769D Off-Highway Truck: Steering System Introduction

Hydraulic Schematic

(1) Steering cylinders
(2) Steering check and relief valve
(3) Backup relief valve for the secondary steering system
(4) Backup relief valve for the primary steering system
(5) Secondary steering relief valve
(6) Primary steering relief valve
(7) Steering pump pressure switch
(8) Pressure reduction valve
(9) Shuttle Valve
(10) Load sensing valve
(11) Secondary steering section
(12) Parking brake release section
(13) Brake release and secondary steering pump
(14) Breaker relief valve
(15) Steering system filter
(16) Steering hydraulic tank
(17) Steering pump case drain filter
(18) Steering piston pump
(19) Pressure and flow compensator valve(20) Steering metering pump

Location of Components

Top view of the steering oil lines
(1) Steering cylinders
(2) Steering check and relief valve
(9) Shuttle valve
(18) Steering piston pump
(19) Pressure and flow compensator valve(20) Steering metering pump

Side view of the steering oil lines
(1) Steering cylinders
(14) Breaker relief valve
(15) Steering system filter
(16) Steering hydraulic tank
(17) Steering pump case drain filter
(18) Steering piston pump(20) Steering metering pump

Top view of the secondary steering lines
(9) Shuttle valve
(13) Brake release and secondary steering pump
(14) Breaker relief valve(16) Steering hydraulic tank

(14) Breaker relief valve
(15) Steering system filter
(16) Steering hydraulic tank(17) Steering pump case drain filter

(18) Steering piston pump
(19) Pressure and flow compensator valve(21) Gear pump for the hoist and the brake

(2) Steering check and relief valve
(3) Backup relief valve for the secondary steering system
(4) Backup relief valve for the primary steering system
(5) Secondary steering relief valve
(6) Primary steering relief valve(8) Pressure reduction valve

Primary Steering System Operation

The steering system is hydraulic. There is no mechanical connection between the steering wheel and steering cylinders (1) that move the front wheels. The steering system is a closed center system. The steering system is independent of all other hydraulic systems. Oil for the steering system is stored in steering hydraulic tank (16) .

Steering piston pump (18) is a pressure compensated piston type pump. The steering piston pump is mounted to the left rear face of the flywheel housing. Steering piston pump (18) is driven by the flywheel. Steering piston pump (18) operates only when the engine is running. The output of the steering piston pump is controlled by pressure and flow compensator valve (19). Metering pump (20) receives signal oil from the pressure and flow compensator valve. The signal oil flows through shuttle valve (9) and to the steering metering pump. The signal oil determines the pressure and the flow of the steering piston pump. The pressure and flow compensator valve is fastened to the steering piston pump. The flow compensator valve in the pressure and flow compensator valve causes the steering piston pump to change from low pressure standby to full flow. The high pressure cutoff valve limits the maximum steering pressure.

Internal leakage from steering piston pump (18) is returned to steering hydraulic tank (16) through steering pump case drain filter (17). Return oil from the steering system flows through steering oil filter (15). The steering oil filter is bolted to the steering hydraulic tank. Normally, oil flows through steering oil filter (15) and clean oil is returned to steering hydraulic tank (16).

If the filter element for steering oil filter (15) becomes full of debris, the restriction to flow causes a pressure increase. The bypass valve for steering oil filter (15) will open when the pressure increases above the setting of the bypass valve. When the bypass valve opens, oil is allowed to bypass the filter. The oil goes directly to steering hydraulic tank (16). When the oil bypasses the filter element, debris in the oil will cause damage to other system components. Proper maintenance must be used to ensure that the filter element does not stop the flow of clean oil to the hydraulic system.

Steering hydraulic tank (16) supplies oil to steering piston pump (18). The steering piston pump (18) pumps oil to primary steering relief valve (6). Primary steering relief valve (6) is mounted in steering check and relief valve (2). The primary steering check valve prevents oil from flowing back through steering piston pump (18) if the engine stops. From primary steering relief valve (6), the pressure oil flows to the steering metering pump (20). When the steering wheel is turned, steering metering pump (20) sends a fixed amount of oil to steering cylinders (1). The speed of the turn is determined by the rate of the steering wheel rotation. The steering linkage is designed in order to use the head end of each cylinder as a mechanical stop.

