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| Illustration 1 | g00946868 |
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Service and Secondary Brake Control Valve (1) Ports to tank (2) Ports from accumulators (3) Ports to the service brakes | |
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| Illustration 2 | g00946870 |
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Section B-B (4) Piston (5) Spring (6) Spring (7) Spring (8) Retainer (9) Ball (10) Spacer (11) Upper valve spool (12) Passage to outlet for the right service brakes (13) Outlet port to tank (14) Supply port from accumulator (15) Passage in upper valve spool (16) Cavity (17) Drain line to ports to tank (18) Lower valve spool (19) Passage to outlet for the left service brakes (20) Passage in lower valve spool (21) Spring (22) Cavity | |
The brake control valve is a dual pressure reducing valve with two independent output pressures. The valve is actuated by the pedal for service and secondary brakes.
Modulation of pressurized oil in the accumulators to the service brakes is controlled by the service brake control valve. The position of the 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 brake pedal is depressed, piston (4) is pushed down. The piston puts a force on springs (5), (6) and (7). The force of the springs moves retainer (8) and ball (9). Movement of the retainer and the ball causes lower valve spool (11) to move away from the seat in spacer (10).
The movement of upper valve spool (11) causes movement of lower valve spool (18) and the compression of spring (21). When the upper and lower valve spools move, oil flow to the tank through passages (12) and (19) is blocked.
This allows pressurized oil from accumulator ports (14) to flow through passages (12), (15), (19) and (20). The oil then flows through passages (12) and (19) in order to engage the service brakes. Pressurized oil also flows in cavities (16) and (22) by way of passages (15) and (20). The oil pressure at the service brakes is proportional to the oil pressure in cavities (16) and (22).
Oil pressure in cavity (22) and the force of spring (21) act to balance lower valve spool (18) against the force that is created by pressure in cavity (16). In the same manner, pressure in cavity (16) creates a force against upper valve spool (11), which is balanced by the force of springs (5) and (6).
The force of springs (5), (6) and (7) is balanced by the force that is applied to the pedal. Upward movement of valve spools (11) and (18) closes accumulator ports (14). In this way, the level of pressure in the independent valve sections create a feedback force which allows the operator to modulate pressure to the brakes.
Upper valve spool (11) and lower valve spool (18) will modulate between the tank port and the brake port. This is done in order to maintain brake pressure until the pedal position changes.
If the brake pedal is depressed further, piston (4) moves down farther. The piston puts more compression on springs (5) and (6). Valve spools (11) and (18) move down farther. This permits more pressure to outlets (12) and (19). This results in a higher oil pressure in cavities (16) and (22). The higher oil pressure keeps valve spools (11) and (18) in balance.