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| Illustration 1 | g00927999 |
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Brake control valve (side view of valve) (1) Parking brake solenoid valve (2) Secondary brake solenoid valve (3) Parking and secondary brake valves (4) Check valve (5) Passage (6) Accumulator piston (7) Reducing spool (8) Pressure feedback chamber (9) Orifice (10) Supply chamber (11) Pilot pressure chamber (12) Proportional solenoid valve (13) Drain (14) Shutoff valve (15) Drain (16) Passage to brake (17) supply oil from pump (18) Pilot pressure chamber (19) Passage (20) Hole (21) Spool (22) Pilot valve | |
The brake control valve is installed on the top of the bevel gear case. The brake control valve is operated electrically by the electronic control module (ECM). The ECM responds to the operator's movement of the service brake pedal.
The service brake pedal modulates the engagement of both of the brakes in order to stop the machine. The parking brake switch engages both of the brakes in order to prevent movement of the machine.
The brakes are engaged with spring force. Hydraulic pressure is required to disengage the brakes. If hydraulic pressure is lost the brakes fully engage due to the action of the spring force.
The brake control valve includes a valve body and a manifold. The valve contains a separate manifold for the service brakes. The valve also contains parking or secondary valves for the brakes. One pilot valve (22) is used for both brakes. Proportional solenoid valve (12), accumulator piston (6), and reducing spool (7) are used to operate the pilot valve.
These components control the brake pressure for both brakes. In addition, the brake control includes shutoff valve (14). If the pressure from pilot valve (22) drops suddenly, the shutoff valve gradually drains the brake pressure. This shutoff valve prevents sudden brake engagement due to an electrical failure. At the same time, the operator can rapidly apply the brakes.
The brakes also have parking and secondary brake valves (3) operated by two on/off solenoids (1) and (2). These valves are operated by parking brake solenoid valve (1) and secondary brake solenoid valve (2). Parking brake solenoid valve (1) is connected to the parking brake switch. Secondary brake solenoid valve (2) is connected to the service brake switch (end of travel). The two solenoids are controlled by these two switches and by the ECM. The parking brake valve and the secondary brake valve enable the operator to apply the brakes immediately without modulation by the shutoff valve.
Proportional solenoid valve (12) is controlled by the ECM. This solenoid sets the pressure in pilot valve (22).
Pilot valve (22) regulates the pilot pressure in chamber (18), in passage (19), and in pilot pressure chamber (11). The applied force from solenoid (12) determines the pilot pressure in the passage and in the chambers. The applied force from solenoid (12) is determined by the position of the brake pedal. This solenoid also determines the pressure setting of reducing spool (7).
Reducing spool (7) regulates the flow of oil to the brakes. The pressure that is maintained is slightly lower than the pilot pressure in chamber (11) which is located at the left end of the spool. This pressure differential is due to a spring, which ensures that the brake pressure can be reduced to zero.
Accumulator piston (6) reduces fluctuations in pilot pressure. Also, the accumulator piston sends oil to shutoff valve (14).
The manifold group supplies oil to the brake control valve. Part of the oil from the manifold group flows to the brake control valve. Part of the oil flows to the modulating valves for the transmission.
Oil to the brake control valve flows into the supply port and through orifice (9). Oil from the supply port also flows through a screened orifice to pilot pressure chamber (18). The pressure in pilot pressure chamber (18) is regulated by pilot valve (22). The screened orifice separates the pilot pressure from the supply pressure.
Pressure in pilot pressure chamber (18) increases until the poppet in pilot valve (22) is forced open. The open poppet sends oil from pilot pressure chamber (18) into drain (13) in order to maintain a constant pressure in pilot pressure chamber (18). The pressure that is required to open the poppet in pilot valve (22) is determined by the force that is applied to the poppet by solenoid (12).
Oil from pilot pressure chamber (18) flows into chamber (11) (through passage (19) in shutoff valve (14) for brake control). Also, the brake pressure in the passage to brake (16) flows through passage (5) and into pressure feedback chamber (8).
When the pilot pressure in chamber (11) is greater than the brake pressure in chamber (8) plus a small amount determined by the spring at the right end of the spool, reducing spool (7) moves to the right. This movement opens the passage from supply chamber (10) into the passage to brake (16). The oil then flows into the brakes.
When the brake passages are full of oil and when the chambers are full of oil, the pressure in pressure feedback chamber (8) increases. The increased pressure moves reducing spool (7) to the left. The passage from supply chamber (10) to the brakes closes.
When the brake pressure reaches the preset pressure under the control of pilot pressure, reducing spool (7) sends oil into the passage to brakes (16). The oil maintains the brake pressure. The reducing spool supplies an allowance for leakage.
