Abstract:
A brush replacement kit for use with a dynamo electric machine of the kind having a yoke, apertures in the yoke through which brushes of the machine are accessible, and a brush holder assembly for supporting the brushes in a predetermined alignment in relation to a commutator or slip-ring assembly of the machine, the kit comprising replacement brushes of a length exceeding the length of the conventional brushes for the machine, and a cover component comprising a cylindrical band for attachment around the exterior of the yoke to close said brush access apertures in use, said band including radially outwardly extending protrusions to be positioned over said apertures of the yoke in use and thereby to accommodate the additional length of the replacement brushes.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present invention is based on U.S. Provisional Patent Application Ser. No. 60/353,763, filed on Jan. 31, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a kit for improving the service life of a dynamo electric machine, and to a dynamo electric machine modified by the use of such a kit. 
     The term “service life” is used herein to indicate a recommended operational period between maintenance times at which the machine is withdrawn from service for scheduled maintenance. 
     The invention is particularly, but not exclusively, concerned with dynamo electric machines for use in conjunction with aircraft gas turbine engines, particularly starter-motor/generator units. The present inventor has recognised that recent improvements in the design and manufacture of such dynamo electric machines has produced a situation in which the service life is governed by the wear rate of the brushes of the machine and has recognised that if the useful life of the brushes of the machine can be extended then the service life can be extended by a corresponding amount up to the point at which the service life of some other component of the machine is reached. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there is provided a kit for use with a dynamo electric machine of the kind having a yoke, apertures in the yoke through which brushes of the machine are accessible, and a brush holder assembly for supporting the brushes in a predetermined alignment in relation to a commutator or slip-rings of the machine, the kit comprising replacement brushes of a length exceeding the length of the conventional brushes for the machine, and a cover component comprising a cylindrical band for attachment around the exterior of the yoke to close said brush access apertures in use, said band including radially outwardly extending protrusions to be positioned over said apertures of the yoke in use and thereby to accommodate the additional length of the replacement brushes. 
     Preferably the kit further includes replacement springs for attachment to respective brush holders and engagable with the outer ends of the replacement brushes to urge the brushes radially inwardly of the machine in use throughout the increased wear length of the replacement brushes. 
     Conveniently said replacement springs comprise constant force springs together with clip means for anchoring the constant force springs to their respective brush holder. 
     Desirably the kit further includes replacement brush holders shaped to receive said replacement springs and said replacement brushes. 
     In another aspect the invention resides in a brush replacement kit for use with a dynamo electric machine of the kind having a yoke, apertures in the yoke through which brushes of the machine are accessible, and a brush holder assembly for supporting the brushes in a predetermined alignment in relation to a commutator or slip-ring assembly of the machine, the kit comprising replacement brushes of a length exceeding the length of the conventional brushes for the machine, replacement springs for attachment to respective brush holders and engagable in use with outer ends of respective replacement brushes to urge the brushes radially inwardly of the machine in use throughout the increased wear length of the replacement brushes, said replacement springs comprising constant force springs together with clip means for anchoring the constant force springs to a respective brush holder, replacement brush holders shaped to receive said replacement springs and said replacement brushes, and a cover component comprising a cylindrical band for attachment around the exterior of the yoke to close said brush access apertures in use, said band including radially outwardly extending protrusions to be positioned over said apertures of the yoke in use and thereby to accommodate the additional length of the replacement brushes. 
     The invention also resides in a dynamo electric machine fitted with a kit of the kind defined above. 
     In a further aspect of the invention there is provided a dynamo electric machine of the kind having a yoke, apertures in the yoke through which brushes of the machine are accessible, a brush holder assembly for supporting the brushes in a predetermined alignment in relation to a commutator or slip-ring assembly of the machine, brushes of a length exceeding the length of the conventional brushes for the machine, spring arrangements attached to respective brush holders and engaging with outer ends of respective brushes to urge the brushes radially inwardly of the machine in use throughout the increased wear length of the brushes, and a cover component comprising a cylindrical band for attachment around the exterior of the yoke to close said brush access apertures in use, said band including radially outwardly extending protrusions to be positioned over said apertures of the yoke in use and thereby to accommodate the additional length of the brushes. 
