797F Off-Highway Truck: Operation of Hydraulic Tank (Brake, Hoist, Steering)

View A of steering solenoid and backup relief valve (23)
(7) Steering control valve
(19) Backup relief valve
(23) Steering solenoid and backup relief valve
(25) Tap for the Auxiliary Power Unit (APU)
(35) Oil supply hose for the steering system
(36) Pressure tap for the steering system
(37) Pressure tap for the steering accumulators(38) Sampling valve for the steering oil

Hydraulic tank (28) supplies oil to steering piston pump (14). Steering piston pump (14) pumps oil through oil supply hose (35) to steering check valve (11) that is located in steering solenoid and relief valve (23). Steering check valve (11) separates the steering accumulator pressure from the low steering pressure that is generated by the pump during low-pressure standby. Steering check valve (11) also prevents oil from flowing back through steering piston pump (14) 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.

The supply pressure from the steering pump can be measured at pressure tap (36). The pump supply pressure will increase until the pump outlet pressure approximately equals the cutout pressure. Then, the pump supply pressure will decrease to the low pressure standby setting.

The steering accumulator pressure can be measured at pressure tap (37). The steering accumulator pressure will increase until the pressure approximately equals the cutout pressure. Because of the normal internal leakage in the steering system, the pressure in the accumulators will gradually decrease to the cut-in pressure. The pump will cycle between the cutout pressure and the cut-in pressure in intervals of 30 seconds or more during the following conditions: LOW IDLE, NEUTRAL and NO STEER

To operate the steering circuit on a disabled truck, connect an Auxiliary Power Unit (APU) to tap (25). The APU will supply oil in order to charge the steering accumulators. Steering capability is then available to tow the truck.

Note: For more information on the APU, refer to the following material: Special Instructions, SEHS8715, "Using 1U-5000 Auxiliary Power Unit Arrangement (APU)", Special Instructions, SEHS8880, "Using the 1U-5525 Attachment Group " and Operation and Maintenance Manual, "Towing With Inoperable Engine".

Oil samples of the steering system can be taken at sampling valve (38) .

Steering solenoid and relief valve (23) acts as a manifold for inlet oil and for return oil. Full pressure oil is supplied to steering accumulators (8) until the steering accumulators are full. When steering accumulators (8) are charged, accumulator pressure sensor (1) will send a signal to chassis ECM (2). Chassis ECM (2) will send a signal to displacement solenoid (4). The displacement solenoid will then move a spool within pressure and flow compensator valve (5). This will cause steering piston pump (14) to destroke to low pressure standby pressure. During low-pressure standby, steering piston pump (14) will only pump enough oil in order to compensate for normal internal leakage.

When the steering wheel is turned, oil from steering accumulators (8) is used causing the pressure in steering accumulators (8) to drop. When accumulator pressure drops to the cut-in pressure, the accumulator pressure sensor will signal chassis ECM (2). Chassis ECM (2) will then send a signal to displacement solenoid (4). The displacement solenoid will then move a spool within pressure and flow compensator valve (5) causing steering piston pump (14) to upstroke to full flow.

Backup relief valve (19) for the steering system is mounted in steering solenoid and relief valve (23). Backup relief valve (19) protects steering piston pump (14) and all other system components from pressure spikes if pressure and flow compensator valve (5) fails to destroke steering piston pump (14). Backup relief valve (19) also limits the maximum pressure if the high-pressure cutoff valve for the steering pump does not open.

Steering solenoid valve (17) is mounted in steering solenoid and relief valve (23). 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 steering solenoid valve (17) is activated, pressure oil is released from steering accumulators (8) preventing the machine from being turned when the machine is not being used.

Note: The battery disconnect switch and the engine shutdown control do not activate steering solenoid valve (17) .

Steering metering pump (22) works with steering control valve (7) in order to turn the machine. Pressure oil in steering control valve (7) is blocked internally until the steering wheel is turned. Steering metering pump (22) provides a fixed amount of pilot oil to steering control valve (7) 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 (6). 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 eye end of each cylinder as a mechanical stop.


View B of steering control valve (7)

Steering control valve (7) also contains two steering check and relief valves. When steering control valve (7) is in the neutral position, the two check and relief valves protect the cylinder lines from large shocks.

(9) Low oil pressure switch
(12) Pressure reducing valve
(39) Return hose to the hydraulic tank
(40) Supply hose from the steering system(41) Signal hose to the piston pump for the fan drive and the brake cooling drive pump

Steering oil flows through oil supply hose (40) to pressure reducing valve (12). The pressure reducing valve reduces the steering pressure to a lower signal pressure. Excess steering oil flows to the hydraulic tank through oil return hose (39). The reduced oil pressure signal flows to the piston pump for the fan drive and the brake cooling drive pump through hose (41). The piston pump for the fan drive and the brake cooling drive pump use the oil pressure signal to destroke to minimum flow at start-up and during cold temperatures.


(1) Accumulator pressure sensor

Two pressure switches and a temperature sensor monitor the condition of the steering system. Low oil pressure switch (9) monitors the output of the steering piston pump. The low oil pressure switch monitors the supply of the steering pump while the system is in low-pressure standby. Steering oil temperature sensor (20) monitors the temperature of the oil in the steering hydraulic tank.

Accumulator pressure sensor (1) is the main input to chassis ECM (2) for controlling the steering system. The sensor is used to determine when the accumulators need to be charged. The accumulator pressure sensor also controls the cutout pressure and the cut-in pressure.

MARYGAR