Travel Motor
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Illustration 1 | g00819872 |
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Illustration 2 | g00819874 |
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Illustration 3 | g00819877 |
The travel motor is an axial piston pump with an infinitely variable swashplate. The output torque and the speed are functions of the pressure in the travel circuit.
The travel motor contains an output shaft, a swashplate, operating pistons, a rotary group, a shift piston, and a travel brake valve.
Travel on Level Surface
When the travel pedal is pushed forward oil enters the travel motor through port (A). The pressure oil shifts the shuttle valve in order to allow high pressure oil to flow to the shift piston for the travel motor and the destroke chamber for the servo piston. When the pressure oil for travel is below 26000 kPa (3770 psi) the shift piston will not shift and the spring force keeps the spool in the left position. When the pressure oil for travel is below 26000 kPa (3770 psi), the travel motor is at the minimum displacement angle. The pressure oil acts on the small area of the servo piston and the motor is kept at the angle for the minimum displacement. The pressure oil moves the spool of the counterbalance valve downward and the return passage (B-side) in the spool opens. The oil flows through the spool and through the open makeup valve to port (B) and back to the main valve block. Oil flows through the motor and the machine moves forward.
Uphill Travel
When the pressure oil for travel reaches 26000 kPa (3770 psi), the spring force in the shift piston for the travel motor is overcome. The shift piston starts shifting to the right. When the pressure oil is above 27000 kPa (3916 psi) the shift piston is fully shifted to the right. When the shift piston is shifted oil enters the large piston area of the servo piston. The servo piston moves to the left and the travel motor upstrokes to the maximum displacement angle. When the pressure oil for travel drops below 26000 kPa (3770 psi) the spring force in the shift piston overcomes the force of the pressure oil. The shift piston shifts to the left. The oil from the large area of the servo piston is drained to the tank. The pressure on the small piston area will move the servo piston to the right. When the servo piston moves to the right, the travel motor destrokes to the minimum displacement angle.
Downhill Travel
Note: The valve for overspeed protection only works when the machine is moving in the forward direction.
There are two situations when the valve for overspeed protection works.
- The operator releases the travel pedal when the machine is going downhill.
When the operator releases the travel pedal, oil flow from port (A) is stopped. The counterbalance valve shifts to the center position and the return passage to port (B) is blocked. When the machine is on a slope, the travel motor becomes a pump. The pressure at port (B) increases. When the pressure at port (B) reaches 32000 ± 3000 kPa (4640 ± 435 psi), the valve for overspeed protection is shifted to the right and oil enters the upstroke chamber of the servo piston. The upstroke piston shifts to the left and the travel motor upstrokes to an intermediate displacement angle. When the pressure reaches 42000 kPa (6090 psi), the cross over line relief valves open. The increased displacement of the travel motor that is combined with the setting for the cross over line relief valve provides the increased braking action. The machine will slow down gradually and the machine will stop.
- The travel pedal is activated and the machine travels downhill. The machine exceeds the nominal speed for the position of the travel pedal.
When the machine travels downhill on a steep grade, the machine speed may exceed the nominal speed for the position of the travel pedal. The travel motor will begin functioning as a pump. When the pressure difference between high pressure port (A) and low pressure port (B) is below 3000 kPa (435 psi), the spool (1) for the counterbalance valve shifts to the center position. The spool starts to block the return passage for port (B). The pressure at port (A) increases.
Creeper Speed
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Illustration 4 | g00819879 |
When the function for creeper speed is selected, pilot oil enters through port (X). The pressure will shift the spool for the shift piston to the right. High pressure oil from either port (B) or port (A) will enter the upstroke chamber of the servo piston. The high pressure oil will move the servo piston to the left and the travel motor upstrokes to the maximum displacement angle. The travel motor will remain at the angle for the maximum displacement, when the function for creeper speed is activated.
Travel Counterbalance Valve
The double-acting travel brake valve is made up of the makeup valves, a shuttle valve, a control spool, check valves, low pressure valves, logic valves, and a return oil check valve.
Travel
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Illustration 5 | g00820141 |
In the TRAVEL position, the oil flow from the main control valve goes around the closed makeup valve through the shuttle valve to the shift piston in the travel motor regulator.
At the same time, the oil pressure moves the left logic valve off the valve seat. The oil flows through the left logic valve, through the travel motor, through the right logic valve and to the center of the control spool and the oil flow is blocked.
The check valve in the control spool allows oil to flow to the left side of the control spool. As the travel pressure increases, the control spool moves to the right against the centering spring.
The center passage of the control spool opens and the return oil flow opens the right makeup valve and flows to the travel control valve and back to the tank.
The low pressure valve is an orifice check valve that works with the other orifice check valve in the control spool in order to allow the spool to open fast. At the same time, the closing of the spool is dampened.
Overspeed
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Illustration 6 | g00820131 |
If the machine is driven downhill, the travel system pressure decreases. As the pressure decreases, the control spool moves toward the center position in order to partially block the return oil to the tank. The flow through the motor creates a hydraulic restriction in the motor in order to slow the machine.
The return oil opens the left makeup valve and the oil enters the supply passage to the motor in order to prevent cavitation.