Abstract:
An improved housing, particularly one for an automotive differential assembly, is provided. The housing has a cast portion formed from a first material and a preformed insert member formed from a second material which is cast into the cast portion. The insert member is strategically placed in an area of the housing to selectively enhance or alter the properties of the housing at that location. The insert member may be encased in a wall of the cast portion or may extend partially through a wall of cast portion depending upon the characteristics which are being enhanced or altered. The properties which may be enhanced or altered include, for example the strength, electric or thermal conductivity, magnetic potential, chemical compatibility or the coefficient of friction.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to cast fabrications and more particularly to a cast fabrication having a cast portion and an insert of a material dissimilar to that of the cast portion. 
     2. Discussion 
     The design criteria for many types of housings typically includes considerations for the strength as well as the overall weight of the housing. This is particularly true for housings designed for use in highway vehicles, such as automobiles and trucks. One such component employed by such vehicles is a differential housing which supports the axles and drive shaft of a vehicle. Conventionally, differential housings have been formed out of lightweight materials, such as aluminum, to reconcile the competing design criteria of strength and weight. 
     While this strategy has provided satisfactory results with the engine and drive train combinations known in the art, the modern trend of vehicle manufacturers is toward vehicles having more power and torque. In some instances, conventional differential housings have not been satisfactorily integrated into such new vehicles with sufficient durability and failures have been noted in high-stress areas, primarily in the area where the drive shaft is supported for rotation within the differential housing. Attempts to improve the durability of the prior art differential housings have included various measures ranging from the fabrication of the differential housing from different materials having improved load carrying characteristics to the complete redesign of the axle system. Neither solution, however, has proven to be entirely satisfactory. 
     The use of a different material, such as cast iron, for example, entails not only a substantial weight penalty, but also substantial increases in the fabrication piece costs due to increased variability in the casting processes where the aluminum differential housing had been die cast. Additionally, conversion to a different material may necessitate the purchase of new equipment designed specifically to machine the new material. Examples of such equipment could range from new spindles, feed screws and tool resharpening equipment, to new machine tools designed to accommodate the specific characteristics of the material to be machined. In addition to being extremely costly, this approach may not be implementable in a given facility due to floor space limitations where production of conventional differential housings is still required. 
     The second alternative, redesigning the complete axle system, is also extremely costly, typically requiring vast resources to design, model and test the new axle system. Furthermore, the final design of many components is based on criteria established by “concurrent engineering” groups which utilize input from several disciplines, such as those associated with the casting, machining, assembly and servicing of the axle system. This additional criteria permits the component to be fabricated and serviced in a reliable and cost-effective manner. While such “concurrent engineering” efforts generally produce high quality, robust components, it is frequently difficult and costly to accommodate even the key fabrication and servicing concerns where the component is subjected to loads generating high stresses. 
     SUMMARY OF THE INVENTION 
     It is therefore one object of the present invention to provide an improved housing fabricated from two dissimilar materials. 
     It is a more specific object of the present invention to provide an improved housing which selectively employs an insert member of a material dissimilar to the primary material from which the housing is formed to provide the housing with one or more enhanced properties in a predetermined area. 
     An improved housing, particularly one for an automotive differential assembly, is provided. The housing of the present invention includes a cast portion formed from a first material and a preformed insert member formed from a second material which is cast into the cast portion. The insert member is strategically placed in an area of the housing to selectively enhance or alter the properties of the housing at that point. The insert member may be encased in a wall of the cast portion or may extend partially through a wall of cast portion depending upon the characteristics which are being enhanced or altered. The properties which may be enhanced or altered include, for example the strength, electric or thermal conductivity, magnetic potential, chemical compatibility or the coefficient of friction. 
     Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of an exemplary housing constructed according to a preferred embodiment of the present invention; 
     FIG. 2 is a cross-sectional view of an exemplary housing constructed according to an alternate embodiment of the present invention; 
     FIG. 3 is a functional illustration of a motor vehicle equipped with a differential housing constructed in accordance with the teachings of the present invention; 
     FIG. 4 is a cut-away perspective view of the differential housing shown in FIG. 3; and 
     FIG. 5 is a section view of a portion of the differential housing shown in FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the housing of the present invention is generally indicated by reference numeral  10 . Housing  10  includes a cast portion  14  and an insert member  18 . Cast portion  14  is formed from a first material having a first set of material properties. Insert member  18  is formed from a second material having a second set of material properties. 