Accumulator piston (6) accumulates oil at pilot pressure. The pressure in pilot pressure chamber (11) moves accumulator piston (6) to the left. The movement increases the supply of the oil to reducing spool (7). Fluctuations in pilot pressure due to the movement of reducing spool (7) are reduced. A supply of oil is provided for operation of shutoff valve (14) for brake control.
When the operator calls for increased brake engagement, the ECM lowers the current to solenoid (12). The reduced current allows pilot pressure in chamber (18) to open the poppet in pilot valve (22). The open pilot valve relieves the pressure from chambers (11) and (18). The higher brake pressure in the passage to brake (16) and pressure feedback chamber (8) moves reducing spool (7) to the left.
This movement to the left opens the passage to brake (16) to drain (15). Oil flows from the brakes to drain (15) until pilot pressure in chamber (11) and brake pressure in pressure feedback chamber (8) are again balanced. Then, reducing spool (7) closes the passage to drain (15).
If an electrical failure causes pilot valve (22) to suddenly lower the pressure in pilot pressure chamber (18), shutoff valve (14) reacts in order to maintain the pressure in pilot pressure chamber (11). One example follows: the pilot pressure in chamber (18) is lowered faster than the rate that is possible by modulation of the service brake pedal by the operator.
The higher pressure in pilot pressure chamber (11) moves spool (21) to the left. The spool movement covers drilled hole (20) which reduces the flow out of pilot pressure chamber (11). This action of the control valve produces a gradual reduction in pressure in pilot pressure chamber (11). Also, the machine brakes gradually.
When the switch (end of travel) for the service brake pedal makes contact, secondary brake solenoid (2) is connected directly to the battery. When both the parking brake switch and the key start switch are in the ON position, parking brake solenoid (1) is connected directly to the battery.
These solenoids are also connected to the ECM. The ECM performs diagnostics on the solenoids. Also, the ECM provides a redundant supply of voltage to solenoids (1) and (2). Solenoids for the parking and secondary brake valves are connected to the service brake switch if the sensor for the pedal is at the maximum position.
When either solenoid (1) or (2) is ON, the corresponding parking or secondary brake valve (3) opens and the valve drains the pilot oil in chamber (11). Reducing spool (7) for the brakes immediately relieves all pressure to the brakes. The brakes are fully engaged.
This method of brake engagement is in addition to the control of proportional brake solenoid (12) which is provided by the ECM.
A calibration procedure is used in order to adjust the settings for the brake pressure. For a description of the calibration procedure, refer to the Testing and Adjusting section in this module.
Note: If any of the following items are replaced or if any of the following items are serviced, recalibrate the brakes:
- Brake control valve
- Solenoids
- Brake components
- ECM
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| Illustration 2 | g01381671 |
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Service brakes engaged. (1) Parking brake solenoid (2) Secondary brake solenoid (7A) Pressure reducing spool for the left brake (7B) Pressure reducing spool for the right brake (12) Proportional brake solenoid (16A) Passage to right brake (16B) Passage to left brake | |
When the operator pushes the service brake pedal toward the floor, the rotary sensor sends a signal to the electronic control module (ECM). This signal tells the ECM the brake pedal position. The electronic control module (ECM) decreases the current to proportional solenoid valve (12) relative to the brake pedal position.
The decreased current reduces pilot oil pressure, which causes reducing spools (7A) and (7B) to move upward. This movement relieves oil pressure in brake circuit (16A) and in brake circuit (16B) by partially opening the brake circuits to the tank.
The current to proportional solenoid valve (12) is dependent upon the position of the brake pedal. The solenoid converts pilot pressure oil to reduced pilot pressure oil. The pressure in the brake circuits is directly affected by the position of the brake pedal.
When the brake pedal is partially depressed the pilot pressure is reduced. The machine brakes lightly. When the brake pedal is fully depressed, the pilot pressure is reduced until the pressure reaches zero kPa (0 psi). The oil is drained from the brake circuit and the machine brakes fully.
Once the pedal is to the floor, the switch (end of travel) for the service brake opens. The switch connects secondary solenoid valve (2) to the battery. The energized solenoid valve opens the secondary brake valve (3), which exhausts any remaining pilot pressure to the drain. When the pilot oil is drained the machine brakes fully.
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| Illustration 3 | g01381678 |
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Service brakes released. (1) Parking brake solenoid (2) Secondary brake solenoid (7A) Pressure reducing spool for the left brake (7B) Pressure reducing spool for the right brake (12) Proportional brake solenoid (16A) Passage to right brake (16B) Passage to left brake | |
When the operator releases the service brake pedal, the rotary sensor sends a signal to the electronic control module (ECM). This signal tells the ECM that the brake pedal is released. The electronic control module (ECM) increases the current to proportional solenoid valve (12).
The increased current increases pilot oil pressure, which moves reducing spools (7A) and (7B) downward. The movement allows the oil to flow into brake circuit (16A) and into brake circuit (16B). The oil pressure acts on the brake piston and compresses the belleville springs, which releases the brakes.