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Hydraulic and Pneumatic Systems

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					                           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




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                             Directional valves
 1. Spool valves or poppet valves:
 Spool valves                      Poppet valves


    have always leakages                        can be totally sealed
  need relatively large displacements           need smaller displacements to let
  because a positive overlap is needed for      fluid through
  sealing

 preferred in all other cases because of       preferred for large flow rates
 their flexibility and lower price



   need radial and axial
       pressure relief,



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                                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.




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                        Directional valves
 3. Number of ports and positions:
                                                            Basic symbols



                                                Two positions Three positions




                                                4/3 valve with ports, operation
                                                positions and governing elements




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                          Directional valves
 4. Kind of governing and positioning unit:
    By hand, electrically or electrohydraulically


    Cross-
    section:


   Symbol:




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 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.




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                                             Servo valves
 Double nozzle – impinging plate (bridge circuit):
                                                                                                     
                    s0-s            s0+s                                                             
                                                                                  1           1      
                                            s0  s  s0        p  p0                2
                                                                                                   2 
                                                                                   s         s 
                                                                            1  1   1  1   
                                                                            s              s 
                           s                                                       0         0  
     Q1                                        Q2
               QU                    QU                                            p     Linear part in
                           User                                                    p0     the middle
    Q01                                           Q01
       p1                                    p2
                               p0
                                                                             0,8


                                                                  -1
                                                                                           1    s
                                                                                                s0

                                                                                   -0,8
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                                    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 are also three-stage servo valves.
  They are used for very large powers.


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                   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.



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                                           Accessories
 Liquid reservoir:

  Tasks:  Storing of necessary liquid volume

                Conducting heat away from the system
                De-airing of the liquid (large surface, long stay of the liquid)
                Separation of condensed water
                Carrying element of other system components (pumps, valves, pipes,
                 etc.)



 Pipes:
                Steel of flexible hoses.
                Key point: sealing



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                                    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.




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                                System concepts
 Delivering of liquid by:
                Flow sources
                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
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                                  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)




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                                 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.
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               System concepts
   Drives




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                           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

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                         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 VW4 and
                                 VSZ1 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.
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                         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          d)
        pushes the cylinder as long to the right as there
        is no equilibrium with the spring force.
 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.
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                           Big pictures
   End of normal presentation
   Beginning of big pictures




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                      Directional valves
 Spool valves – pressure relieves




               1 – axial, 2 – axial

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                   Directional valves
 Spool valves - overlaps




           O>0            O=0                         O<0




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                               Directional valves
 Spool valves
   With two switching positions and latch                         Spool types




     Centred by springs
               Pneumatically operated
               Hydraulically operated




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                              Directional valves
 Spool valves
      Electro-hydraulicaly
      governed, centred by
      spring




      Electro-hydraulicaly
      governed , centred by
      pressure




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




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                Directional valves
 Spool valves




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                Directional valves
 Spool valves




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               Directional valves
 Spool
 valves
 -
 spool
 types




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               Directional valves
 Spool
 valves




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                Directional valves
 Spool valves




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               Directional valves
 Spool
 valves




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                Directional valves
 Spool valves




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                         Directional valves
 Poppet valves
                                  Electrically governed 3/2 poppet valves




      Ball Taper Plate




                                        With one ball                           With two ball


                          1 - ball          2 - spring    3 - poppet
                          4 - housing       5 – lever ?   6 – operating pin ?
                          7 - ball          8 - poppet


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




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                 Directional valves
 Poppet valves




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                 Directional valves
 Poppet valves




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                         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




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                 Directional valves
   5/3 valves:




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                         Directional valves
   Number of ports and positions:




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                         Directional valves
   Number of ports and positions:




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                          Directional valves
   Kind of governing and positioning unit:




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                          Directional valves
   Kind of governing and positioning unit:




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                          Directional valves
   Kind of governing and positioning unit:




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                          Directional valves
   Kind of governing and positioning unit:




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                          Directional valves
   Kind of governing and positioning unit:




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                          Directional valves
   Kind of governing and positioning unit:




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                          Directional valves
   Kind of governing and positioning unit:




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                          Directional valves
   Kind of governing and positioning unit:




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   Construction
                         Servo valves
                                   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
                          Servo valves
   Positioning with
     spring




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                     Servo valves
   Positioning
   with spring




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                          Servo valves
   Positioning with
     spring




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                               Servo valves
   Positioning with inductive device




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                   Servo valves
   Three
   stage
   servo
   valve




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                                 Servo valves
   Three stage servo valve




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                         Proportional valves
  Proportional directional valve




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




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                      Proportional valves
  Proportional
  directional valve




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                           Proportional valves
  Proportional flow rate
  valve




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                           Proportional valves
  Proportional flow rate
  valve




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                         Proportional valves
  Proportional pressure valve




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                 Proportional valves
  Proportional
  pressure
  valve




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                   Accessories
   Filters




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                   Accessories
   Filters




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                   Accessories
   Filters




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                   Accessories
   Filters




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                   Accessories
   Filters




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                   Accessories
   Filters




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                   Accessories
   Filters




2005/2006 I.   Hydraulic and Pneumatic Systems   68
                         System concepts
Positioning drives




       Against bumpers    Drift-free holding of            Positioning out of
                              the cylinder                  creeping motion
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                         System concepts
Positioning drives




       Against bumpers    Drift-free holding of            Positioning out of
                              the cylinder                  creeping motion
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  Circuit plan

                   Velocity drives




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                        Velocity drives
Directional valve positions




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                     Velocity drives
Modification for
very low
working speeds




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                              Force drives
Without adjustment of force




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                            Force drives
Force adjustable with
pressure relief valve




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                   Force drives
                                             Force is adjustable in two
                                             stages




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                              Force drives
Weight balancing, not adjustable




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                              Force drives
Weight balancing with two-stage adjustability




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