No Turn
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| Illustration 1 | g00906188 |
Steering circuit (no turn) (1) Steer and charge pump (2) Unloading valve (3) Priority valve (4) Orifice (5) Steering load sensing port line (6) Steering control unit (SCU) (7) Tank (8) Load sensing priority line (9) Cylinders (10) Orifice (dynamic bleed) (11) Orifice (12) Charge supply line (13) Check valve | |
Oil from steering and charge pump (1) flows to priority valve (3). The pressure signal in steering load port (5) from SCU (6) acts against the top of priority valve (3). Orifice (4) limits pressure surges to the end of priority valve (3). The charge pressure signal in load sensing priority line (8) acts against the bottom of priority valve (3). Orifice (11) limits pressure surges to one end of priority valve (3). The oil pressure on the bottom of the priority valve (3) overcomes the force (spring force and oil pressure) on the top of priority valve (3). The spool in priority valve (3) blocks oil to SCU (6). Steering oil is not a priority. The steering oil flows to the brake and shift valve through line (12) .
Blocked oil in the line from SCU (6) and trapped oil in steering cylinders (9) maintain steering control of the machine.
Bleed orifice (10) serves as a flushing passage for the spool in priority valve (2) .
Turn operation
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| Illustration 2 | g00906281 |
Steering circuit (no turn) (1) Steer and charge pump (2) Unloading valve (3) Priority valve (4) Orifice (5) Steering load sensing port line (6) Steering control unit (SCU) (7) Tank (8) Load sensing priority line (9) Cylinders (10) Orifice (dynamic bleed) (11) Orifice (12) Charge supply line (13) Check valve | |
Oil from steering and charge pump (1) flows to priority valve (3). The pressure signal in steering load port (5) from SCU (6) acts against the top of priority valve (3). Orifice (4) limits pressure surges to the end of priority valve (3). The charge pressure signal in load sensing priority line (8) acts against the bottom of priority valve (3) . Orifice (11) limits pressure surges to one end of priority valve (3) .
In a turn, the operator turns the steering wheel. As the steering wheel is turned, resistance from steering cylinders (9) causes pressure in lines (5) and (8). As the pressure signal in load sensing line (5) increases, steering oil becomes prioritized. The oil pressure and the spring force pushes the spool downward. The oil flows through priority outlet line (8). The oil flows to SCU (6) after coming out of priority outlet line. The oil flow now goes to the appropriate end of steering cylinders (9). The pressure in load sensing lines (5) and (8) is limited to 18950 kPa (2750 psi) by unloading valve (2) .
The oil flow from the steering circuit returns. The oil flow enters the charge supply line (12). Check valve (13) serves as a makeup valve when the engine is turned off. Check valve (13) also serves as a unloading valve when the engine is turned off.
The steering control unit controls the direction and the speed of the turn.
Oil flows through bleed orifice (4). The orifice serves as a flushing passage for the spool in priority valve (3). This flushing oil minimizes the "bottoming out" of the spool in priority valve (3) against the pump housing during pressure surges in the steering circuit. During operation, the priority valve spool oscillates.
Left turns follow the same hydraulic flow. The spool in SCU (6) shifts to the opposite end of the valve. High pressure and charge pressure lines to steering cylinders (9) are reversed.