Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to U.S. Provisional Application Ser. No. 60/286,969, filed on Apr. 30, 2001, the entire contents of which are incorporated herein by reference. 
    
    
     DESCRIPTION OF THE INVENTION 
     The present invention proposes a new brush holder for metal fiber and metal foil brushes. It is designed to guide the brush in axial direction as it wears even while a constant light brush pressure is exerted and a large current is conducted to or from the brush at very low electrical resistance. The invention is depicted in FIGS. 1A to  1 E and is called the “tubular brush holder.” 
     In the version of FIGS. 1A and 1B, fiber brush  27  is conductively fastened to metal baseplate  28  and is pushed forward in electrically conductive brush box  29  by constant force spring  37  so as to load brush  27  against rotor  2  with the desired brush force. At least one of the four long sides of the inside of brush box  29  is lined in an electrically conductive manner with resilient multi-contact metal material  47 , i.e., in the form of metal velvet, metal felt, strands of fine metal fibers, metal fibers combined in the manner of textiles, e.g., through weaving or knitting or any other. At least one metal guide  38  is rigidly, electrically conductive attached to base plate  28  in such a manner that it is parallel to at least one side of the brush box that is lined with resilient multi-contact metal material  47  and is disposed so that it guides fiber brush  27  along the inside of brush box  29  while the brush wears. 
     Low-resistance electrical contact is established between the at least one guide  38  and at least one multi-contact metal material by means of at least one compression spring  54  (in this case depicted as helical springs  54 ( 1 ) and  54 ( 2 ) extending between guide  38  and a thin low-friction plate  39  that is flexibly hinged to baseplate  28  disposed so that it is parallel to the at least one metal guide  38 . Teflon may be a particularly suitable material for hinged low-friction plate  39 . For further stabilization of the brush motion, the remaining inner surfaces of the brush box may be provided with low-friction liner or the edges of base plate  28  may be lined with a low-friction material such as Teflon. Such a liner is indicated by number  56 . 
     The spring action between guide  38  and hinged low-friction plate may be provided by at least one conventional spring, e.g., a helical spring, or by at least one constant force spring. The spring force is adjusted to compare with the brush force exerted on fiber brush  27  by means of constant force spring  37 . The friction force due to the described elastic compression of the multi-contact metal material  47  and metal guide  38  will reduce the brush force on fiber brush  27  by about 30% of the force imposed by constant force spring  37 , for the reason that the coefficient of friction between multi-contact metal material and smooth metals is in the range of 0.2 to 0.3. 
     The contact resistance between brush box  29  and fiber brush  27  via the resilient multi-metal material  47  will be about one half of the electrical resistance between the brush and rotor  2  since static multi contacts have about one half of the resistance of similar sliding contacts under same pressure (compare C. M. Adkins III and D. Kuhlmann-Wilsdorf, “Devleopment of high-performance Metal Fiber Brushes II—Testing and Properties”, Electrical Contacts—1979 (Proc. Twenty-Fifth Holm Conf. On Electrical Contacts, III. Inst. Techn., Chicago, Ill., 1979), pp. 171-184, the entire contents of which is incorporated by reference herein.), provided that the surfaces are clean. This is a requirement that must be fulfilled, either by operating in a protective atmosphere such as humidified CO 2  or making the contact surface between multi contact metal material and slider of noble metal or plating with a noble metal. 
     In cases of high packing density of brush holders, e.g., as may be the case in homopolar motors, the outside of the brush box should be coated with a nonconductive paint or lacquer, e.g., red stop-off lacquer. 
     For a particular application, the tubular brush holder designed in FIGS. 1A and 1B may be too costly or requires too much space. In fact restricted space for bushes is a perennial problem for homopolar motors since their appeal derives from their potentially large power to weight ratio and, due to the typically low voltage per current turns, they require large numbers of brushes. FIGS. 1C to  1 E therefore show simplifications of the concept of the tubular brush holder according to the present invention that are designed both to save space and cost. 
    
    
     In FIG. 1C, the dual function of guiding the brush axially with minimal friction while it wears and conducting current of the brush at very low resistance, is performed by the releasable conductive clamping mechanism  42   a  that comprises a flexible metal ring with spring action  58  that is lined with resilient multi-contact metal material. An optimum compromise between friction and electrical resistance can be achieved by adjusting the wing nut  24 , since tightening it will decrease resistance but increase friction and vice versa. 
    
    
     However, this is just one example of how to construct a releasable clamping mechanism. For example, the clamping mechanism  42   a  may apply to a stem extending from the base plate or some metal guide as in FIGS. 1A and 1B. Also a matter of choice will be the means whereby clamping mechanism  42   a  may be releasably but rigidly mechanically fastened to the stator. One possibility would be to provide a screw thread at the upper end of  42   a  such that it does not interfere with the spring action due to the wing nut acting on the flexible metal ring  58  and a matching threaded hole in the stator. Another would be to similarly provide a dovetail at clamping device  42   a  and a female receptacle for such a dovetail in the stator. 
     Still another version of the tubular brush holder is shown in FIG. 1D where the brush is guided by a cavity in the stator that is lined with resilient multi-contact metal material. Here that guidance is optionally supplemented by a stem  59  stem extending in axial direction from the brush base plate  28  and guided in hole  60  in the stator that may be optionally lined with resilient multi-contact metal material. In this example the brush force is applied by two constant force springs. The advantage of this version of the tubular brush holder is its still greater simplicity, but it lacks the possibility of adjusting the pressure between the multi-contact material and the brush base plate. 
     The simplest tubular brush holder is shown in FIG. 1E, wherein the brush is guided by stem  59  in hole  60  lined with resilient multi-contact metal material  47  and is subject to the brush pressure applied by constant force spring  37 . 
     A common feature of all tubular brush holders, shown in FIGS. 1A to  1 E and any other modifications, is that the current is transferred between stator and brush by means of low-friction, low resistance resilient multi-contact metal material that is electrically connected to the current supply or load and is mechanically rigidly attached to the stator.

Technology Category: 5