797F 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 in the NEUTRAL position
(1) Port to tank
(2) Left turn port
(3) Right turn port
(4) Pressure oil port
(5) Stator
(6) Metering section
(7) Control section
(8) Centering springs
(9) Rotor
(10) Spool
(11) Pin
(12) Drive
(13) Sleeve(14) Thermal leakage passages

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

Control section (7)
Metering section (6)

The two sections are connected by hydraulics and by mechanics.

Pump oil from the steering control valve goes through pressure oil port (4) into control section (7). When the steering wheel is turned, control section (7) sends oil to metering section (6). Metering section (6) produces a specific amount of oil flow. Metered oil from metering section (6) is then directed by control section (7) to either left turn port (2) or right turn port (3). This oil becomes pilot oil for the steering control valve. As the steering wheel is turned faster, there is an increase in the flow of pilot oil. The steering cylinders will move farther with increased speed.

Oil Flow

Steering system oil flow

Pump supply oil from the accumulators flows through the steering control valve to the steering metering pump.

Control section (7) 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 (13) and the passages in spool (10). The steering metering pump directs the oil back to the hydraulic tank through port (1). However, a small amount of pump oil from pressure oil port (4) 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 pilot oil lines to the steering control valve and the steering metering pump warmed during cold weather operations.

Right Turn

Steering metering pump during a right turn
Steering metering pump in the NEUTRAL position
(1) Port to tank
(2) Left turn port
(3) Right turn port
(4) Pressure oil port
(5) Stator
(6) Metering section
(7) Control section
(8) Centering springs
(10) Spool
(11) Pin
(12) Drive
(13) Sleeve(15) Passage

Spool and sleeve
(10) Spool
(13) Sleeve
(16) Oil groove(17) Oil holes

When the steering wheel is turned clockwise, spool (10), pin (11) and drive (12) start to turn. Sleeve (13) will not turn because spool (10) will turn inside sleeve (13). Oil holes (17) in sleeve (13) will be in line with oil grooves (16) in spool (10). The internal thermal leakage is blocked when spool (10) and sleeve (13) rotate.

Pump oil from port (4) goes through oil holes (17) in sleeve (13) into oil grooves (16) in spool (10). The oil in grooves (16) goes out through oil holes (17) in sleeve (13) and into thermal leakage passages (14). Oil flows through passages (14) into metering section (6). Then, the oil is directed into a space between stator (5) and rotor (9) .

Rotor (9) is splined to drive (12). As drive (12) turns, rotor (9) pumps oil through passage (15). The metered oil flows back into sleeve (13) through oil holes (17) and into oil grooves (16) in spool (10). This oil then leaves sleeve (13) through oil holes (17). Then, the oil flows into right turn port (3). Pilot oil from port (3) goes to the directional spool in the steering control valve.

When the steering wheel is not turned, spool (10), pin (11), drive (12) and rotor (9) will not turn. Centering springs (8) will bring spool (10) and sleeve (13) back to the NEUTRAL position. Oil holes (17) in sleeve (13) are not in alignment with oil grooves (16) in spool (10). Metered oil to right turn port (3) stops. This causes the wheels to stay in the current position.

Pump gears in metering section (6)
(5) Stator
(9) Rotor
(11) Pin(12) Drive

If the steering metering pump is disassembled, assemble the steering metering pump with the proper relationship between rotor (9) and pin (11). Use Illustration 6 for a reference. Erratic steering will result if Illustration 6 is not used as a guide.

Exploded view of the steering metering pump
(18) Alignment holes(19) Spring pin

Also, refer to Illustration 7 in order to ensure that spring pin (19) is correctly installed in the gerotor assembly through alignment holes (18). Alignment holes (18) are marked with the number "6".

Load Sensing

The steering metering pump has a load sensing port. The load sensing port is connected to pressure oil port (4) through an orifice. Oil pressure in port (4) is felt in the load sensing line. This signal pressure is communicated to the priority spool in the steering control valve. Refer to Systems Operation, "Steering Control Valve" for a more complete explanation on the operation of the steering metering pump and the steering control valve.

MARYGAR