Two hydraulic lines connect the steering pump to the steering motor. For a right turn, one line is for pressure oil and the other line is for return oil. For a left turn, the line that was the high pressure side of the closed loop circuit for a right turn becomes the return side of the closed loop circuit. Likewise, the line that was the return side of the closed loop circuit becomes the high pressure side of the closed loop circuit.
There is a multifunction valve in each circuit of the closed loop circuit. Therefore, when one valve is in the pressure circuit, the other valve is in the return circuit. The multifunction valve performs different functions. If the valve is in the pressure circuit, the valve is a relief valve and the valve is a high pressure cutoff valve. If the valve is in the return circuit, the valve is a makeup valve.
High Pressure Cutoff
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| Illustration 1 | g00487376 |
(1) Adjustment screw (2) Spring (3) Spool (4) Valve chamber (5) Outlet port (AA) Pressure oil (BB) Reduced pressure oil (EE) Charge oil | |
The multifunction valve is in the high pressure side of the closed loop circuit. The valve sends oil to the steering pump's servo cylinder in order to destroke the steering pump. This protects the system from damage due to high pressure.
The oil pressure in the high pressure side of the closed loop circuit is 36510 ± 1000 kPa (5300 ± 145 psi) above charge pressure. This pressure is present in chamber (4). The oil pressure in chamber (4) depresses spool (3) against spring (2). Pressure oil flows around spool (3). This oil flows through port (5) to the steering pump's servo cylinder. Then, the servo cylinder destrokes the steering pump.
The steering pump destrokes in order to maintain the high load on the pump. The force that is required to unseat spool (3) can be adjusted. This is done with adjustment screw (1) and spring (2).
Pressure Relief
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| Illustration 2 | g00487378 |
(1) Adjustment screw (2) Spring (3) Spool (4) Valve chamber (5) Outlet port (6) Spring (7) Spool (AA) Pressure oil (BB) Reduced pressure oil (EE) Charge oil | |
The multifunction valve is in the high pressure side of the closed loop circuit. The valve sends oil to the steering pump's servo cylinder in order to destroke the steering pump. The valve also sends oil through the hydraulic displacement control valve to the drain. This protects the system from damage due to pressure spikes. Pressure spikes may occur when the machine encounters a large external force.
The oil pressure in the high pressure side of the closed loop circuit is 36510 ± 1000 kPa (5300 ± 145 psi) above charge pressure. This pressure is present in chamber (4). The oil pressure in chamber (4) depresses spool (3) against spring (2). Pressure oil flows around spool (3). This oil flows through port (5) to the steering pump's servo cylinder. Then, the servo cylinder destrokes the steering pump.
Spool (3) cannot relieve enough pressure. The pressure in the circuit unseats spool (7). Spool (7) compresses spring (6). Oil flows around spool (7) to the charge circuit. Oil in the charge circuit flows through the hydraulic displacement control valve to the drain.
The pressure is relieved in the high pressure side of the closed loop circuit when the pressure is 3450 kPa (500 psi) above the high pressure cutoff.
Makeup Operation
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| Illustration 3 | g00487379 |
(4) Valve chamber (6) Spring (7) Spool (8) Spool (EE) Charge oil (JJ) Return oil (LL) Sump oil | |
During makeup operation, the multifunction valve transfers oil from the charge circuit to the return side of the closed loop circuit. This prevents cavitation of oil in the steering pump. Pressure oil in the charge circuit is provided by the charge pump.
The multifunction valve is in the return side of the closed loop circuit. Oil from the charge pump applies pressure on the outside of spool (8). This pressure pushes spool (8) to the left. The pressure of the return oil is present in chamber (4). This pressure acts against the inside of spools (7) and (8). This pressure pushes spools (7) and (8) to the right. This pressure keeps spool (8) seated.
The pressure in the return side of the closed loop circuit becomes less than the pressure in the charge circuit. The pressure in the charge circuit moves spool (8) to the left. Spool (8) moves spool (7) to the left. Spool (7) compresses spring (6). Oil flows from the charge circuit to the return side of the closed loop circuit in order to compensate for flow losses. Flow losses are created by normal leakage through the pump components and the flushing valve in the steering motor.