789D Off-Highway Truck: Steering Control Valve

Steering control valve

The steering control valve is located on the front frame rail under the engine.

A steering control valve is placed in the NEUTRAL position.
(1) Steering control valve
(2) Steering check and relief valve
(3) Holes
(4) Port for left turn "CL"
(5) Port for return oil to tank "HT"
(6) Back pressure valve
(7) Internal tank passage
(8) Port for right turn "CR"
(9) Port for return oil from steering metering pump "T"
(10) Holes
(11) Holes
(12) Springs
(13) Passage
(14) Amplifier spool
(15) Internal cavity
(16) Orifices (four)
(17) Pilot oil left turn port "L".
(18) Spring
(19) Passage
(20) Passage
(21) Port "P" for pump oil to steering metering pump
(22) Passage
(23) Port "HP" for inlet oil from the accumulator.
(24) Priority spool
(25) Passage
(26) Plug
(27) Port for load sensing oil "LS"
(28) Spring
(29) Check valve spool
(30) Orifice
(31) Passage
(32) Pin
(33) Spring
(34) Right turn pilot oil port "R".
(35) Passage
(36) Orifices (six)
(37) Passage
(38) Orifices (eight)(39) Directional spool

The steering control valve is operated with pilot oil through five hydraulic lines that are connected to the steering metering pump. The steering metering pump is connected to the steering wheel. The oil in the five pilot lines from the steering metering pump shifts the spools in the steering control valve. The spools control the amount of pressure oil that is sent to the steering cylinders. The steering control valve (1) is made of the following components:

Priority spool (24)
Amplifier spool (14)
Check valve spool (29)
Directional spool (39)
Steering check and relief valves (2)
Back pressure valve (6)

Oil Flow In Neutral

Pressure oil from the accumulators flows through port (23). The oil fills passages (22), (25), (19) and (20). Oil flows by priority spool (24). Passage (19) is blocked by amplifier spool (14). Pressure oil is communicated to the steering metering pump through port (21). Once all passages fill with pressure oil, priority spool (24) shifts to the left. Priority spool (24) remains open between passages (19) and (22). This allows a small amount of oil flow to the steering metering pump.

With the truck in the NO STEER position, the steering metering pump vents the left and right pilot oil ports to return oil port (9). No pilot oil acts on directional spool (39). Directional spool (39) is maintained in the center position by springs (12). Any resistance that acts on the steering cylinders will create a pressure increase in port (4) or (8). The pressure increase acts upon the steering check and relief valve (2) in the port.

Steering Check and Relief Valves

Steering check and relief valve (2) in the RELIEF position
(2) Steering check and relief valve
(3) Holes
(10) Holes
(40) Poppet
(41) Orifice(42) Dump valve

If the pressure increase exceeds approximately 28500 kPa (4135 psi), pressure oil in holes (3) forces poppet (40) to the left. This creates a pressure drop across orifice (41). The pressure drop causes dump valve (42) to move to the left. This allows oil flow through holes (10) into internal tank passage (7).

Steering check and relief valve (2) in the MAKEUP position
(2) Steering check and relief valve
(10) Holes
(11) Holes
(43) Check valve(44) Seat for the check valve

After pressure oil is dumped into internal tank passage (7), the oil flows across the internal tank passage and back pressure valve (6). This oil enters holes (11) of steering check and relief valve (2). This pressure oil creates a pressure difference of 48 kPa (7 psi) between the internal tank passage (7) and the cylinder port with the lowest pressure. This pressure difference forces check valve (43) to the right which unseats the seat (44). The pressure oil that is dumped flows through holes (10) into the cylinder port with the lowest pressure. Back pressure valve (6) helps check valve (43) to open by allowing a pressure of 172 kPa (25 psi) to be felt in internal tank passage (7). A greater pressure will force back pressure valve (6) open and the oil will drain to the tank.

A steering control valve is placed in the RIGHT TURN position.
(1) Steering control valve
(2) Steering check and relief valve
(3) Holes
(4) Port for left turn "CL"
(5) Port for return oil to tank "HT"
(6) Back pressure valve
(7) Internal tank passage
(8) Port for right turn "CR"
(9) Port for return oil from steering metering pump "T"
(10) Holes
(11) Holes
(12) Springs
(13) Passage
(14) Amplifier spool
(15) Internal cavity
(16) Orifices (four)
(17) Pilot oil left turn port "L".
(18) Spring
(19) Passage
(20) Passage
(21) Port "P" for pump oil to steering metering pump
(22) Passage
(23) Port "HP" for inlet oil from the accumulator.
(24) Priority spool
(25) Passage
(26) Plug
(27) Port for load sensing oil "LS"
(28) Spring
(29) Check valve spool
(30) Orifice
(31) Passage
(32) Pin
(33) Spring
(34) Right turn pilot oil port "R".
(35) Passage
(36) Orifices (six)
(37) Passage
(38) Orifices (eight)(39) Directional spool

