| |
| Illustration 1 | g01119973 |
|
(1) Tank port. (2) Pressure port. (3) Tank port. (4) Spool. (5) Return spring. (6) Cylinder port. (7) Cylinder port. (8) Pilot port. (9) Pressure reducing valve. (10) Backup relief valve. (11) Cylinder crossover relief valve. (12) Shuttle valve. (13) Cylinder port. (14) Cylinder port. (15) Pilot oil. | |
| |
| Illustration 2 | g03670545 |
|
Schematic for the steering control valve in neutral position (4) Spool. (8) Pilot oil line. (9) Pressure reducing valve. (10) Backup relief valve. (11) Cylinder crossover relief valve. (12) Shuttle valve. (15) Pilot oil line. (16) Load sensing signal line. (17) Return line. (18) Pressure line. (19) Reduced pilot oil line. | |
The flow of oil to either end of valve spool (4) is blocked when the rotation of the steering wheel is stopped. With no pilot oil flow against either end of the valve spool, centering spring (5) will keep the spool in the NEUTRAL position.
When the steering control valve is in the NEUTRAL position, oil from the steering pump is blocked by valve spool (4). The pump then destrokes due to the decreased demand for flow. The pressure in the cylinders is communicated through shuttle valve (12) to the compensator valve. The pump will destroke in order to limit the pump flow so that the pump outlet pressure is equal to the pressure at the compensator valve. If the cylinder pressure is equal to tank pressure, then the pump will destroke to the low pressure standby setting of
In the NEUTRAL position, the valve spool also blocks the oil in the lines to the steering cylinders. When the lines to the steering cylinder are blocked, the steering wheel stays in position. If the pressure in one cylinder line exceeds
As an example, a left turn is made and the valve spool is returned to the NEUTRAL position. A force which causes the machine to turn to the right is placed on the machine. Since the oil is trapped in the cylinder lines, this force causes the pressure in the cylinder lines to increase. When the pressure exceeds
| |
| Illustration 3 | g00631305 |
|
(4) Shifted spool for right turn. (5) Compressed centering spring. (20) Return oil to hydraulic tank. (21) Pressurized oil to the steering cylinders. (22) Pressurized pilot oil. | |
When the steering wheel is turned to the right, pilot oil is sent through a check valve (steering valve) to pilot port (8) and (15) on the steering control valve. Pressure increases in the pilot chamber and the spool shifts against the force of the spring. As the spool shifts, pump inlet (2) is opened to the cylinder port, to the rod end of the right steering cylinder, and to the head end of the left steering cylinder.
During a turn, the pressure inside the steering cylinders is sensed at the compensator for the steering pump. This system is called a load sensing system. The pump compensator adjusts the outlet flow of the pump so that the pressure at the pump outlet is higher than the pressure in the steering cylinders by
The oil from the head end of the right steering cylinder flows through the cylinder port. The oil from the rod end of the left steering cylinder flows through the cylinder port. The cylinder port is located in the steering control valve. The oil then exits through tank port (1). The machine will turn to the right until the steering wheel is returned to the original rotational position. The steering pilot control valve will then shut off the flow to the steering control valve and the pressure in the pilot oil chamber will decrease. Spring (5) will push the spool to the NEUTRAL position and the machine will stop articulating.
The machine turns at a speed that depends on the rotational position of the steering wheel. The turning speed of the machine does not depend on the turning speed of the steering wheel.
The shaft of the pilot valve rotates when you turn the steering wheel. The shaft of the pilot valve opens the pilot valve. When the pilot valve is in the OPEN position, pilot oil is allowed to flow. The oil pressure from the pilot valve shifts the steering control valve. The steering control valve sends steering pump flow to the steering cylinders.
As the machine begins to turn, the pilot valve begins to rotate to the CLOSED position. The turning speed of the machine is reduced as the shaft of the pilot valve approaches the CLOSED position. The position of the steering wheel must be steadily adjusted if you desire a constant steering speed. This action is necessary in order to keep the pilot valve open as the machine turns. In order to increase the turning speed of the machine, the rotational position of the steering wheel must be adjusted. In order to decrease the turning speed of the machine, the rotational position of the steering wheel must be adjusted.
The steering control valve for a left turn is similar to the steering control valve for a right turn, except that the pilot oil enters the steering control valve at the other pilot port. The flow of pilot oil causes the valve spool to shift. The pump oil flows through the pump inlet port (2) in the steering control valve and the pump oil flows outward through the cylinder port. Oil is then routed to the rod end of the left steering cylinder and to the head end of the right steering cylinder. Pressure in the cylinders increases until the machine turns to the left. Oil from the head end of the left steering cylinder and oil from the rod end of the right steering cylinder flows through the cylinder port in the control valve. This oil then exits through the tank port.