789D Off-Highway Truck Steering System

Hydraulic Schematic

(1) Steering accumulators
(2) Pressure switch for high-pressure oil from the steering piston pump
(3) Accumulator charging valve
(4) Steering cylinders
(5) Steering solenoid and relief valve
(6) Steering check valve
(7) Steering control valve
(8) Steering pump case drain filter
(9) Steering filter
(10) Steering hydraulic tank
(11) Steering metering pump
(12) Steering piston pump
(13) Breaker relief valve
(14) Pressure switch for low-pressure oil from the steering piston pump
(15) Orifice(16) Pressure compensator valve

Location of Components

Top view of steering oil lines
(1) Steering accumulators
(2) Pressure switch for high-pressure oil from the steering piston pump
(3) Accumulator charging valve
(4) Steering cylinders
(5) Steering solenoid and relief valve
(7) Steering control valve
(8) Steering pump case drain filter
(10) Steering hydraulic tank
(12) Steering piston pump(14) Pressure switch for low-pressure oil from the steering piston pump

Side view of steering oil lines
(1) Steering accumulators
(2) Pressure switch for high-pressure oil from the steering piston pump
(4) Steering cylinders
(7) Steering control valve
(8) Steering pump case drain filter
(10) Steering hydraulic tank
(11) Steering metering pump
(12) Steering piston pump
(14) Pressure switch for low-pressure oil from the steering piston pump
(15) Orifice
(16) Pressure compensator valve(AA) Pressure tap for low-pressure oil from the steering piston pump

(5) Steering solenoid and relief valve(6) Steering check valve

(3) Accumulator charging valve
(7) Steering control valve(BB) Pressure tap for high-pressure oil from the steering piston pump

Operation

(12) Steering piston pump
(16) Pressure compensator valve
(17) Pump drive(AA) Pressure tap for low-pressure oil from the steering piston pump

The steering system is hydraulic. There is no mechanical connection between the steering wheel and steering cylinders (4) 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 (10) .

Steering piston pump (12) is a pressure compensated piston type pump. The steering piston pump, the hoist and rear brake cooling gear pump, and the transmission gear pump are driven by pump drive (17). Pump drive (17) is on the right-hand frame rail near the rear of the flywheel housing. Pump drive (17) is driven by a drive shaft. The drive shaft is connected between the pump drive and the engine. Steering piston pump (12) operates only when the engine is running. The pressure of the steering piston pump is controlled by the accumulator charging valve (3). Accumulator charging valve (3) limits the maximum outlet pressure to approximately 18300 kPa (2650 psi). Pressure compensator valve (16) is fastened to steering piston pump (12). The high-pressure cutoff valve in pressure compensator valve (16) is set to approximately 20000 kPa (2900 psi).

(8) Steering pump case drain filter
(9) Steering filter
(10) Steering hydraulic tank(13) Breaker relief valve

Internal leakage from steering piston pump (12) is returned to steering hydraulic tank (10) through steering pump case drain filter (8). Steering pump case drain filter (8) is bolted to steering hydraulic tank (10). Return oil from the steering system flows through steering filter (9). Steering filter (9) is located inside the steering hydraulic tank. Normally, oil flows through these filters and clean oil is returned to steering hydraulic tank (10). Any debris in the oil is stopped by steering pump case drain filter (8) and the steering filter (9) .

If the filter element for the steering pump case drain filter (8) or for the steering filter (9) becomes full of debris, the restriction to flow causes a pressure increase. The bypass valve for the steering pump case drain filter (8) will open at a pressure of approximately 170 kPa (25 psi). The bypass valve for the steering filter (9) will open at a pressure of approximately 238 kPa (35 psi). When the bypass valves open, oil is allowed to bypass the filters. The oil goes directly to steering hydraulic tank (10). When the oil bypasses the filter element, debris in the oil will damage 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.

(5) Steering solenoid and relief valve
(6) Steering check valve
(18) Steering relief valve(19) Steering solenoid valve

Steering hydraulic tank (10) supplies oil to steering piston pump (12). Steering piston pump (12) pumps oil through steering check valve (6). Steering check valve (6) is mounted to the steering solenoid and relief valve (5). This oil then enters the steering solenoid and relief valve (5). Steering check valve (6) separates the high steering pressure from the low steering pressure that is generated by the pump during low-pressure standby. The high steering pressure is the pressure that is required by the steering accumulators. Steering check valve (6) also prevents oil from flowing back through steering piston pump (12) if the engine stops. Blocking the oil in the lines keeps all of the oil stored in the steering accumulators at the time of engine shutdown. If the engine start switch key is not in the OFF position, the oil will be available to steer the moving machine while you try to stop.

Steering solenoid and relief valve (5) acts as a manifold for inlet oil and return oil. Full pressure oil is supplied to steering accumulators (1) until an approximate pressure of 18300 kPa (2650 psi) is reached. When steering accumulators (1) are charged, accumulator charging valve (3) will work with pressure compensator valve (16). This will cause steering piston pump (12) to destroke to standby pressure. The maximum standby pressure is 3600 kPa (522 psi). At this time, steering piston pump (12) will pump enough oil in order to satisfy normal leakage.

When the steering wheel is turned, oil from steering accumulators (1) is used. This causes the pressure in steering accumulators (1) to drop. When steering accumulator pressure drops below 16470 kPa (2388 psi), accumulator charging valve (3) will signal pressure compensator valve (16). This causes steering piston pump (12) to stroke up to full flow.

Steering relief valve (18) is mounted to the bottom of the steering solenoid and relief valve (5). Steering relief valve (18) protects steering piston pump (12) and all other system components if accumulator charging valve (3) fails to destroke steering piston pump (12) .

Steering solenoid valve (19) is mounted to the side of steering solenoid and relief valve (5). When the engine is shut off with the engine start switch key, the electronic control for the steering accumulator activates the steering solenoid valve. When the steering solenoid valve is activated, pressure oil is released from the steering accumulators (1). This prevents the machine from being turned when the machine is not active.

Note: The battery disconnect switch and the engine shutdown control do not activate the steering solenoid valve (19) .

Two pressure switches monitor the condition of the steering system. The pressure switch for low-pressure oil from the steering piston pump (14) monitors the output of the steering piston pump. The pressure switch for high-pressure oil from the steering piston pump (2) monitors the steering accumulator pressure in the secondary steering system.

Steering control valve (7) directs pressure oil to steering cylinders (4) when the steering wheel is turned. Steering control valve (7) also contains two steering check and relief valves. When steering control valve (7) is in the neutral position, the two steering check and relief valves protect the cylinder lines from large shocks.

Steering control valve (7) works with the steering metering pump (11) in order to turn the machine. Pressure oil in steering control valve (7) is blocked internally until a turn is made. The steering control valve (7) receives a fixed amount of pilot oil from the steering metering pump (11) for each revolution of the steering wheel. The pilot oil shifts the spools in steering control valve (7). Steering control valve (7) now allows a corresponding amount of pressure oil to steering cylinders (4). 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.

MARYGAR