Straight Line Operation
No Steering
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| Illustration 1 | g00488516 |
No steering (1) Servo cylinder (2) Hydraulic displacement control (3) Steering control (4) Filter and bypass valve (5) Line to the charge circuit for the implement hydraulic pump (6) Diverter valve (7) Tandem charge pump (8) Case drain for the steering pump (9) Charge pump (10) Common sump (11) Steering pump (12) Swashplate (13) Multifunction valves (14) Servo relief valves (15) Charge relief valve (16) Steering motor (17) Line to the differential lube (EE) Charge oil (FF) Oil from the large section of the tandem charge pump (LL) Sump oil | |
Illustration 1 shows hydraulic displacement control (2) and steering pump (11) during straight line operation.
Hydraulic displacement control (2) works with the signal pressure from steering control (3) in order to control the amount and direction of oil flow from steering pump (11) .
The signal pressure from steering control (3) shifts hydraulic displacement control (2). This allows pilot oil pressure to flow to servo cylinder (1) .
The pump output of steering pump (11) is controlled by the angle of swashplate (12). The angle of swashplate (12) is ultimately controlled by varying the amount of pilot oil pressure in servo cylinder (1) .
During straight line operation, the steering wheel is centered so that the signal lines from steering control (3) are open to the sump.
When there is no signal pressure to the spring chambers at each end of hydraulic displacement control (2), the control spool stays centered in the neutral position.
Charge oil is blocked by the centered control spool in hydraulic displacement control (2). The passages to servo cylinder (1) are open to case drain (8) for the steering pump across the centered control spool.
When the passages for servo cylinder (1) are open to case drain (8), swashplate (12) is held in the neutral position by two centering springs in steering pump (11) .
When swashplate (12) is in this position, steering pump (11) does not produce flow. If there is no pump output to turn steering motor (16), the machine moves in a straight line.
The pressure of the oil in both the lines to steering motor (16) will be approximately the value of the charge relief valve setting. The function of the makeup valve in multifunction valves (13) keeps the closed loop circuit full.
Right Turn Forward or Left Turn Reverse
Upstroke
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| Illustration 2 | g00488649 |
Upstroke (1) Servo cylinder (2) Hydraulic displacement control (6) Diverter valve (8) Case drain for the steering pump (9) Charge pump (11) Steering pump (12) Swashplate (13) Multifunction valves (14) Servo relief valves (15) Charge relief valve (17) Line to the differential lube (A) Inlet port for signal oil from the steering control (B) Control spool (C) Pilot oil passage (D) Passage for charge oil (E) Passage for charge oil supply (F) Passage to the steering control (G) Line from the steering filter (H) Passage from the steering motor (J) Passage to the steering motor (AA) High pressure oil (DD) Pilot oil (EE) Charge oil (FF) Oil from the large section of the tandem charge pump (JJ) Reduced charge oil (LL) Sump oil | |
When the steering wheel is turned to the right, signal oil from the steering control is routed to port (A). The signal oil enters the spring chamber at the left end of hydraulic displacement control (2). Next, the signal oil is routed to control spool (B) .
Control spool (B) moves a little to the right against the force of the spring on the right end. The movement of control spool (B) opens passage (C) for servo cylinder (1) to the supply passage for charge oil (E) .
The movement of control spool (B) sends metered charge oil pressure through passage (D) to servo cylinder (1). Metered charge oil pressure on servo cylinder (1) causes swashplate (12) to begin to move against the centering spring. When swashplate (12) begins to move, steering pump (11) upstrokes. The output from steering pump (11) is sent through passage (J) to the steering motor.
Constant Flow
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| Illustration 3 | g00488873 |
Constant flow (1) Servo cylinder (2) Hydraulic displacement control (8) Case drain for the steering pump (9) Charge pump (11) Steering pump (12) Swashplate (13) Multifunction valves (14) Servo relief valves (15) Charge relief valve (17) Line to the differential lube (A) Inlet port for signal oil from the steering control (B) Control spool (F) Passage to the steering control (G) Line from the steering filter (H) Passage from the steering motor (J) Passage to the steering motor (AA) High pressure oil (DD) Pilot oil (EE) Charge oil (FF) Oil from the large section of the tandem charge pump (JJ) Reduced charge oil (LL) Sump oil | |
If the steering wheel is held in a steady position, the signal pressure from the steering control is routed through port (A). Next, the oil is routed to the left end of control spool (B) in hydraulic displacement control (2). The signal pressure remains constant.
