Directional valves

1
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

2
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,
3
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.
4
Directional valves

• 3. Number of ports and positions

Basic symbols
Two positions Three positions
4/3 valve with ports, operation positions and
governing elements
5
Directional valves

• 4. Kind of governing and positioning unit

By hand, electrically or electrohydraulically
Cross-section
Symbol
6
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.
7
Servo valves

• Double nozzle impinging plate (bridge circuit)

8
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.
9
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.
10
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
11
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.
12
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
13
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)

14
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.
15
System concepts

• Drives

16
Constant pressure sources

• Position drives

With the adjustable resistance the velocity can
be controlled

1. Is a simple circuit. Problem is drifting in the
   holding position since the spool valve has a
   leakage.

• 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.
• 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

17
Constant pressure sources

• Velocity drives

VW directional valve EV fast forward ER
fast backward AV working forward AR working
backward VSZ flow control valve VDr
restriction valve
Directional valve positions
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.
18
Constant pressure sources

• Force drives

Holding of work pieces, lifting of weights,
forming of materials
a)
b)
c)

1. 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.
2. Force adjustable with a pressure relief valve.
3. Large force. Force is adjustable in two stages.
4. Weight balancing, not adjustable.
5. Weight balancing with two-stage adjustability.

d)
e)
19
Big pictures

• End of normal presentation
• Beginning of big pictures

20
Directional valves

• Spool valves pressure relieves

1 axial, 2 axial
21
Directional valves

• Spool valves - overlaps

O gt 0
O 0
O lt 0
22
Directional valves

• Spool valves

Spool types
With two switching positions and latch
Centred by springs
Pneumatically operated Hydraulically operated
23
Directional valves

• Spool valves

Electro-hydraulicaly governed, centred by spring
Electro-hydraulicaly governed , centred by
pressure
Back
24
Directional valves

• Spool valves

25
Directional valves

• Spool valves

26
Directional valves

• Spool valves

27
Directional valves

• Spool valves
• -
• spool types

28
Directional valves

• Spool valves

29
Directional valves

• Spool valves

30
Directional valves

• Spool valves

31
Directional valves

• Spool valves

32
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
Back
33
Directional valves

• Poppet valves

34
Directional valves

• Poppet valves

35
Directional valves

• Poppet valves

36
Directional valves

• Number of ports and positions

1. 2/2
2. 3/2
3. 4/3
4. 5/3
5. 5/3
6. 4/3
7. 6/3

37
Directional valves

• 5/3 valves

38
Directional valves

• Number of ports and positions

39
Directional valves

• Number of ports and positions

40
Directional valves

• Kind of governing and positioning unit

41
Directional valves

• Kind of governing and positioning unit

42
Directional valves

• Kind of governing and positioning unit

43
Directional valves

• Kind of governing and positioning unit

44
Directional valves

• Kind of governing and positioning unit

45
Directional valves

• Kind of governing and positioning unit

46
Directional valves

• Kind of governing and positioning unit

47
Directional valves

• Kind of governing and positioning unit

48
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
49
Servo valves

• Positioning with spring

50
Servo valves

• Positioning with spring

51
Servo valves

• Positioning with spring

52
Servo valves

• Positioning with inductive device

53
Servo valves

• Three stage servo valve

54
Servo valves

• Three stage servo valve

55
Proportional valves

• Proportional directional valve

56
Proportional valves

• Proportional directional valve

57
Proportional valves

• Proportional directional valve

58
Proportional valves

• Proportional flow rate valve

59
Proportional valves

• Proportional flow rate valve

60
Proportional valves

• Proportional pressure valve

61
Proportional valves

• Proportional pressure valve

62
Accessories

• Filters

63
Accessories

• Filters

64
Accessories

• Filters

65
Accessories

• Filters

66
Accessories

• Filters

67
Accessories

• Filters

68
Accessories

• Filters

69
System concepts

• Positioning drives

Positioning out of creeping motion
Drift-free holding of the cylinder
Against bumpers
70
System concepts

• Positioning drives

Positioning out of creeping motion
Drift-free holding of the cylinder
Against bumpers
71
Velocity drives

• Circuit plan

72
Velocity drives

• Directional valve positions

73
Velocity drives

• Modification for very low working speeds

74
Force drives

• Without adjustment of force

75
Force drives

• Force adjustable with pressure relief valve

76
Force drives

• Force is adjustable in two stages

77
Force drives

• Weight balancing, not adjustable

78
Force drives

• Weight balancing with two-stage adjustability