General Information

Reference: For the Hydraulic Systems Schematics, see the Schematic For 416B, 426B, 428B, 436B, 438B Backhoe Loaders Hydraulic System, Form No. SENR5810.

Reference: For the Hydraulic Pump and Pressure And Flow Compensator Valve; see the Specifications, Systems Operation, and Testing & Adjusting For 416B, 426B, 428B, 436B, 438B Backhoe Loaders Hydraulic System, Form No. SENR5808.

Component Location
(1) Steering cylinder. (2) Hydraulic tank. (3) Metering pump (steering). (4) Check valve. (6) Inlet manifold and priority valve (part of loader control valve). (10) Hydraulic pump. (11) Crossover relief valve (if equipped).

The steering system is a part of the hydraulic system which is a load sensing, pressure compensated, closed-center system. A load sensing, pressure compensated system changes output flow as required to maintain output pressure slightly above load pressure.

Steering System Schematic
(1) Steering cylinder. (2) Hydraulic tank. (3) Metering pump (steering). (4) Check valve. (5) Signal line to priority valve. (6) Inlet manifold and priority valve (part of loader control valve). (7) Final resolver [part of (6)]. (8) Line to pump. (9) Pressure and flow compensator valve [part of (10)]. (10) Hydraulic pump. (11) Crossover relief valve (if equipped). (12) Oil cooler. (13) Oil filter.

Pump (1) has a compensator valve that automatically keeps pump supply pressure and flow at a level needed to fulfill the system load and flow needs. Pump outlet pressure is kept at approximately 2100 kPa (305 psi) above work port pressure. The compensator valve also has a pressure limiting ability that prevents pump and system overloads. When work port pressure goes over a set pump pressure of 19 000 kPa (2750 psi) 426B or 20 700 kPa (3000 psi) for the 416B, 428B, 436B and 438B, the pressure limiter part of the compensator will extend over (override) the load sensing part of the compensator and lower the pump output. For a more complete explanation of the hydraulic pumps, make reference to Systems Operation, Testing And Adjusting Backhoe Loaders Hydraulics, Form No. SENR5809.

The main components of the steering system are hydraulic tank & screen (2); hydraulic pump (10), which is shared with the entire hydraulic system; inlet manifold and priority valve (6); metering pump (steering) (3); crossover relief valve (11) and steering cylinder (1).

Hydraulic tank (2) provides oil for both the steering and implement hydraulic systems. The oil that returns to the tank is filtered by filter element (13) mounted near the transmission. Return oil flows through oil cooler (12) before returning to tank (2).

Oil from the hydraulic tank is sent to either the loader control valve group or the backhoe control valve group by the hydraulic pump. The hydraulic pump is a variable displacement, axial piston, slipper-type pump that is controlled by a load sensing, pressure compensated control valve (9). The control valve is mounted to the pump head end. The pump is located in front of the engine below the hydraulic tank, and is mounted to a bracket that is bolted to the front of the engine. The pump is driven by a drive shaft connected to the crankshaft pulley.

The loader control valve group is located on the right frame rail. Hydraulic pump (10) sends oil to the inlet manifold of the loader control valve group. The inlet manifold houses a priority spool, a signal pressure limiting valve and final resolver (7). Hydraulic pump (10) sends oil to inlet manifold and priority valve (6).

When a steering demand occurs, the priority spool shifts and sends oil to metering pump (steering) (3). Oil pressure in the metering pump is limited to a maximum of 14 000 kPa (2030 psi) by a pressure limiting valve in the inlet manifold and priority valve (6).

The metering pump (steering) is located beneath the operator station floor plates. It is connected to the steering column. Pump supply pressure oil coming from the inlet manifold priority valve enters the metering pump through inlet check valve (4).

When the steering wheel is not turning, there is a small constant bleed flow of oil from the metering pump to the tank of approximately 1.9 liter/min (.5 U.S. gpm). This bleed flow keeps the metering pump and priority spool active and thus improves response, particularly in cold weather or when the steering is used only sporadically. When the wheel is turned, oil is sent to the cylinder and the bleed path is blocked.

