Check Valves Check Valves are the simplest form of directional control valves, but they can also be used as pressure controls.

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1 Check Valves Check Valves are the simplest form of directional control valves, but they can also be used as pressure controls. Standard Check Valve The standard check valve permits free flow in one direction but blocks flow in the reverse direction. Although not normally shown on the print, the valve does have a 1-3 PSI spring inside to hold it closed when the hydraulics are turned off. Standard check valves are made in an inline and a right angle design. The inline check valve is used for systems where the flow does not exceed 100 GPM. The right angle check valve is used in systems up to 300 GPM. Basic Hydraulic Troubleshooting Page 3-27

2 Example No. 1 Pump Isolation A well designed hydraulic circuit will incorporate a check valve immediately downstream from the hydraulic pump. The check valve will prevent system pressure spikes from damaging the pump. In cases where the pump is located below the cylinders or motors, the check valve will maintain oil in the lines. When used with an accumulator and the pump is turned off, the check valve will prevent the fluid in the accumulator from rotating the pump backwards. (Circuits with accumulators should have a method of dumping the accumulator when the pump is turned off. See the accumulator section). Example No. 2 Motor Protection The check valve used in the circuit shown keeps fluid supplied to the motor inlet when the directional valve is de-energized and the motor coasts to a stop. If this check valve were not in the line, no oil would be directed to the inlet port. The motor would then run dry due to lack of lubrication. The motor would fail pre-maturely. Page 3-28 Basic Hydraulic Troubleshooting

3 Spring Loaded Check Valves The spring loaded check valve operates just as the standard check valve does in allowing free flow in one direction and blocking flow in the opposite direction. Because of the spring rating, the pressure must build to 65 PSI before the oil can free flow through the check valve. The valve is normally used as a pressure control when used with a spring pressure above 3 PSI. Check valves that look alike may be different because of the spring rating. The part number should be checked closely to verify that the valve that s being replaced is the same as the original. Example No. 1 Relief Valve A common use of a spring loaded check valve is to protect a cooler. When the cooler plugs up the pump flow will bypass through the check valve. Damage to the cooler is prevented. Basic Hydraulic Troubleshooting Page 3-29

4 Example No. 2 Tank Line Check Valve The purpose of this check valve is to keep fluid in the lines when the directional valve is de-energized. If air is in the line, condensate will form when the system cools down. The oil could also gravity flow out of the hydraulic motor if this check valve were not used. The next time the motor is driven there would be air in the motor possibly causing motor damage. Pilot Operated Check Valves Pilot to Open The most common pilot operated check valve is the pilot to open. The valve operates the same as a standard check valve when there is no pilot pressure. That is, free flow is permitted in one direction but blocked in the other. Page 3-30 Basic Hydraulic Troubleshooting

5 When pilot pressure is applied to the pilot port, reverse flow is allowed through the valve. The most common application of the pilot to open check valve is on suspended loads. A standard closed center directional valve will allow a cylinder to drift down because of leakage between the valve spool and housing. The pilot operated check valve has little or no leakage. Float center valves are normally used with the Pilot Operated check valve. This allows the check valve to close and hold the load by draining any pressure in the pilot line back to tank. Basic Hydraulic Troubleshooting Page 3-31

6 When the directional valve is in the de-energized condition (Figure 1) the pilot operated check valve locks fluid in the rod side of the cylinder. Any pressure in the pilot line is bled back to tank through the directional valve. To lower the cylinder, (Figure 2) the directional valve is energized in the B position. Pressure is directed to the full piston side of the cylinder. At the same time pilot pressure builds at the bottom of the pilot piston. The area that the pilot pressure acts upon is much larger than the area on top of the check valve. The difference in area is usually 3:1. This is known as the pilot ratio. Originally there was 900 PSI acting downward on top of the check valve. Figure 1 Let s say for example the area that the 900 PSI is acting upon is 1 square inch. We can find the force holding the valve closed as follows: Force = PSI x Area Force = 900 x 1 square inch Force = 900 lbs. Page 3-32 Basic Hydraulic Troubleshooting

7 With 900 lbs. of force holding the valve closed, then slightly more than 900 lbs. of force is required to shift the check valve open for reverse flow. If the square inch area of the pilot piston is 3 square inches, we can find the pressure required to open the check valve. PSI = Force Area PSI = 900 lbs. 3 sq. in. PSI = 300 With a 3:1 pilot ratio, slightly more than 300 PSI will shift the valve open. Figure 2 Basic Hydraulic Troubleshooting Page 3-33

