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| Illustration 1 | g06375735 |
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(1) Line to front service brakes
(2) Brake control valve (3) Line to rear service brakes (4) Pressure sensor for the brake system (5) Brake pedal (6) Accumulators (7) Shuttle valve (8) Pressure sensor for brake charging (9) Check valve (10) Orifice (11) Screen (12) Orifice (13) Braking and hydraulic fan pump (14) Strainer (15) Tank (16) Hydraulic oil filter (17) Oil cooler (18) Motor for the axle oil cooler (19) Standard fan motor (if equipped) (20) Makeup valve (21) Demand fan solenoid valve (22) Orifice (23) Flow control valve for the fan (24) Valve for brake charging (25) Priority valve for the brake | |
The braking and hydraulic fan system contains fixed displacement hydraulic pump (13). The braking system has priority over the hydraulic fan system.
When the engine is started, hydraulic oil is drawn from tank (15) by brake and fan drive pump (13). Relief valve (12) is closed. Hydraulic oil flows from pump (13) to priority valve (25). Then, the hydraulic oil flows through priority valve (25) to the right side of the priority valve through brake accumulator charging valve (24). Oil also flows to the left side of the priority valve. The hydraulic oil and the spring pressure counter the pressure of the pilot oil that flows to the left side of the priority valve. This holds the priority valve to the left as the accumulators charge.
The priority valve directs hydraulic oil to screen (11). The oil flows through orifice (10) to check valve (9). As the hydraulic oil pressure increases, the check valve opens. The hydraulic oil flows to brake accumulator charging valve (24). Then, the oil flows to shuttle valves (7). The spring keeps brake accumulator charging valve (24) open. The shuttle valves direct hydraulic oil to the accumulator with the lowest pressure first. As the hydraulic oil pressure to the first accumulator increases, the shuttle valve moves. This allows both accumulators to charge at equal pressures. Hydraulic oil also flows to relief valve (12) through brake accumulator charging valve (24).
As the accumulators charge, hydraulic oil pressure increases to relief valve (12). When the hydraulic oil pressure overcomes the spring pressure on the relief valve, the relief valve moves down. The hydraulic oil to the right side of the priority valve drains to the inlet side of the brake and fan drive pump (13). Then, the oil pressure on the left side of the priority valve moves the priority valve to the right. This directs most of the flow of hydraulic oil to the fan drive system. A small amount of hydraulic oil flows to the right side of the priority valve and to the relief valve. Because the relief valve is open, the oil flows to the inlet side of the pump.
The flow of hydraulic oil from brake accumulators (16) is blocked at brake control valve (2). When brake pedal (5) is pressed, the brake control valve routes hydraulic oil to rear service brakes (3) and to front service brakes (1). This engages the service brakes. Pressure feedback is sensed by each spool in the brake control valve. This allows the operator to feel the application of the service brakes. The pressure to the brake lines varies. As the brake pedals are depressed, the hydraulic oil pressure in the brake lines increases. Due to the spring below the bottom brake valve, the maximum pressure to the front brakes is lower than the maximum pressure to the rear brakes.
When the accumulator charge pressure falls below the spring pressure on the relief valve, the relief valve moves up. This is the cut-in pressure. Then, the accumulator charging cycle begins again.
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| Illustration 2 | g06375793 |
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(19) Standard fan motor
(20) Makeup valve (21) Demand fan solenoid valve | |
The speed of the fan is limited by demand fan solenoid valve (21). The demand fan solenoid valve is controlled by the machine ECM. The control of the fan speed is based on the value of the following conditions:
- Inlet manifold temperature
- Transmission oil temperature
- Hydraulic oil temperature
- Engine coolant temperature
When the machine ECM requires more air flow for cooling, the machine ECM will decrease the amount of electric current that is sent to demand fan solenoid valve (21) to close the demand fan solenoid valve. When the demand fan solenoid valve closes more, the pressure of the pilot oil that is acting on flow control valve (23) for the fan increases and the pressure drop across orifice (22) will decrease. When the pressure drop across orifice (22) decreases, flow control valve (23) for the fan closes to restrict the flow of oil through the flow control valve for the fan. When the flow of oil through the flow control valve for the fan is restricted, the flow of oil from braking and hydraulic fan pump (13) must flow through fan motor (19). This will cause the fan motor to turn faster. The speed of the fan depends on the following inputs:
- The amount of current that is sent to the demand fan solenoid valve
- The amount of pump flow that is available
When the machine ECM does not require more air flow for cooling, the machine ECM will increase the amount of electric current to the demand fan solenoid valve to shift the demand fan solenoid valve more open. When the demand fan solenoid valve shifts more open, the pilot oil that acts on flow control valve (23) for the fan is sent to the tank. The pressure drop across orifice (22) increases. When the pressure drop across orifice (22) increases, flow control valve (23) for the fan will shift more open to increase the flow of oil through the flow control valve for the fan. When the flow of oil through the flow control valve for the fan is increased, the flow of oil from the braking and hydraulic fan pump will flow through the flow control valve for the fan instead of flowing to the fan motor. This will cause the fan motor to turn more slowly. The speed of the fan depends on the amount of current that is sent to the demand fan solenoid valve.