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The 994F Wheel Loader is equipped with a new service brake control valve. The valve has two individual brake ports. The upper brake port is for the front service brakes and the lower brake port is for the rear service brakes. With the service brake valve, the pressure at the ports for the service brake ports will be equal to the pressure developed by the two plunger springs. Also, the spring force will be proportional to the plunger movement.

The brake control valve is equipped with check valves. The top check valve prevents spikes in the tank port from entering the cavity with the plungers springs and acting on the the plunger. The two lower check valves also prevent spikes in the tank line from acting on the upper and lower spools and eventually transferring to the brake pedal.

The brake control valve is also equipped with shims that are between the ball retainer and the springs. These shims are used to adjust the maximum pressure that is directed to the service brakes.

Plunger Return Spring Ball

Upper Spool

Front Brake Port

Return Spring

Orifice

Rear Brake Port Lower Spool Orifice Shims Lower Piston Upper Piston Tank Port Plunger Springs Check Valve Check Valve Retainer System Pressure Port

System Pressure Port

Ball Retainer Check Valve

Tank Port

SERVICE BRAKE VALVE

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Service Brake Valve ON

In order to initiate the operation of the service brake valve, the operator depresses the brake pedal. The plunger is pushed in the downward direction against the plunger springs. The springs push the ball retainer, ball, the upper spool, and the upper piston down against the retainer and the lower spool. The front brake port will be blocked from the upper tank port. The front brake port will then be open to flow from the system pressure port (from the front brake accumulator). Also, the system oil flows through the orifice into the cavity between the upper spool and the upper piston.

The upper spool, upper piston and retainer moves the lower spool downward compressing the lower return spring and the lower piston is bottomed out. The rear brake port will then be open to flow from the system pressure port (from the rear brake accumulator). Also, the oil flows through the orifice into the cavity that is between the lower spool and the lower piston.

Return Spring Ball

Upper Spool Front Brake Port

Return Spring

Orifice

Rear Brake Port Lower Spool Orifice Shims Lower Piston Upper Piston Tank Port Check Valve Check Valve Retainer System Pressure Port

System Pressure Port Ball Retainer

Check Valve

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Service Brake Valve Balanced

In the upper section, the oil pressure in the cavity is equal to the pressure at the service brake port. Due to the area of the upper piston, the upper spool is shifted upward compressing the plunger springs. The upper spool moves up to block the flow of oil between the upper pressure port and the front brake port. This creates a balance between the force of the plunger springs and the front brake port pressure.

At the same time, the pressure in the lower cavity is equal to the pressure at the rear brake port. Due to the area of the lower piston, the lower spool is pushed up and the lower spool blocks the flow of oil between the lower system pressure port and the rear brake port This creates a balance between the upper piston force an the lower brake port pressure.

Increasing the downward movement of the plunger will increase the spring force and cause pressure at the service brake ports to increase until maximum pressure is reached.

Decreasing the downward movement of the plunger will decrease spring force and cause pressure at the service brake ports to decrease. The return springs move the upper and lower spools up when the pedal is fully released opening the service brake ports up to the tank ports.

Plunger Return Spring Ball

Upper Spool Front Brake Port

Return Spring

Orifice

Rear Brake Port Lower Spool Orifice Shims Lower Piston Upper Piston Tank Port Plunger Springs Check Valve Check Valve Retainer System Pressure Port

System Pressure Port Ball Retainer

Check Valve

Tank Port

SERVICE BRAKE VALVE

This illustration shows the service brake accumulators and the check valves. Accumulator (2) and check valve (4) are in the circuit for the front service brakes. Accumulator (1) and check valve (6) are in the circuit for the rear service brakes.

The accumulators are piston type that are charged with dry nitrogen. The charge pressure for the accumulators at 160° C (70° F) is 5520 ± 280 kPa (800 ± 40 psi).

Also, hose (3) is the supply line that is feeding the two accumulators. The hose is installed between the brake pump (not shown) and the divider block (5).

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This illustration shows the location of the brake pump (4) and the check valve (9) on the rear pump drive (6). The brake pump is a pressure compensated piston pump that is adjusted to supply 16000 ± 345 kPa (2300 ± 50 psi) pressure. The check valve has a cracking pressure of 448 ± 55 kPa (65 ± 8 psi).

Also shown is the steering and brake hydraulic tank (1) and brake cooling tank (5). Installed on the steering and brake hydraulic tank are the temperature sensor (3) and the liquid level

switch (2) (the liquid level switch is located on the opposite side of the tank). The temperature sensor for the steering and brake hydraulic oil (3) communicates with the VIMS module. The liquid level switch (2) for the oil in the steering and brake hydraulic tank communicates with the VIMS module.

Also shown are the steering and brake oil cooling pump (8) and the hose (7) that is connected to the dividing block for the brake accumulators (not shown).

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