     Preferably said spring arrangements comprise constant force springs together with clip means for anchoring the constant force springs to a respective brush holder, and said brush holders being shaped to receive said constant force springs together with clip means, and said brushes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a diagrammatic cross-sectional view of a conventional dynamo electric machine; 
     FIG. 2 is a diagrammatic perspective view of a brush holder of a kit in accordance with one example of the present invention; 
     FIG. 3 is an exploded perspective view of part of a dynamo electric machine equipped with the brush holders of FIG. 2; 
     FIG. 4 is a perspective view of part of a dynamo electric machine equipped with the brush holders of FIG. 2 containing brushes and replacement springs; 
     FIG. 5 is a diagrammatic perspective view of a brush and a brush spring forming part of the kit in accordance with one example of the present invention; 
     FIG. 6 is a diagrammatic perspective view illustrating one stage in the insertion of replacement brushes of the kit in accordance with one example of the present invention into a brush holder of the kind shown in FIG. 2 positioned within a dynamo electric machine; 
     FIG. 7 is a view similar to FIG. 6 illustrating a later stage in the application of the kit; and 
     FIGS. 8 and 9 are diagrammatic perspective views of alternative yoke covers forming part of the kit in accordance with one example of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring first to FIG. 1 of the accompanying drawings, it can be seen that the conventional dynamo electric machine which, in this case is a direct current motor, but could be starter-motor/generators or other similar machine, has an outer, cylindrical, ferromagnetic yoke  11  internally supporting stator poles  12  carrying stator windings  13  in known manner. Rotatable within the assembly of stator poles  12  is a rotor  14  including an elongate shaft  15  journalled at its opposite ends respectively for rotation in bearings mounted in end caps secured to the yoke  11  in use. Adjacent one end the shaft  15  carries a commutator  16  (or a slip ring assembly) engaged at spaced points around its circumference by brushes  17  slidably received in brush holders  18  electrically insulated from the case of the machine. Electrical connections are made by conductive braids or twisted strands in conventional manner to the brushes  17  and springs (not shown) urge the brushes radially inwardly to contact the commutator or slip ring assembly  16 . The yoke  11  has a plurality of apertures  19  through which access may be obtained to the brush holders  18  to facilitate replacement, in a radial direction, of the brushes  17 . The apertures  19  are normally closed in use by a plain cylindrical band, formed from steel or other suitable material, and clamped around the exterior of the yoke in conventional manner. 
     The present inventor has recognised that the longevity which can now be achieved in the component life of dynamo electric machines results in the wear rate of the brushes  17  determining the service life of the dynamo electric machine, that is to say determines the period during which the dynamo electric machine is operational between scheduled service intervals at which the brushes are replaced. The inventor has therefore recognised that by increasing the length of the brushes, in a direction generally radially of the dynamo electric machine, then the life of the brushes can be extended by comparison with brushes of conventional length as shown in FIG. 1, and thus the service life of the machine can be extended since the remaining components of the dynamo electric machine can greatly outlive the service life of the brushes  17 . Accordingly the inventor has invented a kit for use at a service interval whereby increased length brushes may be accommodated without major modification to the structure of the remainder of the dynamo electric machine. 
     Referring now to FIGS. 2 to  8  inclusive the kit comprises a plurality of replacement brush holders  21  as illustrated in FIG. 2, a plurality of extra length brushes  22 , a corresponding plurality of constant force spring assemblies  23  (FIG. 5) and a cylindrical cover band  24  (FIG.  8 ). 
     The dynamo electric machine illustrated in FIG. 3 has been fitted with four brush holders  21  of the kind shown in FIG.  2 . Each brush holder  21  is a stainless steel casting and has through passages  25  for slidably receiving first and second brushes in use. As is apparent from FIG. 3 the dynamo electric machine includes an end cap assembly  26  having fins  27  for guiding the air flow through the yoke of the machine generated by a fan of the machine in use. 
     The end cap  26  is arranged to be secured to the yoke  11  of the dynamo electric machine by a plurality of axially extending screws (not shown) which pass through a peripheral ring of the end cap  26  and are threaded into the thickness of the yoke  11 . The brush holders  21  are formed with integral mounting lugs  29  through which the brush holders  21  can be secured in the appropriate position, by means of screws or bolts, to the inner face of the end cap  26 . Thus at a service interval of the dynamo electric machine the end cap  26  is disconnected from the yoke  11 , the existing brush holders are disconnected from the end cap  26  and are discarded, and the replacement brush holders  21  are secured to the end cap. The assembly of the end cap and brush holders is then replaced on the yoke  11  and secured in position with the brush holders  21  exposed through windows  19  in the yoke  11 . 
     Replacement brushes  22  of significantly greater length than the original brushes which they replace, are inserted into the passages  25  of the respective brush holders  21 . Each brush is of rectangular cross-section and is a loose sliding fit within the passage  25  of its respective brush holder. Furthermore, at its radially outermost end each brush  22  carries a stainless steel saddle  32  connected to the conductive material of the brush and seated on an inclined end face  33  of the brush. 
     Associated with each brush  22  is a constant force spring and clip assembly  23  which replaces a conventional “clock spring” arrangement customarily used to urge the original brush radially inwardly. Each constant force spring and clip assembly  23  includes a clip  34  bent from stainless steel strip and comprising first and second generally parallel limbs  34   a ,  34   b  (FIG. 5) interconnected at one end by an integral bend  35 . The limb  34   a  is shorter than the limb  34   b  and terminates at its end remote from the bend  35  in a perpendicular flange  36  having an aperture  37  for receiving a screw in use. The end of the limb  34   b  remote from the bend  35  has a perpendicular flange  38  (seen in FIG. 5 but omitted from FIGS. 4 and 7) bent to extend in the opposite direction to the flange  36 . Intermediate its ends the limb  34  is formed with an outwardly projecting tongue  39  (FIG.  7 ). Secured to the face of the limb  34   b  remote from the limb  34   a  is an elongate constant force spring  41  in the form of an elongate strip of spring material, for example stainless steel. The strip of material forming the spring  41  is secured to the limb  34   b  conveniently by welding or rivetting, at a point adjacent the bend  35 . The strip-like spring  41  is rolled along its length, and if unrolled, against its inherent resilience, will lie along the length of the limb  34   b  as illustrated in FIG.  5 . However, in a rest configuration the spring  41  curls into a roll or scroll adjacent its point of connection with the limb  34   b . The flange  38  overlies the scroll  41   a  of the spring in order to protect the spring during servicing. The flange may be coated with an electrically insulating material such as epoxy resin to reduce the risk of electrical arcing in use. 
