Abstract:
A brake drum is provided having an inner drum defining an outboard, radially inwardly extending flange. The flange defines a first plurality of apertures configured to receive fasteners for coupling the inner drum to a wheel. The inner drum further defines a braking surface. The brake drum further includes an outer drum configured to receive the inner drum therein. The outer drum defines an outboard, radially inwardly extending flange defining a second plurality of apertures aligned with the first plurality of apertures and configured to receive the fasteners for coupling the outer drum to the wheel. The inner drum is comprised of a first material such as iron while the outer drum is comprised of a second material such as aluminum.

Description:
BACKGROUND OF THE INVENTION 
     a. Field of the Invention 
     This invention relates to brake drums. In particular, the invention relates to a brake drum having nested inner and outer brake drums made from different materials to take advantage of different material properties. 
     b. Background Art 
     A conventional drum brake includes a brake drum that rotates with a wheel or wheels proximate to one end of an axle. The drum defines a radially inner braking surface. A brake spider is disposed about the axle and a pair of brake shoes are pivotally mounted at one end to the brake spider. The opposite end of each brake shoe is engaged by an actuating member such as a cam or hydraulic piston or wedge to move the brake shoes between positions of engagement and disengagement with the braking surface of the brake drum. 
     Conventional brake drums have a number of drawbacks. First, conventional brake drums often contribute to brake fade. Brake fade occurs when the brake actuator nears the furthest extent of its possible travel. In this situation, the force output of the brake actuator decreases as well as the resulting brake torque. Conventional brake drums are subject to ovalization which causes the brake actuator to travel further in order to maintain contact between the brake shoes and brake drum while also creating non-uniform contact with the brake linings. Conventional brake drums also retain excessive heat resulting in thermal expansion of the drum and requiring further travel of the actuator. Excessive temperatures also reduce the friction of brake lining materials, further contributing to brake fade. 
     Second, various portions of conventional brake drums heat and cool at different rates. This causes thermal stress on the drum which, along with mechanical stress, leads to cracks in the wall of the brake drum. 
     Third, conventional brake drums have sections that are relatively thick. When the braking friction surface of such a section is heated rapidly, its expansion is constrained by cooler portions of the section causing the warmer surface to yield in compression. When the section cools to a more uniform temperature, contraction of the formerly warm surface is again constrained by the remainder of the section causing the generation of surface cracks. The constrained expansion and contraction of the friction surface in conventional brake drums leads to shallow surface cracks or “heat checks.” 
     Fourth, conventional brake drums are relatively heavy. Most conventional brake drums are machined from gray iron castings. Graphite flakes in gray iron provide material damping and provide relatively stable friction and wear properties, but graphite flakes have planar discontinuities that result in low strength, brittleness and low stiffness. As a result, a relatively heavy design weight is required for gray iron brake drums. Further, conventional brake drums require a relatively large mass to act as a heat sink and control the temperature of the brake and surrounding structures. Materials including ductile iron, vermicular compacted graphite iron, steel, aluminum and aluminum matrix composites have been used in brake drums in an effort to overcome these deficiencies, but each of these materials have their own disadvantages in terms of poor friction and wear properties, low material damping and stiffness, high thermal stress and low strength. 
     The inventor herein has recognized a need for a brake drum that will minimize and/or eliminate one or more of the above-identified deficiencies. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention relates to brake drums. In particular, the invention relates to a brake drum having nested inner and outer brake drums made from different materials to take advantage of different material properties. 
     A brake drum in accordance with one embodiment of the present invention includes an inner drum. The inner drum defines an outboard, radially inwardly extending flange defining a first plurality of apertures configured to receive fasteners for coupling the inner drum to a wheel. The inner drum also defines a braking surface. The inner drum is comprised of a first material. The brake drum further includes an outer drum configured to receive the inner drum therein. The outer drum defines an outboard, radially inwardly extending flange defining a second plurality of apertures aligned with the first plurality of apertures and configured to receive the fasteners for coupling the outer drum to the wheel. The outer drum is comprised of a second material. 
     A brake drum in accordance with another embodiment of the present invention includes an inner drum defining a braking surface. The inner drum comprises iron. The brake drum further includes an outer drum comprising aluminum. The outer drum is configured to receive the inner drum therein and is engaged with the inner drum in an interference fit such that the inner drum may be removed and replaced with a replacement inner drum. 
     A method of manufacturing a brake drum in accordance with one embodiment of the present invention includes the steps of forming an outer drum of a first material and forming an inner drum of a second material. The inner drum defines a braking surface. The method further includes the steps of heating the outer drum and inserting the inner drum into the outer drum whereby the outer drum and the inner drum engage in an interference fit upon cooling of the outer drum. 
