Basic Hydraulic Training

Understand the principals of hydraulic control

 

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Last Updated
2014-07-04

 


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This section should give you a basic understanding of hydraulics.

You can experiment with different loads and areas.

See how the flow through and orifice effects pressure.

Understand how pressure on spool areas is used to control valves.

Experiment with different fundamental hydraulic concepts

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Cylinder area and mass effect pressure

1 Adjust the pump supply pressure until you have balanced the cylinder at mid-stroke.

2 Use the grey arrow buttons to adjust the mass on the cylinder and then observe how this effects the pressure.

3 Now adjust the cylinder area with the yellow arrow buttons and again observe how this changes the pressure.

4 Actual mass and area values can also be entered directly into the formula boxes to calculate the pressure generated.

Constant pressure over an orifice (click FWD arrow)

11 The flow through an orifice is dependent on its area and the pressure drop across it.

12 Adjusting the pump supply pressure changes the pressure dropped and therefore the flow and cylinder speed.

13 Increasing the load pressure reduces the pressure dropped across the orifice and therefore lowers the flow to the cylinder.

14 With a constant supply pressure and load, the pressure drop across the orifice will remain the same. Reducing the orifice area (by clicking the arrow buttons above it) will reduce the flow through the orifice and lower the cylinder speed.

Calculate pressure drop across an orifice (click FWD arrow)

21 With a constant flow supply the pressure drop can be calculated from the size of the orifice.

22 Changing the orifice size will change the pressure dropped across it.

23 The supply pressure is dependent on the load pressure plus the orifice pressure drop.

24 You can re-set the cylinder to its lower position by clicking on the flashing button.

Non return check valves

31 A check valve will only allow flow in one direction.

32 Raise the cylinder to its mid-position and then lower the supply pressure to see how the check valve works.

33 Change the load on the cylinder and observe how the line pressure, and the pressure on top of the check valve poppet, change.

34 Poppet style check valves should hold the load with only a very small leakage of typically less that 1 drop per minute.

35 The spring will ensure that the poppet closes positively and is often used to provide a small up stream pressure.

Pilot operated PO relief valve

41 To test the pilot operated relief valve first balance the cylinder to its mid-position by adjusting the pump supply pressure.

42 Lower the pilot relief setting until the small ball valve opens.

43 The small flow across the pilot generates a pressure difference across the control orifice in the main spool. This opens the main valve against the spring to relieve the supply line pressure to tank.

44 The pressure drop across the main spool is normally only about to 3-4 bar.

45 Although the main relief valve section has a poppet nose, leakage can still occur between the spool section and the tank line. This valve cannot therefore be used to hold a static load for any period of time.

Pressure reducing valve

51 To test the pressure reducing valve first apply a load to the cylinder and increase the supply pressure.

52 Lower the pressure reducing valve spring setting until the ball valve opens.

53 The pilot flow across the main spool's orifice will open the valve against the spring.

54 As the spool opens the supply line is restricted and therefore the pressure in the cylinder line is maintained at the reducing valve setting even though the main inlet supply pressure is higher.

Pilot to open logic valve

61 A pilot to open, pilot to close logic valve can be used to isolate a hydraulic pipe line.

62 When the pilot vents the top of the spool to tank a 'pilot to open' logic valve will not allow flow in either direction.

63 When pilot pressure is applied to the spool section the poppet valve opens, allowing flow in both directions.

64 Typically the pilot ratio for logic valves is approximately 2:1 so pilot pressure must be at least half the load pressure to operate the valve.

65 The seal on the main spool section will ensure that the valve leakage is less than 5 drops per minute.

Pilot to close logic valve

71 A pilot to close logic valve is also used to isolate hydraulic pipe lines.

72 The choice between pilot to open or pilot to close is normally made by considering the safest failure condition.

73 When the top of a 'pilot to close' logic valve is vented to tank the cylinder is free to move in either direction.

74 When pilot pressure is applied to the top of the valve the logic element is closed and no flow is possible.

75 Typically, the pilot ratio for logic valves is about 2:1 so pilot pressure must be at least half the load pressure to operate the valve.

Balanced pilot to close logic valve

81 In the balanced logic design the poppet areas at the working ports are effectively equal

82 The pilot pressure only acts against a spring force equivalent to approximately 14 bar and is independent of the load pressure.

83 The pilot pressure acts only against the spring which allows the valve to work independent of load.

84 With this design the load pressure may exceed the pilot pressure.

Experiment with different component layouts

91 Use the sequence arrow keys to experiment with different circuit designs.

 


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