Patent ID: 7264093

Claim:
A combined service and parking brake apparatus comprising: a piston disposed in a cylinder in such a manner as to be nonrotatable about a cylinder axis and movable along the cylinder axis to thereby form a hydraulic chamber within the cylinder; a brake lining to be moved under pressure by the piston and engaged with a rotary member to be braked, when the piston is moved under pressure of brake fluid fed into the hydraulic chamber, thereby effecting a braking operation on the rotary member; a second piston disposed within the hydraulic chamber in such a manner as to be rotatable about the cylinder axis and movable along the cylinder axis between its home position and its non-home position to thereby divide the hydraulic chamber into a first hydraulic chamber and a second hydraulic chamber, movement of the second piston in a piston-projecting direction being restricted to the home position, and the second piston being urged, by biasing means, along the cylinder axis from the non-home position toward the home position; a first stationary screw element provided integrally with the piston within the first hydraulic chamber, extending along the cylinder axis, and having a predetermined lead; a first movable screw element having a thread surface in opposition to a thread surface of the first stationary screw element, provided integrally with the second piston, and meshed with the first stationary screw element with a predetermined thread-to-thread clearance present therebetween along the cylinder axis; a friction clutch capable of engaging the second piston and the cylinder with each other and disengaging the second piston and the cylinder from each other, the friction clutch being brought in a disengaged condition and allowing rotation of the second piston at the time of the second piston being held at the home position to thereby be disengaged from the cylinder, and the friction clutch being brought into an engaged condition and disabling rotation of the second piston at the time of the second piston moving from the home position toward the non-home position and engaging with the cylinder; and a changeover valve provided in a hydraulic branch line allowing feed and drainage of brake fluid therethrough to and from the second hydraulic chamber and branched from a hydraulic line allowing feed and drainage of brake fluid therethrough to and from the first hydraulic chamber, the changeover valve being able to establish and shut off communication between the first hydraulic chamber and the second hydraulic chamber.