Expansion Spring Brake (ESB)

The expansion spring brake (ESB) is used to hold the piston in the 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. The 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 activated 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 the 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.