Check Valve
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| Illustration 1 | g01063173 |
The purpose of a check valve is to permit oil flow in one direction, and to prevent oil flow in the opposite direction. The check valve is sometimes called a one-way check valve.
Most check valves consist of a spring and a tapered seat valve as in Illustration 1 above; however, a round ball is sometimes used instead of the tapered seat valve. In some circuits, the check valve may be free floating.
When the pump oil pressure overcomes the oil pressure in back of the check valve plus the check valve slight spring force, the check valve opens and allows the oil to flow to the implement. This is shown by the valve on the left.
In the valve on the right, when the pressure of the pump oil is less than the oil pressure in the implement, the check valve closes. This prevents implement oil flow back through the valve.
Pilot Operated Check Valve
The pilot operated check valve is most often used in operations where load drift is a problem. The pilot operated check valve allows load drift to be held to a very close tolerance.
The pilot operated check valve differs from the simple check valve. The difference is that the pilot operated check valve will allow oil flow through the valve in the reverse direction.
Forward Flow
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| Illustration 2 | g01063174 |
Illustration 2 shows a pilot operated check valve. The pilot operated check valve consists of a check valve, a pilot valve, and a rod. The pilot operated check valve allows free flow from the control valve to the cylinder in the same manner as a simple check valve, as shown in Illustration 2.
Flow Blocked
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| Illustration 3 | g01063177 |
When oil flow from the control valve ceases without pilot oil being applied, the check valve seats. This is shown on the right of Illustration 3. The oil flow from the cylinder to the control valve is blocked at the check valve in the same manner as a simple check valve.
Reverse Flow
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| Illustration 4 | g01063178 |
The valve that is shown in Illustration 4, shows how the oil flows from the cylinder to the control valve.
When flow is required, pilot oil is sent to the pilot valve oil chamber. Pilot oil pressure moves the pilot valve and rod to the right. The pilot oil pressure unseats the check valve. The cylinder oil flows through the check valve, to the control valve, and then to the tank.
The pressure ratio between the load pressure and the pilot pressure is designed into the valve. For example, consider a valve that has a pressure ratio of 3:1. The pressure that is needed to open the check valve is equal to one-third of the load pressure. A load pressure of 4134 kPa (600 psi) requires a pilot pressure of 1378 kPa (200 psi) to open the check valve.
Lock Check Valves
Some pilot operated check valves are called lock check valves. This function will allow oil flow in the reverse direction. The difference is that the lock check valve uses the pressure created by the load being moved rather than an independent pilot source to open the lock check valve. This allows oil to be returned from the opposite side of the actuator.
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| Illustration 5 | g01063180 |
Lock check valves are used on both cylinders and motors to ensure no movement in either direction.
Illustration 5 shows a lock check valve. The check valves on both sides of the actuator are closed or blocked. The length of the extensions on each end of the piston are shorter than the distance between the two seated check valves. Both valves are being held on their seat by the blocked oil from the actuator.
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| Illustration 6 | g01063192 |
Illustration 6 shows a lock check valve. The left side is pressurized to move the load. The flow has opened the check valve on the left side. The pressure has moved the piston and its extensions to the right, pushing the check valve on the right side open to allow reverse flow back through the check valve to the control valve.
The flow for opening the left valve is from the directional control valve. The reverse flow oil from the right valve returns through the directional control valve to the tank.
Note: Lock check valves are used in most circuits of G-Series motor graders.
Inverse Shuttle
The inverse shuttle is similar in construction to the lock check valve. The check valve on either end traps oil from flowing back into the valve. The following items are differences between the inverse shuttle and the lock check valve:
- The inverse shuttle has a single source of oil that is supplied into the center chamber.
- The center rod between the two check valves is longer than the distance between the two seated check valves. Thus, one or both of the check valves are always held open.
- The inverse check valve is always used with an accumulator in each valve outlet to ensure that the accumulators are equally charged. The inverse check valve is normally used in the brake system to ensure that equal pressure is available for front brakes and rear brakes.
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| Illustration 7 | g01063217 |
Illustration 7 shows an inverse shuttle that has equally charged accumulators. Note that the check valves are open on both ends of the valve in order to provide equal oil flow to both accumulators and the front and rear brake systems.
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| Illustration 8 | g01063218 |
Illustration 8 shows an inverse shuttle valve. The accumulator on the left end has a higher pressure than the accumulator on the right end. The pressure from the left accumulator has pushed the left check valve against the center chamber. The longer rod is holding the right check valve open. The cut-in valve is now sensing the lower accumulator pressure to actuate the cut-in function. This permits flow from the cut-in valve to flow to the right side until the pressure is equalized.
The pressure in the front and rear brake circuits can become unequal due to greater wear in the brakes of one end, unequal pre-charge of the accumulators, or leakage in one of the brake systems.
Note: Inverse shuttle valves are used in the brake systems of small and medium wheel loaders.
Check Valve ISO Symbols
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| Illustration 9 | g01063220 |
In Illustration 9, symbol (A) and symbol (B) represents the simple check valve in the OPEN position and CLOSE positions.
Symbol (C) represents the shuttle valve. The shuttle valve (resolver) senses the higher pressure (circuit 2), blocks flow to the lower pressure (circuit 1), and sends the higher pressure oil as supply oil to a third circuit (circuit 3). This will keep the two separate circuits isolated from each other and direct the higher pressure oil downstream.
Symbol (D) represents the pilot operated check valve.
Symbol (E) represents the lock check valve
Symbol ( F) represents the inverse check valve.
Makeup Valve
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| Illustration 10 | g01063221 |
The makeup valve in Illustration 10 looks similar to the check valve. The makeup valve is normally positioned in the circuit between the implement and the tank. During normal operations, the pump or cylinder oil fills the area behind the makeup valve. The pressure in the cylinder keeps the valve CLOSED. When the cylinder pressure is approximately 14 kPa (2 psi) lower than the tank pressure, the makeup valve will OPEN. The tank oil bypasses the pump and flows directly through the makeup valve to the cylinder.
The makeup valve is used to prevent cavitation. For example, when a loader bucket is RAISED and the operator moves the control to the FULLY LOWER position, the gravitational force on the bucket is transmitted through the cylinder piston to the return oil. The increased pressure on the return oil increases the flow from the cylinder. When the cylinder piston displaces the return oil faster than the pump sends the oil to displace the piston, a vacuum is formed in the cylinder and the lines. A vacuum can cause the cylinder and lines to cavitate. When the pressure in the cylinder and lines decreases to 14 kPa (2 psi) less than tank pressure, the makeup valve will open. This will allow the tank oil to flow through the makeup valve to the lines and to the cylinder. This procedure prevents cavitation in the cylinder and the line.
Makeup Valve ISO Symbol
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| Illustration 11 | g01081327 |
The operation of the makeup valve is the same operation as the check valve. Therefore, the ISO symbol for the makeup valve is the same symbol as the ISO symbol for the check valve.