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Fluid Power Valves Guide
- 1. 1 TSM363 Fluid Power Systems Valves Tony Grift Dept. of Agricultural & Biological Engineering University of Illinois
- 2. 2 Agenda • Directional Control Valves (DCV’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. 3 The valve family tree has three main branches PRESSURE FLOW DIRECTION CONTROL VALVES FCV PRV DCV
- 4. 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. 5 Directional Control Valves • Check • Two way • Three way • Four Way • Proportional (not discussed)
- 6. 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. 7 Numbering system • 4-3 means 4 ports, 3 positions • What is the numbering of this valve ?
- 9. 9 Check valves in parallel
- 11. 11 Check valves in series
- 12. 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. 13 Two Way Valves 2-2 Valve, normally closed
- 15. 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. 16 4-3 Directional Control Valve
- 18. 18 Some examples of symbols Push buttton, spring return Push buttton, spring return, detented Cam operated, spring return Solenoid operated, spring return
- 19. 19 Some examples of symbols Hydraulically operated, spring return Hydraulic over electric, spring return Hydraulic over electric with manual override , spring return
- 20. 20 Center conditions Closed center Tandem center Floating center Open center
- 21. 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. 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
- 24. 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. 25 2 way Flow control valve
- 26. 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. 27 A pressure drop (across an orifice)only exists if there is flow: No flow-> No Pressure drop
- 28. 28 If there is flow, there will be a pressure drop across the orifice
- 29. 29 Pressure drop across the orifice is proportional to the flow rate 2 o o p q C A
- 30. 30 Orifice Equation 2 o o p q C A 2 2 3 Determined experimentally 1 Pressure drop proportional to flow rate Area orifice, adjusted to control pressure and flow rate Flow rate Density of fluid (petr o o C N p m A m m q s 3 oleum based 850-950 kg m
- 31. 31 Flow Through an Orifice: Laminar Flow Through an Orifice Turbulent Flow Through an Orifice
- 32. 32 Turbulence in hydraulics to be avoided at all times
- 33. 33 Turbulent and laminar flow s Pa s m s kgm s s m s m m kg m D s m v m kg D v N * 1 viscosity Absolute pipe) of diameter (here dimension stic Characteri Fluid of Velocity fluid of Density 1 number Reynolds Re * * Re 2 2 3 3
- 35. 35 Homework Pressure Compensated Flow Control valve i p Hand knob Outlet port Inlet port Manual orifice Am Automatic orifice Aa o p x p q q
- 36. 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
- 39. 39 Meter In Circuits Do Not Control Runaway (overrunning) Loads
- 40. 40 Locations for Meter-In Applications
- 42. 42 Locations for Meter-Out Applications
- 43. 43 Pressure Intensification in Meter-Out Circuits
- 44. 44 Load pressure adds to intensification pressure in meter-out circuits
- 46. 46 Locations for bleed-off applications
- 47. 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 @ 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
- 49. 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. 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. 51 Pressure Relief Valve (PRV) • To limit the maximum pressure at the system input • Ideal (does not exist), Direct Acting or Pilot Operated
- 52. 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. 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. 54 Direct Acting Pressure Relief To Reservoir System Pressure
- 55. 55 Direct Acting Pressure Relief To Reservoir System Pressure
- 56. 56 Pilot Operated Pressure Relief Valve (PRV)
- 57. 57 Balanced Piston PRV: System pressure below preset pressure
- 58. 58 System pressure equals preset pressure
- 59. 59 System Pressure Flow through Relief Valve Direct Acting Relief Valve Pilot Operated Relief Valve Cracking Pressure Full Flow Pressure Pilot Override in a PRV
- 60. 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. 61 Unloading a pump An unloading valve can directly connect the pump to tank to save energy
- 62. 62 Unloading a Pump using Remote Venting on a balanced piston PRV
- 63. 63 Unloading Valves (shutoff-counteracting valve)
- 64. 64 Fig 10.15 Direct acting pressure control valve
- 65. 65 Fig 10.16 Direct acting pressure control circuit
- 66. 66 Sequence Valves Control the order of operations in branch circuits
- 67. 67 Sequence valve using balanced piston concept A B
- 68. 68 First Clamp, then Drill Circuit
- 69. 69 Counterbalance Valves Direct Pilot Optional Remote Pilot Unloading valve is set to 10% above the system pressure required to hold the load.
- 70. 70 Direct Pilot Determine the unloading valve setting required to hold the load? 90 kN 300 cm2 150 cm2 Counterbalance Valves
- 71. 71 Answer Pressure required to hold the load: MPa p p cm kN A F 6 2 150 90 10 10 Set the unloading valve for a 10% higher pressure: MPa 6 . 6 p
- 72. 72 Multi purpose Valve • One valve that can be configured as • Direct acting PRV • Sequence valve • Unloading valve • Counterbalance valve
- 73. 73 Direct acting PRV • Sensing? • Up/downstream • Int. / Ext. Drain? • External control?
- 74. 74 Unloading valve • Sensing? • Up/downstream • Int. / Ext. Drain? • External control?
- 75. 75 Sequence valve • Sensing? • Up/downstream • Int. / Ext. Drain? • External control?
- 76. 76 Sequence valve with integral check (free flow return)
- 77. 77 Counter balance valve • Sensing? • Up/downstream • Int. / Ext. Drain? • External control?
- 78. 78 Which one is which ? Counter Balance Sequence Unloading PRV
- 79. 79 TSM363 Fluid Power Systems Valves The End Dept. of Agricultural & Biological Engineering University of Illinois