Hydraulic System

Location of Components
(1) Oil cooler. (2) Check valve. (3) Return filter gauge. (4) Steer pump. (5) Return filter. (6) Check valve (CP/CS-433B only). (7) Hand metering unit (HMU). (8) Right steer cylinder. (9) Left steer cylinder - not shown. (10) Steer pivot.

Introduction

The steer system hydraulics allow the operator to steer the machine by turning a steering wheel. The main components of the steering system are steer pump (4), hand metering unit (HMU) (7), and two steer cylinders (8) and (9).

Steer Pump (4).

Hydraulic oil flow for the steering system is provided by the steer pump (4). The steer pump is a fixed-displacement vane-type pump, which is driven by the machine engine. On the CP/CS-433B the steer pump is mounted to the vibratory pump. On the CS-431B the steer pump is mounted to the tandem pump used for the propel system and the vibratory system.

Steer Cylinder (8) and (9).

The force needed to steer the machine is provided by two hydraulic cylinders (8) and (9). The cylinders are connected to the rear frame and the yoke across a center-point, single articulation steer pivot (10).

Hand Metering Unit (HMU) (7).

Hydraulic oil from the steer pump is routed to the HMU. The operator steers the machine by turning a steering wheel, which is mechanically connected to the HMU. The HMU controls the flow of hydraulic oil to the steer cylinders. When the operator turns the steering wheel, oil is routed through the HMU to the steer cylinders. As the cylinders extend and retract, the machine scissors left or right at the pivot point, causing the machine to turn.

The HMU is an open-center, no-reaction type. When the steering wheel is not being turned, oil passes through the HMU and is routed back to the hydraulic oil tank. Oil in the steer cylinders is blocked. This maintains the desired straight or turned position of the drum.

Oil Cooler Assembly
(1) Oil cooler. (2) Check valve.

Return oil from the steer system is routed to the oil cooler (1) and check valve (2). If the return oil is cold, pressure in the return line will be higher than normal. Oil will open the check valve (2), bypassing the oil cooler (1). If the return oil is hot, pressure in the return line will be less, and the check valve will remain closed. Return oil will pass through the oil cooler for cooling.

Hydraulic Return Filter (5)

After passing through the oil cooler or check valve, return oil is routed to the hydraulic return filter (5). The filter removes contaminants from the return oil, then routes the oil to the hydraulic oil tank.

Reference: Return oil from the propel system, the vibratory system, the parking brake, and the leveling blade (if applicable) also goes through the oil cooler and the hydraulic return filter. Refer to the appropriate module for more information.

Pump

The steer pump provides hydraulic oil flow to the steer system, the parking brake system, and the leveling blade system (if applicable). The pump has a priority valve located in the front cover section. This provides a nearly constant flow to the steering system (and the parking brake system on CP/CS-433B). Remaining flow is available to the leveling blade system (parking brake on CS-431B), or is routed back to the hydraulic oil tank.

Pump Components
(1) Front cover section. (2) Rotor and vane assembly. (3) Supply oil inlet. (4) Drive shaft.

The steer pump is mounted to the vibratory pump on CP/CS-433B. The steer pump is mounted to the tandem pump on CS-431B. Splines on drive shaft (4) mesh with splines of vibratory pump or tandem pump shaft. Drive shaft rotates the rotor and vane assembly (2) at engine rpm.

Hydraulic oil from the tank enters the pump at supply oil inlet (3), and is ported to the rotor and vane assembly (2). As the rotor and vane assembly rotates, high-pressure output oil is created. This high-pressure oil is ported to the front cover section (1).

Front Cover Section Components
(5) Pump discharge area. (6) Spool. (7) Metering area. (8) Secondary outlet. (9) Cavity. (10) Cavity. (11) Relief valve. (12) Orifice. (13) Spring. (14) Cavity. (15) Metering area. (16) Primary outlet. (17) Relief outlet.

