797F Off-Highway Truck: Steering Control Valve

Location of the steering control valve

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

Steering control valve in the NO STEER position
(1) Steering check and relief valve
(2) Port to the steering cylinder for a left turn "CL"
(3) Port to the steering hydraulic tank for return oil "HT"
(4) Port to the steering cylinder for a right turn "CR"
(5) Back pressure valve
(6) Steering check and relief valve
(7) Directional spool
(8) Port for the pilot oil from the metering pump for a left turn "L"
(9) Port for pilot oil from the metering pump for a right turn "R"
(10) Amplifier piston
(11) Combiner and check valve spool
(12) Priority spool
(13) Port for load sensing oil from the metering pump "LS"
(14) Port for inlet oil from the steering accumulators "HP"(15) Port for pump oil to steering metering pump "P"

The steering control valve is controlled with pilot oil. The steering metering pump is connected to the steering control valve with five pilot lines. Four pilot lines are working lines. The four pilot lines have the following designations: oil supply, tank, right turn and left turn. The fifth pilot line is used for load sensing. 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 in the steering control valve control the amount of pressure oil that flows to the steering cylinders. The steering control valve is made of the following components:

Priority spool (12)
Amplifier piston (10)
Combiner and check valve spool (11)
Directional spool (7)
Steering check and relief valves (1)
Steering check and relief valves (6)
Back pressure valve (5)

Oil Flow In Neutral

Pressure oil from the accumulators flows through port (14). Oil flows by priority spool (12). Pressure oil is communicated to the steering metering pump through port (15). Once all passages fill with pressure oil, priority spool (12) shifts to the left. With the machine in the NEUTRAL or NO STEER position, all four working pilot lines are vented to the tank through the steering metering pump.

With the machine in the NO STEER position, no pilot oil acts on directional spool (7). Directional spool (7) is maintained in the center position by the centering springs. Any resistance that acts on the steering cylinders will create a pressure increase in port (2) for the left steering cylinder or in port (4) for the right steering cylinder. The pressure increase acts upon the steering check and relief valve within that port. Either steering check and relief valve (1) or steering check and relief valve (6) will sense the pressure increase.

Steering Check and Relief Valves

Steering check and relief valve in the RELIEF position
(16) Orifice
(17) Port
(18) Holes
(19) Relief poppet(20) Dump valve

High pressure oil of the steering cylinders is felt through holes (18). If the pressure increase from the steering cylinders exceeds the relief pressure, pressure oil in holes (18) forces poppet (19) to the left. This creates a pressure drop across orifice (16). The pressure drop causes dump valve (20) to open. This allows oil to flow through port (17) to the internal tank passage.

Steering check and relief valve in the MAKEUP position
(21) Check valve
(22) Ports
(23) Opening(24) Seat

After pressure oil is dumped into the internal tank passage, the oil will flow across the internal tank passage and back pressure valve (5). This oil enters ports (22) of the steering check and relief valve. If the pressure difference between the internal tank passage and the cylinder port with the highest pressure gets too high, check valve (21) will move to the right. This moves seat (24) in order to allow oil flow. The oil flows through opening (23) into the cylinder port with the lowest pressure. Back pressure valve (5) helps check valve (22) to open by holding some pressure in the internal tank passage. The oil pressure must shift back pressure valve (5) in order to drain to the steering hydraulic tank.

Oil Flow During A Turn

Steering control valve in the RIGHT TURN position
Steering control valve in the NO STEER position
(1) Steering check and relief valve
(2) Port to the steering cylinder for a left turn "CL"
(3) Port to the steering hydraulic tank for return oil "HT"
(4) Port to the steering cylinder for a right turn "CR"
(5) Back pressure valve
(6) Steering check and relief valve
(7) Directional spool
(8) Port for the pilot oil from the metering pump for a left turn "L"
(9) Port for pilot oil from the metering pump for a right turn "R"
(10) Amplifier piston
(11) Combiner and check valve spool
(12) Priority spool
(13) Port for load sensing oil from the metering pump "LS"
(14) Port for inlet oil from the steering accumulators "HP"
(15) Port for pump oil to steering metering pump "P"
(25) Stabilizing orifice
(26) Pilot orifice
(27) Stabilizing orifice
(28) Connecting pin hole(29) Connecting pin

When the steering wheel is turned to the right, the steering metering pump stops venting all of the pilot oil to the tank. 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 (13) causes priority spool (12) to move to the right. This allows more oil to flow to the pump inlet of the steering metering pump through passage (15). The load sensing pressure will vary with the amount of resistance to steering. Priority spool (12) will move according to the amount of load sensing pressure. This allows sufficient oil flow through passage (15) to meet steering requirements.

