Priority Valve

1. Turning the steering wheel right changes the oil passages in the orbitrol as shown in the figure. In this case, the oil used for steering assistance flows through the following two separate passages.

• Port CF → Port P of orbitrol → Orifice Ci → Hydraulic motor

• Port CF → Orifice Cdy → Pressure chamber B

→ Orifice Cls → Port LS of orbitrol → Hydraulic motor

2. The pressure oil from the hydraulic motor is routed through port R to the steering cylinder for assisting the rightward steering motion.

3. In the priority valve, the difference in pressure between chambers A and B causes spool to move leftward from the neutral position until the leftward pressure on the spool balances with the force of spring. The opening area of the variable orifice Ci changes so that the flowrate required for a particular steering will be obtained, and the spool is made balance at a stroke position where the flowrate corresponding to the opening area is ensured.

4. In other words, with the steering wheel turned in either direction, the oil is allowed to flow to the orbitrol at an appropriate flowrate preferentially selected for the particular steering, and all the excess oil is available for use by the other actuator circuits.

5. The variable orifice Ci stays closed when the steering wheel is in the neutral position, but begins to open as the steering wheel is turned. The quicker the steering wheel is turned, the wider the orifice opens. When orifice Ci is wide open, the pressure difference between before and after the orifice is small, that is, the pressure difference between pressure chambers A and B is small. As a result, spring forces the spool toward the left, allowing more oil to flow to the orbitrol.