High-pressure oil in the front cover section is ported to the pump discharge area (5). High-pressure is sensed at cavity (14), cavity (9), and cavity (10). Flow into cavity (14) is restricted by orifice (12).

From cavity (14), oil flows past metering area (15) to primary outlet (16). Oil exits pump at primary outlet and is routed as follows:

CP/CS-433B. This oil is routed to the check valve and the parking brake system.

CS-431B. This oil is routed directly to the HMU.

Reference: For information on the CP/CS-433B parking brake, see Parking Brake Systems Operation Testing and Adjusting, Form No. KEBR2375.

If steer system pressure exceeds 13 800 kPa (2000 psi), relief valve (11) opens. Oil flows through relief valve to relief outlet (17). Relief oil exits pump at relief outlet and is routed to the hydraulic oil tank.

During operation, pressure builds up in cavity (9) and cavity (10), due to resistance to flow through orifice (12). This causes spool (6) to shift toward cavity (14) against spring (13). The amount of spool shift is proportional to the pressure differential between cavity (14) and cavity (10).

Flow to primary outlet (16) is kept nearly constant by the metering action of spool (6) at metering area (15). As spool moves against spring (13), excess oil at cavity (9) flows past metering area (7) to secondary outlet (8). Excess oil exits pump at secondary outlet, and is routed as follows:

For CP-433B with leveling blade, excess oil is routed to leveling blade system.

For CS-433B, or CP-433B without leveling blade, excess oil is routed to hydraulic oil tank.

For CS-431B, excess oil is routed to parking brake system.

Reference: For information on the leveling blade, see Leveling Blade Systems Operation Testing and Adjusting, Form No. KEBR2376.

Reference: For information on the CS-431B parking brake, see Parking Brake Systems Operation Testing and Adjusting, Form No. KEBR2374.

Check Valve (CP/CS-433B Only)

Check Valve Components
(1) Spring. (2) Outlet port. (3) Poppet. (4) Inlet port.

Hydraulic oil from the steer pump is routed directly to check valve and to the parking brake system. The check valve ensures that the parking brake system receives sufficient pressure to operate the parking brake.

Hydraulic oil enters the check valve at inlet port (4), and encounters poppet (3). When the pressure of this oil is at least 2070 kPa (300 psi), poppet (3) moves against spring (1). Oil flows past poppet to outlet port (2).

Oil exits the check valve at outlet port (2), and is routed directly to the HMU.

If the pressure of the oil at inlet port (4) is less than 2070 kPa (300 psi), poppet (3) will not move against spring (1). Oil from steer pump will be blocked at check valve. Oil will not be routed to HMU, and steering system will not operate.

NOTE: If pressure of oil is less than 2070 kPa (300 psi), operator will be unable to release the parking brake.

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 two steer cylinders.

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 cylinders, 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 cylinders. 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 cylinders 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 to the hydraulic oil cooler.

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 drum 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 two steer cylinders. Return oil from the steer cylinders 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 to the oil cooler.

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 to the oil cooler.

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 two steer cylinders. Return oil from the steer cylinders 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 to the oil cooler.

Steer Cylinders

Steer Cylinders
(1) Right cylinder. (2) Retract port. (3) Extend port. (4) Hydraulic hose to left turn port of HMU. (5) Left cylinder. (6) Retract port. (7) Extend port. (8) Hydraulic hose to right turn port of HMU.

The right steer cylinder (1) and the left steer cylinder (5) provide the force needed to steer the machine. When the operator turns the steering wheel for a right turn, hydraulic oil from the HMU enters the right cylinder at the retract port (2), and the left cylinder at the extend port (7). This causes the right cylinder to retract, and the left cylinder to extend, turning the machine to the right.

As the cylinders extend and retract, return oil exits the cylinders through port (3) and port (6). This return oil is routed back to the HMU through hose (4).

When the operator turns the steering wheel for a left turn, hydraulic oil from the HMU enters the right cylinder at the extend port (3), and the left cylinder at the retract port (6). This causes the right cylinder to extend, and the left cylinder to retract, turning the machine to the left.