At the same time, the steering metering pump sends pilot oil for right turns through port (9). Pilot oil flows through stabilizing orifice (25) in directional spool (7). Pilot oil moves directional spool (7) to the right. As directional spool (7) moves to the right, pilot oil flows to amplifier piston (10) and to the combiner and check valve spool (11) .

The pilot oil divides at amplifier piston (10). Pilot oil flows through a narrow groove around the combiner and check valve spool (11). Pilot oil is momentarily blocked until amplifier piston (10) moves far enough to the right. Then, oil flows through one of eight pilot orifices (26) .

Also, pilot oil flows through connecting pin hole (28) and stabilizing orifice (27) to the left end of amplifier piston (10). This causes amplifier piston (10) to move to the right. Accumulator oil pressure that is present at the right end of amplifier piston (10) flows through connecting pin (29) to the left side of amplifier piston (10). This also causes amplifier piston (10) to move to the right.

When amplifier piston (10) moves to the right, pressurized oil from the accumulator flows into the inner chamber. This forces the combiner and check valve spool (11) to move to the left. Pressurized oil from the accumulator then flows through seven of the eight pilot orifices (26). At this point, the pilot oil and the pressurized oil from the accumulator then combine. Oil flows across directional spool (7) and through port (4) in order to make a right turn.

When the steering wheel is turned faster, the steering metering pump will send more oil to port (9). When the steering wheel is turned faster, directional spool (7) and amplifier piston (10) will shift farther. A higher flow rate is available to cause the machine to turn faster. The ratio of the pilot oil to the pressurized oil from the accumulator does not change because one pilot orifice (26) is dedicated to pilot flow and seven orifices are dedicated to pressurized oil flow from the accumulator.

Return oil from the steering cylinders flows across directional spool (7) into the internal tank passage. This return oil then flows around steering check and relief valve (1). This pressure opens back pressure valve (5). The oil pressure returns to the steering hydraulic tank through port (3) .

The operation of the steering control valve for a left hand turn is almost identical to the operation of the steering control valve for a right hand turn. The pilot oil from the steering metering pump is felt in port (8). This shifts directional spool (7) to the left. The combined pilot oil and pressurized oil from the accumulator flows into port (2). The machine will now turn to the left.

Striking An Obstacle During a Turn

If a front wheel strikes an object that does not move, the oil pressure in that steering cylinder and in the oil line would increase. Oil flow to the steering cylinder would be reversed. This pressure spike is felt in amplifier piston (10). The combiner and check valve spool (11) moves to the right. Spool (11) blocks the seven pilot orifices (26) that are for the pressurized oil from the accumulator. Spool (10) will move to the left. Spool (10) blocks the one pilot orifice (26) that is for the pilot oil. 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 (1) or steering check and relief valve (6) will dump the pressure oil to the tank. For additional information, see "Steering Check and Relief Valves".

Neutral Position After A Turn

When steering effort stops, the steering metering pump returns to the NO STEER position. The steering metering pump vents the pilot oil ports to the tank. The pilot oil that flows from port (9) to directional spool (7) stops. Oil flow in cylinder ports (2) and (4) is stopped as the centering springs center directional spool (7). Loss of pilot oil to the end of amplifier piston (10) allows the spring to shift amplifier piston (10) to the left. Pressure oil is blocked from entering amplifier piston (10). This allows the spring within the combiner and check valve spool (11) to move the combiner and check valve spool (11) to the right. Orifices and pilot passages allow the oil to move from one end of the amplifier piston to the other end of the amplifier piston. This will happen as the amplifier piston shifts. Loss of load sensing oil from port (13) allows the spring at the end of priority spool (12) to shift priority spool (12) to the right.

MARYGAR