Abstract:
In a process for attaching a shunt wire to a carbon brush body in which a resin-containing cement is tamped into a cavity in the brush body, an improved cement is used, formed from a mixture of polyphenylene sulfide powder having a grain size of 5 to 50 μm and copper-based powder. The brush produced thereby is improved in resistance to alcohol-containing fuels.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to carbon brushes, and in particular, a method for attaching a shunt wire to a brush body, an improved cement for attaching a shunt wire to a brush body, and the brush obtained using this cement. 
   In the production of carbon brushes for electrical motors, it is well known to attach a shunt wire to a brush body by tamping. In this process, known for example from U.S. Pat. No. 4,579,611, a cement mixture containing a conductive powder and a thermosetting resin is tamped around a shunt wire. Subsequently, the resin may be cured by heating or by a chemical reaction. The resin enhances the stability and integrity of the tamping connection. Presently, these resins are mostly phenolic resins. While polyimides have also been used, they are undesirable from an environmental point of view. 
   Tamped brushes that are used in fuel pumps, particularly for automotive applications, are often exposed to the fuel that the pump is conveying. As more cars are being fueled with gasoline that contains various additives and alcohol, the long term stability of the tamping cement is no longer guaranteed, as the alcohol and additives tend to cause deterioration of the resin in the cement. This deterioration manifests itself in increased resistance between the shunt wire and the brush. 
   It is also known to produce tamped brushed from silver-plated copper powder without resin. Such brushes are extremely stable in alcohol-based fuels, but the pull out force is relatively low, especially in shallow tamp holes, as these brushes rely solely on mechanical deformations and stress forces in the copper powder to keep the shunt secured in place. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide electrical brushes suitable for use in corrosive fuels which do not exhibit increase resistance when exposed to the fuel. 
   It is a further object of the invention to provide electrical brushes of increased resistance to physical deterioration when exposed to corrosive fuels. 
   In order to achieve these and other objects, the invention proposes the use of a tamping cement formed from a copper based electrically conductive powder in combination with very fine polyphenylene sulfide (PPS) powder as the resin additive. PPS enhances the strength and seals the connection as other resins do, but is inherently stable in all known additives and alcohols and therefore solves the problem of increases in resistance over the lifetime of the motor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1   a - 1   d  are schematic diagrams of the steps of forming a brush according to the invention; and 
       FIG. 2  is a cross-sectional diagram of a brush according to the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Poly(p-phenylene sulfide) (PPS) is a polymer made up of alternating sulfur atoms and phenylene rings in a para substitution pattern. The highly stable chemical bonds of its molecular structure impart molecular stability toward both thermal degradation and chemical reactivity. Because of its molecular structure, PPS also tends to char during combustion, making the material inherently flame retardant. PPS has not been found to dissolve in any solvent at temperatures below about 200° C. 
   The PPS powder useful in the invention generally has a particle size of 5 to 50 μm. PPS of this particle size is known for formation of brush bodies, from DE 199 00 024 A1, and is available commercially under the trademark “Ryton” from Chevron-Phillips Chemicals International N.V. In prior art brush formation, the PPS has been combined with graphite, compressed, granulated and recompressed, then cured at 275-350° C. 
   According to the invention, a cement is formed containing a copper powder of particle size preferably in the range of 25-150 μm, although a coarser powder of size range 75-250 μm may also be used. The copper powder may be plated, for example with silver or tin, as are commonly used in the art. Other metal plated copper powders are also acceptable. 
   The cement contains PPS powder in an amount of 0.5-6 weight %, and preferably 2-4 weight %, in combination with the aforementioned copper powder. Generally, the finer the copper powder used, the greater the amount of resin that is required. 
   The process for attaching the shunt wire to the brush body is shown in  FIGS. 1   a - 1   d.    
   Initially, as shown in  FIG. 1   a , the shunt wire  10  passes through a tamping needle  12 , which passes though a cup  14  containing the cement mixture of the invention  16 . 
   This structure is lowered such that the tamping needle  12  passes into a cavity  18  in brush body  20 . A knot  22  is initially formed in the wire to retain the wire in the body. The needle then is raised as shown in  FIG. 1   c , allowing the cement  16  to fall into the cavity around the wire. Then, the needle is lowered as shown in  FIG. 1   d  to tamp the cement. The raising and lowering is repeated typically 6-7 times, until the cement rises to the desired height. The final product is shown in  FIG. 2 . 
   Preferably, the brush body may be heated to about 275-350° C. after tamping is complete to melt and fuse the thermoplastic PPS powder, allowing it to flow into crevices in the body, and increase the retention strength of the shunt wire in the cavity. The heating time necessary at the melting temperature of the PPS powder to fully cure the resin is about 10-20 minutes, preferably about 15 minutes, although the complete time for a cycle of furnace heating, melting and cooling is about 1 hours. Moreover, it is possible to partially cure the PPS resin by heating to a lower temperature, for example about 150° C., for a longer period, for example, about 1-3 hours. 
   EXAMPLES 
   Example 1 
   Deterioration of the shunt connection for various tamping cements and various fuels was determined as follows: 
   Brushes according to the invention were produced with 3% by weight PPS, remainder silver plated copper powder (25-150 μm) cement and a nickel-plated shunt wire. Some samples were uncured, some samples were undercured at 150° C. for 2 hours, and some samples were cured normally at 290° C. for 30 minutes in a belt furnace. 
   Comparative samples were prepared from silver plated copper filler (no additives), silver plated copper with a cyanoacrylate adhesive gluing the shunt to the copper powder, and silver plated copper powder with phenolic resin (current standard cement) cured to harden the resin. 
   Each brush type was exposed to three test fuels for periods of up to 500 hours. The test fuels were:
         1) corrosive fuel, containing, by weight, 22% ethanol, 77% gasoline and 1% corrosive water containing 33 mg/l sodium sulfate, 33 mg/l sodium chloride and 33 mg/l acetic acid in distilled water;   2) sulfur-containing fuel, containing 20 mg/l sulfur in gasoline; and   3) corrosive two phase fuel, containing, by weight, 10% ethanol, 43.75% toluene, 43.75% iso-octane and 2.5% corrosive water containing 148 mg/l sodium sulfate and 165 mg/l sodium chloride in distilled water).       

