Sequentially actuated servomotor with a pressure differential limiter

Claim 1

In a servomotor having a wall means for separating a front chamber from a rear chamber, control means for sequentially regulating the communication of vacuum, air at atmospheric pressure and air above atmospheric pressure to the rear chamber to develop an operational force from a pressure differential across the wall means to operate a brake actuator in response to an input force, said control means comprising: first hub means connected to said wall means having a first housing with a first axial bore therein, said first housing having a first passage connecting the first axial bore with the front chamber, said first housing having a second passage connecting the first axial bore with the rear chamber; second hub means connected to said first hub means for transmitting the operational force to said brake actuator from said wall means, said second hub means having a second housing with a second axial bore coaxial to said first axial bore, said second housing having an entrance port through which air above atmospheric pressure is communicated to the second axial bore, a first annular poppet means having a first end fixed to said first hub means and a second end loosely located in said first axial bore for controlling the communication of vacuum through the first passage in a first mode of operation; a second annular poppet having a first end fixed to said first hub means and a second end loosely located in the first axial bore for controlling the communication of air at atmospheric pressure through the first axial bore in a second mode of operation; third valve means located in said second axial bore for controlling the communication of air above atmospheric pressure from the second bore in a third mode of operation; actuator means located in said first and second axial bores for progressively operating the first, second and third valve means in response to said input force, said first valve means upon movement of the actuator means interrupting communication of vacuum to the second passage to allow air at atmospheric pressure present in the axial bore to flow through the second passage into the rear chamber to develop said pressure differential, said second valve means upon further movement of said actuator means interrupting the flow of air at atmospheric pressure, said third valve means upon still further movement of said actuator means allowing air above atmospheric pressure to enter the second passage to develop said pressure differential; a first resilient means located between the actuator means and the first annular poppet means, said first resilient means urging the first annular poppet into contact with the actuator means to allow vacuum to evacuate air from the rear chamber in said first mode of operation, said first resilient means urging the first annular poppet into contact with a vacuum seat on the first hub means in said second mode of operation to allow said pressure differential to develop; and a second resilient means located between said first hub means and said second annular poppet, said second resilient means urging the second annular poppet into contact with a stop on the first hub means during said first and second modes of operation to allow air at atmospheric pressure to freely be communicated to said first valve means and into contact with the actuator means during said third mode of operation to prevent air above atmospheric pressure from escaping through the axial bore into the atmosphere.