Introduction to Pilot Operated Implement Hydraulic System
The pilot operated implement hydraulic system is used on various models of mobile construction equipment. The illustrations that are used in this lesson are for the Caterpillar 928G Wheel Loader.
Implement Hydraulic System
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| Illustration 1 | g01064936 |
The schematic in Illustration 1 shows the implement hydraulic system while the engine is running. All pilot valve control levers are in the HOLD position.
The gray and yellow ISO symbols represent movable components. The yellow portion of the symbol shows the current position of the component. In Illustration 1, the yellow center envelopes of the main control valves show that the valves are in the HOLD position.
Main System HOLD position
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| Illustration 2 | g01064937 |
In the HOLD position, the main implement pump draws oil from the tank. Then, the main implement pump sends the supply oil flow past the test port and the main relief valve to the center envelope of the auxiliary control valve.
The supply oil flows through the following valves:
- The auxiliary control valve
- The center of the tilt control valve
- The center of the lift control valve
- Then returns to the tank
This type of system is called an open-center system.
Pilot System HOLD Position
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| Illustration 3 | g01064940 |
In the HOLD position, the pilot and the steering system pump draws oil from the tank. Then, the pilot and the steering system sends the supply oil flow to the steering system supply port and the pressure reducing valve. The pressure reducing valve controls the maximum pressure in the pilot system. Oil flows from the pressure reducing valve, through the check valve to the accumulator, and to the pilot shutoff valve.
The accumulator serves as an emergency pilot oil supply when the pump is not pumping oil.
When the pilot shutoff valve is in the OFF position, the pilot oil is blocked. The pilot system will not function and the control valves cannot be shifted. When the pilot shutoff valve is in the ON position, the oil flows through the pilot shutoff valve to the auxiliary, tilt, and lift pilot control valves.
When the pilot control valves are in the HOLD position, the pilot oil is blocked at the pilot control valves.
Tilt Pilot Control Valve
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| Illustration 4 | g01064941 |
Illustration 4 shows the major components in the tilt pilot control valve. Except for the coil assembly at the top of the dump section (left side), the parts are the same for both sides of the valve.
While the engine is running and the control lever in the HOLD position, pilot oil enters the supply port (bottom center of valve body) and is blocked by the metering stems. Any oil in the lines to the main control valve is vented to the tank port through the center of the metering stems.
TILT BACK Position
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| Illustration 5 | g01064942 |
When the operator moves the pilot control lever to the TILT BACK position, the force causes the pivot plate to move the following componets: the upper plunger, the lower plunger, the metering spring, the metering stem spring retainer, the metering stem spring and the metering stem down. Oil from the pressure reducing valve flows through the orifice and the center of the metering stem to the main control valve. Return oil from the main control valve flows through the dump metering stem port, to the center of the metering stem, and to the tank port.
Metering Spring
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| Illustration 6 | g01064943 |
The metering stem allows movement of the main control valve spool in proportion to the movement of the pilot valve lever. The metering stem and the metering spring function as a pressure reducing valve. The metering stem and the metering spring control the oil pressure at the end of the main control valve spool.
When the metering stem moves down, pilot oil flows through the orifice, through the center of the metering stem, and out to the main control valve spool. The pilot oil is blocked at the main control valve spool that will cause the pilot pressure to increase. The pressure increase overcomes the main control valve spool spring. The pressure increase moves the main control valve spool. The main control valve spool directs the main system supply oil to the cylinder.
The pressure increase is also sensed against the lower end of the metering stem. When the pressure increase overcomes the applied force, the metering stem moves up and the metering stem compresses the metering spring. The movement restricts pilot oil flow through the metering stem orifice. Restricting the oil flow controls the signal pressure at the main control valve spool. The metering spring therefore adjusts the pressure at the main control valve spool in proportion to the movement of the pilot valve lever.
Control Valve in HOLD Position
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| Illustration 7 | g01064946 |
Illustration 7 shows a 928G main control valve in the HOLD position. The tilt valve, lift valve, auxiliary valve, and the fourth function valve are equipped similarly. The auxiliary valve contains only line relief valves in both work ports.
When the control valve is in the HOLD position, the pump supply oil enters the center passage, flows around the control spool, and through the outlet passages to the next valve. Supply oil also flows through the internal passage to the check valve. Oil flows past the check valve to the main control spool. The main control spool blocks supply oil flow to the tilt passages and to the dump passages. The main control spool also blocks the work ports oil passages from the tank oil passages.
