Abstract:
A method of enhancing the operating life of roller bearings assemblies (B) subject to oil-out conditions is disclosed. The method comprises applying a tribological metal carbide reinforced amorphous hydrocarbon coating layer ( 16 ) over either the sliding surfaces (SS) or the rolling surfaces (RS) of the bearing assembly rolling elements (RE), but is preferably applied at least to the sliding surfaces of the rolling element. The coating has a thickness determined according to the equation: Formula (I), where d=coating thickness P=is the ratio of force applied to the rollers/contact area; n=the number of rollers; (t-t 0 )=the desired operating time after oil-out condition starts; and K′ is the Archard constant. 
     
       
         
           
             
               
                 
                   
                     
                       d 
                       
                         K 
                         ′ 
                       
                     
                     = 
                     
                       
                         Pv 
                         n 
                       
                        
                       
                         ( 
                         
                           t 
                           - 
                           
                             t 
                             0 
                           
                         
                         ) 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   1 
                   )

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent App. No. 60/663,229 filed Mar. 18, 2005, and which is incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    This invention relates to rolling element bearings, and in particular to a method of protecting a bearing against oil-out conditions and a bearing made in accordance with the method. 
         [0004]    Adhesive wear in mechanical components results when elements in relative motion come into contact under loads and speeds sufficient to generate micro welding of asperities on the contacting surfaces. When a mechanical system suffers from a catastrophic loss of lubrication, adhesive wear between the elements of the mechanical components leads to galling of the elements and the seizure of the system. The catastrophic loss of lubrication in aerospace systems such as rotorcraft gearboxes and turbine engine bearings is referred to as an oil-out condition. An oil-out condition in gearboxes and turbine engines usually precipitates a bearing failure, which renders the system inoperative. 
         [0005]    Depending upon operating conditions such as load and speed, the amount of time from oil-out to bearing failure can be a few seconds to many minutes. Roller bearings in new turbine engine designs must endure intermittent oil-out conditions under full loads and speeds for at least thirty seconds. It is desirable for roller bearings in rotorcraft gearboxes to continue to operate for more than thirty minutes following an irreversible loss of lubrication. 
       SUMMARY OF THE INVENTION 
       [0006]    Briefly stated, a method of enhancing the operating life of roller bearings assemblies subject to oil-out conditions is disclosed. As is known, roller bearings assemblies comprise inner and outer races and rolling elements positioned between the inner and outer races. The rolling elements having sliding surfaces and rolling surfaces. The method comprises applying a tribological metal carbide reinforced amorphous hydrocarbon coating layer over critical surfaces of roller bearings. The tribological coating layer can be applied to either the sliding surfaces or the rolling surfaces of the rolling element, but is preferably applied at least to the sliding surfaces of the rolling element. In a tapered roller bearing, the tribological coating layer is applied at least over the large end of the roller element to provide wear protection at the roller end/rib interface. 
         [0007]    In accordance with one aspect of the invention, the wear byproduct of the coating is graphite. 
         [0008]    A tribological coating containing tungsten carbide (WC) has been found to work well to increase the life of the bearing assembly when operating under oil-out conditions. The WC is in a crystalline form dispersed in an amorphous carbon or hydrocarbon matrix. The WC crystals are less than about 10 nm in size, and WC can comprise less than 50% of the tribological coating layer. 
         [0009]    In accordance with another aspect of the invention, an adhesion layer is coated over the bearing surface and the tribological coating layer is applied over the adhesion layer. 
         [0010]    The depth d of the tribological coating layer affects how long the bearing can operate in an out-of-oil condition. The depth or thickness of the tribological coating layer is determined according to the Archard equation: 
         [0000]    
       
         
           
             
               
                 d 
                 
                   K 
                   ′ 
                 
               
               = 
               
                 
                   Pv 
                   n 
                 
                  
                 
                   ( 
                   
                     t 
                     - 
                     
                       t 
                       0 
                     
                   
                   ) 
                 
               
             
             , 
           
         
       
     
         [0011]    where d=coating depth
       P=is the ratio of force applied to the rollers/contact area;   n=the number of rollers;   (t-t 0 )=the desired operating time after oil-out condition starts; and   K′ is the Archard constant.       
 
