  20 . A bore  22  extends downwardly from the interior cavity  20  to the bottom of the head  14 . The body  16  and cap  18  include aligned recesses  24  and  26 , respectively, which each receive a bearing assembly  28 . Circumferential channels  30  can be formed in the recesses  24 ,  26  to receive O-rings  32  which form seals around the outer circumference of each bearing assembly  28 . A preload device  17  configured to exert an axial preload on each bearing assembly  28  is fitted in the recess  24 , between the bearing assembly  28  and the cap  18 . 
     In an air-driven handpiece, a driven member  34 , such as a turbine or impeller as shown in  FIG. 2 , is received in the cavity  20 . Correspondingly, for a motor-driven handpiece, the driven member  34  is a gear. The head  14  receives a spindle shaft  36 , to which a rotary tool (or bit) can be attached, which extends through the head bore  22  into the cavity  20  to be received by each of the bearings  28  and engaged with the driven member  34  so that the driven member can rotationally drive the spindle and attached tool during use. 
     Turning to  FIG. 3 , an illustrative bearing assembly  28  made in accordance with the present disclosure consists of an inner ring  40  and outer ring  42 , both of which can be made of stainless steel. The inner ring  40  has an outer circumferential surface  44 , with a curved or arcuate channel  46  formed in the surface  44 . The curved channel  46  defines an inner raceway. The outer ring  42  has an inner circumferential surface  48  having a first portion  48   a  and a second portion  48   b . The first and second portions  48   a  and  48   b  are both generally cylindrical, with the portion  48   b  having a larger diameter than the portion  48   a . The two portions are joined by a curved or arcuate section  48   c  which defines an outer raceway. 
     The outer ring  42  further includes an integrated shield  50  at one axial end of the ring which extends radially inwardly toward the inner ring  40 . The shield  50  has a length equal to at least 50% of the radial distance between the inner and outer rings. The radial distance between the inner and outer rings is defined as the difference between the outermost surface of the inner ring and innermost surface of the outer ring when the two rings are positioned coaxially. Preferably, the shield  50  extends substantially the full width of this radial distance to effectively close the axial end of the bearing assembly  28  without actually contacting the other ring during normal operation. That is, the shield  50  as defined in this disclosure is not a contact seal, such as an elastomeric lip seal which can be designed to contact and slide against a shaft. 
     A circumferential groove  52  is formed on the second portion  48   b  of the inner circumferential surface  48 , proximate to the opposite axial end of the outer ring  42  from the integrated shield  50 . A second shield  54  is received in the groove  52  to close the second axial end of the bearing assembly  28 . The second shield  54  is generally L-shaped, and includes a radially inward extending leg  54   a  and an axially extending head portion  54   b  sized and shaped to be received in the circumferential groove  52 . Preferably, the second shield  54  is a “snap” shield adapted to be snapped into place in the circumferential groove  52  as the shield head portion  54   b  mechanically interlocks with the groove  52 . This mechanical interlocking as shown defines a “snap” shield design in this disclosure, and is will be understood to refer to a shield component which may elastically deform from an operating configuration or shape during installation, enabling the shield to be distorted from a rest configuration by the bearing ring component during installation, and to “snap” back to the rest configuration upon proper positioning adjacent the groove  52  The shield leg  54   a  is sized similarly to the first shield  50 , and thus has a length equal to at least about 50% of the radial distance between the inner and outer rings, and preferably, has a length substantially the full width of this radial distance to close the second axial end of the bearing assembly  28 . As with shield  50 , the shield leg  54   a  effectively closes the axial end of the bearing assembly  28  without actually contacting the opposite ring during normal operation. That is, the shield  54  as defined in this disclosure is not a contact seal, such as an elastomeric lip seal which can be designed to contact and slide against a shaft. 
     As can be seen from  FIG. 2 , the shields  50  and  54  substantially close the opposite axial ends of each bearing assembly  28  contained within the head  14 . The shields  50  and  54  are sized to allow clearance of the inner ring, and hence, although it is preferred that the shields extend at least about 50% of the width of the space between the inner and outer rings, and preferably as much of the distance as possible, the shields will not cover 100% of the annular gap between the inner and outer rings  40 ,  42 . Although the second shield  54  is shown the be mounted to the inner circumferential surface of the outer ring  42 , those of ordinary skill in the art will recognize that the second shield  54  could be mounted to the outer circumferential surface  44  of the inner ring  40  in substantially the same manner, without departing from the scope of the present disclosure. 