   The brushes were tested by connecting the shunt and brush in a direct current circuit at 1 ampere, with stainless steel contacts on the brush portion and the shunt portion connected to a voltmeter to measure the voltage drop at the shunt/brush junction. A measurement of the voltage drop for each brush was made before soaking in fuel, and then measurements were made at 50, 100 and 500 hours. 
   30 pieces of each sample were soaked in a fuel at 60° C. Test results were as follows, with voltage drop reported in millivolts): 
   
     
       
             
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
             
           
         
             
                 
             
             
                 
                 
                 
               Invention 
               Invention 
                 
                 
                 
             
             
                 
                 
               Invention 
               (under- 
               (fully 
             
             
                 
                 
               (uncured) 
               cured) 
               cured) 
                 
                 
               Silver 
             
             
                 
                 
               Silver 
               Silver 
               Silver 
                 
               Silver 
               plated 
             
             
                 
                 
               plated 
               plated 
               plated 
               Silver 
               plated 
               copper 
             
             
                 
                 
               copper 
               copper 
               copper 
               plated 
               copper 
               with 
             
             
               Fuel type 
               Hours 
               w/phenolic 
               w/phenolic 
               w/phenolic 
               copper 
               with glue 
               phenolic 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               corr. 
               0 
               1.30 
               1.34 
               1.73 
               1.34 
               1.54 
               4.91 
             
             
                 
               50 
               1.24 
               1.43 
               1.80 
               1.37 
               2.17 
               9.37 
             
             
                 
               100 
               1.27 
               1.52 
               1.73 
               1.42 
               2.09 
               11.35 
             
             
                 
               500 
               1.23 
               1.40 
               1.93 
               1.51 
               2.61 
               13.46 
             
             
               sulfur 
               0 
               1.27 
               1.52 
               1.66 
               1.36 
               1.58 
               5.58 
             
             
                 
               50 
               1.37 
               1.78 
               1.93 
               1.82 
               2.12 
               6.81 
             
             
                 
               100 
               1.16 
               1.75 
               2.05 
               2.10 
               2.12 
               9.87 
             
             
                 
               50 
               1.56 
               1.96 
               2.24 
               2.45 
               2.69 
               10.68 
             
             
               corr. 2- 
               0 
               1.29 
               1.43 
               1.64 
               1.42 
               1.69 
               5.35 
             
             
               phase 
               50 
               1.71 
               1.94 
               2.49 
               2.22 
               2.87 
               17.05 
             
             
                 
               100 
               1.87 
               2.05 
               2.58 
               2.35 
               3.07 
               20.30 
             
             
                 
               500 
               2.38 
               2.77 
               3.19 
               3.49 
               3.47 
               22.22 
             
             
                 
             
           
        
       
     
   
   The largest voltage drops were observed for all fuels with the prior art cement containing phenolic, which is known to degrade with exposure to fuel. The smallest voltage drops were observed with uncured PPS-based cement. 
   Example 2 
   Brush samples prepared as in Example 1 were used to test the the force necessary to pull out the shunt wire, initially and after soaking for 500 hours in a designated fuel. Thirty samples of each brush were tested. 
   Average pull out force in Newtons was determined for each brush. Results are in the table below (including standard deviation): 
   
     
       
             
             
             
             
             
             
             
           
         
             
                 
             
             
                 
                 
               Ag-plated 
                 
                 
               Invention 
               Ag-plated 
             
             
                 
               Ag-plated 
               copper 
               Invention 
               Invention 
               fully 
               copper 
             
             
               Soak 
               copper 
               w/glue 
               uncured 
               undercured 
               cured 
               w/phenolic 
             
             
                 
             
           
           
             
               not soaked 
               29.0 ± 8.7 
                46.3 ± 11.8 
               48.8 ± 6.8 
               49.4 ± 8.5 
                61.9 ± 10.9 
               54.4 ± 23.0 
             
             
               500 hours 
               37.05 ± 5.9  
               32.25 ± 8.5  
               63.3 ± 7.4 
               58.15 ± 10.9 
               70.9 ± 8.4 
               60.1 ± 19.5 
             
             
               corrosive 
             
             
               fuel 
             
             
               500 hours 
               35.75 ± 10.0 
               34.8 ± 8.2 
                 62 ± 11.4 
               60.6 ± 8.7 
                 67 ± 7.5 
               60 ± 22 
             
             
               sulfur 
             
             
               fuel 
             
             
               500 hours 
                45.3 ± 8.15 
               40.45 ± 10.6 
               69.35 ± 10.7 
               78.95 ± 6.2  
               82.05 ± 8.2  
               59.55 ± 17.9  
             
             
               2-phase 
             
             
               corrosive 
             
             
               fuel 
             
             
                 
             
           
        
       
     
   
   In all cases except the copper powder with glue, the pull force actually increased with soaking in fuel. This is thought to be due to swelling of the cement. 
   The best results were obtained with the cements according to the invention, and the best results among these were with the fully cured cement. 
   It can be seen from the above results that the use of the cement of the invention provides the best combination of properties, with the least deterioration in the electrical properties and the greatest stability in the physical junction.