769D Off-Highway Truck: Metering Pump (Steering)

Location of steering metering pump

The steering metering pump is located on the base of the steering column under a cover at the front of the cab. The resilient mounting of the metering pump will prevent vibration damage and noise.

Steering metering pump
NEUTRAL position
(1) Inner spool
(2) Drain outlet to tank
(3) Left turn port
(4) Outer sleeve
(5) Right turn port
(6) Inlet
(7) Passage
(8) Stator
(9) Centering springs
(10) Rotor
(11) Pin
(12) Drive
(13) Passage
(A) Control section(B) Metering section

The steering metering pump is made up of the following two sections:

Control section (A)
Metering section (B)

The two sections are connected by hydraulics and by mechanics.

Oil from the steering piston pump goes through inlet port (6) 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 left turn port (3) or right turn port (5). This oil flows to the steering cylinders. As the steering wheel is turned faster, there is an increase in the flow of metered oil. The steering cylinders will move farther with increased speed.

Oil Flow

Control section (A) of the steering metering pump is a closed center type. When the steering wheel is in the NO STEER position, there is no alignment between the holes in sleeve (4) and the passages in spool (1). However, a small amount of pump oil from inlet (6) is allowed to flow through the center position of the steering metering pump. This small amount of oil flow (internal thermal leakage) keeps the steering metering pump full. This will give a response time that is quick to steering demands. The internal thermal leakage also helps keep the oil lines to the steering metering pump warmed during cold weather operations.

Right Turn

Steering metering pump
Right turn
(1) Inner spool
(4) Outer sleeve
(5) Right turn port
(6) Inlet
(7) Passage
(8) Stator
(9) Centering springs
(10) Rotor
(11) Pin
(12) Drive
(13) Passage
(A) Control section(B) Metering section

(1) Inner spool
(4) Outer sleeve
(14) Slots for centering springs
(15) Grooves for oil flow
(16) Holes for pin (11)(17) Small holes for oil flow

When the steering wheel is turned clockwise, spool (1), pin (11) and drive (12) start to turn. Sleeve (4) will not turn because spool (1) will turn inside sleeve (4). Small holes (17) in sleeve (4) will be in line with grooves (15) in spool (1). The internal thermal leakage is blocked when spool (1) and sleeve (4) rotate.

Pump oil from inlet (6) goes through small holes (17) in sleeve (4) into grooves (15) in spool (1). The oil in grooves (15) goes out through other small holes (17) in sleeve (4) and into passage (13). Oil flows through passage (13) into metering section (B). Then, the oil is directed into a space between stator (8) and rotor (10) .

Rotor (10) is splined to drive (12). As drive (12) turns, rotor (10) pumps oil through passage (7). The metered oil flows back into sleeve (4) through small holes (17) and into other grooves (15) in spool (1). This oil then leaves sleeve (4) through small holes (17). Then, the oil flows into right turn port (5). Oil from right turn port (5) goes to the steering cylinders in order to turn the wheels.

If the steering wheel turns faster than 10 rpm, spool (1) turns farther inside sleeve (4). Now, different holes (17) in sleeve (4) will be in line with different grooves (15) in spool (1). Now, some of the pump oil from inlet (6) will not flow through metering section (B). This oil will flow into right turn port (5). Oil from right turn port (5) goes to the steering cylinders in order to turn the wheels. This allows the wheels to be turned faster for quick steering response.

When the steering wheel is not turned, spool (1), pin (11), drive (12) and rotor (10) will not turn. Centering springs (9) will bring spool (1) and sleeve (4) back to the NEUTRAL position. Small holes (17) in sleeve (4) are not in alignment with grooves (15) in spool (1). Metered oil to right turn port (5) stops. This causes the wheels to stay in the current position.

Left Turn

Steering metering pump
Left turn
(1) Inner spool
(3) Left turn port
(4) Outer sleeve
(6) Inlet
(7) Passage
(8) Stator
(9) Centering springs
(10) Rotor
(11) Pin
(12) Drive
(13) Passage
(A) Control section(B) Metering section

The operation is in the opposite direction. When the steering wheel is turned counterclockwise for a left hand turn, spool (1), pin (11) and drive (12) start to turn. Oil comes in inlet (6). Since spool (1) and sleeve (4) are turned, a different set of holes and grooves will line up. Oil flows through spool (1) and sleeve (4) into passage (7) and metering section (B). Metered oil now flows through passage (13), spool (1), sleeve (4), and out left turn port (3). Metered oil goes to the steering cylinders in order to turn the wheels.

Pump gears in metering section (B)
(8) Stator
(10) Rotor
(12) Drive(11) Pin

If the steering metering pump is disassembled, assemble the steering metering pump with the proper relationship between rotor (10) and pin (11). Use Illustration 6 for reference. Erratic steering will result if Illustration 6 is not used as a guide.

Steering Cylinder Check and Relief Valves

Two steering cylinder check and relief valves are located in the steering metering pump. Steering cylinder check and relief valves prevent damage from high pressure oil in the steering cylinder circuit. This occurs when the steering wheel is stationary and an outside force on a front wheel suddenly moves a cylinder rod into a cylinder or out of a cylinder. When the front wheels strike a large object, pressure oil from a steering cylinder will open one steering cylinder check and relief valve. This will allow oil flow to the low pressure side of the steering cylinders.

Load Sensing

The steering metering pump has a load sensing port. The load sensing port is connected to inlet (6) through an orifice. Oil pressure in inlet (6) is felt in the load sensing line. This signal pressure is communicated to the pressure compensator valve and to the secondary steering solenoid valve. Refer to Systems Operation, "Steering System" and System Operation, "Steering Piston Pump"for a more complete explanation on the operation of the steering metering pump, the pressure compensator valve, and the secondary steering solenoid valve.

MARYGAR