Steering System Without Blade
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| Illustration 1 | g02102135 |
Steering Hydraulic Schematic With Steering Wheel in HOLD Position (1) Left steering cylinder (2) Right steering cylinder (3) Line relief valves (4) Makeup valve (5) Relief valve (6) Orifice (7) Orifice (8) Makeup valve (9) Control spool (10) Steering control unit (11) Metering pump (12) Anticavitation check valve (13) Filter bypass valve (14) Charge filter (15) Pressure switch (16) Line to propel system and vibratory system (17) Suction screen (18) Steering pump (19) Priority valve | |
Before the engine is started, the spring in priority valve (19) forces the valve to the right, and the left envelope of the valve is active. Because steering pump (18) is a positive displacement pump and the pump is driven by the engine, the pump produces flow as the engine is being started. Also, the pump produces flow while the engine is running. When the pump is operating, oil is drawn from the hydraulic tank through suction screen (17) .
When pump (18) is producing flow, supply oil flows across priority valve (19). Downstream from the priority valve, oil is directed through an orifice (7). Downstream from orifice (7) oil is directed to relief valve (5) through another orifice (6), and out port "PR" of the pump. The pressure between the priority valve and orifice (7) acts against one side of the priority valve. Downstream from second orifice (6) oil acts against the priority spool in conjunction with the spring.
From port "PR" of steering pump (18), supply oil flows into port "P" of steering control unit (10). When the steering wheel is not being rotated, the center envelope of control spool (9) is active. In this case, all supply oil flows across the control spool to port "T" of the steering control unit. Oil that flows out of the outlet of the steering control unit is directed to charge filter (14) .
The pressure differential between the two sides of orifice (6) on the downstream side of the priority valve causes the priority valve to shift. This shift directs some pump oil out of port "EX" of steering pump (18). Oil that is flowing from port "EX" is directed to charge filter (14). Eventually, the priority valve reaches an equilibrium position. As a result, the steering control unit receives 34.1 ± 2.6 L/min (9.0 ± 0.7 US gpm) of flow. Also, the remainder of the oil flow from the pump is directed out of port "EX".
Orifice (6), that is shown at the left end of priority valve (19), modulates the reaction time of the priority valve. This orifice prevents the priority valve from "hunting".
Charge filter (14) contains filter bypass valve (13). If the pressure differential across the filter is greater than 350 kPa (51 psi), the bypass valve opens and oil is sent to the hydraulic tank.
Line relief valves (3) limit the maximum pressure in left steering cylinders (1) and right steering cylinders (2). These relief valves act as shock valves for the steering system. The valves are set at the factory to open when the pressure in the steering cylinder reaches 22500 kPa (3263 psi). The line relief valves are not adjustable.
Anticavitation check valve (12) and makeup valve (4) and makeup valve (8) allow for limited manual steering when the engine is not operating. The check valve prevents the backflow of oil to steering pump (18) during manual steering of the machine.
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| Illustration 2 | g02102136 |
Steering Hydraulic Schematic is Shown During a Right Turn (1) Left steering cylinder (2) Right steering cylinder (3) Line relief valves (4) Makeup valve (5) Relief valve (6) Orifice (7) Orifice (8) Makeup valve (9) Control spool (10) Steering control unit (11) Metering pump (12) Anticavitation check valve (13) Filter bypass valve (14) Charge filter (15) Pressure switch (16) Line to propel system and vibratory system (17) Suction screen (18) Steering pump (19) Priority valve | |
When the steering wheel is rotated to the right, the top envelope of control spool (9) is active. The control spool is a tandem control valve. Steering wheel rotation determines the flow of the oil through the control spool. The amount of oil that is required for steering flows across the control spool to metering pump (11). Oil leaving the metering pump flows to the rod end of right steering cylinder (2) and to the head end of left steering cylinder (1). The control spool directs the excess oil that is not required for steering out of port "T" of the steering control unit.
Oil that flows into the steering cylinders causes the machine to turn right. Oil in the head end of right steering cylinder (2) and in the rod end of left steering cylinder (1) flows out of steering control unit (10) through port "T". Oil that flows out of the outlet of the steering control unit is directed to charge filter (14) .
As the operator turns the steering wheel faster, metering pump (11) increases the flow to the steering cylinders. As a result, the machine turns faster. The metering pump directs flow to the steering cylinders until the operator stops turning the steering wheel. When the steering wheel stops turning, the centering springs center control spool (9). Oil is blocked at the steering cylinders, and the steering angle does not change until the operator moves the steering wheel again.
If the operator continues to turn the steering wheel after the steering cylinders reaches the end of the cylinder's stroke, relief valve (5) will open. This valve maintains pressure in the steering system. Also, this valve directs the excess pump flow back to the inlet side of the pump. The priority valve continues to send oil through the system until the operator releases tension on the wheel.
Note: The relief flow that returns to the pump inlet will cause a drop in flow to the charge system for the propel pump and the vibratory pump. This may cause a low charge pressure fault.
Steering System With Blade Option
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| Illustration 3 | g02102137 |
Steering Hydraulic Schematic With Blade in Hold (1) Right steering cylinder (2) Line relief valve (3) Makeup valve (4) Control spool (5) Anticavitation check valve (6) Steering control unit (7) Metering pump (8) Inlet check valve (9) Relief valve (10) Orifice (11) Orifice (12) Priority valve (13) Left steering cylinder (14) Line relief valve (15) Makeup valve (16) Blade cylinder (17) Makeup valve (18) Line relief valve (19) Blade control spool (20) Makeup valve (21) Differential pressure valve (22) Blade valve (23) Filter bypass valve (24) Charge filter (25) Pressure switch (26) Line to propel system and vibratory system (27) Suction screen (28) Steering pump | |
Flow from port "EX" of steering pump (28) enters blade valve (22) through port "IN". Priority valve (12) controls the flow to the blade valve. The priority valve makes sure that sufficient flow is available to the steering system at the expense of the flow for operating the blade.