         [0016]    Preferably, d and K′ are both small numbers. In a preferred embodiment of the coating, K′ is less than 1×10-16 Pa-m and d is less than 5 μm. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0017]      FIG. 1  is a cross-sectional view of a bearing assembly; 
           [0018]      FIG. 2  is a schematic cross-sectional drawing of the coating of the present invention applied to a bearing element surface; 
           [0019]      FIG. 3  is a graph charting test data of the effectiveness of different coatings to extend the life of a bearing operating under an oil-out condition; and 
           [0020]      FIG. 4  is a plot of a mathematical analysis of the results shown in  FIG. 3  in terms of the Archard analysis of Equation 4 below. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    The following detailed description illustrates the invention by way of example and not by way of limitation. This description will clearly enable one skilled in the art to make and use the invention, and describes several embodiments, adaptations, variations, alternatives and uses of the invention, including what we presently believe is the best mode of carrying out the invention. Additionally, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
         [0022]    As is known, bearing assemblies B include an outer race OR, an inner race IR and a plurality of roller elements RE positioned between the two races. The roller elements, which are shown in  FIG. 1  to be tapered roller bearings, are typically held in place relative to each other by means of a cage C. In a bearing assembly, the roller elements have sliding surfaces SS and rolling surfaces SR. In tapered roller bearings, such as shown in  FIG. 1 , the rolling element side surface is the rolling surface and the end surfaces, and in particular the large end surface, are the sliding surfaces. 
         [0023]    This invention describes the use of tribological coatings applied to critical surfaces, and in particular, to the sliding surfaces, of roller bearings to temporarily defeat the adhesive wear associated with oil-out conditions, thereby increasing the time to failure of the most critical components of a system. As shown in  FIG. 2 , a coating  10  made in accordance with the present invention comprises a metallic adhesion layer  12  that is applied to the sliding surface  14  of a bearing element. This sliding surface  14  comprises the opposed end surfaces of rolling element bearings, including tapered rollers, cylindrical rollers, spherical rollers, and needle rollers. In a ball bearing, the surface of the ball bearing is both a sliding surface and a rolling surface. A tribological coating layer  16  is applied over the metallic adhesion layer  12 . The tribological coating layer comprises a metal carbines contained in an amorphous carbon or hydrocarbon matrix (MC/aC or MC/aC:H). 
         [0024]    Metal carbide reinforced amorphous carbons or hydrocarbon coatings are nanocomposite materials, comprised of metal carbide particles (with dimensions typically less than 10 nm) contained in amorphous matrices. The particles are dispersed in the matrices which enhances the toughness and hence the fracture resistance of the coating. Another important aspect of these coatings is that they do not adhesively interact with steel, which makes them effective barriers to the adhesive wear that arises during oil-out conditions. The adhesion layer  12  strongly adheres to the steel substrates, and the tribological coating layer adheres strongly to the adhesion layer. The adhesion layer can comprise metals such as Cr and Ti. 
         [0025]    The coating  10  can be applied by means of sputtering or chemical vapor deposition. Although the drawing of  FIG. 2  shows a sharp transition between the adhesion or metal transition layer  14  and the tribological coating layer  16 , in reality, there is a more gradual transition forming a metal carbide gradient between the two layers. Towards the metal layer side of the transition portion, the metal carbide content is low, and the metal carbide content increases towards the tribological coating layer  16 . 
         [0026]    When the metal carbide is a tungsten carbide, the WC coating comprises WC crystals in an amorphous matrix. The matrix can be an amorphous carbon matrix (aC) or an amorphous hydrocarbon matrix (aC:H). The WC crystals make up less than 50% of the coating, and preferably less than 20% of the coating. The WC crystals are small, having a crystal size of less than 10 nm. 
         [0027]    Suppose that during an oil-out event, the time to failure of a given roller bearing is defined as t 0 . The wear coefficient of a tribological coating can be expressed in terms of the Archard equation as: 
         [0000]    
       
         
           
             
               
                 
                   K 
                   = 
                   
                     HVn 
                     Fl 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0028]    Where H is the hardness of the coating (in N/m 2 ), F/n is the applied load/roller (in N), I is the sliding distance (in m), and V is the volume of material worn away (in m 3 ). Many times the Archard equation is expressed in terms of K′=K/H. If the sliding distance/can be equated to the product of the sliding speed and the time (t′) during which the bearing operates under an oil-out condition, and if V can be expressed in terms of the contact area (A) and the worn depth of the coating (d′), then Eq. 1 becomes 
         [0000]    
       
         
           
             
               
                 
                   
                     K 
                     ′ 
                   
                   = 
                   
                     
                       nAd 
                       ′ 
                     
                     
                       Fvt 
                       ′ 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0029]    Solving for t′, 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       t 
                       ′ 
                     
                     = 
                     
                       
                         nd 
                         ′ 
                       
                       
                         PvK 
                         ′ 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0030]    where the substitution P=F/A has been used. 
         [0031]    If the coating  10  is applied to the critical elements (i.e., sliding surfaces) of a bearing that are otherwise responsible for bearing failure during an oil-out event, the life of that bearing will be extended while the coating is wearing away. Once the coating is worn away, the bearing will fail. If d is the total thickness of the coating, then t′ describes the additional life provided by a tribological coating to a component in an oil-out condition. Hence the life of the bearing in an oil-out event will be equal to 
         [0000]    
       
         
           
             
               
                 
                   t 
                   = 
                   
                     
                       
