Abstract:
A high temperature and energy capacity friction material for wet brake and clutch applications comprising a resin impregnated carbon fabric. Preferably, the fabric is a woven cloth and the resin is a thermoset such as phenolic resin. The invention involves the controlled impregnation of the fabric so as to leave the fabric with a high compressive strength, effectively impervious to internal oil flow, but with macroscopic voids between the tows of yarn to afford local reservoirs of oil at the working surface and flow of oil across the working surface.

Description:
BACKGROUND OF THE INVENTION 
     Certain machinery, particularly in large mining, construction and like vehicles, severe conditions can be imposed on friction materials used in transmissions and brakes. To dissipate friction generated heat in these applications, it is known to circulate cooling oil across the faces of the friction material and opposing plate. Known premium friction products for these applications have included so-called “paper” based materials. Paper friction materials in wet applications are capable of handling energy densities of, for example, 150 to 200 joules/cm 2 . As equipment gets larger, heavier, and/or faster, there is a need for higher performance friction materials. 
     SUMMARY OF THE INVENTION 
     The invention provides a friction material composite that affords much greater performance than has been achieved with other known commercial materials. In the disclosed embodiment, the composite material is a carbon fiber fabric preferably woven and critically saturated with a resin such as phenolic resin. 
     It has been found that, surprisingly, when such fabric is saturated with the appropriate relative amount of resin, energy density capacity is far greater than has been experienced with prior commercial materials. In accordance with the invention, the resin loading is sufficient to wet the fibers and adhere them to any contacting or closely adjacent fibers. The resulting composite material is relatively impervious to any flow of cooling oil through it, but has sufficient macro surface irregularities to support pockets of cooling oil at its working surface. Since the friction material of the invention is several times more effective than prior materials, drive or brake assemblies using this material can be reduced in size and weight and/or much longer service life can be obtained where desired. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a face view of a friction disc assembly constructed in accordance with the invention; 
     FIG. 2 is a fragmentary cross-sectional view of the friction disc assembly taken in the plane indicated at  2 — 2  in FIG. 1; 
     FIG. 3 is a scanning electron micro photograph of unsaturated carbon fiber woven cloth prior to resin impregnation; 
     FIG. 4 is a scanning electron micro photograph of strands in tows of the yarn woven into the cloth of FIG. 3 prior to resin impregnation; 
     FIG. 5 is a scanning electron micro photograph of carbon fiber woven cloth saturated with resin in accordance with the invention; and 
     FIG. 6 is a scanning electron micro photograph of strands in tows of the yarn woven into the resin impregnated cloth of FIG.  5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIGS. 1 and 2 there is shown a friction article  10  in the form of a clutch or brake disc. The illustrated disc  10  is generally conventional in shape and includes an annular base or core  11  preferably stamped from sheet steel or other metal and friction material  12  in an annular pattern on both faces of the core. The illustrated friction material is carbon fiber fabric saturated with a thermosetting resin. More specifically, the fabric is preferably a plain woven fabric formed by strands of twisted yarn bundles, in turn, made from spun carbon fibers. The fabric weight, unsaturated, is between about 6.0 oz. and about 17 oz. per square yard. The thickness of the material  12  is generally proportional to the weight of the cloth. A suitable carbon-based friction material of the type described is available from Textron Specialty Corp., Lowell, Massachusetts. 
     The carbon fiber fabric, in accordance with the invention, is saturated in a critical manner with a suitable resin such as phenolic resin. It has been found that for woven carbon fiber fabric and phenolic resin, the final composite friction material  12  should comprise resin, in a dried state, in an amount where the resin weight represents from about 41% to about 60% of the weight of the resin and fabric together. The fabric is preferably saturated with resin by immersing it in a bath of solvented resin. Where the resin is a phenolic, a suitable solvent such as an alcohol, for instance ethanol or isopropyl alcohol, can be used. 
     The solvent/resin ratio can be regulated to control the final weight percentage content of resin in the friction material. The higher the relative solvent content in the bath in which the fabric is immersed, the lower the amount of resin content that will be produced in the finished friction material and vice versa. The resin saturated fabric is dried by removal of the solvent. Depending on the weight of the fabric used, the friction material, comprising a single fabric ply, can have a thickness of between about 0.50 mm to about 1.3 mm. The fabric is cut to shape before or after it is largely heat cured. The cut shape of the friction material  12  can be a complete circumferentially continuous annulus or may be sections that are assembled into an annulus corresponding to the annulus shape of the friction material  12  in FIG.  1 . 
     The friction material  12  is bonded to the metal core  11  preferably in the following manner. A coating of phenolic or epoxy resin is applied to each side of the steel core to a thickness of between about 0.003 to 0.0015 in. This coating of resin is advanced or “B” staged. A film adhesive such as a phenolic film in sheet form blanked to an annular shape corresponding to the shape of the friction material  12  is positioned between the coated core  11  and friction material  12  on each side of the core. The resulting sandwich formed by the two outside layers of friction material  12 , two adjacent layers of the film adhesive and the central core  11  with resin coated on both sides is then bonded together under heat and pressure. Before being bonded to the core, the friction material is nearly fully cured, for example, by as much as 90 or 95%. 
     After the friction material  12  is bonded to the core  11 , the resulting assembly can be cooled and suitably grooved. Grooves  13  are cut across the face of the friction material  12  at both sides of the article  10 . Such grooves, for oil distribution, can be cut with a suitable cutter wheel. The grooves are square or rectangular in cross-section and have a depth up to about half the thickness of each layer of friction material. The following table gives examples of suitable groove or slot patterns and sizes for various size friction discs. FIG. 1 illustrates a Wagon Track pattern. 
     
