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| Illustration 1 | g00411199 |
Service Brake Control Valve (arrow) | |
The service brake control valve is located on the front right side, and underneath the operator's platform. The brake valve is a pressure reducing valve. A mechanical link connects the two pedals to each other.
Modulation of the pressurized oil in the accumulators to the service brakes is controlled by the service brake control valve. The position of either brake pedal causes a specific pressure at the service brakes. As the position of the pedal changes, the pressure at the service brakes also changes.
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| Illustration 2 | g00373996 |
Service Brake Control Valve (A) Outlet port to service brakes. (B) Inlet port from accumulators. (C) Hystat pump signal. (D) Outlet for the pump signal to the hydraulic tank. (E) Outlet for the brakes to the hydraulic tank. | |
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| Illustration 3 | g00449474 |
Released Service Brake Cross section of the Service Brake Control Valve (AA) Supply oil. (BB) Hydraulic oil (tank pressure). (CC) Blocked oil (hystat pump signal) (1) Piston. (2) Spring. (3) Spring. (4) Spring. (5) Retainer. (6) Ball. (7) Upper valve spool. (8) Cavity to the service brake port. (9) Supply oil from accumulators. (10) Cavity. (11) Lower valve spool. (12) Cavity. (13) Hystat pump signal. (14) Spring. (15) Cavity. (16) Passage. (17) Outlet for the pump signal to the hydraulic tank. (18) Upper valve spool passage. (19) Outlet for the brakes to the hydraulic tank. | |
When the brake pedal is not depressed, the brake valve is in the position that is shown in Illustration 3. The piston (1) is in the up position. Springs (2) , (3) and (4) hold the retainer (5) and ball (6) against the upper valve spool (7) . The upper valve spool (7) is held in the position shown in Illustration 3. The upper valve spool (7) blocks the supply oil (AA) from the port (9) to flow to the service brakes (8) . Therefore, the supply oil (AA) from the accumulators does not flow to the service brakes (8) . The upper valve spool (7) allows the hydraulic oil (BB) from the service brake port (8) to flow to the hydraulic tank port (19) . This releases the pressure behind the piston in the service brakes and the brakes are released. For additional information on the service brakes, refer to Systems Operation, SENR6772-01, "Service Brakes". At the same time, the hystat pump signal from port (13) is blocked to the hydraulic tank port (17) . Cavities (12) and (15) are always connected to the hydraulic oil tank (17) , which prevents a hydraulic lock.
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| Illustration 4 | g00373892 |
Applied Service Brake Cross section of the Service Brake Control Valve (AA) Supply oil. (BB) Hydraulic oil to the service brakes. (CC) Hydraulic oil (tank pressure). (1) Piston. (2) Spring. (3) Spring. (4) Spring. (5) Retainer. (6) Ball. (7) Upper valve spool. (8) Cavity to the service brake port. (9) Supply oil from accumulator for the service brakes. (10) Cavity. (11) Lower valve spool. (12) Cavity. (13) Hystat pump signal. (14) Spring. (16) Passage. (17) Outlet for the pump signal to the hydraulic tank. (18) Upper valve spool passage. (19) Outlet for the brakes to the hydraulic tank. | |
When the brakes are fully applied, the brake valve is in the position that is shown in Illustration 4. When the brake pedal is depressed, the linkage to the brake valve pushes down on top of piston (1) . Piston (1) puts a force on springs (2) , (3) and (4) . The force of springs (2) and (3) moves retainer (5) and ball (6) . Movement of retainer (5) and ball (6) causes the upper valve spool (7) to move downward.
The movement of upper valve spool (7) causes the movement of lower valve spool (11) and the compression of spring (14) . When valve spools (7) and (11) move, the service brake port (8) is blocked to the hydraulic oil tank (19) .
This allows supply oil (AA) from port (9) to flow to the service brakes (8) in order to apply the service brakes. The hydraulic oil to the service brake (BB) is reduced oil pressure from the pump supply oil (AA) . At the same time, hydraulic oil (BB) flows through upper valve spool passage (18) into cavity (10) . The oil pressure at the service brakes has the same pressure as the oil in cavity (10) .
The hystat pump signal (13) flows through passage (16) to the hydraulic oil tank (17) . As the lower valve spool (11) moves down, the hydraulic oil (CC) flows out of chamber (14) to the hydraulic oil tank (17) . Chamber (15) is shown in Illustration 3. The oil from chamber (15) flows through the cavity (12) in the lower valve spool to passage (16) and exits to the hydraulic tank (17) .
If the brake pedal is depressed, piston (1) moves down. Springs (2) , (3) and (4) are compressed more. Springs (4) and (5) will move valve spool (7) . This will open supply oil (AA) , which increases the service brake pressure (8) . The oil pressure also increases in cavity (10) . The oil pressure in cavity (10) acts upward on the upper valve spool (7) . This allows the operator to feel the pressure increase at the service brakes.
The lower valve spool is balanced by the forces on the top and the bottom of the spool. The force of spring (14) act on the bottom of the lower valve spool (11) . The oil pressure in cavity (10) acts on the top of the lower valve spool (11) .
Inching Mode
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| Illustration 5 | g00451193 |
Inching Mode Cross section of the Service Brake Control Valve (AA) Pump supply oil. (BB) Blocked oil. (CC) Hydraulic oil (tank pressure). (1) Piston. (2) Spring. (3) Spring. (4) Spring. (5) Retainer. (6) Ball. (7) Upper valve spool. (8) Cavity to the service brake port. (9) Supply oil from accumulator for the service brakes. (10) Cavity. (11) Lower valve spool. (12) Cavity. (13) Hystat pump signal. (14) Spring. (15) Cavity. (16) Passage. (17) Outlet for the pump signal to the hydraulic tank. (18) Upper valve spool passage. (19) Outlet for the brakes to the hydraulic tank. | |
The inching mode is standard on all machines. The inching mode is operational during the first 25 to 50 mm (1.0 to 2.0 inch) of travel of either service brake pedal. This causes the valve spools to shift downward. Initially, the upper valve spool (7) shifts downward. The service brake port (8) is opened to the hydraulic oil tank port (19) . Further movement of the upper valve spool blocks the service brakes (8) to the hydraulic tank port (19) . This is shown in Illustration 5. The service brakes are also blocked to the pump supply (9) .
The lower valve spool (11) shifts downward. The downward movement opens the hystat pump signal (13) to the hydraulic tank (17) through passage (16) . This decreases the pressure in the hystat pump signal. The flow from the hystat pump to the motor is redirected to the hydraulic oil tank, which decreases the speed of the machine. The pressure decrease in the hystat pump signal also signals the pump to gradually destroke. Full engine rpm is also maintained.
During normal operation, the inching mode could be used to load material. The inching mode could be used to unload material. When the brake pedals are released, the inching mode is deactivated.