1. TSM363 Fluid Power Systems Valves Tony Grift
Dept. of Agricultural & Biological Engineering University of Illinois

2. Agenda Directional Control Valves (DCV’s) Flow Control Valves (FCV’s)
Check valves (non-return valves)
Two way valves
Three way valves
Four way valves
Actuation methods
Center conditions
Flow Control Valves (FCV’s)
Throttle
Pressure Compensated Flow Control Valve
Orifice equation derivation
Turbulence prevention
Metering methods
Pressure control valves
Pressure relief valve (PRV)
Direct acting, Pilot operated
Pressure control valve
Sequence valve
Unloading valve
Counterbalance valve

3. The valve family tree has three main branches
PRV
FCV
DCV

4. Video 10: Valves (3:12)
Power control section makes sure the system function as designed: Interface between power supply and drive unit
Valves open and close flow paths
Control direction (DCV)
Control flow rate and pressure (PRV)
Types
Poppet valves (tight sealing 2 and 3 way valves)
Sliding valves (spool type) do not seal tightly, only up to 315 bar
Slide valve open/close multiple flow paths at once
Actuation by
lever
foot pedal
mechanically
spring return
electrically (solenoid valves)

5. Directional Control Valves
Check
Two way
Three way
Four Way
Proportional (not discussed)

6. JIC Directional Valve Envelope
Each block represents a position. The center condition is usually drawn with port connections.
A, B : Cylinder/motor connections
P : Pressure port
T : Tank port

7. Numbering system 4-3 means 4 ports, 3 positions
What is the numbering of this valve ?

8. Check Valve
2-1 Valve

9. Check valves in parallel

11. Check valves in series

12. Video 12: Non-return valves (Check Valves), (1:59)
Permit flow in only one direction
Constructed as poppet valves (tight sealing, no leakage)
Can hold a load in position (platform lift)
Unlocking function releases the load

13. 2-2 Valve, normally closed
Two Way Valves
2-2 Valve, normally closed

14. Three-way Valves
3-2 Valve
normally open

15. Four Way Valves A four-way valve pressurizes and exhausts two ports.
It can have more than four actual port connections for supply, exhaust, and two outlet ports.

16. 4-3 Directional Control Valve

17. Actuation Methods

18. Some examples of symbols
Push buttton, spring return
Push buttton, spring return,
detented
Cam operated, spring return
Solenoid operated, spring return

19. Some examples of symbols
Hydraulically operated, spring return
Hydraulic over electric, spring return
Hydraulic over electric with
manual override , spring return

20. Center conditions Closed center Tandem center Floating center
Open center

21. Exercise
Modify the design of this basic circuit with a solenoid activated DCV so that the motor can turn in both directions and normally is stopped.

22. Video 11: Directional Control Valves (10:39)
Valves control flow path of the hydraulic oil
Numbering system
4-3 means four ports and 3 switching positions
3-2 valve used for single acting plunger (ram) cylinder
4-3 valves used for double acting cylinders
Positive piston overlap: all ports blocked in idle
Negative piston overlap: all ports connected in idle
Positive and negative in one valve
P-T connected valve, no pump pressure buildup
Differential circuit (regenerative) make extension and retraction speeds the same

23. Flow Control Valves

24. Types of flow control Throttle valves
Flow depends on pressure drop across valve
Pressure Compensated Flow Control Valves
Maintains a constant flow, independent of pressure drop
Temperature compensation

25. 2 way Flow control valve

26. Video 6:Bernoulli’s Law & Cavitation (2:10)
Laminar flow if velocity is low
Higher flow velocity > lower pressure (Bernoulli’s law)
Pressure can drop so low it creates bubbles: This can cause Cavitation or even spontaneous combustion> premature aging of the oil
Proper dimensioning can alleviate these effects

27. A pressure drop (across an orifice)only exists if there is flow: No flow-> No Pressure drop

28. If there is flow, there will be a pressure drop across the orifice

29. Pressure drop across the orifice is proportional to the flow rate

30. Orifice Equation

31. Flow Through an Orifice:
Laminar Flow Through an Orifice
Turbulent Flow Through an Orifice

32. Turbulence in hydraulics to be avoided at all times

33. Turbulent and laminar flow

34. Rule of thumb

35. Homework Pressure Compensated Flow Control valve

36. Methods of Flow Control
Meter-In
By metering the fluid supplied to the actuator
Meter Out
By metering the fluid returned from the actuator
Bleed-off
By bleeding the excess fluid off to the reservoir

37. Meter-In Circuit Meter-in (Fig 11-1,2,3)
Meter the total amount of fluid in a circuit.
Can’t be used for overrunning loads.
Pump operates at internal PRV setting.