     The second shield  54  is made by molding or machining an engineering plastic, composite or other material that can withstand repeated sterilization, autoclaving, and application temperatures without degradation of mechanical properties. The engineering plastic, composite, or other material is selected to remain rigid and strong enough to function as a barrier to preserve shield integrity at any application or sterilization temperature which may be experienced by the handpiece  10  during normal usage. An example autoclaving sterilization temperature is ˜134° C., and the preferred embodiment materials can accommodate this temperature level. The thermal expansion properties of the engineering plastic, composite, or other material are selected to ensure proper shield fit or retention occurs at any temperature within the normal operating range. One exemplary engineering plastic for forming the second shield  54  consists primarily of an amorphous polyetherimide, with or without glass filler, such as that sold under the registered trademark ULTEM containing no fillers. 
     Within the interior space of the bearing assembly  14 , axially between the first and second shields  50 ,  54 , a plurality of balls  60  are positioned within the curved channels  46  and  48   c  which define the inner and outer raceways. The curvatures of the raceways  46  and  48   c  each define a radius, with the radius of each being substantially the same, and as seen in  FIG. 3 , the curvature of the raceways corresponds substantially to the curvature of the balls  60 . The curvature of the raceway  46  subtends an arc of between about 80° and about 90°, and preferably about 85°. The curvature of the raceway  48   c , on the other hand, subtends an arc of between about 40° and about 50°, and preferably about 45°. The two arcs begin at approximately the same axial position. That is, one end of the arcs are the same distance from an axial end of the rings. The plurality of balls  60  are preferably formed from a ceramic such as silicon nitride (Si 3 N 4 ) or steel, and are held in place between the raceways and relative to each other by a retainer or cage  62  having optimized dimensions and clearances for the intended operating conditions of the application. 
     The bearing assembly  28 , including the balls  60 , is preferably lubricated with a grease consisting of a mineral base oil and polyurea thickener. Nominal properties of a preferred grease or lubricant include a specific gravity of 0.9, a dropping point temperature of 470° F., a viscosity (cSt at 104° F.) of 110, penetration (unworked) of 195, and penetration (worked after 60 strokes) of 280. Those of ordinary skill will recognize that different greases or lubricants having desired properties may be utilized without departing from the scope of the present disclosure. 
     As seen in  FIG. 3 , the bearing assembly  28  has an angular contact design, whereby one of the rings is relieved for assembly and the one piece retainer or cage  62  is fitted that fully encompasses the equator of each ball. The retainer cannot be assembled into the bearing after the rings and balls are assembled, and the retainer is necessarily of the “non-crown” or “non-pronged” design. 
     As with the second shield  54 , the retainer or cage  62  is designed to withstand repeated sterilization, autoclaving, and application temperatures without degradation of mechanical properties. The retainer can be made from an engineering plastic base material such as polyamide-imide (PAI), such as that sold under the registered trademark TORLON, or from polyetheretherketone (PEEK). In either case, the material preferably contains fillers, including carbon (graphite and or carbon fibers) and fluoropolymer particles such as polytetrafluoroethylene (PTFE, such as the trademarked TEFLON). The carbon filler is preferably present in amounts greater than or equal to about 10 wt %, while the fluoropolymer particle filler is preferably present in amounts greater than or equal to 1 wt % if the retainer is made from a PAI, or in amounts greater than or equal to about 5 wt % if the retainer is made from PEEK. 
     An exemplary composition of the retainer or cage  62  having a PAI base such as that sold under the registered trademark TORLON, consisting of about 20 wt. % carbon (graphite or fiber), about 3 wt. % fluoropolymer (such as polytetrafluoroethylene), and balance (about 77%) polyamide-imide-type polymer with respect to the total weight of the article. 
     An exemplary composition of the retainer or cage  62  having a PEEK base consists of about 15 wt. % PTFE, about 15 wt. % carbon fiber, and balance (70 wt. %) PEEK with respect to the total weight of the article. 
     The following table sets forth an example of nominal properties suitable for an embodiment of the second shield  54  and retainer  62 : 
     