Inlet check valve (4) prevents steering kickback from external forces. In the metering pump control section there is another check valve between pump supply and return oil ports. The check valve provides steering capability, when the engine is not running, by allowing oil to recirculate between the metering pump and the steering cylinder. When equipped, crossover relief valve (11) is mounted to the metering pump. See topic Steering Crossover Relief Valve.

Steering cylinder (1) is located on the front axle in front of the rear trunnion mount. The cylinder has one piston and is double ended with two rod ends. When the steering wheel is turned, the metering pump directs oil to one side of the piston while draining the opposite side to the tank.

2-WD Front Axle

The front axle consists of a fabricated axle (two wheel drive units), body, two spindles, two hubs, steering cylinder, two short tie rods and necessary ball joints.

The axle body (two wheel drive units) consists of three break formed steel parts welded together to form a box section to which all other parts of the axle assembly are attached. Two forged steel spindle yokes are welded to each end, the steering cylinder housing is welded to the rear center and the trunnion mounts are welded to the front and rear center.

The steering mechanism consists of a double acting steering cylinder connected to the pitman arms of the spindles through two short tie rods and necessary ball joints. These parts are on the rear of the axle and are well protected. Steering lines routing is simplified due to the single cylinder design.

4-WD Front Axle

Front Drive Axle
(1) Left planetary housing. (2) Left tie rod end. (3) Left tie rod end adjuster. (4) Steering cylinder. (5) Differential. (6) Side gear thrust washers (left and right). (7) Differential side gears (left and right). (8) Spider shafts (two). (9) Right drive axle. (10) Differential spider gears (four). (11) Spider gear thrust washers (four). (12) Planetary carrier and hub. (13) Left planetary ring gear. (14) Planetary gears (three). (15) Sun gear and shaft. (16) Hub seal. (17) Left steer knuckle housing. (18) Upper kingpin. (19) Kingpin shims. (20) Left drive axle. (21) Differential bearings. (22) Differential carrier. (23) Differential bevel gear. (24) Pinion. (25) Pinion nut. (26) Pinion depth shims. (27) Collapsible spacer. (28) Drive flange. (29) Drive joint end play shims. (30) Left wheel hub nut. (31) Outer drive joint seal. (32) Left hub bearing. (33) Lower kingpin. (34) Left drive joint. (35) Inner drive joint seal. (36) Drive axle housing.

The 4-WD front axle is connected to the transmission by a drive shaft. Drive input enters the front drive axle at drive flange (28). The power flow is through pinion (24) and ring gear (23) to differential (5). The differential utilizes four spider gears (10) and two side gears (6) to transmit the power flow to left and right axles (20) and (9). These axles are splined to left and right drive joints (34), which enables flexibility (turning) while transmitting power. The outer end of drive joint (34) is splined to sun gear and shaft (15) to drive three planetary gears (14). The three planetary gears are also engaged with nonrotating ring gear (13) that is connected to steer knuckle housing (17).

When power is transmitted through drive joint (34) to sun gear and shaft (15), planetary gears (14) walk around the inside of ring gear (13). The planetary carrier and hub (12) will turn, supplying the front wheel drive.

Inlet Manifold And Priority Valve

(1) Return line to tank. (2) Pump supply line. (3) Metering pump supply line. (4) Metering pump signal line. (5) Signal line to pump control valve.

The inlet manifold and priority valve is located on the end of the loader control valve group which is mounted inside the right frame rail under the operator platform.

The inlet manifold provides SAE straight thread ports for connecting pump supply and return lines, metering pump supply line, metering pump signal line and the pump signal line to the valve sections. It also houses the steering priority valve and the final resolver which compares the signal pressures of the metering pump and the loader valve group.