Two steering cylinder check and relief valves are located inside steering metering pump (20). Steering cylinder check and relief valves prevent damage from high pressure oil in the cylinder circuit. This occurs when the steering wheel is stationary and an outside force on a front wheel suddenly moves a cylinder rod into a cylinder or out of a cylinder. When the front wheels strike a large object, pressure oil from a steering cylinder will open one of the steering cylinder check and relief valve. This will allow oil flow to the low pressure side of steering cylinders (1) .

The backup relief valve for the primary steering system (4) is located in steering check and relief valve (2). The backup relief valve for the primary steering system protects the primary steering system if the high pressure cutoff valve in pressure and flow compensator valve (19) fails to limit the maximum pressure of the system.

Pressure reducing valve (8) protects the steering pump pressure switch (7) from high pressure oil. High pressure oil will damage the switch. Pressure reducing valve (8) modulates the flow of oil when the pressure of the steering system is above the pressure setting of the pressure reducing valve. Steering pump pressure switch (7) now senses the pressure setting of pressure reducing valve (8). The pressure reducing valve shifts when the steering system pressure drops below the setting of the pressure reducing valve. The steering pump pressure switch (7) now senses the steering system pressure. Steering pump pressure switch (7) monitors the output of the steering piston pump.

Secondary Steering System Operation

(13) Brake release and secondary steering pump(26) Plug for the main relief valve on the secondary steering system

(22) Action light
(23) Secondary steering indicator
(24) Secondary steering alert indicator(25) Brake release and secondary steering switch

Steering hydraulic tank (16) also supplies oil to brake release and secondary steering pump (13). Brake release and secondary steering pump (13) is located on the frame behind the right front suspension cylinder. The brake release and secondary steering pump provides oil in order to steer the front wheels if the normal flow is lost from the primary steering system or if the engine stops.

Brake release and secondary steering pump (13) operates according to the position of brake release and secondary steering switch (25) that is located in the cab. During normal machine operation, brake release and secondary steering switch (25) is placed in the AUTO position. The brake release and secondary steering pump will only operate when additional pressure oil is needed in the steering system.

When the engine is started, the Power Train Electronic Control Module (Power Train ECM) activates a relay that is located behind the cab. The relay behind the cab activates a larger relay that is located on the inside of the left frame rail next to the engine. Then, the larger relay activates brake release and secondary steering pump (13). The brake release and secondary steering pump will operate for three seconds. Secondary steering indicator (23) will turn on when the pump is operating.

Brake release and secondary steering switch (25) is moved to the MANUAL position only when the machine is towed. This switch sends a signal to the Power Train ECM. The Power Train ECM then activates brake release and secondary steering pump (13) and the secondary steering indicator.

The monitoring system is designed to alert the operator of an immediate problem in the steering system or in other machine systems. The monitoring system also alerts the operator about an impending problem in the steering system or in other machine systems.

If the steering system pressure decreases to the deactuation pressure of the pressure switch, steering pump pressure switch (7) will send an electrical signal to the Power Train ECM. Secondary steering warning indicator (24) will flash when the steering pressure is low. Secondary steering indicator (23) will turn ON when the pump is operating. The Power Train ECM will then activate brake release and secondary steering pump (13) .

Load sensing signal pressure oil will flow through load sensing valve (10) and shuttle valve (9) to the steering metering pump. The load sensing signal pressure moves the spool of the load sensing valve. The load sensing valve controls the flow of oil from the brake release and secondary steering pump to steering check and relief valve (2) .

Secondary steering check valve (5) is located in steering check and relief valve (2). Secondary steering check valve (5) prevents pressure oil from the primary steering system from flowing back through brake release and secondary steering pump (13). Secondary steering check valve (5) receives pressure oil from the brake release and secondary steering pump. Pressure oil opens secondary steering check valve (5) and flows to the primary steering check valve (6). The pressure oil does not open the primary steering check valve (6), but the oil flows behind the primary steering check valve (6). The operations of the secondary steering system and of the primary system are identical after the pressure oil flows past the primary steering check valve (6) .

The main relief valve on the secondary steering system is also located in load sensing valve (10) of brake release and secondary steering pump (13) behind plug (26). The main relief valve on the secondary steering system controls the maximum pressure of the secondary steering system.

The backup relief valve for the secondary steering system (3) is located in steering check and relief valve (2). The backup relief valve for the secondary steering system protects the secondary steering system if the main relief valve for the secondary steering system fails to limit the maximum pressure of the secondary steering system.

MARYGAR