Introduction

Location of Components
(1) Front bolster group. (2) Steer pump. (3) Hydraulic cylinder. (4) Hand metering unit (HMU).

The steering system hydraulics allow the operator to steer the machine by turning a steering wheel. The main components of the steering system are steer pump (2), hand metering unit (4), and hydraulic cylinder (3).

Steer pump (2) provides hydraulic oil pressure to the steering system. The steer pump is a fixed-displacement, vane-type pump which is located by the front of the engine on the right side of the machine. Pump (2) is belt-driven by a pulley, which is connected to the engine crankshaft.

Hydraulic oil from the steer pump is routed to HMU (4). The operator steers the machine by turning a steering wheel, which is mechanically connected to the HMU. When the operator turns the steering wheel, oil is ported through HMU (4) to hydraulic cylinder (3).

Hydraulic cylinder (3) provides the force needed to steer the machine. The piston end of the cylinder is fastened to the frame of the machine. The rod end of the cylinder is connected to front bolster group (1). High-pressure oil from HMU (4) enters the cylinder, causing it to extend or retract. This causes front bolster group (1) to pivot on bearings, steering the machine.

Pump Drive

Pump Drive
(1) Steer pump. (2) Belt. (3) Engine. (4) Drive pulley. (5) Driven pulley. (6) Pump support bracket.

Steer pump (1) is bolted to pump support bracket (6). The pump is located in front of engine (3) on the right side of the machine. Driven pulley (5) is keyed to the pump drive shaft.

Drive pulley (4) is bolted to the crankshaft pulley on the front of the engine. When the engine is operating, drive pulley (4) is rotated at engine rpm. Belt (2) transmits power from pulley (4) to driven pulley (5), driving the steer pump.

The two pulleys (4) and (5) have the same operating diameter. This causes the steer pump to be driven at engine rpm any time the engine is operating.

Pump

Pump Components
(1) Outlet passage. (2) Reservoir. (3) Supply oil inlet. (4) Drive shaft. (5) Vanes. (6) Rotor. (7) Inlet passage. (8) Return oil inlet. (9) Control section.

The steer pump provides hydraulic oil flow to the steering system. The pump has an integral reservoir (2) which supplies oil to the pump. Control section (9) meters output oil from the pump. This provides a nearly constant flow of oil to the steering system. Remaining flow is ported back to reservoir (2).

Drive shaft (4) is connected to the pump drive assembly. Drive shaft (4), rotor (6), and vanes (5) are driven at engine rpm any time the engine is running.

Oil from reservoir (2) enters the pump through supply oil inlet (3). This oil is ported to rotor (6) and vanes (5) through inlet passage (7). Rotation of rotor (6) traps oil between vanes (5). The vanes carry the oil around to outlet passage (1). Oil is forced into the outlet passage, creating a flow of high-pressure oil. This high-pressure oil is ported to control section (9).

Control Section
(10) Spring. (11) Secondary port. (12) Metering area. (13) Passage. (14) Chamber. (15) Relief valve. (16) Spring. (17) Primary port. (18) Orifice. (19) Spool.

High-pressure oil enters the control section of the pump through passage (13). This oil passes through orifice (18), and is ported to primary port (17) and chamber (14). Oil exits the pump through primary port (17), and is routed to the hand metering unit (HMU). Return oil from the HMU re-enters the reservoir through return oil inlet (8).

Oil at chamber (14) is ported to relief valve (15). If steering system pressure exceeds 6900 kPa (1000 psi), relief valve (15) moves against spring (16). Oil flows through the relief valve to secondary port (11). Relief oil is routed through the secondary port, back to reservoir (2).

The pressure of the oil from passage (13) tends to push spool (19) against spring (10). Oil flow through primary port (17) is kept nearly constant by the metering action of spool (19) against spring (16). As the spool moves to the left, excess oil flows past metering area (12) to secondary port (11). Excess oil is ported through the secondary port back to reservoir (2).

Hand Metering Unit (HMU)

HMU Components
(A) Control section. (B) Metering section. (1) Spool. (2) Sleeve. (3) Tank port. (4) Left turn port (hidden) (5) Check valve. (6) Pressure port. (7) Right turn port (hidden). (8) Passage. (9) Passage. (10) Gerotor. (11) External pump gear. (12) Centering springs. (13) Pin. (14) Passage. (15) Body. (16) Drive.

The hand metering unit (HMU) has two main sections: control section (A), and metering section (B). These two sections work together to send hydraulic oil to the steer cylinder.

Oil from the steer pump enters the HMU at pressure port (6), and is routed to the control section (A) of the HMU. When the steering wheel is turned, the control section sends the oil to and from the metering section (B). The metering section pumps the oil out of the HMU to the steer cylinder, turning the machine.

The metering section is a small hydraulic pump, powered by the steering wheel. It controls (meters) the oil that goes to the steer cylinder. The direction and speed of a turn are controlled by the metering section. As the operator turns the steering wheel faster, oil flow to the steer cylinder is increased. This increased flow causes the machine to make a faster turn.

HMU Operation

HMU Components
(A) Control section. (B) Metering section. (1) Spool. (2) Sleeve. (3) Tank port. (4) Left turn port (hidden). (5) Check valve. (6) Pressure port. (7) Right turn port (hidden). (8) Passage. (9) Passage. (10) Gerotor. (11) External pump gear. (12) Centering springs. (13) Pin. (14) Passage. (15) Body. (16) Drive.

