The steering system on the 777F is similar to the 777D except a steering disable solenoid valve has been added and some of the component locations have changed.
When energized, the steering disable solenoid valve stops the oil flow coming from the steering pump. This prevents the front wheels from turning to allow servicing to be conducted safely in the front wheel area.
The steering system uses a load sensing, pressure compensated pump. Minimal horsepower is used by the steering system when the truck is traveling in a straight path. Steering hydraulic horsepower requirements depend on the amount of steering pressure and flow required by the steering cylinders.
This illustration shows the following main steering components:
- Steering pump (1)
- Steering disable valve and steering valve (2)
- HMU (3)
- Steering tank (4)
- Secondary steering pump (5)
The steering system tank is located on the right platform
Check the steering system oil level at the sight gauge (1).
The steering system oil filter (2) is located on the side of the steering tank.
The steering system uses a pressure compensated piston type pump. Case drain oil from the steering pump returns to the hydraulic tank through a case drain filter (3) on the side of the steering tank.
Before removing the cap to add oil to the steering system, depress the pressure release button (4) on the breather to release any remaining pressure from the tank.
The steering system filter base and the case drain filter base have bypass valves that allow the steering oil to bypass the filters if they are plugged.
The 777F Trucks are equipped with a load sensing, pressure compensated, piston-type pump.
The steering pump operates only when the engine is running and provides the necessary flow of oil for steering system operation. The steering pump contains a load sensing controller with two valves. The high pressure cutoff valve (1) functions as the primary steering system relief valve.
The flow compensator valve (2) is used to adjust the low pressure standby setting. When the truck is traveling in a straight path, virtually no flow or pressure is sent to the steering cylinders, and the pump destrokes to low pressure standby.
When the truck is traveling in a straight path, the steering cylinders require virtually no flow or pressure. The HMU provides a very low pressure load sensing signal to the flow compensator in the load sensing controller.
Pump oil (at low pressure standby) flows to the swashplate piston and past the lower end of the displaced flow compensator spool to the actuator piston. The actuator piston has a larger surface area than the swashplate piston. The oil pressure at the actuator piston overcomes the spring force and the oil pressure in the swashplate piston and moves the swashplate to destroke the pump. The pump is then at minimum flow, low pressure standby.
Pump output pressure is equal to the setting of the flow compensator plus the pressure required to compensate for system leakage.
During a turn, when steering pressure and flow are required, pressure increases in the HMU load sensing signal line. The pressure in the signal line is equal to the pressure in the steering cylinders. The pump load sensing controller is spring biased to vent the actuator piston pressure to drain. Venting pressure from the load sensing controller and the actuator piston positions the spring biased swashplate to maximum displacement (maximum flow).
As pressure increases in the HMU load sensing signal line, pump supply pressure is sensed on both ends of the flow compensator. When pressure is present on both ends of the flow compensator, the swashplate is kept at maximum angle by the force of the spring in the pump housing and pump discharge pressure on the swashplate piston. The pistons reciprocate in and out of the barrel and maximum flow is provided through the outlet port. Since the pump is driven by the engine, engine rpm also affects pump output.
The steering disable valve (1) is located behind the shock on the right frame rail.
When the steering disable solenoid valve (2) is energized, oil flow from the steering pump to the steering valve is blocked by the steering disable valve, which allows servicing behind the front wheels with the machine running.
When the machine lockout switch, located under a panel on the left stairway, is toggled, a signal is sent to the Transmission/Chassis ECM. The Transmission/Chassis ECM energizes the steering disable solenoid allowing service to be performed behind the front wheels safely.
Also located on the steering disable valve is a pressure tap (3) for checking the load sensing signal to the pump, and an S•O•S tap (4).
Steering oil flows from the pump through the steering disable valve to the steering valve (1) located on the frame behind the right front suspension cylinder. The primary steering pressure switch (2) monitors the output of the steering pump. The steering pressure switch provides input signals to the Transmission/Chassis ECM which sends a signal to the monitoring system to inform the operator of the steering system condition. A steering system warning is displayed if the pressure is too low.
The steering pressure switch cannot tolerate high steering system pressures. A pressure reducing valve (not visible) reduces the steering system pressure to the steering pressure switch.
Two relief valves are located on the left side of the steering valve. The top relief valve (3) is a back-up relief valve for the secondary steering system. The secondary steering back-up relief valve protects the secondary steering system if the relief valve on the secondary steering pump malfunctions.
The lower relief valve (4) is a back-up relief valve for the primary steering system. The primary steering back-up relief valve protects the primary steering system if the high pressure cutoff valve on the steering pump malfunctions. Primary steering pressure is first controlled by the high pressure cutoff valve located on the steering pump.
Check valves are used to separate the primary and secondary steering systems. The secondary check valve (5) is behind the left plug, and the primary check valve (6) is behind the right plug.
Steering system pressures can be measured at the steering system pressure tap (7).
This illustration shows the location of the HMU (arrow) for the 777F. Serviceability has improved for the HMU on the 777F due to the redesigned walkways. The HMU is connected to the steering wheel and controlled by the operator.
The HMU meters the amount of oil sent to the steering cylinders by the speed at which the steering wheel is turned. The faster the HMU is turned, the higher the flow sent to the steering cylinders, and the faster the wheels will change direction.
The steering system is referred to as "Q-amp" which means flow amplification. During a sudden steering change (steering wheel speed greater than 10 rpm), additional steering pump oil flow will bypass the gerotor pump in the HMU and flow directly to the steering cylinders.
Steering oil flow to the cylinders is equal to the gerotor pump oil flow plus the bypass oil flow from the steering pump. The steering oil flow is amplified up to 1.6 to 1. The purpose of the flow amplification is to provide quick steering response when sudden steering changes are needed.
