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| Illustration 1 | g01060064 |
(1) Steering cylinder (2) Metering pump (3) Priority valve (4) Test port (5) Test port (6) Gear pump (7) Hydraulic tank (A) Hystat charge oil and pilot supply oil (B) Supply to the implement circuit (C) Return line from the couplers (D) Supply to the charge pump (E) Return line from the hydrostatic drive system (F) Return line from the implement main control valve (G) Return line from the implement pilot control valve | |
The steering system is supplied by the front section of the gear pump (6). Oil flows from the gear pump to the priority valve (3) which is one of the valve sections on the bank valve. The priority valve supplies oil to the metering pump (2) in order to turn the machine. The steering system has priority over the work tool system. If there is no demand for oil by the metering pump, the priority valve allows the oil to flow to the control valves of the bank valve. From the metering pump, oil flows to the steering cylinder (1). The extension and retraction of the steering cylinder turns the machine. Return oil flows from the cylinder, through the metering pump, and back to the tank.
Priority Valve
Supplying the Work Tool System
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| Illustration 2 | g01060068 |
(3) Priority valve (H) Oil pressure from the load sensing port on the metering pump (J) Oil pressure to the metering pump (K) Supply oil from the front section of the gear pump (L) Return oil to the tank | |
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| Illustration 3 | g01060065 |
(3) Priority valve (8) Spring (9) Spool (H) Oil pressure from the load sensing port on the metering pump (J) Oil pressure to the metering pump (K) Supply oil from the front section of the gear pump (L) Return oil to the tank (M) Port for the relief valve | |
When the operator is not turning the steering wheel, the passage in the steering supply port on the metering pump (2) becomes blocked. Oil from port (H) flows to the tank through the metering pump (2). The pressure in port (J) begins to build up. This pressure on the spool (9) causes the spool to shift to the position that is shown in illustration 3 against the spring (8). Oil can then flow into the remaining valve sections in the bank valve.
When the operator turns the steering wheel, oil in port (H) cannot flow to the tank. The oil pressure in port (H) builds up until the pressure in port (H) equals the load pressure. The pressure also builds up in port (J). This causes the spool (9) to shift. When the spool shifts, the oil can flow to port (J). Port (J) supplies the steering system. When the spool is shifted in order to provide oil to the steering system, the flow is blocked to the other functions in the bank valve. The priority valve is constantly moving in order to accommodate the demand for oil in the steering system. The work tool system is supplied when the demand for oil in the steering system has been met.
Supplying the Steering System
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| Illustration 4 | g01060069 |
(3) Priority valve (H) Oil pressure to the load sensing port on the metering pump (J) Oil pressure to the metering pump (K) Supply oil from the front section of the gear pump (L) Return oil to the tank | |
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| Illustration 5 | g01060067 |
(3) Priority valve (8) Spring (9) Spool (10) Passage (11) Passage (H) Oil pressure to the load sensing port on the metering pump (J) Oil pressure to the metering pump (K) Supply oil from the front section of the gear pump (L) Return oil to the tank (M) Port for the relief valve | |
When the operator turns the steering wheel, the steering supply port on the metering pump is opened. This frees the pressure that was trapped between port (J) and the metering pump. Supply oil from the gear pump (6) flows into the valve through port (K). The oil is blocked by the spool (9). The oil flows through passages (10) and (11) and out of the valve into the metering pump through port (J). Oil that flows into the supply port flows to the steering cylinder (1) in order to turn the machine.
Metering Pump
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| Illustration 6 | g01060070 |
Metering Pump (12) Centering springs (13) Outlet to tank (14) Internal check valve (15) Inlet port (16) Passage (17) Passage from gerotor (18) Drive shaft (19) Spool (20) Pin (21) Passage to the left turn port (22) Passage to the right turn port (23) Sleeve (24) Passage to gerotor (25) Gerotor (26) Selector valve (27) Gerotor (A) Control section (B) Metering section | |
The steering wheel is connected to the metering pump by a mechanical linkage.
