DIRECTIONAL CONTROL VALVES

1
Chapter 10

• DIRECTIONAL CONTROL VALVES

2
Directional Control Valves

• Consists of a body with internal passages which
  are connected and disconnected by a moveable
  part.
• Moveable part is usually a spool.

3
4-way Directional Valve

• Consists of a pump passage, tank passage and two
  actuator passages.
• Has four distinct passages within its body.
• Its main function is to reverse action in a
  cylinder.
• Moving parts have two extremes shown
  symbolically the two squares are connected
  together but when in a circuit only one is
  connected in the circuit.

4
3-way Directional valve

• Consists of valve-body pump passage, tank
  passage, and one actuator passage.
• Function is to pressurize an actuator when the
  spool is in one extreme position and to exhaust
  when its in the other.
• Used to operate single acting actuators like rams
  or spring-returns Fluid is directed into the cap
  end side of the cylinder. When in the other
  position the flow to actuator is blocked and the
  actuator passage within the body is connected to
  tank.
• Not found very often in industrial hydraulic
  systems.

5
2-way Directional Valve

• Consists of two passages that are connected and
  disconnected.
• Flow through valve in one extreme position and
  blocked in the other.
• Gives an on/off function used as a safety
  interlock and to isolate and connect various
  system parts.

6
Valve Size and Ratings

• Come in some basic sizes ¼, 3/8, ½, 3/4 and 1¼
  inches.
• Common to rate valves on their nominal flow
  rating. Example 3-10 gpm, 10-20 gpm, 40 gpm,
  80 gpm, and 160 gpm.
• With ratings at the higher end the pressure
  differential pressure between P to A or B to T is
  approximately 40 psi.

7
Directional Spool valves

• Different valves have different amount of lands
  on their spools.
• With three large diameters spaced
• equally apart connected with a
• shaft such as used in 4-way
• directional valves are known as
• 3-land spool.
• Most industrial valves have 2 or 4
• land spools.
• The small flow ratings usually use the 2-land
  spools.
• The large flow ratings generally use the 4-land
  spools.

8
Subplate-mounted 4-way valves

• The valves given as examples have had the tank
  and pump passages situated on one side and the
  actuator passages on the opposite side just like
  the schematic symbols are drawn.
• In most systems, they are actually mounted on a
  subplate that has the valve body mounted to it
  and all the valve passages on the bottom surface.

9
Directional Valve Actuators

• Spools are moved by either mechanical,
  electrical, hydraulic, pneumatic, or human
  energy.
• Those moved by human energy are known as manually
  actuated valves. Ex levers, pushbuttons, and
  pedals
• Used when actuators whose operation must be
  sequenced and controlled at operators discretion
• When shifting of the valve must occur at the same
  time an actuator reaches a specific condition.

10
Pilot Operation

• Spools are often shifted with fluid pressure
  either air or hydraulic.
• Pressure is applied to the two extreme spool
  lands if it is a 4-land spool, or to separate
  pistons if it is a 2-land spool.

11
SOLENOID

• An electromechanical device which converts
  electric power into linear mechanical force and
  motion which is a common way to operate a
  directional valve. Its counterpart in hydraulic
  system is a cylinder.
• Come in two types Air gap and wet armature.

12
Air Gap Solenoid

• Older of the two designs that consists of a
  T-plunger, wire coil, and C frame.
• How it works
• Electric current passes through a wire, setting
  up an electric field around the wire. This
  current is made stronger by the wire coiling.
• An iron path is setup around the coil by the C
  frame, and another iron path in the center of the
  coil by the T-plunger, which is moveable.
• At rest the T-plunger is partially out of the
  coil, when current is applied the plunger is
  attracted into the center causing the plunger to
  hit a pushpin, in turn shifting the spool in the
  valve.
• With the spool shifted the plunger fully seats
  within the coil resulting in the magnetic field
  traveling completely through an iron path.
• As the plunger moves in the air gap gradually
  decreases, thus increasing the solenoid force.

13
Wet Armature Solenoid

• Been more accepted due to their increased
  reliability resulting from better heat transition
  characteristics and elimination of pushpin seals.
• Consists of a coil, rectangular frame armature
  and tube.
• The coil is surrounded by the plastic
  encapsulated rectangular frame with a hole that
  runs through the coil center and two sides of the
  frame.
• The tube fits within this bore and screwed into
  the directional valve body.
• The tube houses an armature which is bathed by
  system fluid with in the valve.

14
Wet Armature Solenoid

• How it works
• An electric current creates a magnetic field
  around the coils, which is magnified by the iron
  rectangular frame and the armature.
• The armature is partially out of the coil as the
  coil begins to receive current. As it increases
  the armature is pulled into the coil.
• The directional valves shifts as the armature
  hits the pushpin.
• When the spool shifts the armature is fully
  centered within the coil and the coil magnetic
  field is moved completely through an iron path.
• As the armature moves in the gap gradually
  decreases thus increasing the solenoid force.

15
AC Hum

• Current moves from zero to positive, through zero
  to negative and back to zero at 60 Hz known as
  Alternating current (AC).
• Magnetic field and solenoid force is greatest
  when current is at positive and negative peaks.
• As current goes through zero the solenoid force
  decreases, this action pushes out the plunger.
• As the force raises again, the plunger is pulled
  back in.
• This in and out motion causes a humming, buzzing,
  or chattering known as AC Hum.

