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| Illustration 1 | g00547915 |
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| Illustration 2 | g00818578 |
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| Illustration 3 | g00559729 |
Wheel brake valve (1) Spring (2) Control element (3) Spring (4) Spool (5) Spool (6) Power brake valve (7) Needle pin (8) Needle pin (9) Needle pin (10) Shuttle valve (11) Spool (12) Spool (13) Accumulator charging valve (14) Check valve (15) Pilot control piston (16) Pressure adjuster (17) Pressure balance (18) Spring | |
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| Illustration 4 | g00559743 |
(1) Wheel brake valve (13) Accumulator charging valve | |
The wheel brake valve is a combination valve that includes accumulator charging valve (13), shuttle valve (10) and dual-circuit power brake valve (6) .
The power brake valve is controlled by the wheel brake pedal that is connected to the single-acting cylinder with a spring return. The single-acting cylinder is activated by the oscillating axle and operating brake valve that is placed in position III. This position is for parking the excavator during work.
Accumulator charging valve (13) consists of pressure balance (17), spring (18), check valve (14) and pilot control piston (15). The shift point of the piston can be adjusted via pressure adjuster (16) .
Shuttle valve (10) includes spools (11) and (12), and needle pin (9) that is installed between the spools.
Power brake valve (6) consists of spools (4) and (5) with needle pins (7) and (8), and control element (2) with springs (1) and (3) .
The front wheel brakes and the rear wheel brakes are separate systems. When the brake pedal is applied, the front circuit and the rear circuit are activated. If a failure occurs in either the front or the rear brakes, the other circuit will stop the machine.
Accumulator Charging Valve
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| Illustration 5 | g00559784 |
Accumulator charging valve (charging position) (13) Accumulator charging valve (14) Check valve (15) Pilot control piston (16) Pressure adjuster (17) Pressure balance valve (18) Spring (19) Port "T" (20) Port "SP" (21) Spring (22) Head end of pilot control piston (23) Passage (24) Edge "T" of spool (25) Edge "P" of spool (26) Spring (27) Oil passage (28) Port "P" of brake pump (29) Port "N" (30) Oil passage | |
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| Illustration 6 | g00559932 |
(1) Wheel brake valve (16) Pressure adjuster | |
Accumulator charging valve (13) receives oil from the brake pilot pump. This valve is used to switch the pump delivery flow to the brakes or the pilot manifold. The pump delivery flow will depend on the pressure in the accumulators.
When the pressure in the accumulators drops to the approximate cut-in pressure of 12000 kPa (1740 psi), the accumulator charging valve shifts to the filling position. Now, oil flows from the valve through port "SP" (20) and into the two-wheel brake accumulators through the shuttle valve.
The valve will shift when the approximate cut out pressure of 15000 kPa (2175 psi) is reached. The oil from the pump goes from port "P" through port "N" (29) to the pilot manifold.
The accumulator charging valve includes a pressure adjuster (16), pressure balance valve (17) and check valve (14) .
Operation of the Brake Accumulator Valve
The brake pilot pump delivers oil to port "P" (28) regardless of the position of the accumulator charging valve. From port "P", oil flows to check valve (14) .
Check valve (14) opens against the pressure of spring (21). In this way, the oil flow can continue through port "SP" (20) and through the line that connects the wheel brake accumulators.
At the same time, oil from the brake pilot pump flows through oil passage (30) to pilot control piston (15). The position of the spool allows oil to pass by edge "P" (25) of the spool (15) and to oil passage (27). The oil pressure is applied on the spring side of valve (17). Since the same pressure is applied on both sides of the pressure balance valve (17), spring (18) holds the valve in position in order to block flow from port "P" (28) to port "N" (29). Oil pressure builds up in passage (23) and the oil pressure pushes on the head end of pilot control piston (22) against the force of spring (26). This regulates the oil flow.
Oil from the brake pilot pump flows to port "SP" (20) until the pressure in the accumulator increases to 15000 kPa (2175 psi). This causes pilot control piston (15) to shift against spring (26). The oil flow from passage (30) to oil passage (27) at edge "P" (25) of spool (15) is blocked. The oil pressure on the spring side of pressure balance valve (17) passes by edge "T" (24) of spool (15) and to the tank through port "T" (19) .
Oil from the brake pilot pump that enters port "P" of brake pump (28) pushes pressure balance valve (17) against the force of spring (18). This establishes a connection between ports "N" (28) and "P" (29). The pressure on the pump side of check valve (14) decreases. Check valve (14) is closed by the force of spring (21) and by the pressure in the wheel brake accumulators. The accumulator charging is stopped when the cut out pressure of 15000 kPa (2175 psi) is reached.
When the pressure in the wheel brake accumulators drops to the cut-in pressure of 12000 kPa (1740 psi), pilot control piston (15) is pushed down by spring (26). The spool now moves so that edge "T" of spool (24) blocks the connection between the spring side of pressure balance valve (17) and port "T" (19) to tank. Edge "P" of spool (25) allows pump oil from passage (30) to enter oil passage (27) and the chamber of spring (18). Pressure balance valve (17) now closes the connection between port "P" (28) and port "N" (29) and opens check valve "14". Oil from the brake pilot pump flows into the wheel brake accumulators until the cut out pressure of 15000 kPa (2175 psi) causes the accumulator charging valve to shift to the neutral position.