     A brake drum in accordance with the present invention represents an improvement relative to conventional brake drums. The use of different materials for the outer and inner drum enables the drum to take advantage of the different properties of the different materials. For example, in certain embodiments of the invention, the outer drum is made from aluminum with an outboard flange made from a metal matrix composite material. The composite material helps to resist ovalization of the drum while the aluminum provides an effective heat sink to reduce the temperature of the brake drum thereby reducing brake fade. The non-bonded interface between the inner and outer drums provides relief from thermal stress thereby reducing cracks in the wall of the brake drum. The use of two relatively thin drums as opposed to a single thick drum of a single material further reduces thermal stress and heat checks. Finally, the use of multiple drums of different materials allows the use of relatively low weight materials for portions of the brake drum. 
     The foregoing and other aspects, features, details, utilities, and advantages of the present invention will be apparent from reading the following description and claims, and from reviewing the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a brake drum in accordance with one embodiment of the present invention. 
         FIG. 2  is a flow chart illustrating method for manufacturing a brake drum in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views,  FIG. 1  illustrates a brake drum  10  cross-section in accordance with one embodiment of the present invention mounted to a conventional wheel  12 . Drum  10  is particularly adapted for use in heavy trucks. It should be understood, however, that drum  10  may be used on a wide variety of vehicles and in non-vehicular applications. Drum  10  may include an inner drum  14  and an outer drum  16  made from different materials in accordance with one aspect of the present invention. 
     Inner drum  14  defines a braking surface  18  on a radially inward side that may be selectively engaged by brake linings on conventional brake shoes that are moved into and out of engagement with drum  14  by a conventional brake actuator. In accordance with one aspect of the invention, drum  14  may be made from iron and, in particular, vermicular graphite iron. The use of vermicular graphite iron provides friction and wear properties compatible with conventional brake lining materials. It also provides material damping of low-displacement vibrations, torque transmission to wheel  12  and thermal isolation from wheel  12 . 
     Drum  14  is annular and configured for rotation with wheel  12  about a central axis  20 . Drum  14  defines a central aperture  22  centered about axis  20  and aligned with a corresponding aperture  24  in wheel  12 . Apertures  22 ,  24  are configured to receive one end of an axle (not shown) disposed about axis  20 . Drum  14  further defines a radially inwardly extending flange  26  at an outboard end of drum  14 . Flange  26  defines a plurality of apertures  28  that are aligned with corresponding apertures  30  in wheel  12  and configured to receive fasteners such as bolts for coupling drum  14  to and associated wheel  12  and hub (not shown). Drum  14  includes a bridge section  32  extending between flange  26  and an inboard section  34  that defines surface  18 . The inner and outer diameters of bridge section  32  may increase moving from flange  26  to inboard section  34  (i.e. drum  14  widens moving from flange  26  to section  34 ) and may increase at the same rate such that the width of section  32  remains constant throughout its length. The inner and outer diameters of section  34  may be constant over the length of section  34  and the width of section  34  may also remain constant throughout its length. 
     Outer drum  16  provides a means for transferring heat away from inner drum  14 , resisting ovalization of drum  10  and relieving thermal stress on drum  10 . In accordance with one aspect of the invention, drum  16  may be made from aluminum. The use of aluminum in drum  16  is advantageous because it is relatively lightweight and provides an effective heat sink due to its high specific heat and high thermal conductivity. Drum  16  is sized to receive inner drum  14  and a radially inner surface of drum  16  is shaped complementary to a radially outer surface of drum  14 . The interface between drums  14 ,  16  and the use of different materials for drums  14 ,  16  provides relief from thermal stress and Coulomb damping (dissipation of energy through sliding fiction) of higher-displacement vibrations. 
     Drum  16  is annular and configured for rotation with wheel  12  and inner drum  14  about axis  20 . Drum  16  defines a central aperture  36  aligned with aperture  24  in wheel  12  and aperture  22  in drum  14  and configured to receive one end of an axle (not shown) disposed about axis  20 . Drum  16  defines a radially inwardly extending flange  38  at an outboard end of drum  16 . Flange  38  defines a plurality apertures  40  that are aligned with apertures  30  in wheel  12  and apertures  28  in drum  14  and that are configured to receive fasteners such as bolts for coupling drum  16  to wheel  12  and drum  14 . Flange  38  has sufficient strength to resist transient vibration-induced loads. Drum  16  includes a bridge section  42  extending between flange  38  and a central section  44  and disposed radially outwardly of bridge section  32  in inner drum  14 . The inner and outer diameters of bridge section  42  may increase moving from flange  38  to central section  44  (i.e. drum  16  widens moving from flange  38  to section  44 ) and may increase at the same rate such that the width of section  42  remains constant throughout its length. Although the inner diameter of section  44  may remain constant over the length of section  44 , the outer diameter of section  44  may increase moving in an inboard direction such that the width or thickness of section  44  of drum  16  narrows moving from an inboard end of drum  16  towards an outboard end of drum  16 . This configuration helps to direct conduction of heat outboard towards the exposed end of the drum  10 , rather than inboards towards wheel  12 . Section  44  may also define a plurality of radially outwardly extending fins  46  that increase in radial height moving away from the outboard end of drum  16  and towards the exposed inboard end of drum  16 . Fins  46  aid convection of heat from drum  10  to the atmosphere. 
