Hydraulic Assembly: Description and Operation

The hydraulic modulator controls hydraulic pressure to the front calipers and rear wheel cylinders by modulating hydraulic pressure to reduce the tendency of wheel slip. The basic hydraulic modulator configuration consists of gear subassemblies, ball screws, nuts, pistons, and hydraulic check valves. The motor pack consists of three motors, three drive gears, and three Expansion Spring Brakes (ESB)s.




ESB Operation
An Expansion Spring Brake (ESB) is used on each front motor and the rear pistons to hold the pistons in their upmost (or "home") position. An ESB is a spring that is retained in a housing at a close tolerance. One end of the spring is in contact with the motor drive dog and the other end is in contact with the pinion drive dog. In normal braking, brake pressure is present on the top of the piston, applying a downward force. This force applies a counterclockwise torque to the motor pinion which tries to rotate the spring counterclockwise. The counterclockwise torque expands the spring outward within the housing and prevents gear rotation.

When the motor is turned ON and tries to drive the ball screw nut, the end of the ESB in contact with the motor drive dog rotates inward causing the spring to contract in its housing allowing the motor to rotate the modulator gear. The most common application of this principle is in window crank mechanisms, where a small amount of force on the crank handle allows the window to be lowered or raised, but the weight of the window or force on the window will not allow the window to move downward. For an ABS ESB, brake pressure on the top of the pistons corresponds to the weight of the window and the motor corresponds to the window crank handle.