The two sections (A) and (B) are connected by hydraulics and by mechanics. The two sections work together in order to provide oil to the steering cylinder. Pump oil goes through inlet (15) into control section (A). When the steering wheel is turned, control section (A) sends oil to metering section (B). Metering section (B) produces a specific amount of oil flow. Metered oil from metering section (B) is then directed by control section (A) to either the passage to the left turn port (21) or the passage to the right turn port (22) .
Metering section (B) consists of two small hydraulic motors and a selector valve (26). The hydraulic motors produce a metered amount of oil flow. Then, the metered oil is directed by the control section (A) to the steering cylinder.
The direction and the speed are controlled by the metering pump. As the steering wheel is turned faster, there is an increase in the flow of oil from the steering metering pump to the cylinder. The increased flow causes the cylinder to move faster. This turns the machine faster.
When the steering wheel is not being turned, control section (A) is in the NEUTRAL position. The metering pump is a closed center valve. This means that there is no alignment between the passages in the spool and the orifices in the sleeve in the NEUTRAL position. Only a small amount of oil flows through the center position. This keeps the spool slightly open and ready to respond quickly to any steering demands.
When the steering wheel is turned for a right turn, pump oil flows into the metering pump through inlet (15) and passage (16). Oil flows from passage (16), through passage (24) and into gerotor (25). Oil also flows through the selector valve (26) and into gerotor (27). While the steering wheel is turned, gerotors (25) and (27) turn. The gerotors also pump oil while the steering wheel is turned. The oil flows outward through passage (17). This oil is directed to the steering cylinder through the right turn port.
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| Illustration 7 | g01060066 |
Spool and sleeve (19) Spool (20) Pin (23) Sleeve (28) Slots for springs (29) Grooves for oil flow (30) Holes for pin (31) Small holes for oil flow | |
When the steering wheel started to turn, spool (19), pin (20), and drive shaft (18) also started to turn. The sleeve (23) did not start to turn at the same time as the spool. The holes in the sleeve (30) are slightly larger than the pin (20). The difference allows the spool (19) to turn inside the sleeve. This aligns the small holes in the sleeve (31) with the grooves in the spool (29). The alignment of the holes and of the grooves provides the path for oil flow to gerotors (25) and (27), and to the steering cylinder (1) .
Centering springs (12) are compressed when the spool moves in relation to the sleeve. When the steering wheel has stopped turning, the springs will return the spool and the sleeve to a NEUTRAL position. When the steering action stops, the wheels will remain in the present position. The steering wheel must be turned in the opposite direction in order to straighten the wheels. The steering wheel must also be turned in the opposite direction in order to make a left turn. For a right turn, oil flows to the cylinder through the passage to the right port (22). While this happens, return oil from the cylinder comes back into the metering pump through the passage from the left port (21). This oil is sent to the tank through outlet (13) .
The same components that move for a right turn also move for a left turn. The components which are performing a left turn move in the opposite direction of the components which are performing a right turn. Oil still enters inlet (15) and flows to gerotors (25) and (27) through passage (16) and through passage (24). The oil exits gerotors (25) and (27) through passage (17) and flows through the sleeve spool combination. The sleeve spool combination is turned in the opposite direction of the right hand turn. This allows a different set of holes to align. This also provides a path for oil to the left turn port (21). The oil flows from the left turn port to the steering cylinder (1) .
When the machine is off, the internal check valve (14) allows limited manual steering. Within the metering pump, selector valve (26) cuts off flow to gerotor (27). Since gerotor (27) no longer receives flow, a smaller amount of oil is displaced through gerotor (25). The smaller displacement allows the steering wheel to be turned with less effort, which aids in manual steering. The metering pump contains crossover relief valves and makeup valves. External forces cause pressure spikes. The valves protect the system from pressure spikes. The metering pump has a load sensing port. The load sensing port is connected to inlet (15) through an orifice. Oil pressure in inlet (15) is felt in the load sensing line. This signal pressure is communicated to the priority valve.
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| Illustration 8 | g01060063 |
Pump gears in the metering section (18) Drive shaft (20) Pin (32) Stator (33) Rotor | |
The rotor and the stator are part of the gerotors (25) and (27) .
Note: The proper relationship between rotor (33) and pin (20) within the metering pump is shown in Illustration 8 for reference only.