Introduction

Location of Components
(1) Front bolster group. (2) Steer pump. (3) Hydraulic cylinder. (4) Return filter. (5) Hand metering unit (HMU). (6) Hydraulic tank.

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 (5), and hydraulic cylinder (3).

Steer pump (2) draws oil from hydraulic tank (6). The pump puts this oil under pressure and provides oil to the steering system. The steer pump is a fixed-displacement, gear-type pump which is bolted to the front of the engine on the left side of the machine. Pump (2) is driven by a gear inside the engine.

Hydraulic oil from the steer pump is routed to HMU (5). 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 (5) 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 (5) enters the cylinder, causing it to extend or retract. This causes front bolster gorup (1) to pivot on bearings, steering the machine.

Return oil exits cylinder (3), and is routed back through HMU (5) to return filter (4). The filter removes contaminants from the return oil, then routes the oil to hydraulic tank (6).

Pump

Pump Components
(1) Drive shaft. (2) Drive gear. (3) Flow divider. (4) Supply oil inlet (5) Pressure chamber. (6) Supply chamber. (7) Idler gear. (8) Passage.

The steer pump provides hydraulic oil flow to the steering system. The pump has an integral flow divider (3). This provides a nearly constant flow of oil to the steering system. Remaining flow is routed back to the hydraulic oil tank.

The pump is mounted to the front of the engine on the left side of the machine. A gear is keyed to the end of drive shaft (1). This gear engages another gear located inside the engine. Supply oil inlet (4) is connected to the hydraulic tank through a hose.

Drive shaft (1) is driven any time the engine is running. This causes drive gear (2) and idler gear (7) to rotate. Oil enters the pump through supply oil inlet (4). This oil is ported through passage (8) to supply chamber (6). Rotation of the gears carries the oil around to pressure chamber (5). Oil is forced into the pressure chamber, creating a flow of high-pressure oil. This high-pressure oil is ported from pressure chamber (5) to flow divider (3).

Flow Divider Section
(9) Relief passage. (10) Relief valve. (11) Metering area. (12) Primary port. (13) Spring. (14) Secondary port. (15) Metering area. (16) Orifice holes. (17) Balancing hole. (18) Piston. (19) Pressure inlet.

High pressure oil from pressure chamber (5) is ported to the flow divider through pressure inlet (19). This oil flows around the outside of piston (18), and passes through orifice holes (16) to the inside of the piston. This oil is ported to primary port (12), and to relief valve (10). Oil exits the pump through primary port (12), and is routed to the hand metering unit (HMU).

If steer system pressure exceeds 6900 kPa (1000 psi), relief valve (10) opens. Oil flows through the relief valve to relief passage (9). Relief oil is routed back to supply oil inlet (4), and is recirculated through the pump gears.

Oil from pressure inlet (19) also passes through balancing hole (17), to the bottom of piston (18). Oil pressure at the bottom of the piston and below metering area (15) tends to push piston (18) up against spring (13).

Oil flow through primary port (12) is kept nearly constant by the metering action of piston (18) against spring (13). As the piston moves up, metering area (11) restricts the flow of oil through primary port (12). At the same time, excess oil flows past metering area (15) to secondary port (14). Excess oil exits the pump through secondary port (14), and is routed through the return filter, back to the hydraulic tank.

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 through the return filter to the hydraulic tank.

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 through the return filter to the hydraulic tank.

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 through the return filter to the hydraulic tank.

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 through the return filter to the hydraulic tank.

Steer Cylinder

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

The 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.

Return Filter

Return Filter
(1) Inlet port. (2) Bypass valve. (3) Spring. (4) Outlet port. (5) Element.

Return oil from the HMU and excess oil from the steer pump are both routed through the return filter before returning to the hydraulic tank. During normal operation, oil enters the filter at inlet port (1) and passes through element (5). The element traps any debris that is in the oil. Oil then exits the filter through outlet port (4), and is routed to the hydraulic tank.

If the element becomes clogged with debris, the restriction to the flow of oil causes a pressure increase outside the element. If the pressure differential across the element reaches 172 kPa (25 psi), the pressure of the oil causes bypass valve (2) to move against spring (3). When this happens, oil passes directly through the bypass valve and exits the filter through outlet port (4).

When the oil does not go through the element, the debris in the oil will cause damage to the components of the steering system. Correct maintenance must be used to make sure that element (5) does not become clogged, stopping the flow of clean oil to the system.

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.