Neutral

When the steering wheel is not being turned, the HMU is in the neutral position. Oil from the steer pump enters the HMU through pressure port (6), and is ported through passage (8) to sleeve (2). Since the HMU is open-centered, a passage is open through sleeve (2) and spool (1). Oil flows through this passage and is ported to tank port (3). Oil exits the HMU through tank port, and is routed back to the hydraulic reservoir on the steer pump.

When the HMU is in neutral, oil is not ported to the metering section (B) of the HMU. The right turn port (7) and the left turn port (4) are blocked. This maintains the machine in the original straight-ahead or turned position.

Right Turn

The steering wheel is connected to spool (1) by a shaft assembly and splines. When the steering wheel is turned, spool (1), pin (13), and drive (16) turn a small amount until centering springs (12) are compressed. This allows spool (1) to turn inside of sleeve (2) enough to put small holes (19) in the sleeve in line with grooves (20) in the spool. This provides a passage from control section (A) to metering section (B) of the HMU.

Spool and Sleeve
(1) Spool. (2) Sleeve. (17) Slots for springs. (18) Holes for pin. (19) Small holes for oil flow. (20) Grooves for oil flow.

Oil from the steer pump enters the HMU at pressure port (6), and is ported through passage (8) to sleeve (2). Oil flows through holes (19) in sleeve and grooves (20) in spool (1), to passage (9). Oil is routed from passage (9) to gerotor (10).

Metering Section Components
(10) Gerotor. (11) External pump gear. (13) Pin. (16) Drive.

With the steering wheel turning, drive (16) causes gerotor (10) to rotate inside gear (11). This rotation of the gerotor sends a controlled (metered) flow of oil back through body (15). This metered oil is ported to the right turn port (7).

Oil exits the HMU at right turn port (7), and is routed to the steer cylinder. Return oil from the steer cylinder enters the HMU at left turn port (4). This oil is ported through the HMU to tank port (3). Return oil exits the HMU at tank port, and is routed back to the hydraulic reservoir on the steer pump.

If the steering wheel rotation is stopped, springs (12) will move sleeve (2) back in alignment with spool (1). This will close passages between the metering section and control section, and the HMU will be in the neutral position.

Excess oil from the steer pump, not needed by the HMU, opens check valve (5). Oil passes through check valve to tank port (3). Oil exits the HMU through tank port, and is routed back to the hydraulic reservoir on the steer pump

Left Turn

HMU operation for a left turn is similar to that for a right turn. The same components move, only in the opposite direction. Oil flow through the HMU is also in the opposite direction of a right turn.

Oil from the steer pump still enters the HMU through pressure port (6), and is ported through passage (8) to sleeve (2) and spool (1). When the steering wheel is turned for a left turn, spool (1) turns inside of sleeve (2) enough to put the small holes (19) in sleeve in line with the grooves (20) in the spool.

Oil flows through sleeve and spool, to passage (14), and is ported to gerotor (10).

With the steering wheel turning, drive (16) causes gerotor (10) to rotate in the opposite direction than for a right turn. Rotation of the gerotor sends a metered flow of oil through body (15) to the left turn port (4).

Oil exits the HMU at left turn port (4) and is routed to the steer cylinder. Return oil from the steer cylinder enters the HMU at right turn port (7). This oil is ported through the HMU to tank port (3). Return oil exits the HMU through tank port (3), and is routed back to the hydraulic reservoir on the steer pump.

Steer Cylinder

Cylinder Components
(1) Clevis. (2) Extend port. (3) Chamber. (4) Piston. (5) Chamber. (6) Retract port. (7) Rod. (8) Clevis.

The steer cylinder is a double-acting hydraulic cylinder, used to transmit steering force to the front bolster group. Clevis (1) is connected to the frame of the machine. Clevis (8) is connected to the front bolster group. Retract port (6) is connected to the right turn port of the HMU. Extend port (2) is connected to the left turn port of the HMU.

When the operator turns the steering wheel for a right turn, hydraulic oil from the HMU enters the steer cylinder at retract port (6). This causes piston (4), rod (7), and clevis (8) to retract, turning the machine to the right.

When the operator turns the steering wheel for a left turn, hydraulic oil from the HMU enters the steer cylinder at extend port (2). This causes piston (4), rod (7), and clevis (8) to extend, turning the machine to the left.

Reference: The steer cylinder has two grease fittings which must be lubricated for correct operation. Refer to the Operation and Maintenance Guide for lubrication information.

Front Bolster Group

Front Bolster Group
(1) Bolster. (2) Bearings. (3) King pin housing. (4) King pin. (5) Clevis mount.

The front bolster group contains the five front wheels of the machine. These are non-drive wheels, and they have no brakes installed. The front bolster group turns the force of the steer cylinder into the actual steering motion of the machine.

The rod end of the steer cylinder is connected to clevis mount (5). King pin (4) is connected to bolster (1) with a pivot pin. King pin (4) is held inside king pin housing (3) by bearings (2).

When the operator turns the steering wheel to the left or the right, the steer cylinder extends or retracts. This causes king pin (4), bolster (1), and the attached wheels to pivot on bearings (2). This provides the steering of the machine.

Reference: The front bolster group has one grease fitting which provides lubrication to bearings (2). Refer to the Operation and Maintenance Guide for lubrication instructions.