Directional valves Classification schemes 1 Spool valves or

Directional valves Classification schemes: 1. Spool valves or poppet valves 2. Switching or continuously adjustable valves 3. Number of ports and positions 4. The kind of governing and positioning unit 2005/2006 I. Hydraulic and Pneumatic Systems 1

Directional valves 1. Spool valves or poppet valves: Spool valves Poppet valves have always leakages can be totally sealed need relatively large displacements because a positive overlap is needed for sealing need smaller displacements to let fluid through preferred in all other cases because of their flexibility and lower price preferred for large flow rates need radial and axial pressure relief, 2005/2006 I. Hydraulic and Pneumatic Systems 2

Directional valves 2. Switching or continuously adjustable valves: Directional valves work either in certain switching positions or they are continuously adjustable. Latter by electrohydraulic or electric governing units. 2005/2006 I. Hydraulic and Pneumatic Systems 3

Directional valves 3. Number of ports and positions: Basic symbols Two positions Three positions 4/3 valve with ports, operation positions and governing elements 2005/2006 I. Hydraulic and Pneumatic Systems 4

Directional valves 4. Kind of governing and positioning unit: By hand, electrically or electrohydraulically Cross-section: Symbol: 2005/2006 I. Hydraulic and Pneumatic Systems 5

Electrically governed continuously adjustable valves § § Servo valves Proportional valves Servo valves: They are able to govern a very large output power with a very small input power. Large masses have to be moved in a short time against varying forces to precise positions. These valves are very expensive because they have to be manufactured with very high accuracy. Because of this, their operation is also expensive, because the fluid has to be very well filtered. They have usually two (for very large flow rates three) amplifying stages. 2005/2006 I. Hydraulic and Pneumatic Systems 6

Servo valves Double nozzle – impinging plate (bridge circuit): s 0 -s s Q 1 QU Q 01 p 1 s 0+s Q 2 User p 0 Linear part in the middle QU p 2 Q 01 0, 8 -1 1 -0, 8 2005/2006 I. Hydraulic and Pneumatic Systems 7

Servo valves Operation: If there is no current, there is no displacement, the valve body is kept in the middle. If there is current, the iron core is displaced and the impinging plate that is rigidly coupled with it is also displaced and a pressure drop is created. Note that there is a small negative overlap, the manufacturing must be very good. There must always be a flow. Disadvantage: it is difficult to hold the zero position, because of dry friction or contamination. Solution: There also three-stage servo valves. They are used for very large powers. 2005/2006 I. Hydraulic and Pneumatic Systems 8

Continuously adjustable valves Proportional valves: Continuously adjustable proportional magnets are able to adjust continuously and precisely the position of a directional valve. The magnetic force is proportional to the current and nearly independent of the position. The core reaches out far from the coil. They might be connected with a position control, where hysteresis effects are smaller. They are often used for flow rate control but all pressure valves can work with a proportional magnet instead of a spring. They are more precise. 2005/2006 I. Hydraulic and Pneumatic Systems 9

Accessories Liquid reservoir: Tasks: v Storing of necessary liquid volume v Conducting heat away from the system v De-airing of the liquid (large surface, long stay of the liquid) v Separation of condensed water v Carrying element of other system components (pumps, valves, pipes, etc. ) Pipes: Steel of flexible hoses. Key point: sealing 2005/2006 I. Hydraulic and Pneumatic Systems 10

Accessories Filter: • Surface filter • Volume filter – an order of magnitude better but not cleanable It can be before the pump, after the pump, before every sensitive elements or in the back flow line. 2005/2006 I. Hydraulic and Pneumatic Systems 11

System concepts Delivering of liquid by: v Flow sources v Pressure sources In case of flow sources normally for each actuator a separate source. One pressure source can deliver several actuators parallel. Pressure source should not be controlled by restriction valves because the losses will be then too high. * Better is to control by adjusting the displacement volume of the actuator. It can happen with a hydromotor easily but for a hydraulic cylinder one needs a hydraulic transformer. This is called secondary control. * → exp. on next slide 2005/2006 I. Hydraulic and Pneumatic Systems 12

System concepts Cont. * Basically there are two main subgroups: 1. The pressure is always constant In this case the main task is to find a control system to change the flow rate for the oscillating flow rate needs of the actuators. Various control mechanisms can be used here 2. The pressure is not constant - Secondary control (see previous page) - Load sensing control (much more complicated) 2005/2006 I. Hydraulic and Pneumatic Systems 13

