1. 2005/2006 I. Hydraulic and Pneumatic Systems1 Pressure reducing valves Task: They have the task to provide a constant secondary pressure p 2, independently of the primary pressure p 1 and of the flow rate Q. Applied for example if a pump delivers fluid to two different partial systems with two different pressures. The reduced pressure is often used for auxiliary functions such as governing. The pressure reducing valves are directly operated, pilot operated or three-way valves.

2. 2005/2006 I. Hydraulic and Pneumatic Systems2 Pressure reducing valve Directly operated: Pilot operated: similar but we shall not deal with it. In the neutral position the valve is fully open. If p 1 rises and exceeds the spring force then the valve starts to close. There is a balance so that the valve opening is exactly as large that the spring force holds balance with the wanted p 2 pressure. The loss power is (p 1 -p 2 )Q so that it should not operate very long. C p1p1 p2p2

3. 2005/2006 I. Hydraulic and Pneumatic Systems3 Other pressure control valves Pressure difference valves: Has the task to hold the pressure difference between inlet and outlet independently of the pressures and the flow rate. Similar to the pressure reducing valve, except that the spring side is not connected to the discharge but to one of the working pressures. Pressure ratio valves: Have the task to keep the pressure ratio between inlet and outlet constant, independently of the pressures and the flow rate. They have no spring, only a piston with different surface area on both sides. Is seldom applied.

4. 2005/2006 I. Hydraulic and Pneumatic Systems4 Flow valves Basics: The velocity of hydraulic cylinders can be influenced by the flow rate. This can be done by an adjustable pump or by a flow rate control valve. The flow rate is changed by changing an orifice surface area or by changing the pressure difference on a resistance or dividing the flow rate. Advantages of flow valves are:  simplicity  safe operation  good dynamic behaviour  precision Disadvantage:  the relatively high energy losses Types: - Restrictor valves (throttle valves) - Flow control valves - Flow dividing valves

5. 2005/2006 I. Hydraulic and Pneumatic Systems5 Flow valves Restrictor valves: These are constant or variable flow rate resistances. If it is variable then the restriction area is varied. As far as it is possible the resistance should be independent of the Reynolds number. This is easier to do with orifice-type restrictors. (d >> l) Most common restrictor forms are: Circle Rectangle Triangle Ring

6. 2005/2006 I. Hydraulic and Pneumatic Systems6 Flow valves Some aspects of a good cross section: Restrictor valves can be:  Compact (not sensitive to contamination), best is circle  Progressive growth of surface area, especially for small cross-sections (triangle)  Simple variability of the cross section (all, except circle)  in series before (bad because of cavitation and heat),  in series after  parallel to the actuator Restrictor valves have a bad efficiency. Should be used only for small powers and short times.

7. 2005/2006 I. Hydraulic and Pneumatic Systems7 Flow valves Flow control valves: They consist of a pressure difference valve combined with a restriction valve. The pressure difference valve holds the pressure drop constant over the restriction valve so that the flow rate depends only on the effective flow area or the valve displacement x. The movement of the actuator becomes independent of the load. They can be either in series or parallel. Parallel construction has a better efficiency. a) Series – two-way valveb) Parallel – three-way valve

8. 2005/2006 I. Hydraulic and Pneumatic Systems8 Flow valves Flow dividing valves: They divide a flow in two parts with a previously given ratio, independently of the two user pressures. This ensures a controlled speed of two actuators independently of the load. Over the flow restriction 1 and 2 there is a pressure drop  p. In equilibrium p 1 =p 2, the valve is in the middle. If p 3 is increased, e.g. because of a higher load, then the flow rate Q 1 will decrease.  p 1 increases  valve is displaced to the right  Q 1 increases until p 1 =p 2 again. The openings 3 and 4 are amplified here to a great extent. Other dividing ratios can be created by asymmetry. Q Q2Q2 Q1Q1 p2p2 p1p1 Δp = p-p 1 = p-p 2 p p3p3 p4p4 1 2 4 3 This is a 1:1 flow dividing valve.

9. 2005/2006 I. Hydraulic and Pneumatic Systems9 Flow valves Flow dividing valves: Dynamic behaviour of the valves is difficult. There are problems at start. There are errors due to the compressibility of the fluid and of the pipes. Between flow dividing valve and user there should be as small fluid volume as possible and there should not be any valves with leakage. Check valves: They have the task to allow the flow in one direction only. In the other direction the flow is prevented from flowing usually by a conical or spherical element (poppet valves). There are also spool valves but they do not seal so well. The element is pressed to the seat by a spring force. Usually needs metallic surfaces with very accurate machining. Valve seats with elastic surface did not prove to be very good.