                         t 
                         0 
                       
                       + 
                       
                         t 
                         ′ 
                       
                     
                     = 
                     
                       
                         t 
                         0 
                       
                       + 
                       
                         nd 
                         
                           PvK 
                           ′ 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0032]    K′ is a value that can be determined experimentally on laboratory tribometers, and P, n and v are determined by the application and/or component design. Inspection of Eq. 4 implies that thick coatings with small K′ values applied to the critical (i.e., sliding) surfaces of roller bearings will lead to the largest values of t′. In practice, several aspects of the application such as the contact stress, geometrical tolerances, and thermal resistance limit the thickness of tribological coatings for example. Additionally, hard coatings (small K′) typically have large elastic moduli, which mean that they can have very large internal stresses. It is the nature of hard coatings that the contact stress increases with coating thickness. Hence, there exists a compromise between K′ and d for most tribological coatings. Finally, it is important that the coating wears in a molecular or atomic fashion as opposed to fracture and delamination. 
         [0033]    The family of metal carbide reinforced amorphous carbons and hydrocarbons can be synthesized to have high values of d (˜5 μm), low values of K′ (˜10 −17  m 2 /N), and exhibit molecular or atomic wear without fracture and delamination. Additionally, the wear byproduct of these types of coatings is graphite, a very good solid lubricant. 
         [0034]    In summary, the important attributes of tribological coatings utilized to extend the life of roller bearings in oil-out conditions are (1) strong adhesion to the steel substrate; (2) barrier to adhesive interactions with steel; (3) wear atomically or molecularly and not through fracture and delamination; and (4) a large ratio of d/K′. Intuitively the friction coefficient was assumed to be the most desirable attribute of a tribological coating in order to extend bearing life under oil-out conditions. Surprisingly, this turns out not to be the case. 
         [0035]    Table 1 lists three types of metal carbide reinforced amorphous carbon and hydrocarbon coatings along with their friction coefficients (μ), nominal thicknesses (d), and d/K′ values. These coatings were applied by reactive and non-reactive sputtering, a physical vapor deposition process, to the large ends of rollers available from The Timken Company under its M86600 series. Eighteen (n=18) coated rollers were then assembled into bearings, and the bearings were tested under oil-out conditions with a specified load of 4448 N (157 MPa) contact stress and sliding speed of 2700 rpm (1.2 m/s) between large roller end and rib face. The results of the testing are shown in  FIG. 3 . Standard bearings lasted about 6 minutes under these conditions, while bearings with TiC/aC, TiC/aC:H, and WC/aC:H coatings lasted approximately 4, 9, and 56 times longer. Thus, while the TiC coatings worked acceptably to allow operation of the bearing under an oil-out condition, the bearing having the WC coating lasted significantly longer than the bearings having the Ti coating. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Properties of some MC/aC:H Coatings 
               
             
          
           
               
                   
                 Coating 
                 μ 
                 d (μm) 
                 K′ (Pa-m) 
               
               
                   
                   
               
               
                   
                 TiC/aC 
                 0.07 
                 1 
                 1 × 10 −16   
               
               
                   
                 TiC/aC:H 
                 0.15 
                 2 
                 9 × 10 −17   
               
               
                   
                 WC/aC:H 
                 0.20 
                 3 
                 2 × 10 −17   
               
               
                   
                   
               
             
          
         
       
     
         [0036]    The data graphed in  FIG. 4  can be analyzed in terms of Eq. 4 with t plotted as a function of d/K′. A linear fit to the data yields 
         [0000]    
       
         
           
             
               
                 
                   t 
                   = 
                   
                     270 
                     + 
                     
                       9.9 
                       × 
                       
                         10 
                         
                           - 
                           8 
                         
                       
                        
                       
                         
                           ( 
                           
                             d 
                             
                               K 
                               ′ 
                             
                           
                           ) 
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0037]    Comparison of Equations 5 and 4 indicates that t 0 =270 seconds, and (n/Pv)=9.9×10−8 (s/Pa-m). It is calculated that a thrust load of 4448 N generates about 157 MPa of contact stress, and 2700 rpm corresponds to 1.2 m/s of roller end/rib face speed in Timken M86600 series bearings. The values of the fitted slope (9.9×10 −8  s/Pa-m) and the calculated value (9.2×10 −8  s/Pa-m) of (n/Pv) are in good agreement. Eq. 4 may need a scaling factor to account for additional wear modes or other effects not included in the Archard analysis. However, the model of Eq. 4 appears to be very effective in terms of calculating the additional life that a coating on a roller end provides to a tapered roller bearing in an oil-out condition. 
         [0038]    The applicability of Eq. 4 need not be limited to the large roller ends of tapered roller bearings. The coating can be applied to the small end of tapered roller bearings, one or both ends of cylindrical roller bearings, and the bodies of rollers for cylindrical and spherical roller bearings. Since it is difficult to apply the metal carbide reinforced amorphous carbon and hydrocarbon coatings to spherical ball surfaces, and since these coatings sometimes appear to promote early ball bearing failure when they are applied to the raceways of ball bearing rings, other types of coatings may be better suited for ball bearing applications. 
         [0039]    It will be appreciated that roller bearings treated in accordance with the method outlined above can advantageously be used in rotorcraft gearbox bearings and turbine engine bearings. 
         [0040]    As various changes could be made in the above constructions without departing from the scope of the 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.