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 movable member provided 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, movement of the movable member in a piston-returning direction being restricted to the home position, the movable member being urged, by first biasing means, along the cylinder axis from the home position toward the non-home position;
- a first stationary screw element provided integrally with the cylinder within the hydraulic chamber, extending along the cylinder axis, and having a predetermined lead;
- a first movable screw element having a thread surface engaged with a thread surface of the first stationary screw element, provided integrally with the movable member, and meshed with the first stationary screw element with a predetermined thread-to-thread clearance present therebetween along the cylinder axis;
- a second stationary screw element disposed coaxially with the first stationary screw element within the hydraulic chamber, provided integrally with the piston, extending along the cylinder axis, and having a lead shorter than the lead of the first stationary screw element;
- a second movable screw element having a thread surface engaged with a thread surface of the second stationary screw element, provided within the hydraulic chamber in such a manner as to be rotatable about the cylinder axis and movable along the cylinder axis, meshed with the second stationary screw element with a predetermined thread-to-thread clearance present therebetween along the cylinder axis, and urged along the cylinder axis against the first biasing means by second biasing means having biasing force greater than that of the first biasing means;
- a friction clutch capable of engaging the second movable screw element and the movable member with each other and disengaging the second movable screw element and the movable member from each other, the friction clutch being brought in a disengaged condition and allowing relative rotation between the second movable screw element and the movable member at the time of the second movable screw element moving away from the movable member against the second biasing means, and the friction clutch being brought into an engaged condition and restricting relative rotation between the second movable screw element and the movable member at the time of the second movable screw element being urged by the second biasing means and engaging with the movable member;
- and
- a drive device capable of moving the movable member and the second movable screw element along the cylinder axis against the second biasing means from their respective home positions toward their respective non-home positions.