When blade control spool (19) is in the HOLD position, oil flows across the open center of the spool. This oil flows to charge filter (24) through port "OUT" of blade valve (22). When in the HOLD position, the blade control spool also blocks the rod end and blocks the head end of blade cylinder (16). In this case, the blade is held in position.
If the blade encounters an obstruction that forces the blade upward, line relief valve (18) will allow oil from the head end of blade cylinder (16) to flow through port "OUT" of blade valve (22). Makeup valve (17) near the relief valve will provide oil to the head end of the cylinder when the blade returns to the original position.
Charge filter (24) directs charge oil to the propel system and the vibratory system.
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| Illustration 4 | g02102138 |
Steering Hydraulic Schematic With Blade in Lift (1) Right steering cylinder (2) Line relief valve (3) Makeup valve (4) Control spool (5) Anticavitation check valve (6) Steering control unit (7) Metering pump (8) Inlet check valve (9) Relief valve (10) Orifice (11) Orifice (12) Priority valve (13) Left steering cylinder (14) Line relief valve (15) Makeup valve (16) Blade cylinder (17) Makeup valve (18) Line relief valve (19) Blade control spool (20) Makeup valve (21) Differential pressure valve (22) Blade valve (23) Filter bypass valve (24) Charge filter (25) Pressure switch (26) Line to propel system and vibratory system (27) Suction screen (28) Steering pump | |
Flow from port "EX" of steering pump (28) enters blade valve (22) through port "IN". Inside the blade valve, supply oil is sent to the following locations that are on the same line: differential pressure valve (21), makeup valve (20), inlet check valve (8) and blade control spool (19) . Supply pressure seats the makeup valve and the supply pressure acts against the differential pressure valve. This pressure then acts against a spring and the force due to the oil pressure at port "OUT" of the valve.
When the blade control pedal is in the LIFT position, supply oil flows through inlet check valve (8) and across blade control spool (19). The blade control spool directs oil to the rod end of blade cylinder (16). Oil from the head end of the blade hydraulic cylinder is directed through the blade valve to charge filter (24).
During blade operation, differential pressure valve (21) maintains pressure in the blade system while the valve allows the excess pump flow to enter the charge circuit.
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| Illustration 5 | g02102140 |
Steering Hydraulic Schematic With Blade in Lower (1) Right steering cylinder (2) Line relief valve (3) Makeup valve (4) Control spool (5) Anticavitation check valve (6) Steering control unit (7) Metering pump (8) Inlet check valve (9) Relief valve (10) Orifice (11) Orifice (12) Priority valve (13) Left steering cylinder (14) Line relief valve (15) Makeup valve (16) Blade cylinder (17) Makeup valve (18) Line relief valve (19) Blade control spool (20) Makeup valve (21) Differential pressure valve (22) Blade valve (23) Filter bypass valve (24) Charge filter (25) Pressure switch (26) Line to propel system and vibratory system (27) Suction screen (28) Steering pump | |
Flow from port "EX" of steering pump (28) enters blade valve (22) through port "IN". Inside the blade valve, supply oil is sent to the following locations: differential pressure valve (21), makeup valve (20), inlet check valve (8) and blade control spool (19). Supply pressure seats the makeup valve and the supply pressure acts against the differential pressure valve. Also, this pressure acts against the spring and the force due to the oil pressure at port "OUT" of the valve.
When the blade control pedal is in the LOWER position, supply oil flows through inlet check valve (8) and across blade control spool (19). The blade control spool directs oil to the head end of blade cylinder (16). Oil from the rod end of the blade hydraulic cylinder is directed through the blade valve to charge filter (24).
During blade operation, differential pressure valve (21) maintains pressure in the blade system while the valve allows the excess pump flow to enter the charge circuit.
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| Illustration 6 | g02102141 |
Steering Hydraulic Schematic With Blade in Float (1) Right steering cylinder (2) Line relief valve (3) Makeup valve (4) Control spool (5) Anticavitation check valve (6) Steering control unit (7) Metering pump (8) Inlet check valve (9) Relief valve (10) Orifice (11) Orifice (12) Priority valve (13) Left steering cylinder (14) Line relief valve (15) Makeup valve (16) Blade cylinder (17) Makeup valve (18) Line relief valve (19) Blade control spool (20) Makeup valve (21) Differential pressure valve (22) Blade valve (23) Filter bypass valve (24) Charge filter (25) Pressure switch (26) Line to propel system and vibratory system (27) Suction screen (28) Steering pump | |
Flow from port "EX" of steering pump (28) enters blade valve (22) through port "IN". Inside the blade valve, supply oil is sent to the following locations: differential pressure valve (21), makeup valve (20), inlet check valve (8) and blade control spool (19). Supply pressure acts against the differential pressure valve. Also, the supply pressure acts against the spring and the force due to the oil pressure at port "OUT" of the valve.
When the blade control pedal is in the FLOAT position, blade control spool (19) blocks the passage from the inlet check valve and the blade control spool opens a passage for oil to flow to port "OUT" of blade valve (22). When the pedal is in this position, the control valve also opens a passage between the rod end and the head end of blade cylinder (16). In this case, the blade is lowered under slight pressure until the blade contacts the ground. The pressure in the head end of the blade cylinder and the pressure in the rod end of the cylinder is equal. Since the effective area of the head end of the cylinder is greater than the effective area of the rod end of the cylinder, a slight downward force is applied to the blade.