574B Forwarder Power Train Braking System Caterpillar

Oil flows from the implement pump to the combination valve through port (P2). Parking brake control valve (4) is part of the combination valve. Then, the oil flows from port (P1) to brake accumulator charging valve (14) .

Accumulator Charging System

Brake accumulator charging valve (14) controls the amount of pressure oil that flows to brake accumulators (10) and (12). Brake accumulator charging valve (14) contains a load sensing spool, an inverse shuttle section, a check valve, and an orifice.

The orifice reduces the oil flow that enters the accumulator charging valve.

The check valve prevents oil from flowing back through the accumulator charging valve to the implement pump.

The inverse shuttle section of the accumulator charging valve senses the pressure in the brake accumulators. When the oil pressure drops below 10000 kPa (1450 psi), the spool in brake accumulator charging valve (14) shifts so oil can flow to the brake accumulators. This is the cut-in pressure.

When the oil pressure reaches 17500 kPa (2550 psi), the spool in brake accumulator charging valve (14) shifts so oil flow is blocked to the brake accumulators. This is the cutoff pressure.

Brake accumulator charging valve (14) contains pressure switch (15). Pressure switch (15) alerts the operator when inadequate pressure is available to charge brake accumulators (10) and (12). When the oil pressure drops below 6900 kPa (1000 psi), pressure switch (15) alerts the operator. "Brake Failure" will be displayed on the IQAN screen.

Pressurized oil flows from brake accumulator charging valve (14) to the oil end of two brake accumulators (10) and (12). Accumulators (10) and (12) contain a precharge of dry nitrogen. This precharge is approximately 4200 kPa (600 psi).

The oil flows from accumulators (10) and (12) to brake control valve (7) and (9) .

When the load sensing spool in the brake accumulator charging valve opens, a load signal flows through combination valve (2) at port (LSB). Then, the load signal flows from combination valve (2) at port (LSP) to the implement pump. The signal upstrokes the pump in order to charge the brake accumulators.

When the operator turns the key to start the engine, current from the battery flows to solenoid valve (19). The current causes solenoid valve (19) to shift to the right. This blocks the load signal oil from flowing to the implement pump from brake accumulator charging valve (14). The implement pump is prevented from upstroking which decreases the load on the engine. Thus, the engine is easier to start.

When the operator releases the key, current is cutoff to solenoid valve (19). Solenoid valve (19) shifts to the left. This allows the load signal oil to flow to the implement pump from brake accumulator charging valve (14) .

Service Brakes

Each brake application removes some of the oil from brake accumulators (10) and (12). The piston in each accumulator moves toward the oil end of the accumulator until there is a decrease in oil pressure. Brake accumulator charging valve (14) permits oil from the hydraulic pump to flow to brake accumulators (10) and (12) until the pressure again rises.

Brake control valve (7) or (9) controls front service brakes and the rear service brakes. Brake control valve (7) is mounted at the rear of the cab. Brake control valve (9) is mounted at the front of the cab.

The front and the rear service brake pedals are connected by hydraulic hose. When the operator pushes on one of the brake pedals, the service brakes are applied.

When the operator pushes on either of the service brake pedals, the spool in brake control valve (7) or (9) will move. This allows oil from brake accumulators (10) and (12) to flow to the brake cylinders on each axle.

The oil pressure that engages the service brakes also closes brake light pressure switch (18) when the oil pressure exceeds 300 kPa (44 psi). When the pressure switch closes, the brake lights on the rear of the machine illuminate.

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Illustration 3	g00929195
Brake Cylinder

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Illustration 4	g00929395

The oil flows to service brake cylinder (36). The oil pressure will move service brake piston (34) downward. Service brake piston (37) forces wedge (39) into brake disc pack (21). The brake disc pack is engaged in the axle. The heat from the brakes is removed by the oil in the differential housings.

When the operator releases the brake pedal, the spool in brake control valve (7) or (9) returns to the center position. Oil flow from the accumulator is now blocked. Hydraulic oil then flows from brake accumulator charging valve (14) to the hydraulic oil tank. Service brake cylinder (36) can move upward also. This will release the oil pressure to the hydraulic tank.

When the oil pressure drops below 300 kPa (44 psi), brake light pressure switch (18) opens. When the pressure switch opens, the brake lights on the rear of the machine are turned off.

Parking Brake

Oil from the implement pump flows to a pressure reducing valve in parking brake control valve (4). The oil pressure is reduced to 2800 ± 100 kPa (400 ± 15 psi).

When the operator activates the parking brake switch, the electrical signal is turned off to the IQAN.

Then, the IQAN turns off the signal to the solenoid valve on parking brake control valve (4) and ladder control valve (3) .

Then, the solenoid is shifted on parking brake control valve (4). This prevents the oil flow to the parking brake cylinder. Spring (28) presses downward on pistons (27) and (37). Pistons (27) and (37) press wedge (39) into the brake disc pack. This engages the parking brake.

The parking brake is engaged with spring pressure and disengaged with oil pressure.

When the operator deactivates the parking brake switch, an electrical signal is sent to the IQAN. Then, the IQAN sends a signal to the solenoid valve for the parking brake.

Note: Two electrical signals are required to release the parking brake. The parking brake switch must be in the OFF position. The signal from the brake release switch that is located on the accelerator pedal must be received by the IQAN.

Then, the IQAN sends a signal to the solenoid valve on parking brake control valve (4).

Reference: For more information about the IQAN Monitoring and Control System, refer to Systems Operation, RENR6127, "IQAN Monitoring and Control System".

Then, the solenoid shifts. This allows pressure oil to flow to the parking brake cylinder. First, the oil pressure will move parking brake piston (27) against spring (28). Next, spring (28) will compress. This removes wedge (39) from the brake disc pack. This disengages the parking brake.

Parking brake pressure switch (1) alerts the operator when inadequate pressure is available to deactivate the parking brake. When the oil pressure drops below 2300 kPa (335 psi), pressure switch (1) alerts the operator. "Parking brake" will be displayed on the IQAN screen.

Ladder

Oil from the implement pump flows to a pressure reducing valve in ladder control valve (3). The oil pressure is reduced to 2800 kPa (400 psi).

When the operator activates the parking brake, the ladder is lowered. When the operator deactivates the parking brake, the ladder is raised.

When the operator engages the parking brake, an electrical signal is sent to the solenoid valve on ladder control valve (3). This allows oil to flow to the head end of ladder cylinder (8). This causes the ladder to lower.

When the parking brake switch is disengaged, the electrical signal is turned off to the solenoid valve on ladder control valve (3). This allows the oil to flow to the rod end of ladder cylinder (8). This causes the ladder to raise.

An orifice is installed in each oil line to the ladder cylinder. These two orifices prevent the ladder from overspeeding.