       
         
               
               
               
             
               
             
               
               
               
             
               
             
               
               
               
             
               
             
               
               
               
             
           
               
                   
               
               
                 Property 
                 Second “snap” shield 
                 Retainer 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 General 
               
             
          
           
               
                 Density (lb/in 3 ) 
                 0.046 
                 0.054 
               
               
                 Water Absorption, 24 hrs 
                 0.25 
                 0.33 
               
               
                 (%) 
                   
                   
               
             
          
           
               
                 Mechanical 
               
             
          
           
               
                 Tensile Strength (psi) 
                 16,500 
                 19,000 
               
               
                 Tensile Modulus (psi) 
                 475,000 
                 1,130,000 
               
               
                 Flexural Strength (psi) 
                 20,000 
                 30,200 
               
               
                 Flexural Modulus (psi) 
                 500,000 
                 1,060,000 
               
               
                 Compressive Strength (psi) 
                 22,000 
                 17,800 
               
               
                 Compressive Modulus (psi) 
                 480,000 
                 580,000 
               
               
                 IZOD Notched Impact (ft- 
                 0.5 
                 1.6 
               
               
                 lb/in) 
                   
                   
               
             
          
           
               
                 Thermal 
               
             
          
           
               
                 Heat Deflection Temp (° F.) 
                 392 
                 536 
               
               
                 at 264 psi 
                   
                   
               
               
                 Max Operating Temp (° F.) 
                 340 
                 450 
               
               
                   
               
             
          
         
       
     