When the signal pressure is balanced with hydraulic displacement control (2), control spool (B) meters the pilot pressure oil. This is done in order to maintain the pressure in servo cylinder (1) .
The pressure against servo cylinder (1) maintains the angle of swashplate (12). Swashplate (12) stops moving which causes the output of steering pump (11) to remain constant. The pressure keeps both the steering motor and the machine turning at a constant speed if the engine speed and ground speed remain constant.
Destroke
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| Illustration 4 | g00489122 |
Destroking of the pump (1) Servo cylinder (2) Hydraulic displacement control (6) Diverter valve (8) Case drain for the steering pump (9) Charge pump (11) Steering pump (12) Swashplate (13) Multifunction valves (14) Servo relief valves (15) Charge relief valve (17) Line to the differential lube (A) Inlet port for signal oil from the steering control (B) Control spool () (F) Passage to the steering control (G) Line from the steering filter (H) Passage from the steering motor (J) Passage to the steering motor (BB) Reduced high pressure oil (EE) Charge oil (FF) Oil from the large section of the tandem charge pump (LL) Sump oil | |
When the steering wheel is returned to the centered position, both signal lines from steering control (3) are open to the sump. The spring at the right end of hydraulic displacement control (2) moves control spool (B) to the left. The oil from servo cylinder (1) flows to the case drain (8) for the steering pump.
The return oil must flow across an orifice in control spool (B). The orifice controls the rate of flow to the sump. This controls the response of the destroking of the pump. As oil is drained to the case drain (8), centering springs bring swashplate (12) back to the neutral position. The output of steering pump (11) returns to zero as the machine returns to straight line operation.
If the steering wheel is returned to the center position too quickly, control spool (B) may shift in the opposite direction far enough in order to route pilot pressure oil to the opposite servo cylinder (1). Pilot pressure oil destrokes steering pump (11). Metered pilot pressure oil and the centering springs will bring swashplate (12) back to the neutral position.
High Pressure Cutoff
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| Illustration 5 | g00489259 |
High pressure cutoff (1) Servo cylinder (2) Hydraulic displacement control (6) Diverter valve (8) Case drain for the steering pump (9) Charge pump (11) Steering pump (12) Swashplate (13) Multifunction valves (14) Servo relief valves (15) Charge relief valve (17) Line to the differential lube (A) Inlet port for signal oil from the steering control (B) Control spool () (C) Signal passage (F) Passage to the steering control (G) Line from the steering filter (H) Passage from the steering motor (J) Passage to the steering motor (AA) High pressure oil (BB) Reduced high pressure oil (DD) Pilot oil (EE) Charge oil (FF) Oil from the large section of the tandem charge pump (JJ) Reduced charge oil (LL) Sump oil | |
If the steering circuit has a load with a high enough resistance, steering pump (11) begins to destroke.
If a pressure of 36510 ± 1000 kPa (5300 ± 145 psi) above the charge pressure is felt in the high pressure closed loop, the pressure limiter valve in multifunction valve (13) becomes unseated.
High pressure oil from multifunction valve (13) is ported to servo cylinder (1) on the opposite side. High pressure oil that is working on the opposite side of servo cylinder (1) destrokes steering pump (11). The output pressure is reduced in order to protect the components of the steering pump.
The pressure that is used to destroke steering pump (11) is controlled by servo relief valve (14) in signal passage (C) .
Servo relief valve (14) limits the servo pressure to 5200 kPa (750 psi) over pilot pressure. The high pressure oil has a leak path across control spool (B) .
The amount of oil that flows to the sump is controlled by an orifice that is in the passage to the sump.
Oil that is not used to power the destroking of steering pump (11) is returned to the pilot oil circuit across servo relief valve (14) .
The metered charge oil that is used in order to upstroke steering pump (11) is forced back through control spool (B) into the pilot oil circuit.