     The first material is selected based upon the first set of material properties and a plurality of design criteria which frequently include considerations for the weight, strength, cost and processability (i.e., the ease with which the material is cast, machined and finished). The second material is selected based on the second set of material properties and its ability to compliment the properties of the first material to completely satisfy the plurality of design criteria. Note, too, that considerations for the shape and size of insert member  18  are also key to the fulfillment of the design criteria and as such, the scope of the present invention is not limited to an insert of any given size or shape. Accordingly, the present invention is not limited to an insert member  18  having a generally tubular shape as shown in FIG. 1, but also includes other structural shapes which are tailored to meet the plurality of design criteria for a given application. Furthermore, the second material may contribute any number of properties to housing  10 , including tensile strength, sheer strength, electric or thermal conductivity, magnetic potential, chemical resistance or even the coefficient of friction. 
     Housing  10  is formed in a suitable casting process, such as die casting, investment casting (including lost wax and lost foam), and sand casting. Insert member  18  is preformed and positioned in a mold cavity. The first material is then heated to a molten state and introduced into the mold cavity to at least partially encase insert member  18  into a wall  22  of cast portion  14 . 
     As shown in FIG. 1, insert member  18  may be fully encased in wall  22  to permit, for example, an aperture  26  in housing  10  to be formed in a conventional manner (e.g., coring, drilling, reaming, boring) without the use of special tooling including tool bits and equipment which may otherwise be necessary if insert member  18  extended through wall  22  into aperture  26 . In this example, the first material is an aluminum alloy and second material is a steel alloy. The aluminum alloy has good processability characteristics, modest strength and enables housing  10  to be fabricated in a relatively light-weight manner. The steel alloy provides housing  10  with increased strength in an area proximate its location. 
     Alternatively, as shown in FIG. 2, insert member  18 ′ maybe partially encased in wall  22 ′ of cast portion  14 ′ to permit the second material which forms insert member  18 ′ to extend into aperture  26 ′ in housing  10 ′. Construction of housing  10 ′ in this manner incorporates the second set of material properties into a surface  30  of aperture  26 ′. In this example, the first material comprising cast portion  14 ′ is an aluminum alloy and the second material comprising insert member  18 ′ is a plastic material, such as nitrile. The nitrile material provides surface  30  with a reduced coefficient of friction relative to that provided by the aluminum alloy and permits a journal-style bearing to be incorporated into housing  10 ′ during the casting process. 
     A more specific application of the present invention is illustrated in FIGS. 3 through 5. With reference to FIG. 3, a vehicle  40  is schematically shown which is suited for use with the present invention. Vehicle  40  includes a driveline  42  drivable via a connection to a powertrain  44 . Powertrain  44  includes an engine  46  and a transmission  48 . Driveline  42  includes a driveshaft  50 , a rear axle  52  and a plurality of wheels  54 . Engine  46  is mounted in an in-line or longitudinal orientation along the long axis of vehicle  40  and its output is selectively coupled via a conventional clutch to the input of transmission  48  to transmit drive torque therebetween. The input of transmission  48  is commonly aligned with the output of engine  46  for rotation about a rotary axis. Transmission  48  also includes an output and a gear reduction unit. The gear reduction unit is operable for coupling the transmission input to the transmission output at a predetermined gear speed ratio. Driveshaft  50  is coupled for rotation with the output of transmission  48 . Drive torque is transmitted through driveshaft  50  to rear axle  52  where it is selectively apportioned in a predetermined manner to the right and left rear wheels  54   a  and  54   b.    
     Referring now to FIGS. 4 and 5, rear axle  52  is shown to include a differential assembly  72 , a left axle shaft assembly  74  and a right axle shaft assembly  76 . Differential assembly  72  includes a housing  80 , a differential unit  84  and an input shaft assembly  88 . Housing  80  supports differential unit  84  for rotation about a first axis  92  and further supports input shaft assembly  88  for rotation about a second axis  96  that is perpendicular to first axis  92 . 
     Housing  80  is initially formed in a suitable casting process such as die casting, investment casting (lost wax or lost foam) or sand casting, and thereafter machined as required. Housing  80  includes a cast portion  114  and an insert member  118 . Cast portion  114  includes a wall member  122  defining a central cavity  126  having a left axle aperture  130 , a right axle aperture  134 , and an input shaft aperture  138 . Cast portion  114  is formed from a first material, such as an aluminum alloy, to provide housing  80  with modest strength and a relatively low mass. 