38. Meter-In Circuit

39. Meter In Circuits Do Not Control Runaway (overrunning) Loads

40. Locations for Meter-In Applications

41. Meter-Out Circuit Meter-out (Fig 11-5)
Meter the total return flow to the tank.
Can be used with overrrunning loads.
Pump operates at internal PRV setting.
Note that the output flow of a double acting cylinder <> input flow.

42. Locations for Meter-Out Applications

43. Pressure Intensification in Meter-Out Circuits
Pressure intensification (Fig 11-7)
Pressures are higher at the outlet side due to difference in area between the rod and cap end.

44. Load pressure adds to intensification pressure in meter-out circuits

45. Bleed-off circuit Bleed off circuit(Fig 11-9)
Pump operates at pressure required to move the load.
Rule of thumb: do not bleed off more than 50% to tank.

46. Locations for bleed-off applications

47. Video 14: Flow Control Valves (4:23)
FCV’s control the speed of an actuator
Needle valves
Reducing flow to actuator> rest through PRV pressure!
Load reduces the advance speed
True Flow Control valves are pressure compensated
Load does not affect the advance speed
Two ways to control speed
Meter-in (into cylinder) heats up oil
Meter-out (out of cylinder) allows heating up oil. However the pressure is much higher here

48. Pressure Control Valves

49. Pressure control valves
Pressure Relief, Pressure Reducing Valves
To limit or reduce pressure
Sequence Valves
To sequence hydraulic operations
Unloading Valves
To unload the pump during idle cycles
Counterbalance
To prevent a load from free falling

50. Video 13: Pressure Control Valves (4:24)
Pressure relief valve (PRV) limits pressure at the pump, every hydraulic systems has one
Pilot operated (much closer to ideal)
Direct acting
Dynamic situation (when actuator is moving)
Static situation is when the cylinder is stalled
Here the system reaches its highest energy demand!
Pressure Control Valve (regulator)
Controls pressure downstream
Downstream sensing
Normally open valve

51. Pressure Relief Valve (PRV)
To limit the maximum pressure at the system input
Ideal (does not exist), Direct Acting or Pilot Operated

52. A Pressure Relief Valve (PRV) provides overload protection
In the symbol there are
Main pressure lines (solid)
Sense lines (dashed)
Spring return
Adjust arrow
Direction arrow
Pressure and Tank connections

53. Some questions about a PRV
Is this valve normal open or normal closed
Closed / open, think of a door
Where is the pressure sensed, upstream or downstream and why ?
What is the pressure at the tank port ?
Why is a PRV adjustable ?

54. Direct Acting Pressure Relief
To Reservoir
System Pressure

55. Direct Acting Pressure Relief
To Reservoir
System Pressure

56. Pilot Operated Pressure Relief Valve (PRV)

57. Balanced Piston PRV: System pressure below preset pressure
The balanced piston has a land in which a small hole is drilled which acts like an orifice. The function of the orifice is to sense the primary system pressure. The poppet valve in the pilot section will remain closed as long as the system pressure ( times the projected area of the poppet in contact with the fluid) is smaller than the spring force. Note that in this situation there is no flow through the orifice which implies that there is no pressure drop. Furthermore, note that the pilot section is internally drained through the hollow stem of the piston because the dump port is always connected to tank in the Pressure Relief Valve case.

58. System pressure equals preset pressure
As soon as the system pressure ( sensed through the orifice ) times the projected area of the poppet in contact with the fluid is higher than the spring force, the poppet will open. This means that the volume above the piston land is directly connected to tank (through the hollow stem connection). Because of this pressure difference, the piston will rise and a direct connection is opened between the system pressure port and the dump port. Note that there is a small flow through the orifice since there is a pressure drop across it. It is tempting to think that the system pressure drops to zero but it does not, because in that case the poppet would close and pressure would build up again. Instead the pressure oscillates around the point where the poppet alternatingly switches between seated and unseated, in other words the system pressure fluctuates around the preset pressure. If the pressure above the piston land happens to be equal to the pressure underneath it, still the dump valve will remain closed, because of a small spring between the piston and the housing.

59. Pilot Override in a PRV Full Flow Pressure Flow through Relief Valve
Pilot Operated
Relief Valve
Cracking Pressure
Direct Acting Relief Valve
System Pressure

60. The need for unloading a pump
A closed center system in idle will dump all oil from a constant delivery pump to tank through the PRV. This is a complete waste of energy.

61. Unloading a pump
An unloading valve can directly connect the pump to tank to
save energy

62. Unloading a Pump using Remote Venting on a balanced piston PRV
Activation of this valve will force a pressure drop over the piston land and open up the valve completely. The venting can only take place at pressures lower than the preset pressure of the pilot stage, because the over pressure protection is still functional. When the valve is vented the pressure in the system drops ( virtually ) to zero, which means that the pump is unloaded and no energy is wasted. Of course the pressure in the system must be at least high enough to keep the piston lifted against the small spring force.