System concepts Constant pressure sources: - Pump with pressure relief valve: not very effective – lot of lost energy. - The displacement volume or the rotational speed of a pump can be controlled by measuring the pressure. In fact flow rate control but indirectly pressure control. Position drives: VDr = Restriction valve VW = direction valve - All three versions have a 4/3 valve - All three versions have two restriction valves, one constant and one adjustable The constant valve is for safety: it limits the maximum possible flow rate through the system and prevents a breakdown of the pressure. These valves are all between the actuator and the reservoir. This has the advantage that the actuator is between two liquid columns that are both under pressure. 2005/2006 I. Hydraulic and Pneumatic Systems 14

System concepts Drives 2005/2006 I. Hydraulic and Pneumatic Systems 15

Constant pressure sources Position drives: With the adjustable resistance the velocity can be controlled: a) Is a simple circuit. Problem is drifting in the holding position since the spool valve has a leakage. b) Solves this problem by putting a 2/2 directional valve before the 4/3 valve. This seals perfectly. In addition, a double check valve prevents the cylinder from moving. c) Is even better. It allows a very accurate positioning. The directional valve 1 has the job to switch between a creeping mode and a normal speed mode. From the creeping mode it is easy to stop accurately. Hold: 1 Normal speed: 2 Creeping speed: 0 2005/2006 I. Hydraulic and Pneumatic Systems 16

Constant pressure sources Velocity drives: VW = directional valve EV = fast forward ER = fast backward Directional valve positions AV = working forward AR = working backward VSZ = flow control valve VDr = restriction valve Simpler circuits can be derived. If for example no working backward mode is needed then VW 4 and VSZ 1 can be omitted. If the working velocity is so small that the flow rate is around the leakage flow rate, large errors occur. In this case spool valves may not be applied, instead pilot operated check valves. 2005/2006 I. Hydraulic and Pneumatic Systems 17

Constant pressure sources Force drives: Holding of work pieces, lifting of weights, forming of materials a) b) c) a) Simple circuit without adjustment of force. Why is it a constant force drive? Because the pressure pushes the cylinder as long to the right as there is no equilibrium with the spring force. d) b) Force adjustable with a pressure relief valve. c) Large force. Force is adjustable in two stages. e) d) Weight balancing, not adjustable. e) Weight balancing with two-stage adjustability. 2005/2006 I. Hydraulic and Pneumatic Systems 18

Big pictures End of normal presentation Beginning of big pictures 2005/2006 I. Hydraulic and Pneumatic Systems 19

Directional valves Spool valves – pressure relieves 1 – axial, 2 – axial 2005/2006 I. Hydraulic and Pneumatic Systems 20

Directional valves Spool valves - overlaps O>0 2005/2006 I. O=0 Hydraulic and Pneumatic Systems O<0 21

Spool valves Directional valves With two switching positions and latch Spool types Centred by springs Pneumatically operated Hydraulically operated 2005/2006 I. Hydraulic and Pneumatic Systems 22

Spool valves Directional valves Electro-hydraulicaly governed, centred by spring Electro-hydraulicaly governed , centred by pressure 2005/2006 I. Hydraulic and Pneumatic Systems Back 23

Spool valves 2005/2006 I. Directional valves Hydraulic and Pneumatic Systems 24

Spool valves 2005/2006 I. Directional valves Hydraulic and Pneumatic Systems 25

Spool valves 2005/2006 I. Directional valves Hydraulic and Pneumatic Systems 26

Spool valves spool types 2005/2006 I. Directional valves Hydraulic and Pneumatic Systems 27

Spool valves 2005/2006 I. Directional valves Hydraulic and Pneumatic Systems 28

Spool valves 2005/2006 I. Directional valves Hydraulic and Pneumatic Systems 29

Spool valves 2005/2006 I. Directional valves Hydraulic and Pneumatic Systems 30

Spool valves 2005/2006 I. Directional valves Hydraulic and Pneumatic Systems 31

Poppet valves Directional valves Electrically governed 3/2 poppet valves Ball Taper Plate With one ball 1 - ball 4 - housing 7 - ball 2005/2006 I. 2 - spring 5 – lever ? 8 - poppet With two ball 3 - poppet 6 – operating pin ? Hydraulic and Pneumatic Systems Back 32

Poppet valves 2005/2006 I. Directional valves Hydraulic and Pneumatic Systems 33

Poppet valves 2005/2006 I. Directional valves Hydraulic and Pneumatic Systems 34

Poppet valves 2005/2006 I. Directional valves Hydraulic and Pneumatic Systems 35

Directional valves Number of ports and positions: a) 2/2 b) 3/2 c) 4/3 d) 5/3 e) 5/3 f) 4/3 g) 6/3 2005/2006 I. Hydraulic and Pneumatic Systems 36