Full Flow To Loader Section

Inlet Manifold And Priority Valve
(6) Pump supply passage. (7) Metering pump supply passage. (16) Spring. (17) Chamber. (18) Hole. (19) Loader valve passage. (20) Passage. (21) Chamber. (22) Priority valve spool. (BB) Pump oil. (CC) Metering pump supply oil. (DD) Loader supply oil. (EE) Loader signal oil.

When no steering demand occurs there will be no signal pressure in chamber (17). Pump supply pressure in passage (6) and metering pump supply passage (7) enters spool (22) through hole (18) and flows into chamber (21) through axial passage (20). Pressure in chamber (21) will force spool (22) to the right and allow pump supply flow to go to loader valve passage (19). Pressure oil will be between the inlet manifold and metering pump, keeping the steering system full of oil at all times.

Even with no steering demand there is still a small flow of oil through the metering pump to tank, which prevents spool (22) from shifting fully to the right and blocking the passageway between pump supply passage (6) and metering pump supply passage (7). Under this condition, metering pump supply pressure will be approximately 700 kPa (100 psi) as controlled by the spring (16).

Full Flow To Metering Pump

Inlet Manifold And Priority Valve
(6) Pump supply passage. (7) Metering pump supply passage. (8) Passage. (9) Hole. (10) Signal oil passage from loader valve group. (11) Signal oil passage to pump. (12) Metering pump signal port. (13) Final resolver. (14) Spring. (15) Signal pressure limiting valve. (16) Spring. (17) Chamber. (18) Hole. (19) Loader valve passage. (20) Passage. (21) Chamber. (22) Priority valve spool. (23) Passage to tank. (AA) Metering pump signal oil. (BB) Pump oil. (CC) Metering pump supply oil.

The steering system has flow priority over the loader section. When the steering wheel is turned, signal oil from the metering pump is sent to the inlet manifold and priority signal hrough metering pump supply line (7). This signal oil flows through port (12) to passage (8), through hole (9) and into chamber (17). The 700 kPa (100 psi) margin spring (16) in chamber (17) combines with the metering pump signal oil and forces priority valve spool (22) to the left. Pump oil which had been flowing from pump supply passage (6) to loader valve passage (19) is either stopped or metered by valve spool (22). Now pump oil flows from pump supply passage (6) into metering pump supply passage (7), through metering pump supply line (3) to the metering pump.

When the signal pressure in chamber (17) reaches 14 000 kPa (2030 psi), pressure limiting valve (15) moves to the right against the force of spring (14) and signal oil flows into tank passage (23). Primary spool (22) now moves to the right restricting oil flow to the metering pump supply passage (7) limiting the pressure in the steering circuit to the pressure limiting valve.

Partial Flow To Metering Pump

Inlet Manifold And Priority Valve
(6) Pump supply passage. (7) Metering pump supply passage. (10) Signal oil passage from loader valve group. (11) Signal oil passage to pump. (13) Final resolver. (17) Chamber. (18) Hole. (20) Passage. (21) Chamber. (22) Priority valve spool. (AA) Metering pump signal oil. (BB) Pump oil. (CC) Metering pump supply oil. (DD) Loader supply oil. (EE) Loader signal oil.

The pressure in chamber (17) is communicated to the pump control valve through the final resolver (13) and signal line (5). If there is a higher signal pressure in passage (10) from the loader valve group, final resolver (13) shifts, sending the highest resolved signal pressure through passage (11) to the compensator valve on the pump. Pump supply pressure oil in passage (6) increases to 2100 kPa (305 psi) more than signal pressure. Oil entering the pump supply passage (7) goes through hole (18) and passage (20) to chamber (21), creating pressure on the left side of valve spool (22). This will force the spool to the right, creating a restriction between pump supply passage (6) and metering pump supply passage (7). Pump supply oil will flow to the loader section while limiting metering pump supply pressure to 14 000 kPa (2030 psi). This occurs only when the total flow demand is low enough that the pump can supply all required flow.