     When assembled in the handpiece head  14 , each bearing assembly  28  is oriented along the axis of the spindle shaft  36  such that the second shield  54  is positioned between the interior of the bearing assembly (including the balls  60 , retainer  62 , and internal grease) and the driven member  34 . In an air powered device, the second shield  54  prevents air that drives the turbine from entering the bearing assembly and blowing grease out of the bearing assembly  28 . The second shield  54  also prevents ingress of cleaning solutions, foreign material, and contamination into the bearing assembly  28  that can compromise, degrade, dissolve, or otherwise remove the initial lubricant from the bearing. In non-air-turbine powered handpieces, the second shield  54  encloses the interior of the bearing assembly  28  more effectively than prior single shield designs, thus retaining factory-applied grease lubricant more effectively than prior designs. 
     The second shield  54  works in combination with synergies achieved by retainer materials and grease specifications, as highlighted above. The synergies achieved by combining the three elements (i.e., a second shield, the retainer material, and grease selection) cooperatively function to extend the operational life of the bearing assemblies  28  when used in medical, surgical, or dental handpieces  10  subjected to repeated sterilization and autoclaving procedures. However, it is believed that some benefits could be realized by incorporating only a second shield; more benefits could be realized by incorporating the second shield and the retainer material; and the highest benefits could be achieved by incorporating all three elements (i.e., the second shield, the retainer material and the specified grease) as discussed above with respect to handpiece operation in the absence of periodic re-lubrication. Those of ordinary skill in the art will recognized that the bearing assemblies  28  of the present disclosure can be incorporated in older handpieces  10  as a retrofit option to dramatically improve performance of such older handpieces without redesigning the handpiece head  14 . 
     Turning to  FIG. 4 , and alternate embodiment  128  of the bearing assembly is shown. The bearing  128  is generally similar to bearing assembly  28  shown in FIG.  3 , which can be described as an outer ring relieved design. In contrast, the bearing  128 , as will become apparent from the description below, can be described as an inner ring relieved design. In this description of the bearing assembly  128 , only the elements which are different from the corresponding elements in the bearing assembly  28  will be described. 
     The bearing assembly  128  comprises an inner ring  140  and outer ring  142  which can be made of stainless steel. The outer ring  142  has an inner circumferential surface  148  with a curved or arcuate channel  149  formed in the surface  148 . The channel  149  defines an outer raceway. The inner ring  140  has an outer circumferential surface  144  having a first portion  144   a  and a second portion  144   b . The first and second portions  144   a  and  144   b  are both generally cylindrical, with the portion  144   a  having a larger diameter than the portion  144   b . The two portions are joined by a curved or arcuate section  144   c  which defines an inner raceway on the inner ring  140 . 
     The curvatures of the raceways  149  and  144   c  each define a radius, with the radius of each being substantially the same, corresponding substantially to the curvature of the balls  60 . The curvature of the raceway  144   c  subtends an arc of between about 45° and about 55°, and preferably about 50°. The curvature of the raceway  149 , on the other hand, subtends an arc of between about 85° and about 95°, and preferably about 90°. The two arcs begin at approximately the same axial position. That is, one end of the arcs are the same distance from an axial end of the inner and outer rings  140 ,  142 . 
     Like the bearing assembly  28 , the bearing assembly  128  has an angular contact design, whereby one of the rings is relieved for assembly and the one piece retainer  62  is fitted that fully encompasses the equator of each ball  60 . The difference is that in the bearing assembly  28 , the outer ring is relieved and in the bearing assembly  128 , the inner ring is relieved. In each embodiment, the retainer  62  cannot be assembled into the bearing assembly  28 ,  128  after the rings and balls are assembled, and the retainer  60  is necessarily of the “non-crown” or “non-pronged” design. 
     An exemplary test program was conducted to evaluate air turbine driven handpieces  10  incorporating the bearing assemblies  28  (i.e., containing a second shield enclosure  54 ) and other design modifications including new retainer material and grease options. The retainer  62  of the tested bearing  28  was comprised of about 15 wt. % PTFE, about 15 wt. % carbon fiber, and balance (about 70 wt. %) PEEK. The goal was for the bearing characteristic life to exceed 200 hours without additional lubrication in a handpiece test including a cyclic steam sterilization procedure. Prior art bearings were assembled and tested under identical conditions as the bearings  28 . All test bearings passed a torque test prior to life testing. All test bearings were sterilized for 25 cycles in an autoclave at 134° C. prior to life testing. Handpieces  10  containing the test bearings were mounted in special testers with a simulated burr (pin or rotary tool) affixed to the head. Air pressure was first set to achieve a free running speed of 350,000 rpm (air pressure typically around 3 bar). A 170 g normal load was then applied to the simulated burr in a cyclic manner to simulate actual handpiece use by a dentist. After 50 load cycles, the sequence was paused for 30 seconds by shutting off the air pressure that drives the turbine  34  in the head  14 . After the pause, another interval of load cycling began. After each 5,000 cycle period, technicians evaluated the condition of the test handpieces. Every 10,000 load cycles, each handpiece  10  was removed from the testers and sterilized for 10 cycles at 134° C. in an autoclave. The test was stopped once a handpiece  10  was unable to reach a free running speed of 280,000 rpm, was excessively loud, or had excessive run-out of the simulated burr. A 2-parameter Weibull approach was used to calculate characteristic lives of the test bearings. Results are shown in  FIG. 5 . Handpieces  10  incorporating prior art bearing assemblies were unable to meet the 200 hour test life goal, whereas handpieces  10  incorporating the bearing assembly  28  of the present invention surpassed the test goal and exceeded the prior art bearing assembly operational life by more than 10 times. 
     A second test program was conducted to evaluate air turbine driven handpieces  10  incorporating the bearing assemblies  128  (i.e., containing a second shield enclosure) and other design modifications including new retainer material and grease options. The retainer  62  of the tested bearing  128  was comprised of a polyamide-imide (PAI) polymer, such as that sold under the name TORLON®, containing normally about 20% graphite and about 3% fluoropolymer fillers. The goal was for the bearing characteristic life to exceed 200 hours without additional lubrication in a handpiece test including a cyclic steam sterilization procedure. Handpieces  10  included prior art bearing assemblies were tested under identical conditions as handpieces  10  containing the bearing assemblies  128 . All test bearings passed a torque test prior to life testing. All test bearings were sterilized for 25 cycles in an autoclave at 134° C. prior to life testing. Handpieces  10  containing the test bearings were mounted in special testers with a simulated burr (pin or rotary tool) affixed to the head. Air pressure was first set to achieve a free running speed of 350,000 rpm (air pressure typically around 3 bar). A 170 g normal load was then applied to the simulated burr in a cyclic manner to simulate actual handpiece  10  use by a dentist. After 50 load cycles, the sequence was paused for 30 seconds by shutting off the air pressure that drives the turbine  34  in the head  14 . After the pause, another interval of load cycling began. After each 5,000 cycle period, technicians evaluated the condition of the test handpieces. Every 10,000 load cycles, the handpieces  10  were removed from the testers and sterilized for 10 cycles at 134° C. in an autoclave. The test was stopped once a handpiece  10  was unable to reach a free running speed of 280,000 rpm, was excessively loud, or had excessive run-out of the simulated burr. A 2-parameter Weibull approach was used to calculate characteristic lives of the test bearings. Results are shown in  FIG. 6 . Handpieces  10  incorporating the prior art bearing assemblies were unable to meet the 200 hour test life goal, whereas handpieces  10  incorporating the bearing assembly  128  surpassed the test goal and exceeded the prior art bearing life by approximately 10 times. 
     In view of the above, it will be seen that the present disclosure sets forth a bearing assembly  28 ,  128  for use in high-speed medical, surgical or dental handpieces  10  that can withstand routine autoclave steam sterilization (typically around temperatures of 134° C.) and requires no maintenance or re-lubrication during its extended useful life. 
     As various changes could be made in the above constructions without departing from the scope of the claimed invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. For example, although the first shield  50  is shown to be integral with the outer ring, the shield  50  could be integral with the inner ring. Alternatively, the shield  50  could be a snap fitted shield, similar to the second shield  54 , which is snapped into place in the outer diameter surface of the inner ring, the inner diameter surface of the outer ring, or the outer diameter surface of the outer ring. The examples set forth herein, and the accompanying test results, are illustrative only, and do not limit the present disclosure to the specific features or properties set forth therein.