     The spring and clip assembly associated with each brush  22  is introduced into the brush holder  21  after insertion of its respective brush. Each assembly is introduced between the face of the brush and the brush holder, and sits within a lateral extension  25   a  of the respective passage  25 . Each assembly  23  is positioned with the spring  41  presented to the respective brush  22  and the scroll  41   a  of the spring seated on the saddle  32  of its respective brush. As each assembly is inserted into the brush holder the spring unrolls and when the assembly is fully seated in the brush holder the remainder of the scroll  41   a  seated in the respective saddle  32  is exerting a force on the saddle to urge the brush radially inwardly of the dynamo electric machine. 
     When each spring and clip assembly is fully seated in its respective brush holder the flange  36  overlies a respective lug  37   a  integral with the outer face of the respective brush holder and a respective securing screw (not shown) is inserted through the respective aperture  37  and into the lug  37   a  to clamp the flange  36  and therefore the assembly, to the brush holder. It will be recognised that as a replacement brush wears in use the respective spring rerolls under it&#39;s inherent resilience to urge the brush radially inwardly, towards the commutator or slip ring assembly, through the whole wear length of the replacement brush. 
     Although not apparent in the drawings the tongues  39  of the limbs  34   a  of the clips  34  coact with respective undercut shoulders in the brush holders  21  to retain the spring and clip assemblies  23  in position in the brush holder prior to introducing the fixing screws to the apertures  37 . 
     It will be understood that if, when replacing the original brushes, it were possible to substitute the original “clock spring” arrangement for urging the brushes radially, with a “clock spring” arrangement suitable for the increased wear length of the replacement brushes, as may be the case in some dynamo electric machine types, then replacement of the associated brush holder would be unnecessary as a primary reason for replacing the brush holders is to provide holders which accommodate the replacement spring and clip assemblies  23 . 
     As mentioned above the brushes  22  are significantly longer than the conventional brushes which they have replaced. As is apparent from FIGS. 6 and 7 the brushes  22  are of such a length that they protrude radially through the respective apertures  19  in the yoke  11 . Similarly the limbs  34   b  of the clips  34  together with the scrolls of the springs  41  also protrude through the apertures  19 . FIG. 8 illustrates a cover band  24  forming part of the kit, and which is attached to the exterior of the yoke  11  to close the apertures  19 . As is apparent the band  24  comprises four hollow box structures  42  conveniently formed as sheet metal pressings and interconnected by three part circular metal band portions  43 . The box structures  42  are positioned with their open faces presented inwardly, and are spaced apart by 90° around the common axis of the band portions  43 . A further band portion  43  is divided into first and second sections  43   a ,  43   b  each of which has secured thereto a strap  44  having a radially outwardly extending flange  45 . The flanges  45  can be secured together by a nut and bolt or the like so that the assembly of portions  43  and box structures  42  defines a cylindrical band. The internal diameter of the cylindrical band is equal to the external diameter of the yoke  11 , and by detaching the flanges  45  from one another the band can be slid over the axial end of the yoke  11  to overlie the apertures  19 . The cover band  24  is formed with internally projecting lugs  46  which engage within one or more of the apertures  19  to locate the band  24  circumferentially on the yoke  11  with the open faces of the box structures  42  disposed over the apertures  19  and therefore accommodating the protruding portions of the brushes  22  and clip assemblies  23  without touching them. Thus the box structures  42  provide protective covers overlying, but spaced from the brushes and clip assemblies. 
     It will be recognised that the cover band  24  can be fabricated from a plurality of separately formed pressed steel strip components or, if desired, could be formed as an integral pressing in elongate strip which is then bent to a cylindrical form. If desired the cover band  24  could be formed from synthetic resin material since it adds little to the ferromagnetic properties of the yoke at the axial end of the yoke. FIG. 9 shows an alternative to the band of FIG. 8 in which a continuous strip of metal is bent to define a cylindrical band  51  the ends of which are provided with projecting lugs  52  whereby the band can be clamped around the yoke  11  using a pinch bolt or the like. At four equiangularly spaced points the band has rectangular apertures  53  closed by outwardly projecting box formations  54  which can be integrally formed in the strip of band by a pressing, stamping or deep drawing operation or which can be separately formed and then secured to the apertured band by welding or the like, the separately formed formations  54  having integral skirts to facilitate spot welding to the band.