Two stage servomotor

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The braking system 10, shown in the drawing has a fluid pressure servomotor 12 connected to a master cylinder 14 for supplying the front wheel brakes 16 and rear wheel brakes 18 with an operational hydraulic force in response to an input force applied to pedal 20 by an operator.
The fluid pressure servomotor 12 has a first shell 22 joined to a second shell 24 by a twist lock arrangement 26.
A movable wall means 28 is located within the first shell 22 and the second shell 24 to form a first variable volume chamber 30 and a second variable volume chamber 32.
The wall means 28 has central hub 34 with a backing plate 36 extending therefrom.
A diaphragm 40 has a first bead 42 on its periphery which is held between flange 44 and flange 46 on the first shell 22 and the second shell 24, respectively, and a second bead 48 which is snapped into groove 50 adjacent the backing plate 36.
The hub 34 has a rearwardly extending projection 52 which extends through opening 54 in the second shell 24.
The projection 52 has an axial bore 56 into which a control valve means 58 is located and supplied with an operational input from pedal 20 through push rod 60.
The control valve means 58 has a poppet assembly 62 which is sequentially operated upon movement of a plunger 66 by the push rod 60 to interrupt vacuum and allow air to develop a pressure differential across the wall means 28.
The poppet assembly 62 has a face 68 which is separated from a fixed bead 64 by a flexible section 70.
The bead 64 is retained within bore 56 by a retainer 72 which biases the bead 64 against the projection 52.
A first spring 74 connected to shoulder 80 on push rod 60 urges the face 68 toward a vacuum seat 67 adjacent vacuum passage 76.
The vacuum passage 76 connects the first chamber 30 with the interior bore 56 of the hub means 34.
A second spring 87 urges the plunger 66 against stop 89 to allow vacuum to enter the rear chamber 32 through passage 86.
A return spring 88 located between the shell 22 and hub means 34 urges bumper 90 on diaphragm 40 toward shell 24