Shuttle Valve
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| Illustration 7 | g00559934 |
Shuttle valve (10) Shuttle valve (30) Passage (front wheel brake circuit) (31) Passage (rear wheel brake circuit) (32) Spool (33) Passage (accumulator charging valve) (34) Spool | |
Note: The shuttle valve is shown with no applied pressure. Spools (32) and (34) are in a random position. When pressure is applied to the shuttle valve, one spool will always be pressed against the seat.
The oil flow from the brake pilot pump goes to the wheel brake valve and to the port "SP" of accumulator charging valve through the power brake valve and shuttle valve (10). The shuttle valve has the following purposes: distribution of oil to the front wheel brake circuit, distribution of oil to the rear wheel brake circuit and separation of the two brake circuits. The shuttle valve ensures that both circuits are supplied with equal pressure.
Oil from the brake pilot pump enters the shuttle valve at passage (33) and leaves through passages (30) and (31). Oil from passage (30) goes to the wheel brake accumulator for the front wheel brakes. Oil from passage (31) goes to the wheel brake accumulator for the rear wheel brakes.
When the brakes are activated pressure in the wheel brake circuits decreases. Check valves (32) and (34) make sure that both circuits maintain pressure in "S1" and "S2". At the same time, the shuttle valve regulates the flow of oil from passage (33) to the wheel brake accumulator with a lower pressure.
Oil that enters the shuttle valve at passage (33) goes to the check valves. When pressure drops in one brake system, the pressure on the check valve for that system is reduced. The higher pressure in passage (33) will open the check valve. Oil from the brake pilot pump can now flow to the brake system with the lower pressure until the pressure is equalized.
Power Brake Valve
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| Illustration 8 | g00559975 |
Power brake valve (1) Spring (2) Control element (3) Spring (4) Spool (5) Spool (6) Power brake valve (7) Needle pin (8) Needle pin (10) Shuttle valve (13) Accumulator charging valve (19) Port "T" (30) Passage (front service brake circuit) (31) Passage (rear service brake circuit) (35) Passage (36) Passage (37) Return spring (38) Port "B2" (39) Passage (40) Operating piston (41) Port "B1" (42) Return spring | |
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| Illustration 9 | g00560035 |
(1) Wheel brake valve (6) Power brake valve | |
The dual-circuit power brake valve (6) includes control element (2) and the two spools (4) and (5) with needle pins (7) and (8). Each spool controls a separate wheel brake circuit.
The power brake valve regulates the pressure in the brake circuits proportionally to the force that is applied to the wheel brake pedal. If one brake circuit loses pressure, the other circuit remains fully operational due to the mechanical contact of needle pins (7) and (8) of the spools. The force that is exerted on the wheel brake pedal will remain unchanged if the pressure is lost in one circuit.
Pushing down on the wheel brake pedal will operate the wheel brakes. The wheel brakes are supplied oil by the wheel brake accumulators until the available pressure reaches cut-in pressure. When the cut-in pressure is reached, the brake pilot pump supplies flow through the wheel brake valve in order to charge the accumulators. When power brake valve (6) is operated, a metered amount of oil goes through the swivel to the front and rear wheel brakes.
The power brake valve is a three-way pressure reducing valve. The power brake valve provides infinitely variable control of the brake circuit pressure that is proportional to the applied force to the brake pedal.
Pressures of 12000 to 15000 kPa (1740 to 2175 psi) is always available in passages (30) and (31) ahead of spools (4) and (5). When the valve is operated, the pressure drops to the recommended value. Refer to Testing and Adjusting, "Pressure Specifications". This brake pressure goes through ports "B1" (41) and "B2" (38) to the top swivel. From the swivel, the pressure passes to the brakes.
When the power brake valve is not operated, there is no brake pressure to the wheel brakes.
Two pressure switches are mounted on the brake valve. The pressure switch at port "Br" performs the following functions: activation of the stoplights and deactivation of the solenoid valve for travel activation. This switch activates when the brake pressure reaches 500 kPa (72 psi). The other pressure switch at "DS2" activates the low brake pressure alarm and the low brake pressure indicator when the pressure in the system drops below 10000 kPa (1450 psi).
The power brake valve is controlled by the brake pedal and the single-acting cylinder. The single-acting cylinder has a spring that applies pressure to operating piston (40). The piston pushes springs (1) and (3) and spools (4) and (5) left against the force of return springs (37) and (42). The movement prevents oil from returning to the tank.
The movement of spools (4) and (5) allows oil to flow from oil passage (31) through drilled passage (36) to port "B2" (38). The movement also allows oil to flow from oil passage (30) through drilled passage (39) to the rear of port "B1" (41). Pressure that is increasing in the brake lines also acts on spool (5) via the control oil passage against the force of spring (3). Brake pressure increases proportionally to the applied force on the brake pedal.
When the brake pedal is held in position, spools (4) and (5) hold the pressure at ports "B1" and "B2" constant. This applies a constant brake pressure to the wheel brakes.
The design of spools (4) and (5) causes the rear brakes to be applied slightly before the front brakes.
When the brake pedal is released, springs (1), (3), (37) and (42) with the oil pressure in passages (36) and (39) shift spools (4) and (5) to the right. The shift closes the connection from passage (30) to port "B1" (41) and from passage (31) to port "B2" (38). At the same time, a connection is made between passages (30) and (31) to passage (35) of port "T" (19) to the tank. This connection relieves the pressure in the brake circuit so that the wheel brakes are released.
Note: If a brake failure occurs in the front wheel brakes or in the rear wheel brakes, the shuttle valve ensures that full braking power will exist in the unaffected wheel brake circuit.