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| Illustration 1 | g00471324 |
Service Brake Control Valve | |
The service brake control valve is located under the operator's cab just behind the steering shaft.
A mechanical link connects the two brake pedals to each other. When a brake pedal is depressed, a roller on the cross shaft assembly actuates the service brake control valve. The control valve is a dual pressure reducing valve with two independent output pressures.
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| Illustration 2 | g00471325 |
Service Brake Control Valve (B) Service brake ports. (C) Pressure ports (accumulators). | |
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| Illustration 3 | g00471347 |
View A-A of the Service Brake Control Valve (1) Piston. (2) Spring. (3) Spring. (4) Spring. (5) Retainer. (6) Ball. (7) Spacer. (8) Upper valve spool. (9) Plug. (10) Outlet to rear service brakes. (11) Passage. (12) Drain port to tank. (13) Supply from accumulator for the rear service brakes. (14) Upper valve spool passage. (15) Cavity. (16) Lower valve spool. (17) Outlet to tank. (18) Outlet to front service brakes. (19) Passage. (20) Lower valve spool passage. (21) Supply from accumulator for the front service brakes. (22) Spring. (23) Cavity. | |
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.
When the operator pushes on the brake pedal, a roller on the brake pedal assembly forces piston (1) downward. Piston (1) applies 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 upper valve spool (8) to move away from the seat in spacer (7) .
The movement of upper valve spool (8) causes the movement of lower valve spool (16) and the compression of spring (22). When valve spools (8) and (16) move, oil flow through passages (11), and (19) to the hydraulic oil tank is blocked.
This allows pressurized oil from accumulator ports (13) and (21) to flow through passages (11), (14), (19), and (20). The oil then flows through passages (10) and (18) in order to apply the service brakes. At the same time, pressurized oil flows into cavities (15) and (23), and through passages (14) and (20). The oil pressure at the service brakes has the same pressure as the oil in cavities (15) and (23) .
Oil pressure in cavity (23) and the force of spring (22) act to balance lower valve spool (16) against the force of the pressure in cavity (15). In the same manner, pressure in cavity (15) creates a force against the bottom of upper valve spool (8). The upper valve spool is balanced by the force of spring (2) and (3) at the top of spool (8) .
The force of springs (2), (3), and (4) is balanced by the force that is applied to the pedal. Upward movement of valve spools (8) and (16) causes accumulator ports (13) and (21) to be covered. The level of pressure in the independent valve sections creates a force of feedback. This allows the operator to modulate the pressure to the service brakes.
Valve spools (8) and (16) balance between the ports for the hydraulic oil tank and the ports for the service brakes. This is done in order to maintain the brake pressure until the position of the pedal changes. The change in the pedal position means that more brake pressure or less brake pressure is necessary.
If piston (1) is moved downward in order to apply more compression on springs (2) and (3), valve spools (8) and (16) will move. The valve spools move in order to allow more pressure oil to flow to outlets (10) and (19) to the service brakes. This results in a higher oil pressure in cavities (14) and (23). A higher oil pressure is necessary in order to maintain the balance of valve spools (8) and (16).