• When the pilot signal is ON, pilot pressure PI from the control pump acts on travel speed se-lector valve (1) and the capacity is automatically changed by the drive pressure of the travel mo-tor.
Balance of force for travel speed selector valve F1 [Force pushing spool (1a) to right (high-speed side)] = A (area of spool (1a) receiving pressure) × PI
F2 [Force pushing spool (1a) to left (low-speed side)] = A (area of spool (1a) receiving pressure)
× PH (motor drive pressure) + force of spring (1b)
i) When traveling on level ground, if the travel motor drive pressure is low, the condition becomes F1 > F2, and spool (1a) is pushed
hydraulic force from propulsion force Fc of control piston (10). It rotates and holds the moment balance at the high speed surface and is held in position (Mx < Fc, L).
ii) When operating the steering or traveling uphill, if the motor drive pressure is high, and it goes above the set pressure, the condition becomes F1 < F2, and spool (1a) is pushed to the left.
When this happens, the main oil pressure (M1, M2) passes through spool (1a) and is drained to the motor case. This creates the low-speed condition in the same way as when PI = 0.
1-3 Parking brake
The parking brake is a hydraulic release, wet type, multiple disc negative brake. The release pressure employs a self-pressure release method using the travel motor drive pressure through the oil passage in the counterbalance valve spool of the travel brake valve.
When the machine is traveling, the drive pres-sure is sent to port P of the parking brake circuit, acts on the piston in the brake cylinder chamber, overcomes the force of the spring and releases the brake.
When the machine is parked or stopped, the counterbalance valve spool returns to neutral, and the oil in the cylinder chamber is released to the tank through the counterbalance valve spool, so the brake is applied by the force of the spring to mechanically lock the motor shaft and hold the machine in position.
When parking brake is ON
When the travel lever is placed in neutral, the counterbalance valve spool returns to the neu-tral position and parking brake circuit P is closed.
The pressurized oil in chamber e of the brake piston passes through the counterbalance valve spool, is drained to the tank, and brake piston (18) is pushed fully to the left by force Fs of spring (19), and the brake is applied.
When parking brake is OFF
When the travel lever is operated, the pressur-ized oil from the pump actuates the counterbal-ance valve spool, opens the circuit to the parking brake, and flows into chamber e of brake piston (18). It overcomes the force of spring (19), and pushes brake piston (18) to the right.
When this happens, the force pushing plate (17) and disc (16) together is lost, so the plate and disc separate and the brake is released.
1-4 Relief valve
Function
In addition to the pressure control function, this relief valve has a shockless function (2-stage pressure increase function). This acts to reduce the shock generated at the beginning when the speed is reduced (when the machine is stopped).
Operation
1) When relief valve starts to be actuated (1st stage)
The area receiving the pressure which lifts pop-pet (2) becomes area S1 of the poppop-pet seat.
For this reason, it has a much larger pressure-receiving area than the pressure-pressure-receiving area when it is set (S1 – S2), so the relief actuating pressure becomes a low pressure of approx.
15.7 MPa {160 kg/cm2}.
This condition is maintained until the movement of shockless piston (1) is completed (approx.
0.25 sec).
2) When movement of shockless piston is com-pleted (2nd stage)
The pressure in the relief valve spring chamber rises, the pressure on both sides of poppet (2) becomes the same, and it becomes the speci-fied set pressure.
a Set pressure and amount of adjustment Amount of adjustment for one turn:
Approx. 7.8 MPa {80 kg/cm2}
1. Shockless piston 2. Poppet
3. Poppet orifice 4. Relief housing orifice 5. Piston stroke
1-5 Travel brake valve
Function
• The brake valve consists of counterbalance valve (3) and check valve (4).
• The counterbalance valve acts to prevent the piston motor from stopping or overrunning.
• When traveling downhill, the weight of the ma-chine makes it try to travel at a speed faster than the rotation of the motor.
For this reason, if the machine is traveling with the engine running at low speed, the motor will rotate under no load or may run away, which is extremely dangerous.
To prevent this, this valve controls the amount of oil on the return side in order to keep the valve pressure constant, and makes it possible to carry out travel operations which match the oil delivery from the motor.
Specification
Safety valve set pressure: 34 MPa {350 kg/cm2} Counterbalance switching pressure:
1.0 ± 0.1 MPa {10 ± 1 kg/cm2} Check valve switching pressure:
0.02 ± 0.01 MPa {0.2 ± 0.1 kg/cm2} 1. Cap assembly
2. Return spring
3. Counterbalance valve 4. Check valve
5. Check valve spring
Operation when oil is supplied
• When the travel lever is operated, the pressu-rized oil from the control valve is supplied to port P1. It pushes open check valve (5a) and flows from motor inlet port M1 to motor outlet port M2.
However, the motor outlet port is closed by check valve (5b) and spool (3), so the pressure at the supply side rises.
• The pressurized oil at the supply side flows from orifice a in spool (3) to chamber A. When the pressure in chamber A goes above the spool switching pressure, spool (3) is pushed to the left.
As a result, port M2 and port P2 are connected, the outlet port side of the motor is opened, and the motor starts to rotate.
Operation of brake when traveling downhill
• If the machine tries to run away when traveling downhill, the motor will turn under no load, so the pressure at the motor inlet port will drop, and the pressure in chamber A through orifice a will also drop.
When the pressure in chamber A drops below the spool switching pressure, spool (3) is returned to the right by spring (2), and outlet port M2 is throttled.
As a result, the pressure at the outlet port side rises, resistance is generated to the rotation of the motor, and this prevents the machine from running away.
In other words, spool (3) moves to a position where the pressure at outlet port M2 balances the pressure at the inlet port and the force gen-erated by the weight of the machine. It throttles the outlet port circuit and controls the travel speed according to the amount of oil discharged from the pump.