Two crossover relief valves are installed in the top of the HMU. The crossover relief valves are installed in series with the left and right turn ports. If an outside force is applied to the front wheels while the steering wheel is stationary, the crossover relief valves provide circuit protection for the steering lines between the steering cylinders and the HMU. The crossover relief valves allow oil to transfer from one end of the steering cylinders to the opposite end of the cylinders.
To test the right crossover relief valve, install two tees with pressure taps in the right turn steering hose at the steering cylinders. Steer the truck completely to the right against the stops, and shut off the engine. An external pump supply must be connected to one of the pressure taps on the right turn hose. Connect a pressure gauge to the other pressure tap on the right turn hose. Pressurize the steering system, and the reading on the gauge will be the setting of the right crossover relief valve.
To test the left crossover relief valve, install two tees with pressure taps in the left turn steering hose at the steering cylinders. Steer the truck completely to the left against the stops, and shut off the engine. An external pump supply must be connected to one of the pressure taps on the left turn hose. Connect a pressure gauge to the other pressure tap on the left turn hose.
Pressurize the steering system, and the reading on the gauge will be the setting of the left crossover relief valve.
The electric secondary steering pump (1) and motor (2) on the 777F are the same as the 777D, however the location has changed. The pump and motor are now located on the front of the front crossmember. The pump and motor assembly also includes the brake release pump section (3) and the prelubrication (QuickEvac) pump section (4).
The secondary pressure switch (5) is also mounted next to the secondary steering pump. The pressure switch detects if the wheels are being turned via the steering wheel when secondary steering is activated. When the wheel is turned in a secondary steering condition, the pressure switch will signal the Transmission/Chassis ECM and the QuickEvac function will be disabled.
If the primary steering pressure switch signals the Transmission/Chassis ECM that the steering system pressure is low, the ECM will energize the secondary steering relay located behind the cab. The secondary steering relay will then energize a second larger relay located on the frame above the steering valve, which will then energize the secondary steering motor.
The primary relief valve for the secondary steering is accessible through the small allen head plug (6). To check the setting of the secondary steering primary relief valve, do not start the truck. Turn ON the key start switch and depress the secondary steering switch in the cab. Turn the steering wheel hard to the left or right while the secondary steering pump is running. Secondary steering system pressures can be measured at the steering system pressure tap.
Shown is a schematic of the steering hydraulic system in the HOLD position. The primary steering pump pulls oil from the steering tank. All piston-type pumps produce a small amount of leakage to the case drain circuit for lubrication and cooling. The case drain oil flows to the steering tank through a case drain filter.
Steering oil flows from the pump to the steering disable valve. When the steering disable valve is energized, oil is allowed to flow to the steering valve.
In the steering valve, a steering pressure switch monitors the output of the steering pump. The steering pressure switch cannot tolerate high steering system pressures. A pressure reducing valve lowers the steering system pressure to the steering pressure switch.
If the steering pressure switch signals the Transmission/Chassis ECM that the steering system pressure is low, the ECM will then energize the secondary steering motor. Secondary steering supply oil will flow to the steering valve.
Two relief valves are installed in the steering valve. The secondary steering back-up relief valve protects the secondary steering system if the relief valve on the secondary steering pump malfunctions. The primary steering back-up relief valve protects the primary steering system if the high pressure cutoff valve on the steering pump malfunctions.
Two check valves are located on the steering valve. The check valves are used to separate the primary and secondary steering systems.
Steering supply oil flows to the HMU from the steering valve. Return oil from the HMU flows through the steering valve and the steering filter to the steering tank.
The HMU meters the amount of oil sent to the steering cylinders by the speed at which the steering wheel is turned. The faster the HMU is turned, the higher the flow sent to the steering cylinders, and the faster the wheels will change direction.
Two crossover relief valves are installed in the top of the HMU. The crossover relief valves are installed in series with the left and right turn ports. If an outside force is applied to the front wheels while the steering wheel is stationary, the crossover relief valves provide circuit protection for the steering lines between the steering cylinders and the HMU. The crossover relief valves allow oil to transfer from one end of the steering cylinders to the opposite end of the cylinders.
When the Transmission/Chassis ECM energizes the secondary steering motor, load sensing signal oil will flow from the secondary steering load sensing valve through the load sensing resolver to the HMU. The load sensing valve uses the load sensing signal pressure to control the amount of flow from the secondary steering pump to the steering valve.
The 777F Trucks use a dynamic load sensing steering system the same as the late model "D Series" Trucks. In a dynamic system, there is load sensing pressure and flow between the HMU and the steering pumps.
A load sensing pilot signal resolver valve is located in the steering disable valve. The resolver valve allows load sensing signal oil to flow between the HMU and the primary steering pump or the secondary steering pump. In the NO STEER position, oil flows to the HMU. In a LEFT or RIGHT STEER position, oil also flows from the HMU to the resolver valve.
Normally, the secondary steering pump is OFF and the resolver is closed from the HMU to the secondary steering pump. The flow from the primary steering pump holds the resolver open and load sensing pilot signal pressure is present between the HMU and the piston pump flow compensator.
The load sensing signal flow from the primary steering pump is also used for "thermal bleed" through the HMU. The "thermal bleed" is used to keep the HMU temperature the same as the rest of the steering system. Keeping the HMU the same temperature prevents sticking.
See You Soon!!!
MARYGAR
terimakasih banyak untuk infonya. izin untuk mengutip pengertian dan maksud dari artikelnya. ini untuk tugas akhir saya. . . mudah-mudah bisa berbagi lebih banyak lagi untuk materi electrical steeringnya. kalau boleh tau ada rinciannya buat komponen supplemental... kalau tidak keberatan bisa di buatin artikelnya :) terimakasih
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