Servo-motors for vehicle braking systems

A fluid-pressure operated servo-motor comprising a housing an input member, an output member, a movable wall dividing said housing into separate chambers and through which said input member acts on said output member, valve means responsive to an input force applied to said input member for controlling the generation of differential pressure to opposite sides of said movable wall to produce an output force which is adapted to be transmitted to said output member through said wall, and reaction means for transmitting back to said input member a reactive proportion of said output force to provide a reaction whereby operation of the servo-motor can be more readily controlled, wherein said reaction means comprises a single component in the form of a one-piece annular reaction plate of corrugated outline which, in use, is adapted to flex to transmit a load from said movable wall to said output member and to transmit a reactive proportion of said output force back to said input member, wherein said annular reaction plate comprises angularly spaced fingers of segmental outline having inner and outer ends of which alternate fingers are located in a corresponding one of two spaced parallel planes, and radial webs substantially normal to said fingers and extending between said planes integrally interconnect adjacent fingers and said movable wall includes an axially movable piston of substantial diameter, and a face of said piston adjacent to said output member is provided with abutment means which defines a fulcrum for said outer end of said fingers and said output member is provided with abutment means defining a fulcrum with said plate at an intermediate point in the radial length of said plate, and wherein said piston comprises a hub member guided in said housing for sliding movement and having an inner end, a radial flange at said inner end of said hub member provided with a recess, and a first metal pressing retained in said recess accommodates said annular reaction plate, and a second metal pressing clamped against said hub member by said first metal pressing to retain in a groove in said hub member, a tickening or bead at the peripherally innermost end of a flexible diaphragm which is supported by said second pressing and which is sealingly connected to said housing at its outermost peripheral edge, and projection means provided in said first pressing defining said abutment means.