5/3 valves: 2005/2006 I. Directional valves Hydraulic and Pneumatic Systems 37

Directional valves Number of ports and positions: 2005/2006 I. Hydraulic and Pneumatic Systems 38

Directional valves Number of ports and positions: 2005/2006 I. Hydraulic and Pneumatic Systems 39

Directional valves Kind of governing and positioning unit: 2005/2006 I. Hydraulic and Pneumatic Systems 40

Directional valves Kind of governing and positioning unit: 2005/2006 I. Hydraulic and Pneumatic Systems 41

Directional valves Kind of governing and positioning unit: 2005/2006 I. Hydraulic and Pneumatic Systems 42

Directional valves Kind of governing and positioning unit: 2005/2006 I. Hydraulic and Pneumatic Systems 43

Directional valves Kind of governing and positioning unit: 2005/2006 I. Hydraulic and Pneumatic Systems 44

Directional valves Kind of governing and positioning unit: 2005/2006 I. Hydraulic and Pneumatic Systems 45

Directional valves Kind of governing and positioning unit: 2005/2006 I. Hydraulic and Pneumatic Systems 46

Directional valves Kind of governing and positioning unit: 2005/2006 I. Hydraulic and Pneumatic Systems 47

Servo valves Construction Torque motor Permanent magnets Iron core with coils tube Bending tube Double nozzle – impinging plate Return springs Spool valve Constant restrictions User 2005/2006 I. Hydraulic and Pneumatic Systems 48

Positioning with spring 2005/2006 I. Servo valves Hydraulic and Pneumatic Systems 49

Positioning with spring 2005/2006 I. Servo valves Hydraulic and Pneumatic Systems 50

Positioning with spring 2005/2006 I. Servo valves Hydraulic and Pneumatic Systems 51

Servo valves Positioning with inductive device 2005/2006 I. Hydraulic and Pneumatic Systems 52

Three stage servo valve 2005/2006 I. Servo valves Hydraulic and Pneumatic Systems 53

Three stage servo valve 2005/2006 I. Servo valves Hydraulic and Pneumatic Systems 54

Proportional valves Proportional directional valve 2005/2006 I. Hydraulic and Pneumatic Systems 55

Proportional valves Proportional directional valve 2005/2006 I. Hydraulic and Pneumatic Systems 56

Proportional valves Proportional directional valve 2005/2006 I. Hydraulic and Pneumatic Systems 57

Proportional valves Proportional flow rate valve 2005/2006 I. Hydraulic and Pneumatic Systems 58

Proportional valves Proportional flow rate valve 2005/2006 I. Hydraulic and Pneumatic Systems 59

Proportional valves Proportional pressure valve 2005/2006 I. Hydraulic and Pneumatic Systems 60

Proportional valves Proportional pressure valve 2005/2006 I. Hydraulic and Pneumatic Systems 61

Accessories Filters 2005/2006 I. Hydraulic and Pneumatic Systems 62

Accessories Filters 2005/2006 I. Hydraulic and Pneumatic Systems 63

Accessories Filters 2005/2006 I. Hydraulic and Pneumatic Systems 64

Accessories Filters 2005/2006 I. Hydraulic and Pneumatic Systems 65

Accessories Filters 2005/2006 I. Hydraulic and Pneumatic Systems 66

Accessories Filters 2005/2006 I. Hydraulic and Pneumatic Systems 67

Accessories Filters 2005/2006 I. Hydraulic and Pneumatic Systems 68

Positioning drives Against bumpers 2005/2006 I. System concepts Drift-free holding of the cylinder Hydraulic and Pneumatic Systems Positioning out of creeping motion 69

Positioning drives Against bumpers 2005/2006 I. System concepts Drift-free holding of the cylinder Hydraulic and Pneumatic Systems Positioning out of creeping motion 70

Circuit plan Velocity drives 2005/2006 I. Hydraulic and Pneumatic Systems 71

Velocity drives Directional valve positions 2005/2006 I. Hydraulic and Pneumatic Systems 72

Modification for very low working speeds 2005/2006 I. Velocity drives Hydraulic and Pneumatic Systems 73

Without adjustment of force 2005/2006 I. Force drives Hydraulic and Pneumatic Systems 74

Force adjustable with pressure relief valve 2005/2006 I. Force drives Hydraulic and Pneumatic Systems 75

Force drives 2005/2006 I. Force is adjustable in two stages Hydraulic and Pneumatic Systems 76

Force drives Weight balancing, not adjustable 2005/2006 I. Hydraulic and Pneumatic Systems 77

Force drives Weight balancing with two-stage adjustability 2005/2006 I. Hydraulic and Pneumatic Systems 78