TILT BACK Position
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| Illustration 8 | g01064947 |
When the operator moves the pilot control lever to the TILT position, pilot oil (orange) moves the tilt control spool to the right. The control spool blocks the oil outlet passages. The control spool opens the passage from the check valve to the tilt cylinder head end. The control spool opens the passage from the tilt cylinder rod end to the tank return.
When the supply pressure is higher than the pressure in the head end of the tilt cylinder, the supply oil opens the check valve and flows past the control spool to the head end of the tilt cylinder. The return oil flows from the rod end of the tilt cylinder, past the control spool, and to the tank. The bucket begins tilting back.
Combination Line Relief and Makeup Valve
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| Illustration 9 | g01064948 |
Illustration 9 shows the combination line relief and makeup valve. The line relief valve is simply a pilot operated relief valve. However, the line relief valve is not designed to handle maximum implement pump flow.
The oil in the cylinder is connected through lines to the right end of the combination valve. The oil flows through the main valve orifice to the main valve spring chamber. The oil pressure at the front (right) of the valve is the same as the oil pressure in the spring chamber. The oil pressure in the main valve spring chamber plus the force of the spring keeps the valve closed.
Line Relief in RELIEF Position
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| Illustration 10 | g01064950 |
In Illustration 10 the line relief is shown in the RELIEF position. When the oil pressure overcomes the setting of the pilot valve, the pilot valve moves to the left against the spring. The high pressure oil in the main valve spring chamber flows through the pilot valve orifice to the pilot valve spring chamber drain passage. The pressure in the main valve spring chamber decreases. The higher pressure oil at the right side of the main valve moves the main valve to the left. The high pressure oil flows past the main valve, and through the makeup valve dump openings to the tank.
The makeup valve does not move when the line relief valve opens.
MAKEUP Position
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| Illustration 11 | g01064951 |
Illustration 11 shows the combination line relief and makeup valve in the MAKEUP position. The tank pressure is sensed on the effective area of the combination valve at all times. When the oil pressure in the cylinder, the connecting line, and the relief valve spring chamber decreases to 13.78 kPa (2 psi) less than the oil pressure in the tank, the tank pressure moves the makeup valve and the main valve to the left against the main valve spring. The tank oil flows through the newly opened passage to the connecting line and the cylinder.
Lift Control Valve in LOWER Position
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| Illustration 12 | g01064952 |
Illustration 12 shows the lift control valve in the LOWER position.
When the operator moves the lift/lower pilot control lever to the LOWER position, pilot oil (orange) moves the lift control spool to the left. The control valve spool blocks the oil outlet passages. The control valve spool opens the passage from the check valve to the lift cylinder rod end. The control valve spool opens the passage from the lift cylinder head end to the tank return. When the supply pressure is higher than the pressure in the rod end of the tilt cylinder, the supply oil opens the check valve and flows past the control spool to the rod end of the lift cylinder. The return oil flows from the head end of the lift cylinder, past the control spool, and to the tank. The bucket begins to lower.
If the bucket is lowered faster than the pump can supply oil to the rod end, the makeup valve opens and oil from the tank passage flows into the rod of the lift cylinder in order to prevent cavitation.
Lift Control Valve in FLOAT Position
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| Illustration 13 | g01064953 |
Illustration 13 shows the lift control valve in the FLOAT position.
When the operator moves the lift/lower pilot control lever to the FLOAT position, pilot oil (orange) moves the lift control spool completely to the left. The control spool opens the passage from the check valve to the left side outlet. The control spool opens the passage from the lift cylinder head end to the tank return. The control spool also connects the lift cylinder rod end to the tank return. When the pump and both ends of the lift cylinder are connected to the tank, the lift cylinder cannot be hydraulically raised or lowered.
When the machine is moved and the lift/lower control is in the FLOAT position, the implement follows the curvature of the ground.
928G IMPLEMENT HYDRAULIC SYSTEM IN THE HOLD POSITION
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| Illustration 14 | g01064954 |
In the HOLD position, the oil from the steering and pilot pump flows to the steering system supply port and the pressure reducing valve. The pressure reducing valve controls the maximum pressure in the pilot system. Oil flows from the pressure reducing valve, through the check valve to the accumulator, and to the pilot shutoff valve.
The accumulator serves as an emergency pilot oil supply when the pump is off.
When the pilot shutoff valve is in the OFF position, the pilot oil is blocked. The pilot system will not function. When the pilot shutoff valve is in the ON position, the oil flows through the pilot shutoff valve to the auxiliary, tilt back/dump, and lift/lower pilot control valves. When the pilot control valves are in the HOLD position, the pilot oil is blocked at the pilot control valves. This type of system is called a closed center system.