10. 2005/2006 I. Hydraulic and Pneumatic Systems10 Check valves Pilot-operated check valves: With an outside pressure the valve can be opened again. There is a piston with a larger surface which is operated with a lower pressure than the closing pressure the valve can be opened. Main application: holding of loads and when desired, sinking. Pilot operated check valve without drain port Pilot operated check valve with drain port

11. 2005/2006 I. Hydraulic and Pneumatic Systems11 Check valves Sandwich type check valve: Model Z2S Main properties: Nominal size6…25 Maximum permissible flow 300 l/min System pressure315 bar Cracking pressure1,5; 3; 7,5; 10 bar (NG 6 and 10) 2,5; 5; 7,5; 10 bar (NG 16 and 25) Pilot operated check valve Main properties: Nominal size6…150 Maximum permissible flow 6400 l/min System pressure315 bar Typical circuit

12. 2005/2006 I. Hydraulic and Pneumatic Systems12 Check valves Applications: This is a simple circuit. If the sinking function is switched (pilot- operated check valve opens) then the pressure breaks down in the pumping line so in the control line too. The check valve closes again and the procedure starts again. The pressure builds up and it sinks again. For continuous sinking there must be some adjustable restriction valves for braking and ensuring constant speed. P T A B x

13. 2005/2006 I. Hydraulic and Pneumatic Systems13 Check valves Applications: Rectifier (Graetz circuit) Rectifier sandwich plate, model Z4S Application of the model SV pilot operated check valve Application of the model SL pilot operated check valve, the port A is, e.g. with a throttle- check valve pre-stressed

14. 2005/2006 I. Hydraulic and Pneumatic Systems14 Big pictures End of normal presentation Beginning of big pictures

15. 2005/2006 I. Hydraulic and Pneumatic Systems15 Valves Directly operated pressure reducing valve

16. 2005/2006 I. Hydraulic and Pneumatic Systems16 Valves Directly operated pressure reducing valve

17. 2005/2006 I. Hydraulic and Pneumatic Systems17 Valves Directly operated pressure reducing valve 1 - állító elem, 2 - vezérlő vezeték, 3 - nyomórugó, 4 - vezérlő tolattyú, 5 - vezető él, 6 - rugótányér + résolaj vezeték Y, 7 - visszacsapó szelep, 8 - mérőpont 1 – adjusting element 2 – governing line 3 – spring 4 – governing spool 5 – governing edge 6 – spring plate and leakage line 7 – check valve 8 – measuring point

18. 2005/2006 I. Hydraulic and Pneumatic Systems18 Flow control valves Two-way valve

19. 2005/2006 I. Hydraulic and Pneumatic Systems19 Flow control valves Two-way valve

20. 2005/2006 I. Hydraulic and Pneumatic Systems20 Flow control valves Two-way valve

21. 2005/2006 I. Hydraulic and Pneumatic Systems21 Flow control valves Three-way valve

22. 2005/2006 I. Hydraulic and Pneumatic Systems22 Flow control valves Three-way valve

23. 2005/2006 I. Hydraulic and Pneumatic Systems23 Flow control valves Three-way valve

24. 2005/2006 I. Hydraulic and Pneumatic Systems24 Valves Check valve

25. 2005/2006 I. Hydraulic and Pneumatic Systems25 Valves Check valve

26. 2005/2006 I. Hydraulic and Pneumatic Systems26 Valves Pilot-operated check valves, Internally drained, without decompression poppet option 1 - main poppet 3 - spring 4 - piston X - pilot port Y – drain port

27. 2005/2006 I. Hydraulic and Pneumatic Systems27 Valves Pilot-operated check valves, Internally drained, with decompression poppet option 1 – main poppet 2 – decompression poppet 3 - spring 4 - piston X – operating path p St = pilot pressure p 1 = pressure on the port B p 2 = pressure on the port A A 1 = surface of the main poppet A 2 = surface of the decompression poppet A 3 = surface of the pilot piston A K = piston surface at the cylinder A R = ring surface at the cylinder F = load at the piston F F = spring force with friction larger picture

28. 2005/2006 I. Hydraulic and Pneumatic Systems28 Valves Pilot-operated check valves, Externally drained, with decompression poppet option

29. 2005/2006 I. Hydraulic and Pneumatic Systems29 Valves Pilot-operated check valves

30. 2005/2006 I. Hydraulic and Pneumatic Systems30 Valves Sandwich type check valve:

31. 2005/2006 I. Hydraulic and Pneumatic Systems31 Valves Sandwich type check valve:

32. 2005/2006 I. Hydraulic and Pneumatic Systems32 Valves Sandwich type check valve:

33. 2005/2006 I. Hydraulic and Pneumatic Systems33 Valves Sandwich type check valve:

34. 2005/2006 I. Hydraulic and Pneumatic Systems34 Valves Rectifier (Graetz circuit)

35. 2005/2006 I. Hydraulic and Pneumatic Systems35 Valves Rectifier sandwich plate, model Z4S

36. 2005/2006 I. Hydraulic and Pneumatic Systems36 Valves Application of pilot operated check valve

37. 2005/2006 I. Hydraulic and Pneumatic Systems37 Valves Application of pilot operated check valve

38. 2005/2006 I. Hydraulic and Pneumatic Systems38 Valves ???