       
         
               
               
               
             
               
             
               
               
               
               
             
               
             
               
               
               
               
             
           
               
                   
                 TABLE 
               
               
                   
                   
               
               
                   
                 Groove Pattern 
                 Groove Width 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 BELOW 9″ OUTSIDE DIAMETER 
               
             
          
           
               
                   
                 Radial: 
                 6 equally spaced (60° apart) 
                 3.17 mm 
               
               
                   
                   
                 or 
               
               
                   
                 Radial: 
                 6 equally spaced (60° apart) 
                 2.30 mm 
               
               
                   
                   
                 or 
               
               
                   
                 Wagon Track: 
                 two passes - 
                 3.17 mm 
               
               
                   
                   
                 two grooves per pass 
               
               
                   
                   
                 or 
               
               
                   
                 Wagon Track: 
                 two passes - 
                 2.30 mm 
               
               
                   
                   
                 two grooves per pass 
               
             
          
           
               
                 9″  OUTSIDE DIAMETER AND ABOVE 
               
             
          
           
               
                   
                 Radial: 
                 12 equally spaced (30° apart) 
                 3.17 mm 
               
               
                   
                   
                 or 
               
               
                   
                 Radial: 
                 12 equally spaced (30° apart) 
                 2.30 mm 
               
               
                   
                   
                 or 
               
               
                   
                 Wagon Track: 
                 two passes - 
                 3.17 mm 
               
               
                   
                   
                 two grooves per pass 
               
               
                   
                   
                 or 
               
               
                   
                 Wagon Track: 
                 two passes - 
                 2.30 mm 
               
               
                   
                   
                 two grooves per pass 
               
               
                   
                   
               
             
          
         
       
     
     It is believed that when the resin content is between about 41% to about 60% of the total weight of the resin and carbon fiber of the friction material  12 , an optimum balance of strength and oil retention and distribution capacity is obtained and, consequently, maximum energy capacity is obtained. Friction articles made according to the invention have demonstrated energy absorption capabilities 2 to 3 times that of conventional paper friction materials. For example, the friction article  10  described above has operated successfully in the range of 500-600 joules/cm 2  per engagement cycle. 
     FIGS. 3-6 illustrate, by comparison, an example of the degree of resin impregnation that has been found to produce the surprisingly high energy capacity of the invention. FIGS. 3 and 4 are different magnifications of the surface of a typical carbon fiber woven cloth useful with the invention prior to resin impregnation. FIGS. 5 and 6, at magnifications corresponding to FIGS. 3 and 4, respectively, illustrate the same fabric after it has been impregnated with phenolic resin to a degree where the resin weight, in a dried state, is between about 41% to about 60% of the weight of the combined resin and fabric. It can be seen in FIG. 5 that the space between the tows of yarn is largely void so as to leave on a macroscopic level a large number of oil reservoirs at the surface and channels for fluid flow around the tows of yarn. FIG. 6 illustrates that all of the strands of the carbon fiber are coated with resin and the areas between adjacent strands is typically filled with resin so as to effectively eliminate microporosity and the ability of oil to flow through the friction material thereby assuring that oil delivered to the working surface through the grooves  13 , for example, will stay at the working surface of the friction material. 
     The disclosed friction material exhibits performance levels measured by its capacity to sustain repeated cycles of energy absorption, quantified in dissipated energy per area per cycle, far in excess of that produced by conventional known materials such as paper friction materials. The friction material can safely operate at temperatures that exceed the temperature limits of ordinary cooling fluids. The friction material has demonstrated an ability to maintain the operating temperatures of the reaction plates against which it operates at a generally uniform level from inside diameter to outside diameter and thereby reduces the risk of warpage of such plates. Still further, the disclosed friction material resists glazing of its working surface even at severe use so that its performance characteristics do not change during its service life and engineers can confidently use a working design load that is close to the ultimate capacity of the friction material. 
     At resin impregnation weights below that described herein, a friction product can suffer from limited compressive strength and microporosity. High compressive strength obtained with the invention is important in operating at high unit pressures and, consequently, high energy dissipation levels. Microporosity, which is avoided by the resin weights contemplated by the present invention, it is believed, can limit performance by allowing oil to dissipate through the friction material from the working surface where it is most effective. Resin impregnation weights above those described for the invention herein evidence a decrease in performance which is believed to be the result of the macroporosity at the surface being reduced and a tendency of the friction material to clog up with cooling oil debris created when such oil overheats and begins to carbonize. This condition excludes the beneficial existence of pockets of oil that aid in maintaining a sufficient quantity of oil at the working surface to support cooling action and leads to premature failure. 
     It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.