STACK AS 4155207 - Caterpillar

Illustration 1 g01953717
Parking brake (parking brake disengaged) (1) Stopper (2) Spool (3) Spring (4) ValveThe spool (2) of the travel motor is initially held in the NEUTRAL position by the force of spring (3). When pressure oil is supplied through Port A, the oil opens valve (4). The oil then passes through Port C on the suction side of the motor, and the oil begins to turn the motor. At the same time, pressure oil passes through a small hole and Passage a of spool (2). The oil enters Space b. The oil then acts on the end face of the spool and the oil generates a force. This force acts against the force of spring (3), and the spool slides to the left.When the spool moves, a passage (Port D) is formed between the spool and the rear flange of the travel motor. This passage connects the return side of the motor with Port B on the other side of the motor. This action enables return oil to return to the tank, and this allows the motor to rotate.When pressure oil is supplied through Port B, spool (2) and valve (4) move in the opposite direction and the motor rotates in the reverse direction.
Illustration 2 g00842284
Parking brake (parking brake engaged) (1) Stopper (2) Spool (3) Spring (4) ValveWhen pressure oil that is supplied through Port A is stopped during travel, the pressure of the oil against spool (2) becomes zero. The force of spring (3) begins to return the spool to the NEUTRAL position. At this time, the oil in Space b starts to go through Passage a in order to go to the Port A side of the motor. Space b has a throttling effect, which generates back pressure. The back pressure limits the return speed of the spool.When the spool returns to the NEUTRAL position fully, the return passage is shut off completely and the motor stops.When the brake is intentionally applied to stop the machine, the inertia of the rotating motor tries to maintain rotary motion, and the motor tries to suck oil. However, the suction side of the motor is shut off by the spool. Therefore, oil cannot be supplied to the motor, and cavitation (a vacuum condition) may occur. At this point, valve (4) opens and the valve provides an oil passage between the Port A side of the circuit and the Port C side in order to prevent cavitation.When pressure oil is supplied through Port B, the components and the oil flow act in reverse in order to stop the motor.
Illustration 3 g00842295
Parking brake (operation during overrunning) (1) Stopper (2) Spool (3) Spring (4) ValveWhen the machine moves down a steep slope, the machine moves faster. The motor requires an oil flow that is greater than the normal pump flow. This condition is called overrunning.During this condition, cavitation can be generated. However, Space b has a throttling effect, which generates back pressure. The back pressure limits the return speed of the spool. The motor