     Insert member  118  is preformed in a desired structural shape from a second material, such as a steel alloy, to selectively strengthen a portion of housing  80 . Insert member  118  is generally shaped in the form of a hollow cylinder and includes an exterior surface  154 , an interior surface  158  and preferably, a plurality of retaining apertures  162  which extend at least partially through one or both of the exterior and interior surfaces  154  and  158 , respectively. 
     Cast portion  114  is formed around insert member  118  so as to at least partially encase or encapsulate insert member  118  in wall member  122 . During the casting process, while the first material is in a molten state, a portion of the first material flows into the plurality of retaining apertures  162  and mechanically fixes insert member  118  to wall member  122  when the second material solidifies. Preferably, insert member  118  and wall member  122  are sized in a manner which permits input shaft aperture  138  to be formed in a conventional manner, such as coring, drilling, reaming and boring, without the use of special processes, tooling or equipment as compared to a housing which does not include insert member  118 . 
     Left axle shaft assembly  74  includes a first axle tube  164   a  fixed to axle aperture  130  of housing  80  and a first axle half-shaft  166   a  supported for rotation in first axle tube  164   a  about first axis  92 . Similarly, right axle shaft assembly  76  includes a second axle tube  164   b  fixed to axle aperture  134  of housing  80  and which supports a second axle half-shaft  166   b  for rotation about first axis  92 . 
     Differential unit  84  is disposed within central cavity  126  of housing  80 . Differential unit  84  includes a case  170 , a ring gear  174  fixed for rotation with case  170 , and a gearset  176  disposed within case  170 . Gearset  176  includes first and second side gears  178   a  and  178   b  and a plurality of differential pinions  186  rotatably supported on pinion shafts  188  mounted to case  170 . Case  170  includes a pair of trunions  190   a ,  190   b  and a gear cavity  194 . Bearing assemblies  182   a ,  182   b  are shown to support trunions  190   a ,  190   b  from housing  80  for rotation about first axis  92 . First axle half-shaft  166   a  and second axle half-shaft  166   b  extend through left axle aperture  130  and right axle aperture  134 , respectively, where they are coupled for rotation about first axis  92  with first and second side gears  178   a  and  178   b , respectively. Case  170  is operable for supporting first and second side gears  178   a  and  178   b  for rotation within gear cavity  194  about first axis  92 . Case  170  is also operable for supporting the plurality of differential pinions  186  for rotation within gear cavity  194  about one or more axes perpendicular to first axis  92 . First and second side gears  178   a  and  178   b  each include a plurality of teeth  198  which meshingly engage teeth  202  of differential pinions  186 . As noted, ring gear  174  is coupled for rotation with case  170  and includes beveled ring gear teeth  206 . 
     Input shaft assembly  88  extends through input shaft aperture  138  where housing  80  supports it for rotation about second axis  96 . Input shaft assembly  88  includes an input shaft  210 , a pinion gear  214  having pinion teeth  218  meshingly engaging ring gear teeth  206  and bearing assemblies  222  and  224  which cooperate with housing  80  to rotatably support input shaft  210 . Input shaft assembly  88  is coupled for rotation with driveshaft  50  and is operable for transmitting drive torque to differential unit  84 . More specifically, drive torque received by input shaft  210  is transmitted by pinion teeth  218  to ring gear teeth  206  such that drive torque is distributed through the differential pinions  186  to first and second side gear  178   a  and  178   b.    
     With specific reference to FIG. 5, a reaction force is created by the transfer of drive torque between pinion teeth  218  and ring gear teeth  206  which tends to push input shaft  210  toward housing  80  in the direction of arrow A. As such, the reaction force is transmitted through housing  80  in an area proximate bearing assembly  224  where it is ultimately transferred to insert member  118 . The higher strength of insert member  118  relative to wall member  122  permits the reaction force to be absorbed by insert member  118  and/or transmitted to a different area of housing  80  adjacent to insert member  118 , such as gussets  224  and  226 , which minimizes stress in the housing  80  that results from the reaction force. 
     The encapsulation of insert member  118  has been shown molded or cast in-situ in one particular location. However, it will be appreciated that additional insert members can be used in other locations in housing  80  or, for that matter, in case  170  depending on the particular drivetrain application. Thus, while the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the description of the appended claims.