As the cylinders extend and retract, return oil exits the cylinders through port (2) and port (7). This return oil is routed back to the HMU through hose (8).

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

Oil Cooler and Check Valve

Oil Cooler and Check Valve Assembly
(1) Inlet. (2) Tee. (3) Check valve. (4) Hose. (5) Oil cooler. (6) Outlet. (7) Tee.

Return oil from the HMU is routed to the oil cooler and check valve assembly. Return oil enters the assembly at tee (2), and is routed into the oil cooler (5) at inlet (1), and into the check valve (3).

The oil cooler is mounted behind the engine radiator. Oil flows through the oil cooler, and is cooled by forced air from the radiator fan. Cooled oil exits the oil cooler at outlet (6), and enters tee (7).

Check valve (3) has a crack rating of 450 kPa (65 psi). If the oil in the system is cold, pressure at tee (2) will be higher than normal. Check valve (3) will open, allowing some return oil to bypass the oil cooler through hose (4). This oil is routed to tee (7), where it joins the oil that has passed through the oil cooler.

All return oil from tee (7) is routed to the return filter before returning to the hydraulic oil tank.

Reference: Return oil from the propel system, the vibratory system, the leveling blade (if applicable), and the parking brake (CS-431B only) is also routed through the oil cooler and check valve assembly. Oil from these systems flows through the same route as oil from the steering system.

Return Filter

Return Filter Components
(1) Inlet port. (2) Gauge port. (3) Diverter/bypass valve. (4) Outlet port. (5) Element.

Return oil from the oil cooler assembly passes through the return filter before entering the hydraulic oil tank. During normal operation, oil enters the filter at inlet port (1). The diverter/bypass valve (3) directs the oil into the element (5). Filtration takes place as oil passes through the filter media from the inside of the element. Clean return oil exits the filter at outlet port (4), and is routed to the hydraulic oil tank.

As element (5) collects contaminants on its inner surface, the pressure differential across the element increases. If the pressure differential across the element reaches 172 kPa (25 psi), the pressure of the oil causes diverter/bypass valve (3) to open. Return oil passes directly through the diverter/bypass valve, and exits the filter at outlet port (4). When the oil does not pass through the element, debris in the oil will enter the hydraulic oil tank.

Correct maintenance must be used to make sure that element (5) does not become clogged. If the filter is allowed to bypass oil, contaminants could spread to the other hydraulic systems on the machine.

Pressure Gauge (6)

Oil pressure at the diverter/bypass valve (3) is also routed through gauge port (2) to pressure gauge (6), mounted behind the operator's seat. The operator can observe the pressure gauge while operating the machine. If the gauge indicates 172 kPa (25 psi), the filter is bypassing oil, and element (5) should be replaced.

NOTE: It is normal for the pressure gauge (6) to read 172 kPa (25 psi) when starting the engine, particularly during cold weather. As the hydraulic oil warms up, the pressure should drop, unless the element is clogged.

Steer Pivot

Steer Pivot Assembly
(1) Drum yoke. (2) Horizontal pin. (3) Steer pivot housing. (4) Vertical pins. (5) Frame. (6) Bushings. (7) Bushings.

When the operator turns the steering wheel to the left or the right, the steer pivot assembly acts as a center-point scissor. This allows drum yoke (1) to turn to the left or right in relation to machine frame (5). Steer pivot housing (3) is attached to frame (5) with two vertical pins (4). Drum yoke (1) is attached to steer pivot housing (3) with one horizontal pin (2).

Vertical pins (4) are supported by bushings (7). These two pins provide a left/right steering angle of 37° in each direction.

Horizontal pin (2) is supported by bushings (6). This pin allows drum yoke (1) to oscillate in relation to frame (5), to plus or minus 16°.

Reference: The steer pivot assembly has four grease fittings which must be lubricated for correct operation. Refer to the Operation and Maintenance Guide for lubrication instructions.