valve components:
- Priority spool - Amplifier spool with
combiner/check spool
- Directional spool - Relief/makeup valves - Back pressure valve
RELIEF/MAKEUP VALVE
LEFT TURN
CYLINDER BACK PRESSURE
VALVE
LEFT TURN PILOT OIL
AMPLIFIER SPOOL RIGHT TURN PILOT OIL
COMBINER/CHECK
Shown is a sectional view of the steering directional valve. The main components of the steering directional valve are: the priority spool, the amplifier spool with internal combiner/check spool, the directional spool, the relief/makeup valves and the back pressure valve.
Pressure oil from the accumulators flows past the spring biased priority spool and is blocked by the amplifier spool. The same pressure oil flows through an orifice to the right end of the priority spool. The orifice stabilizes the flow to the priority spool and must be present to open and close the priority spool as the flow demand changes. The same pressure oil flows to the HMU. After all the passages fill with pressure oil, the priority spool shifts to the left, but remains partially open. In this position, the priority spool allows a small amount of oil flow (thermal bleed) to the HMU and decreases the pressure to the HMU supply port.
The lower pressure prevents the HMU from sticking.
ports (supply, tank, right turn and left turn) are vented to the tank through the HMU. The directional spool is held in the center position by the centering springs.
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• Steering directional valve during a RIGHT TURN
AMPLIFIER SPOOL BACK PRESSURE VALVE
LEFT TURN PILOT OIL RIGHT TURN PILOT OIL
COMBINER/CHECK SPOOL
When the steering wheel is turned to the RIGHT, the "thermal bleed" and venting of the four work ports to the tank is stopped. The increased supply pressure flows to the HMU and the load sensing pilot line. The load sensing pilot line directs cylinder pressure to the priority spool in the directional valve. Cylinder pressure is present in the HMU because pilot oil combines with accumulator oil in the combiner/check valve spool in the directional valve. The increased pressure in the load sensing line causes the priority spool to move to the right and allows more oil to flow to the HMU through the supply line. The load sensing pump supply pressure varies with the steering load. The priority spool moves proportionally, allowing sufficient oil flow to meet the steering requirements.
Pilot oil flows through a stabilizing orifice to the right turn pilot port of the directional valve and moves the directional spool. Movement of the directional spool allows pilot oil to flow to the amplifier and
combiner/check spools.
• Pilot oil moves directional spool
• Load sensing pilot pressure moves priority spool
amplifier spool narrow groove around the combiner/check spool. The pilot oil is momentarily blocked until the amplifier spool moves far enough to the right to allow partial oil flow through one of eight orifices.
Pilot oil also flows through a connecting pin hole and a stabilizing orifice to the left end of the amplifier spool and causes the amplifier spool to move to the right. Accumulator oil at the spring end (right end) of the amplifier spool flows through a mid-connecting pin to the left end of the amplifier spool and also causes the amplifier spool to move to the right.
When the amplifier spool moves to the right, accumulator oil flows to the inner chamber, forcing the combiner/check spool to the left. Accumulator oil then flows through seven of the eight orifices. Pilot and accumulator oil combine. Oil flows across the directional spool (which has already shifted) for a RIGHT TURN.
The faster the steering wheel is turned, the farther the directional spool and the amplifier spool are shifted. A higher flow rate is available, which causes the truck to turn faster. The ratio of pilot and pump supply oil that combine is always the same because one orifice is dedicated to pilot flow and seven orifices are dedicated to accumulator supply flow.
Return oil from the cylinders flows across the directional spool, around the relief/makeup valve, forces the back pressure valve open and returns to the tank.
During a turn, if a front wheel strikes a large obstruction that cannot move, oil pressure in that steering cylinder and oil line increases. Oil flow to the cylinder is reversed. This pressure spike is felt in the
amplifier spool. The combiner/check spool moves to the right and blocks the seven pump supply oil orifices to the steering cylinders. The
amplifier spool moves to the left and blocks the pilot oil orifice. Pilot oil flow to the steering cylinders stops. The pressure spike is not felt at the HMU. If the pressure spike is large enough, the relief/makeup valve drains the pressure oil to the tank as previously described.
• Turning steering wheel faster provides more flow to cylinders
• Pressure spike moves combiner/check spool and blocks flow to HMU
• Pilot and accumulator oil combine in
combiner/check spool
• HMU (arrow)
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The Hand Metering Unit (HMU) (arrow) is located at the base of the steering column behind a cover at the front of the cab. The HMU is connected to the steering wheel and controlled by the operator.
The HMU meters the amount of oil sent to the steering directional valve by the speed at which the steering wheel is turned. The faster the HMU is turned, the higher the flow sent to the steering cylinders, and the faster the wheels will change direction.
• Meters oil to directional valve
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• HMU in NEUTRAL CONTROL
Shown is a sectional view of the HMU in the NEUTRAL (NO TURN) position. The metering section is a small hydraulic pump which produces a specific (metered) amount of oil flow. This metered oil is then directed by the control section to the left or right turn port. As the steering wheel is turned faster, the flow of oil increases. More oil is sent to the steering cylinders, which allows the cylinders to move faster.
When the steering wheel is in the NEUTRAL position (steering wheel stationary), the holes in the sleeve and the passages in the spool are not aligned. However, a small amount of pump oil from the inlet is allowed to flow through the center of the HMU. This small amount of oil flow (thermal bleed) keeps the HMU full and ready for a quick response to steering demands. The thermal bleed oil also helps keep the HMU warm during cold weather operation.
When the steering wheel is turned, the spool, pin and drive start to turn.
The sleeve does not turn at the same time because the diameter of the holes for the pin in the sleeve is slightly larger than the diameter of the pin. The slight delay in sleeve movement allows the spool to turn far enough inside the sleeve to align the holes in the sleeve with the grooves in the spool. The oil path for thermal bleed is then blocked by the rotation of the spool and the sleeve.
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• HMU during RIGHT TURN
When the steering wheel is turned to the RIGHT and the holes in the sleeve are aligned with the grooves in the spool, pump oil (P) at the inlet flows through the holes in the sleeve and the grooves in the spool. The oil in the grooves goes through other holes in the sleeve and into the lower passage. Oil flows through the lower passage to the metering section and is then directed into a space between the stator and the rotor.
The rotor is splined to the drive. As the drive turns, the rotor turns and directs oil through the upper passage. The metered oil flows through the holes in the sleeve, the grooves in the spool and out of the sleeve through the right turn port (R). Metered oil from the port goes to the steering directional valve.
Return oil from the steering cylinders flows through the tank port (T) in the HMU to the steering tank.