Oil from the main implement pump flows past the test port and the main relief valve to the main control valve.
The implement pump oil flows through the center of the auxiliary valve spool, the center of the tilt back/dump valve spool, the center of the lift/lower valve spool and returns to the tank. This type of system is called an open-center system. The open-center system is also an interrupted series circuit, since the return oil from any cylinder is returned to the tank rather than being routed to the next downstream valve.
928G IMPLEMENT HYDRAULIC SYSTEM IN THE TILT BACK POSITION
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| Illustration 15 | g01064955 |
This schematic shows the flow through the hydraulic system with the tilt back/dump control lever in the TILT BACK position.
When the operator moves the tilt back/dump control lever to the TILT BACK position, pilot oil flows past the tilt back/dump pilot control valve to the end of the tilt back/dump valve spool in the main implement control valve. Pilot oil moves the tilt back/dump valve spool to the TILT BACK position. Oil from the opposite end of the tilt back/dump valve spool flows past the tilt back/dump pilot control valve to the tank.
Movement of the tilt valve spool to the TILT BACK position, blocks the flow of implement pump oil to the tank and directs implement pump oil to the head end of the tilt cylinder. Oil in the rod end of the tilt cylinder is forced back past the tilt valve spool to the tank.
The orifices in the pilot oil lines between the tilt back/dump pilot control valve and the tilt back/dump valve spool causes a restriction to the flow of pilot oil and provides better operator control of the tilt back/dump valve spool.
928G IMPLEMENT HYDRAULIC SYSTEM IN THE LIFT POSITION
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| Illustration 16 | g01064956 |
This schematic shows the conditions in the implement hydraulic system with the lift/lower control lever in the LIFT position.
When the operator moves the lift/lower control lever to the LIFT position, pilot oil flows past the lift/lower pilot control valve to the end of the lift/lower valve spool in the main implement control valve. Pilot oil moves the lift/lower valve spool to the LIFTposition. Oil from the opposite end of the lift/lower valve spool flows past the lift/lower pilot control valve to the tank.
Movement of the lift/lower valve spool to the LIFT position blocks the flow of supply oil to the tank. Supply oil is directed to the head ends of the lift cylinders. Oil in the rod ends of the cylinders is forced past the lift/lower valve spool to the tank.
The orifices in the pilot oil lines between the lift/lower pilot control valve and the lift/lower valve spool causes a restriction to the flow of pilot oil and provides better operator control of the lift/lower valve spool.
928G IMPLEMENT HYDRAULIC SYSTEM IN THE LOWER POSITION
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| Illustration 17 | g01064959 |
This schematic shows the conditions in the implement hydraulic system with the lift/lower control lever in the LOWER position.
When the operator moves the lift/lower control lever to the LOWER position, pilot oil flows past the lift/lower pilot control valve to the end of the lift/lower valve spool in the main implement control valve. Pilot oil moves the lift/lower valve spool to the LOWER position. Oil from the opposite end of the lift/lower valve spool flows past the lift/lower pilot control valve to the tank.
Movement of the lift/lower valve spool to the LOWER position blocks the flow of supply oil to the tank. Supply oil is directed to the rod ends of the lift cylinders. Oil in the head ends of the cylinders is forced past the lift/lower valve spool to the tank.
The orifices in the pilot oil lines between the lift/lower pilot control valve and the lift/lower valve spool causes a restriction to the flow of pilot oil and provides better operator control of the lift/lower valve spool.
928G IMPLEMENT HYDRAULIC SYSTEM IN THE FLOAT POSITION
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| Illustration 18 | g01064960 |
In this schematic, the lift/lower control lever has been moved to the FLOAT position.
When the operator moves the lift/lower control lever to the FLOAT position, pilot oil flows past the lift/lower pilot control valve to the end of the lift/lower valve spool in the main implement control valve. Pilot oil moves the lift/lower valve spool to the FLOAT position. Oil from the opposite end of the lift/lower valve spool flows past the lift/lower pilot control valve to the tank.
Movement of the lift/lower valve spool to the FLOAT position blocks the flow of supply oil to the tank. Supply oil is directed to the rod ends of the lift cylinders and to the tank. Oil in the head ends of the cylinders flows past the lift/lower valve spool to the tank.
When the machine is moved and the implement is in float, the bucket will follow the curvature of the ground. The slight pressure in the rod ends of the lift/lower cylinders will not force the bucket into the ground.