Steering System in a NO TURN Condition
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| Illustration 1 | g03401697 |
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Steering System (No Turn) (1) Cold oil relief valve (2) Steering charge filter and bypass valve (3) Steering charge pump (4) Pump control valve (5) Actuator piston (6) Power train ECM (7) Steering pump (9) Steering lever position sensors (10) Crossover relief valves and makeup valves (11) Passage from hydraulic oil tank (12) Return to hydraulic oil tank (case drain) (13) Charge pressure relief valve (14) Pressure override valve (POR) (15) Left steering loop (16) Pilot oil passage to the valve stack (17) Steering motor (18) Flushing valve (19) Right steering loop (20) Passage from fan (21) Resolver (FF) Activated components (GG) Tank pressure (KK) High pressure (LL) Reduced pump pressure (RR) Charge pressure (SS) Reduced charge pressure | |
Whenever the engine is running, steering charge pump (3) draws oil from the hydraulic oil tank through passage (20). When the oil is cold, the pressure of the oil between steering charge filter (2) and cold oil relief valve (1) may be high enough to open cold oil relief valve (1). As a result, a portion of the charge oil will flow to the suction line through cold oil relief valve (1) until the oil warms. Before the charge oil flows to steering pump (7), the charge oil flows through steering charge filter (2). Charge oil then flows to three components:
- Charge Pressure Relief Valve (13)
- Crossover Relief Valves (10)
- Pump Control Valve (4)
- Pressure override valve (POR)
- Pilot system
Charge pressure relief valve (13) maintains the charge pressure at a specified flow.
Crossover relief and makeup valves (10) supply the steering loop with charge oil. When steering pump (7) is not producing any flow, charge pressure will flow through both makeup valves to both sides of the closed loop until the pressure in the steering loop is equal to charge pressure.
Pump control valve (4) regulates the flow of charge oil to actuator piston (5). Actuator piston (5) controls the angle of the pump swashplate. Pump control valve (4) is shifted by solenoids that receive current from power train ECM (6). Power train ECM (6) receives a signal from steering lever position sensors (9).
When the control lever is not being moved, no current is sent to the solenoids of pump control valve (4). Springs center the pump control valve spool and the charge oil passes through the pump control valve spool to the case drain. Both sides of actuator piston (5) have equal pressure. The actuator piston is centered by springs. The swashplate in steering pump (7) is held in the NEUTRAL position.
Consequently, the steering pump does not produce any flow and the pressure in both sides of the closed drive loop is equal. The pressure of the oil on both sides of steering motor (17) is equal. The steering motor remains stationary and the steering system remains in a NO TURN condition.
Steering System in a LEFT TURN Condition
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| Illustration 2 | g03401698 |
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Steering System (Left Turn) (1) Cold oil relief valve (2) Steering charge filter and bypass valve (3) Steering charge pump (4) Pump control valve (5) Actuator piston (6) Power train ECM (7) Steering pump (9) Steering lever position sensors (10) Crossover relief valves and makeup valves (11) Passage from hydraulic oil tank (12) Return to hydraulic oil tank (case drain) (13) Charge pressure relief valve (14) Pressure override valve (POR) (15) Left steering loop (16) Pilot oil passage to the valve stack (17) Steering motor (18) Flushing valve (19) Right steering loop (20) Passage from fan (21) Resolver (FF) Activated components (GG) Tank pressure (KK) High pressure (LL) Reduced pump pressure (RR) Charge pressure (SS) Reduced charge pressure | |
When the operator moves steering control lever to LEFT TURN, steering lever position sensors (9) send a signal to power train ECM (6). Power train ECM (6) sends a current to the left solenoid on pump control valve (4). The solenoid shifts the pump control valve spool to the right. Because the solenoid is a proportional solenoid, the movement of the pump control valve spool will be proportional to the movement of steering control lever. Therefore, a metered amount of charge oil will flow to actuator piston (5). The angle of the swashplate in steering pump (7) changes to a value that is proportional to the original movement of steering control lever.
When the swashplate of the pump angles, the pump produces flow. The pump draws oil from right steering drive loop (19). Pump output flows to left steering drive loop (15).
The pump output flows to steering motor (17). The flow of the oil turns steering motor (17), which turns the steering differential. Consequently, the steering differential will turn at a rate that is directly proportional to the original input from the steering control lever.
The output oil leaves steering motor (17) and flows into right steering drive loop (19). This oil flows back to the suction side of steering pump (7). The oil goes around the closed drive loop again.
Flushing valve (18) allows some of the oil flow in the low-pressure side of the closed drive loop to flow through the steering motor. This oil performs the following functions:
- This oil cools the steering motor.
- This oil lubricates the internal components of the steering motor.
- This oil flushes the internal components of the steering motor.
- Replaces hot loop oil with cool oil.
This oil is replaced by charge oil through the makeup check valves in crossover relief valves (10).
In a turning condition, resolver (21) will shift. This shift will allow the highest pressure in the drive loop to act on POR valve (14). When the pressure in the closed drive loop becomes higher than the pressure setting of POR valve (14), POR valve (14) will open. As a result, oil from actuator piston (5) will drain to the pump case. This action lowers the pressure of the oil at the controlled side of actuator piston (5). This action causes the pump to destroke in order to maintain POR setting. Temporary pressure spikes are relieved through crossover relief valves (10).
The steering system in a RIGHT TURN condition is the exact opposite of the steering system in a LEFT TURN condition.