Currently most parking brakes are mechanically applied by use of a cable and ratcheting lever assembly. Parking brakes on disc brake units are primarily of two types: an integrated parking brake caliper and the top hat drum style. Alternatively, electric parking brakes are being used on some vehicles. The electric motor can pull on a conventional parking brake cable, or the electric motor can be mounted on the caliper and directly drive the caliper piston to apply force to the brake pads.
FIGURE 32-32
Integrated parking brake operation.
Integrated Mechanical Parking Brake Calipers
The integrated parking brake mechanically forces the disc brake piston outward, forcing the brake pads to clamp the rotor when the parking brake is applied FIGURE 32-32. A lever on the back side of the caliper is pulled by the parking brake cable. The lever converts that motion to rotary motion on a shaft that enters the rear of the caliper cylinder. The shaft uses a seal to prevent fluid leakage from the bore. The shaft has a coarse thread machined into it, which threads into a nut assembly inside the caliper piston. As the shaft is turned by the parking brake lever, the nut causes the piston to be forced outward, applying the brakes. Releasing the parking brake cable allows a spring to unwind the shaft and release the pressure on the brake pads.
Top Hat Design Parking Brake
The top hat design gets it name from the shape of the rotor. The rotor has a deeper offset than normal, giving the appearance of a top hat. The offset portion allows room for a drum surface within the center of the rotor. Drum brake shoes are mechanically forced outward into contact with the inside of the brake drum, which locks the wheel. Releasing the parking brake allows the springs to retract the brake shoes from contact with the drums.
Electric Parking Brake
The electric parking brake uses an electric motor to apply the disc brake assemblies. The cable style uses an electric motor to pull standard parking brake cables, which apply standard integrated mechanical parking brake calipers. The electrically integrated caliper style uses an electric motor mounted on the caliper to directly apply the brakes. Pushing a parking brake button on the dash causes the motor to either tension the cable or directly apply the parking brake. Electric parking brakes also can be integrated with the controller area network bus (CAN-bus) system to provide additional features beyond just holding the vehicle when it is parked. It can be used to automatically hold the vehicle while it is stopped on a hill to prevent it from rolling backward or forward. It also can be automatically released by the vehicle’s electronic control module (ECM) when the throttle is applied for starting to move away from the stop. It also may work with the vehicle’s proximity detector when backing up. If the system detects the vehicle getting too close to an object, the ECM can apply the electric parking brake to stop the vehicle and prevent it from striking the object.