     Drum  16  defines a radially outwardly extending flange  48  at an inboard end. Flange  48  may be relatively thick and, in particular, thicker than section  42  of drum  16 . In accordance with one aspect of the present invention, flange  48  may be made from a metal matrix composite material. In particular, flange  48  may be made from aluminum and ceramic. The form and material composition of flange  48  provide increased stiffness to drum  10  which resists ovalization of both inner drum  14  and outer drum  16 . The lower conductivity and sensitivity to thermal stress of flange  48  relative to the remainder of drum  16  is mitigated by the distance of flange  48  from braking surface  18 . 
     Referring now to  FIG. 2 , a method of manufacturing a brake drum  10  in accordance with one embodiment of the present invention will be described. The method may begin with the step  50  of forming an inner drum  14 . As discussed above, the inner drum  14  may be made from iron and particularly vermicular graphite iron and defines a braking surface  18 . Step  50  may include the substeps  52 ,  54  of casting drum  14  (and specifically, spin casting drum  14 ) and machining the radially outer surface of drum  14 . The method may also include the step  56  of forming an outer drum  16 . As discussed hereinabove, drum  16  may be made from aluminum with an outboard flange  48  made from a metal matrix composite. Step  56  may include the substep  58  of inserting a porous ceramic preform into a mold. Step  56  may further include the substep  60  casting drum  16  around the preform such that the preform is located in a radially outwardly extending flange  48  at the inboard end of drum  16 . High pressure die casting or squeeze casting may be used in substep  60  to allow the aluminum to infiltrate the ceramic preform. Step  56  may further include the substep  62  of machining a radially inner surface of drum  16 . Drum  10  may then be anodized to increase corrosion and wear resistance. As illustrated in  FIG. 2 , steps  50  and  56  (and their associated substeps) may be performed concurrently or simultaneously. It should be understood, however, that the steps  50  and  56  (and their associated substeps) may alternatively be performed in succession. 
     The method may continue with the step  64  of heating outer drum  16  to permit thermal expansion of drum  16 . The method may then include the step  66  of inserting drum  14  into drum  16 . Step  66  may include the substep of aligning apertures  28  in flange  26  of drum  14  with apertures  40  in flange  38  of drum  16 . The method may then include the step  68  of cooling outer drum  16  such that outer drum  16  contracts and inner and outer drums  14 ,  16  engage in an interference fit. Cooling may take place through natural, passive heat dissipation from drum  16  or through active enhancement of the cooling process. The size of drums  14 ,  16  and the high thermal expansion coefficient of aluminum in drum  16  allows the drums  14 ,  16  to be shrunk-fit together with low residual stress. Drums  14 ,  16  are not permanently bonded and during high operating temperatures, a slight clearance may open between the cylindrical sections of drums  14 ,  16  while the brake is released. In accordance with one aspect of the present invention, outer drum  16  can be reheated to allow removal and replacement of drum  14  with a replacement inner drum when wear occurs. The method may conclude with the step  70  of machining the radially inner surface of inner drum  14  including braking surface  18 . 
     A brake drum  10  in accordance with the present invention represents an improvement relative to conventional brake drums. The use of different materials for the inner and outer drums  14 ,  16  enables the drum  10  to take advantage of the different properties of the different materials. For example, in certain embodiments of the invention, the outer drum  16  is made from aluminum with an inboard flange  48  made from a metal matrix composite material. The composite material helps to resist ovalization of the drum  10  while the aluminum provides an effective heat sink to reduce the temperature of the brake drum  10  thereby reducing brake fade. The interface between the inner and outer drums  14 ,  16  provides vibration damping and relief from thermal stress thereby reducing cracks in the wall of the brake drum. The use of two relatively thin drums  14 ,  16  as opposed to a single thick drum of a single material further reduces thermal stress and heat checks. The use of multiple drums  14 ,  16  of different materials also allows the use of relatively low weight materials for the brake drum  10 . Finally, the non-bonded, interference fit between drums  14 ,  16  permits removal and replacement of inner drum  14  when wear occurs. 
     While the invention has been shown and described with reference to one or more particular embodiments thereof, it will be understood by those of skill in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.