797F Off-Highway Truck Steering System: Hydraulic Schematic

Basic steering hydraulic schematic with "No Turn"
(A) To fan motor
(B) To fan pump
(1) Steering accumulators
(2) Pressure sensor (steering pump)
(3) Solenoid and relief vale manifold
(4) Steering pump
(5) Case drain oil filters
(6) Hydraulic tank (steering section)
(7) Return oil filter
(8) Oil cooler
(9) Pressure reducing valve
(10) Hand metering unit
(11) Steering control valve(12) Steering cylinders

Location of Components


Typical bottom view of 797F LAJ1-151 (6) Steering cylinders (7) Steering control valve (8) Steering accumulators (18) Steering and fan drive filter(23) Steering solenoid and backup relief valve


Typical bottom view of 797F XQ WSP1-Up and 797F LAJ152-Up. (6) Steering cylinders (7) Steering control valve (8) Steering accumulators (18) Steering and fan drive filter(23) Steering solenoid and backup relief valve


(7) Steering control valve (19) Backup relief valve(23) Steering solenoid and backup relief valve


Inside of the RH side of the frame (1) Accumulator pressure sensor (12) Pressure reducing valve(23) Steering solenoid and backup relief valve


Inside of the RH side of the frame (14) Steering piston pump (21) Hydraulic tank(24) Pump drive


(22) Steering metering pump


Bottom view of the steering accumulators (1) Accumulator pressure sensor(10) High oil pressure switch

Operation

The steering system is hydraulic and the steering pump is electronically controlled. There is no mechanical connection between the steering wheel and steering cylinders (6) that move the front wheels. Oil for the steering system is stored in hydraulic tank (21). Steering piston pump (14) is a variable displacement piston pump. Accumulator pressure sensor (1) sends the pressure reading of the accumulators to chassis ECM (2). Chassis ECM (2) controls the flow of oil from the steering pump by energizing displacement solenoid (4).




Illustration 9	g01987913
(14) Steering piston pump(24) Pump drive

Pump drive (24) drives the following pumps: the steering piston pump, the piston pump for the fan drive, the brake actuation pump, the gear pump for the motor drive for the rear axle oil pump, the gear pump for hoist and brake cooling and the brake cooling motor drive. Pump drive (24) is on the right-hand frame rail near the rear of the flywheel housing. Pump drive (24) is driven by a drive shaft. The drive shaft is connected between the pump drive and the engine. Steering piston pump (14) operates only when the engine is running.

The steering piston pump will produce flow at high pressure until steering accumulators (8) are charged with oil and the pressure increases to the cutout pressure. When the cutout pressure is reached, accumulator pressure sensor (1) will send a signal to chassis ECM (2). Chassis ECM (2) will then increase the amount of current to displacement solenoid (4) in order to move a spool within pressure and flow compensator valve (5). The pressure and flow compensator valve destrokes the steering piston pump to the low pressure standby condition.

At low-pressure standby, the steering pump operates at the minimum swashplate angle in order to supply oil for lubrication and normal internal leakage. Because of the normal internal leakage within the steering system, the pressure in the accumulators will gradually decrease until the cut-in pressure is reached. When the pressure in steering accumulators (8) decreases to the cut-in pressure, accumulator pressure sensor (1) sends a signal to chassis ECM (2). Chassis ECM (2) then decreases the amount of current to displacement solenoid (4) and the steering pump upstrokes to the maximum displacement.

Engine speed is also an input to chassis ECM (2) that is used to control the output from the steering pump. The engine speed is monitored by engine speed sensor (3) .

The accumulator pressure sensor can have one of the following values:

Table 1
Really low	This value occurs during heavy steering.
Low	This value occurs during light steering or normal internal leakage.
Normal	The desired pressure range of the accumulator.
High	This value occurs when the accumulator pressure has gone above the normal cutout pressure.

Whenever accumulator pressure sensor (1) senses the system pressure to be low or really low, the charging system will activate. The steering pump is activated until the system pressure is high. At that point, the charging system deactivates. The system stays deactivated until normal internal leakage or some amount of steering reactivates the charging system.

The charging system has three modes of operation:

Off
Low
High

For each of the three charging modes, there is a separate map of engine output speed to charging control current. The map is intended to adjust the steering pump output in order to provide more flow at lower engine speeds. The output current has a rate limit check.

MARYGAR