Hollow piston assembly for brakes or clutches
BACKGROUND OF THE INVENTION 
     This invention relates to a piston assembly for brakes or clutches and a method for making the same. 
     A conventional brake package normally comprises a plurality of interleaved friction discs and an annular piston disposed on one axial side of the friction discs to selectively compress the same for braking purposes. The piston is normally formed as a one-piece casting and machined to exhibit the required tolerances for installation in the brake package. The solid piston is weighty and requires substantial cooling time upon fabrication thereof. In addition, heat build-up in the brake package may tend to be retained in the solid piston assembly whereas it is highly desirable to dissipate as much heat as possible therefrom. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to overcoming one or more of the problems as set forth above. 
     The present invention comprises a hollow piston assembly reciprocally mounted in a housing to selectively compress a plurality of interleaved friction discs, such as those employed in a clutch or brake. The piston assembly comprises annular first and second members mounted together in back-to-back relationship to define a chamber therebetween. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Other objects of this invention will become apparent from the following description and accompanying drawing which is a cross sectional view through a brake package mounted within a wheel hub and having a hollow piston assembly of this invention incorporated therein. 
    
    
     DETAILED DESCRIPTION 
     The drawing illustrates a brake package 10 mounted within the confines of an annular wheel hub 11, such as the type mounted on a wheel tractor. An input shaft 12 is adapted to drive the hub via final drive means, not shown, in a conventional manner. The hub is rotatably mounted on a spindle 13 by a pair of axially spaced and annular bearing assemblies 14 (one shown). 
     Brake package 10 comprises a first set of annular friction discs 15 suitably splined to an annular carrier member 16 secured to hub 11. A second set of annular friction discs 17 are suitably splined on a housing 18 and interleaved with the first set of friction discs 15. Housing 18 is suitably secured to stationary spindle 13 whereby selective compression of the friction discs against an annular back-up plate 19 of housing 18 will brake and retard rotation of hub 11 relative to spindle 13. 
     Brake package 10 further comprises a hollow piston assembly 20, embodying this invention, which defines an actuating chamber 21 between it and an outer cover plate 22 of housing 18. The brake is normally held in an &#34;off&#34; condition of operation by a plurality of circumferentially disposed coil springs 23, engaged between back-up plate 19 and piston assembly 20. The brake package is oil-cooled in a conventional manner and an annular sealing assembly 24 of the sliding-type is suitably mounted between the brake package and the wheel hub to prevent the egress of oil thereby. 
     Piston assembly 20 comprises an annular first member 25 having a substantially flat plate portion 26, a cylindrical hub 27 formed on an inner end of a plate portion and a flange 28 formed on an outer end of the plate portion. A first annular recess 29 is defined at the juncture of plate portion 26 and hub 27 whereas a second annular recess 30 is defined at the juncture of the plate portion and flange 28. 
     An annular groove 31 is formed externally on hub 27 of first member 25 and seats an O-ring seal 32 therein. A second annular groove 33 is formed externally on flange 28 of the first member and seats an O-ring seal 34 therein. As shown, the seals maintain sealing contact between the housing and the piston assembly upon reciprocation of the piston assembly between its brake &#34;on&#34; and brake &#34;off&#34; conditions of operation. 
     The piston assembly further comprises an annular second member 35 disposed in back-to-back relationship with respect to first member 25. The second member comprises an annular wall portion 36 having an annular flange 37 secured on an inner end thereof and disposed in recess 29 of the first member. The second member is formed with annular cavity 38 defined therein and has a radial flange 39 formed on the outer end thereof and disposed on plate portion 26 of the first member and further disposed in recess 30 formed thereon. It should be noted that flanges 37 and 39 of the second member engage recesses 29 and 30 of the first member in press-fit relationship therewith and are held in such relationship by springs 23 which engage portions of both members. 
     The piston assembly is formed by the following steps. First member 25 is suitably die cast along with second member 35. The members are press-fitted together to dispose flanges 37 and 39 of the second member within recesses 29 and 30 of the first member, respectively. Brake package 10 is thereafter assembled with piston assembly 20 mounted therein as shown in the drawing and biased leftwardly to the brake &#34;off&#34; condition of operation by springs 23. As mentioned above, springs further function to bias the first member into its press-fit relationship with the second member.