8 To raise the cylinder (Figure 3) the directional valve is energized in the A position. The oil then free flows through the check valve and into the rod side of the cylinder, raising the load. Figure 3 Page 3-34 Basic Hydraulic Troubleshooting

9 Pilot to Close The pilot to close valve is a normally open valve. With the hydraulic system off, the spring holds the valve open. When the system is turned on, the oil can flow in either direction. When pilot pressure is applied to the pilot port the valve shifts to the closed position. Flow is now blocked in both directions. The most common use of the pilot to close check valve is an accumulator dump valve. The application is demonstrated in the accumulator section of this manual. Check Valve Failures Check valves will normally fail in the open position. Contamination can lodge between the ball and seat permitting reverse flow through the valves. On a standard and pilot operated check valve, removing it from the system and visually inspecting it is the best test. Care should be taken that the system pressure is bled down before removing the valve. On pilot to open check valve applications, the load should be fully lowered before removing from the system. Basic Hydraulic Troubleshooting Page 3-35

10 Since spring loaded check valves are normally used for pressure control, the gauge pressure can be checked at the inlet of the valve. The first question that should be answered is whether or not there should be flow through the valve. Most spring loaded check valves have their spring pressure indicated in the part number. 5, 25, and 65 PSI are common spring pressures. In the example of a check valve used as a relief valve (page 3-29), the gauge pressure can be checked and the outlet of the valve can be felt for heat. If the outlet is warm and the gauge pressure is below the spring rating, the valve is stuck open. Logic Valves Logic valves, also called cartridge valves, have become more popular in recent years. The advantage of logic valves is that they are usually mounted inside a manifold, reducing piping, fittings, and leaks. The disadvantage to a logic valve is that because it is mounted in a manifold it is difficult to isolate a single valve for troubleshooting. Although logic valves can be used for pressure and flow control, their most common use is directional control applications. The square inch area on top of the valve piston is equal to the square inch area on the bottom of the piston. For example, if the area on top of the piston is 3 square inches, then the total area on the bottom of the piston is also 3 square inches. The spring acting downward to hold the valve closed is usually less than 100 PSI. Page 3-36 Basic Hydraulic Troubleshooting

11 As long as the hydraulic force acting upward is greater than the hydraulic and spring forces acting downward, oil will flow through the valve in either direction. When the total force pushing downward is higher than the upward force, the valve shifts closed. Flow is then blocked through the valve in both directions. A small directional valve is normally used to control the opening and closing of the valve. The main cause of failure in logic valves is contamination lodging between the piston and seat. Failure of the seals on the outer sleeve can also cause the valve to malfunction. When used with directional valves as the following examples show, problems with the directional valve sticking or bypassing can cause the logic valve to fail open or closed. Example No. 1 In the example, the logic valve is used to lower a vertical load. When the directional valve is de-energized pressure is ported to the top of the logic valve piston. With an area of 3 square inches on top of the piston and a 10 PSI spring we can find the force holding the valve closed. Hydraulic Force = PSI x Area Hydraulic Force = 1000 PSI x 3 sq. in. Hydraulic Force = 3000 lbs lbs lb. spring = 3,010 lbs. The hydraulic force trying to open the valve is acting on an area of 1 square inch. The force trying to open the valve is: Force = 1000 PSI x 1 square inch Force = 1000 lbs. With 3010 lbs. of force exerting downward on the piston and only 1000 lbs. acting upward, the valve remains closed. Basic Hydraulic Troubleshooting Page 3-37

12 To lower the load, the solenoid energizes and shifts the directional valve spool. Pressure from the system is now blocked to the top of the piston. Pilot pressure is ported from the top of the piston back to tank through the directional valve. The fluid from the cylinder now only has to overcome the spring force (10 PSI) to shift the logic valve open. Oil flows out of the cylinder and back to tank through the logic valve. Example No. 2 With the solenoid de-energized, flow from the pump is blocked to the top of the valve piston. At the same time any pressure on top of the piston is vented to tank. The logic valve is open allowing flow from the pump to the hydraulic motor. When the solenoid energizes, pilot pressure from the pump is directed to the top of the valve piston. Initially the same pressure is on the bottom of the logic valve piston. Because of the added force of the spring pushing down, the valve shifts closed. Flow to the motor is now blocked. Page 3-38 Basic Hydraulic Troubleshooting