General Information
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| Illustration 1 | g00854788 |
The steering control valve is the first control valve in the valve stack. The steering control valve has priority over the remaining control valves. This is important so that the tractor has the ability to turn regardless of the load requirements and flow requirements of the implement circuits that are down stream. The steering control valve controls the speed of the steering motor and the direction of rotation for the steering motor. The pump oil flows to the steering control valve from the inlet manifold. The oil flow to the steering control valve is limited to 155 L/min (41 US gpm) by the flow control valve (19). The control valve has control valve spool (1), makeup valves (5) and (9), resolver (11), priority flow control valve (19), and load check valve (22) .
The steering valve is spring centered. Control valve spool (1) is a closed center spool and a manually operated spool with three positions: STEER RIGHT, HOLD and STEER LEFT.
Spring (2) keeps the spool in the HOLD position when the steering circuit is not active. Movement of the spool is controlled manually by the operator. The steering control lever is connected to valve spool (1) by a linkage.
Resolver (11) is a two-way check valve. Resolver (11) sends the signal oil with the highest pressure to the inlet manifold. The inlet manifold sends the signal oil to the compensator valve on the pump. Priority flow control valve (19) gives flow priority to the steering control valve over all of the implement control valves. Descriptions of the resolver and of the priority flow control valve are in this section. These descriptions are given with the steering valve spool in different positions. Descriptions for makeup valves (5) and (9) and for load check valve (22) are in separate sections.
Load Check Valve
Load check valve (22) will not open until pump oil pressure in chamber (20) becomes greater than the force of the spring for the load check valve and the steering oil pressure in passage (21) .
Makeup Valves
Makeup valve (5) or (9) opens when steering oil pressure in passage (6) or (8) drops 14 kPa (2 psi) below the return oil pressure in outlet passage (7). Makeup valves (5) and (9) add return oil in outlet passage (7) to steering oil passages (6) and (8). The additional oil keeps the lines for the steering motor full and the additional oil prevents cavitation (vacuum) and aeration in the steering motor.
Basic Valve Operation
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| Illustration 2 | g00854788 |
When the control valve spool and all implement valve spools are in the HOLD position, pump oil from the inlet manifold flows to inlet passage (18). Then, oil flows through metering holes in priority flow control valve (19). The oil moves priority flow control valve (19) against spring (16). Then, pump oil flows out of outlet passage (17) to the next control valve. Because all implement control valves are in the HOLD position, pump oil fills the parallel oil passages at approximately 3000 kPa (435 psi). Passage (3), resolver (11), passage (12), passage (13), and chamber (14) are drained. The oil that is in steer passages (6) and (8) and passage (21) is blocked.
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| Illustration 3 | g00856034 |
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| Illustration 4 | g00854889 |
When control valve spool (1) is moved to the STEER LEFT position, chamber (14) is closed off from outlet passage (10). Passage (3) is opened to steer left passage (6). Even when priority flow control valve (19) is moved to the right, some pump oil flows through the metering holes in the priority flow control valve. The flow opens load check valve (22) and the pump oil flows around control valve spool (1) to steer left passage (6). The pressure in steer left passage (6) increases and oil is sent through passage (3) to chamber (14). The oil now becomes signal oil. Some of the signal oil flows through passage (15) to the spring chamber that is behind priority flow control valve (19). The signal oil moves priority flow control valve (19) to the left. The remaining signal oil pushes resolver (11) to the side and the signal oil flows through passage (13) to the inlet manifold. The inlet manifold sends the signal oil to the compensator valve on the pump. The signal oil causes the pump to upstroke until system pressure is reached. System pressure is approximately 2100 kPa (305 psi) above the pressure of the signal oil.
As pump oil flow and system pressure increases, the steering oil flow and the pressure through steer left passage (6) increases. The increased oil flow through steer left passage (6) flows through the counterbalance valve to the steering motor and the machine turns left.
Return oil comes to steer right passage (8) from the steering motor and from the counterbalance valve. The return oil flows around control valve spool (1) and through outlet passage (10) to tank.
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| Illustration 5 | g00855979 |
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| Illustration 6 | g00854897 |
When steering valve spool (1) is moved to STEER RIGHT position, chamber (14) is closed off from outlet passage (10). Passage (3) is opened to steer right passage (8). Pump oil flow through priority flow control valve (19) opens load check valve (22) and flows around control valve spool (1) to steer right passage (8). The pressure in steer right passage (8) increases and oil is sent through passage (3) to chamber (14). The oil now becomes signal oil. Some of the signal oil flows through passage (15) to the spring chamber that is behind priority flow control valve (19). Priority flow control valve (19) moves to the left. The remaining signal oil pushes resolver (11) to the side and the signal oil flows through passage (13) to the inlet manifold. The inlet manifold sends the signal oil to the compensator valve in order to upstroke the pump.
As pump oil flow and system pressure increases, the steering oil flow and the pressure through steer right passage (8) increases. The increased oil flow through steer right passage (8) flows through the counterbalance valve to the steering motor and the machine turns right.
Return oil comes to steer left passage (6) from the steering motor and from the counterbalance valve. The return oil flows around control valve spool (1) and through outlet passage (4) to tank.