When the steering wheel is turned to the right, the steering metering pump stops venting all of the pilot oil ports to return oil port (9). This increases the pressure in the pump port of the steering metering pump. At the steering metering pump, the load sensing port is connected internally to the pump port. The increased pressure in the pump port is felt in the load sensing line. The increased pressure from load sensing port (27) causes priority spool (24) to move to the right. This allows more oil to flow to the pump inlet of the steering metering pump through passage (19). Load sensing pressure will vary with the resistance from steering. Priority spool (24) will move accordingly. This will allow oil to flow through passage (19) to meet steering requirements.

At the same time, the steering metering pump sends pilot oil for a right turn through port (34). Pilot oil flows through an internal passage (31) in directional spool (39). Pilot oil moves directional spool (39) to the right. As directional spool (39) moves to the right, pilot oil becomes open to passage (35). This pilot oil flows to amplifier spool (14) .

Pilot oil from passage (35) flows to the left end of amplifier spool (14). This oil then flows through a hole in connecting pin (32) and in stabilizing orifice (30). This causes amplifier spool (14) to move to the right. Oil from the accumulator at the right end of amplifier spool (14) flows through a connecting pin to the left side of the amplifier spool. This oil also causes the amplifier spool to move to the right.

Pilot oil from passage (35) also flows through orifices (36) into passage (37) of check valve spool (29). Pilot oil in passage (37) is momentarily blocked until amplifier spool (14) moves far enough to the right. Oil now flows through one of eight orifices (38) into passage (13) .

When amplifier spool (14) moves to the right, pressure oil in passage (19) flows through orifices (16). This oil then fills cavity (15). This forces check valve spool (29) to the left. Pressure oil flows through seven orifices (38) into passage (13). At this point, pilot oil combines with pressure oil. Oil flows through passage (13) and across directional spool (39) into port (8) for a right turn.

The faster the steering wheel is turned, the more oil the steering metering pump sends through port (34). The faster the steering wheel is turned, the farther directional spool (39) and amplifier spool (14) will shift. A higher flow rate is available to cause the machine to turn faster. The ratio of pilot oil to pressure oil does not change. This ratio is constant because one orifice (38) is dedicated to pilot oil flow, and seven orifices (38) are dedicated to pressure oil flow.

Return oil from the steering cylinders enters port (4), and flows across directional spool (39) into internal tank passage (7). This return oil then flows around steering check and relief valve (2). This forces back pressure valve (6) open, and the return oil returns to the steering tank through port (5) .

If a front wheel would strike something that cannot move, the oil pressure in that steering cylinder and in the oil line would increase. Oil flow to the cylinder would be reversed. This pressure spike is felt in amplifier spool (14). Check valve spool (29) moves to the right, and the spool blocks the seven pressure oil orifices (38). Amplifier spool (14) moves to the left and blocks pilot oil orifice (38). This stops pressure oil flow to the steering cylinders. The pressure spike is not felt at the steering metering pump. If the pressure spike is large enough, steering check and relief valve (2) will dump the pressure oil to the tank as described before.

Left Turn

The operation of the steering control valve (1) is the same for a left hand turn as for a right hand turn. The difference is that the pilot oil from the steering metering pump is felt in port (17). This shifts directional spool (39) left. Pressure oil flows into port (4). The return oil flows out of port (8). The machine will now turn left.

Neutral Position

When steering effort stops, the steering metering pump returns to the NEUTRAL position. The steering metering pump vents the pilot oil ports to return oil port (9). Pilot oil flow from port (34) to directional spool (39) stops. Oil flow in cylinder ports (4) and (8) is closed as springs (12) center directional spool (39). Loss of pilot oil through passage (35) to the end of amplifier spool (14) allows spring (18) to shift it to the left. Pressure oil is blocked from entering amplifier spool (14). This allows spring (33) to shift check valve spool (29) to the right. Orifices and pilot passages allow oil to move from one end of the amplifier spool to the other as the amplifier spool shifts. Loss of load sensing oil from port (27) allows spring (28) to shift priority spool (24) right. This keeps passages (19), (20), (22) and (25) full of oil.

MARYGAR