Metering Pump (Steering)

Metering Pump (Steering)
(1) Outlet to tank. (2) Metering pump inlet check valve. (3) Inlet from pump. (4) Passage. (5) Passage from Gerotor. (6) Drive. (7) Springs. (8) Gerotor. (9) Spool. (10) Pin. (11) Sleeve. (12) Passage to Gerotor. (19) Check valve (internal). (A) Control section. (B) Pump section.

The metering pump (steering) has two main sections; control section A and pump section B. These two sections work together to send oil to the steering cylinders. The metering pump is located just under the front of the operator station floor plates.

The direction and speed of a turn are controlled by the metering pump. As the steering wheel is turned faster, there is an increase in the flow of oil to the steering cylinder. This increased flow causes the cylinder to move farther and faster, which turns the machine faster.

When the steering wheel is not being turned, control section A is in the neutral position. The metering pump is a closed center valve so there is no alignment between the passages in the spool and orifices in the sleeve in this position. Only a small amount of oil flows through the center position to keep the priority spool slightly open and ready for quick response to steering demands.

When the steering wheel is turned for a right turn, pump oil flows into the metering pump through inlet (3) and passage (4). From passage (4) oil goes into Gerotor (8) through passage (12). With the steering wheel turning, Gerotor (8) is turning and pumping oil. The oil flow comes out through passage (5), where it is directed to the steering cylinders through the right turn port. When the steering wheel started to turn; shaft spool (9), pin (10) and drive (6) also started to turn. Sleeve (11) did not start to turn at the same time as the spool because the diameter of the holes in the sleeve are slightly larger than the diameter of pin (10). This allows spool (9) to turn inside of sleeve (11) enough to put the small holes in sleeve (11) in line with the grooves in spool (9). The alignment of these holes and grooves provides the path for oil flow to Gerotor (8) and to the steering cylinder. Centering springs (7) are compressed when the spool moves in relation to the sleeve. When the steering wheel is no longer turning, the springs will bring the spool and sleeve back to a NEUTRAL position. This means that the wheels will stay in the position they were in when the steering effort stopped. The steering wheel must be turned in the opposite direction to bring the wheels back to straight ahead or to make a left turn. As the oil goes out to the cylinder through the right port for a right turn, return oil from the cylinder is coming back into the metering pump through the left port. This oil is sent to tank through outlet (1).

Spool And Sleeve
(9) Spool. (11) Sleeve. (13) Slots for springs. (14) Holes for pin. (15) Small holes for oil flow. (16) Grooves for oil flow.

When the steering wheel is turned to the left, the same components move that do for a right turn, only in the opposite direction. Oil still comes in inlet (3), goes through passage (4) and (12) to Gerotor (8). From Gerotor (8), the oil leaves through passage (5) and goes through the sleeve spool combination. The sleeve spool combination is turned in the opposite direction of the right hand turn. This allows a different set of holes to line up and provides a path for oil to the left turn port. From the left turn port the oil goes to the steering cylinder.

If there is a pump failure or the engine can not be started again, the metering pump can be manually operated. Check valve (19) in the metering pump between the pressure and tank ports, along with check valve (2) in the supply line, allows dead engine steering by trapping oil between the metering pump and steering cylinder.

Pump Gears In The Pump Section
(6) Drive. (10) Pin. (17) Rotor. (18) Stator. NOTE: Rotor and stator are part of the Gerotor.

If the unit is taken apart for any reason it must be put back together with the relationship between pin (10) and rotor (17) as shown.

Steering Crossover Relief Valve (Attachment)

Crossover Relief Valve

The blocked oil in the steering cylinder and in the lines between the steering cylinder and the metering pump prevents a change in direction of the wheels when the steering wheel is not turned. If a front wheel would strike a large obstruction that can not move, the pressure of the oil increases in one end of the cylinder and in the line connected to it. If the increase in oil pressure gets to approximately 21 000 kPa (3050 psi), the crossover relief valve opens. Oil can then go between the two ends of the steering cylinder. The crossover relief valve is mounted to the metering pump.