63. Unloading Valves (shutoff-counteracting valve)

64. Fig 10.15 Direct acting pressure control valve
Pressure Reducing Valves
Sense pressure in a secondary circuit (‘downstream’) and prevent secondary pressure to rise over preset. Note that the pressure reducing valve is normally open.
Direct Acting Pressure Reducing Valves (Fig 10-11) (Spool Type)
Basically normal open valve that uses a pressure controlled variable orifice to limit pressure. After closing a small amount of fluid is kept flowing to prevent pressure build up due to leakage. Leakage is drained to tank separately.

65. Fig 10.16 Direct acting pressure control circuit

66. Sequence Valves
Control the order of operations in branch circuits

67. Sequence valve using balanced piston concept
The sequence valve is quite similar to the PRV. The only difference is the fact that the pilot stage needs to be externally drained because the pressure in the secondary system is not necessarily zero as was the case in the PRV where the dump port was always connected to tank. The function is to allow system pressure from a primary source (A) to enter the secondary system (B) only if the system pressure is higher than a certain preset value. The primary pressure is again sensed through the orifice in the land and as soon as this pressure (times the projected area of the poppet in contact with the fluid) exceeds the spring force the poppet will unseat. This enforces a pressure drop above the piston land and fluid will flow from the primary to the secondary circuit without obstruction. The system pressure can exceed the preset pressure of the pilot stage because there can be a pressure buildup underneath the piston land, whereas in the PRV case this pressure was always zero. In practical applications the stem of the piston is still hollow, however the fluid will never take this path since there is a shortcut to tank through the pilot stage.

68. First Clamp, then Drill Circuit

69. Counterbalance Valves
Optional Remote Pilot
Direct Pilot
Unloading valve is set to 10% above the system pressure
required to hold the load.

70. Counterbalance Valves
300 cm2
Direct Pilot
150 cm2
90 kN
Determine the unloading valve setting
required to hold the load?

71. Answer Pressure required to hold the load:
Set the unloading valve for a 10% higher pressure:

72. Multi purpose Valve One valve that can be configured as
Direct acting PRV
Sequence valve
Unloading valve
Counterbalance valve

73. Direct acting PRV Sensing? Int. / Ext. Drain? External control?
Up/downstream
Int. / Ext. Drain?
External control?
The back pressure valve is in fact a direct acting PRV ( Pressure relief valve) because the secondary circuit is connected to tank. The pressure is sensed upstream, and when the pressure on the small piston (divided by the area) is higher than the required spring force, flow will be diverted to the tank preventing further pressure build up. The pressure drop from the primary to the secondary circuit is constant and determined by the spring setting. The channel drilled through the center of the spool prevents pressure build up caused by leakage past the (small) piston.

74. Unloading valve Sensing? Int. / Ext. Drain? External control?
Sensing?
Up/downstream
Int. / Ext. Drain?
External control?
The unloading valve differs from the back pressure (PRV) valve, in that the pressure is not sensed in the secondary circuit, but externally. It can also be characterized as a remotely operated direct acting pressure relief valve. In normal mode, there is free flow from the input to the output, and as soon as the external pressure rises high enough to overcome the spring force, the valve starts dumping to tank.

75. Sequence valve Sensing? Int. / Ext. Drain? External control?
Sensing?
Up/downstream
Int. / Ext. Drain?
External control?
In a sequence valve, there must be a preset pressure available in the primary circuit, in order to have a pressure in the secondary circuit. The classical example is a piece of material that needs to be clamped before a hole can be drilled. The pressure is sensed in the secondary circuit and the drain is external since neither one of the ports is connected to tank.

76. Sequence valve with integral check (free flow return)
When free flow is required, the sequence valve can be fitted with a check valve that is connected between the primary and the secondary circuit. Note again that the drain is external (see symbol).  

77. Counter balance valve Sensing? Int. / Ext. Drain? External control?
Sensing?
Up/downstream
Int. / Ext. Drain?
External control?
A counter balance valve is used in case of overrunning loads such as a cylinder load falling by gravity or a vehicle with a hydrostatic transmission driving downhill. In the normal situation there must be free flow, therefore a check valve connects the reverse flow circuit. In the forward flow situation, the pressure is controlled as in sequence valve. Note that there are only two connections, there is no drain line connected, the valve merely acts as a variable orifice type pressure regulator.

78. Which one is which ?
Counter Balance Sequence Unloading PRV

79. TSM363 Fluid Power Systems Valves The End
Dept. of Agricultural & Biological Engineering University of Illinois