HARNESS AS- 4866276 - Caterpillar

Illustration 1 g03523605
Hydraulic Schematic for the Brake System
(1) Steering pump
(2) Brake pump
(3) Priority valve
(4) Pressure switch
(5) Accumulators
(6) Brake valve
(7) Rear brakes
(8) Front brakes
(9) Relief valve
(10) Line to fan motor
(11) Check valve The hydraulic pump for the brake system is a gear pump (2) attached in tandem to the steering gear pump (1). The brake pump (2) supplies the hydraulic oil to the brake system. Oil flows from the brake pump (2) to the brake priority valve (3). Some of the oil flows to the pressure switch (4). The pressure switch (4) sends a signal to the light on the dash if the pressure is low.The brake priority valve (3) maintains the pressure in the brake system by charging the accumulators to a Predetermined pressure range. The brake priority valve (3) directs the oil flow to the accumulators until the Pre charge MAX (cut-out) pressure is reached, then excess oil flows to tank or down stream services. After several applications of the brakes, the accumulator pressure will drop below the MIN (cut-in) pressure and the brake priority valve will change state and re fill the accumulators.When in the charging state, oil flows from the brake priority valve (3) to accumulators (5). Both accumulators (5) contain a Pre charge of dry nitrogen. When oil flows into an accumulator, the increase in oil pressure moves the piston inside the accumulator. The piston moves against the charge of dry nitrogen gas. This action compresses the dry nitrogen until the oil pressure reaches the cut out pressure. The oil can also flow to brake valve (6).Depressing either brake pedal from the first 25 mm to 50 mm (1.0 inch to 2.0 inch) shifts both spools inward and activates the inching mode. The further the pedal is depressed, the further the inching mode will reduce the machine travel speed. This situation occurs while commanded engine rpm is maintained by gradually de stroking the hydrostatic pump and up stroking the motor.Brake valve (6) is located between the accumulators and the service brakes and is mounted under the front of the cab. When either brake pedal is pushed, the spool shifts inward. This action causes oil from one accumulator to flow across the spool to the disc brakes in the axle housing. The control valve is a split modulating valve with two independent circuits each supplied by its independent accumulator. If there is a partial or full failure of one circuit, the other circuit will still have normal braking.When, the brake valve (6) is released the lines from the accumulators are blocked. The brake lines between the brake valve and the service brakes are connected to the drain port. This action allows the service brakes to release. The oil from the service brakes returns to the hydraulic oil tank via the brake valve. The pressure of the oil causes the wet disc brakes that are enclosed in each axle to engage. The heat from the friction is dissipated by the oil in the axle housings.