16
Shading Coils

• To minimize AC hum, shaded coils are used.
• Air gap Copper loops attached to the C-frame
  are used.
• Wet armature copper wire ring at the pushpin
  end of the tube.
• Works because current is generated in the shading
  coils that lags behind the applied current which
  is generally sufficient enough to hold the
  plunger/armature in place when the magnetic field
  of the coil is at its lowest value.

17
Eddy Currents

• AC magnetic current fluctuations that cause stray
  currents, which move in tiny circles around the
  iron magnetic paths. They consume power and
  generate heat reducing solenoid force.
• Air Gap - to minimize the C frame and plunger are
  made up of thin metal sheets, insulated from one
  another with an oxide coating which allows the
  magnetic current to flow but the eddy currents
  cannot.
• Wet armature to minimize the square frame is
  made from laminated thin metal sheets. This is
  not practical for the armature, so it is made of
  one piece.

18
Solenoid Inrush current

• AC current is a source of heat generation for a
  solenoid, which over time will cause the coil to
  fail.
• The goal is to achieve sufficient holding and
  shifting force with as little current as
  possible, to avoid the over generation of heat
  eventually leading to coil failure.
• The designing sufficient AC resistance is known
  as impedance.
• The result of pure resistance to electron flow of
  the conductor material. E.g. copper offers less
  resistance than aluminum
• The effect generated by the electromagnetic field
  surrounding the coil which tends to restrict
  current flow the stronger the magnetic field the
  less current flows in solenoid windings.
• As energized the magnetic field is not at peak
  due to the high resistance gap, so resistance
  flow is coming from the pure resistance to the
  electron flow from conductor material
  consequently high inrush current is experienced
  in coil windings. This decreases as the plunger
  becomes fully seated which at this time
  impedance is at its peak and AC current is at is
  minimum.

19
Solenoid Inrush current

• As energized the magnetic field is not at peak
  due to the high resistance gap, so resistance
  flow is coming from the pure resistance to the
  electron flow from conductor material
  consequently high inrush current is experienced
  in coil windings. This decreases as the plunger
  becomes fully seated which at this time
  impedance is at its peak and AC current is at is
  minimum.
• If anything obstructs the plunger/armature large
  amounts of current will rush into the coils
  generating large amounts of heat.
• Causes plastic material on coil ends to melt.
• Causes encapsulated designs to get bubbles on the
  coating.
• Wire insulation deteriorates quickly causing coil
  failure in a minute or two.

20
Continuous duty solenoid

• One that can be held energized indefinitely
  without overheating.
• Its heat dissipating ability is such that it will
  dissipate the heat generated at the coils lower
  holding current.
• Most industrial hydraulic direction valves are
  this.

21
Solenoid Limitations

• Cannot be used in wet or explosive environments.
• When the cycle life of directional valve is
  extremely long, electrically controlled solenoids
  not used.
• The force generated by them to shift a valve is
  limited.

22
Solenoid failure

• Fail because of the heat, which is usually the
  result of
• Blockage something that stops the plunger from
  being completely seated thus causing an inrush of
  current resulting in large heat generation
  eventually resulting in coil failure.
• The valve spool can become blocked from
  contamination of build up between it and the
  valve body and thus not allowing the plunger to
  seat.
• Due to a valve base that is not flat.
• Wear patterns can develop between the plunger and
  the C-frame, overtime not allowing the plunger to
  fully seat.
• High ambient temps since heat must dissipate
  from the coil, if surrounding air temp is high,
  it becomes more difficult and the coil may burn
  out. Such is the case if it is operating in poor
  ventilated area, or the machine is a source of
  heat.
• Low voltage if line voltage drops below 100
  volts for the 60 Hz solenoid it has insufficient
  force to seat the plunger resulting with inrush
  currents eventually causing coil failure.

23
Manual Override

• Air gap solenoids usually have a cover to protect
  them. At the cover end, a small pin is located
  in line with the plunger. Pushing this pin will
  result in manually shifting the plunger and in
  turn the valve spool, overriding the solenoid
  function.
• In wet armatures the pin is located at the end of
  the tube which houses the armature and pushpin.
• Used to check movement of the directional valve
  spool.
• Used to cycle an actuator without energizing the
  complete electrical control system.

24
Solenoid controlled pilot operated spring offset

• Two position directional valve uses a solenoid to
  shift the pilot valve spool to an extreme
  position, the spool is returned via spring.

25
Detents

• Is a locking device which keeps a spool in the
  desired shifted position.
• Used when two actuators are used to shift the
  spool of a two position valve.
• Spool of detented valve has notches and groove.
  With each being a receptacle for a spring-loaded
  moveable part.
• Directional valves equipped with detents are not
  required to keep their actuators energized to
  achieve a shifted position.

26
Center Conditions

• 4-way directional valves are frequently three
  position valves with two extreme positions and a
  center position.
• The two extremes are directed related to the
  actuator motion.
• The center is designed to perform logic or
  satisfy a need or condition of the system.
• Varieties are open, closed, tandem and float
  centers. Accomplished using the same valve body
  with the appropriate spools.

27
Open center

• A directional valve with an open center has all
  passages connected to each other in the center
  position.
• Used in single actuator circuits actuator
  completes its cycle, the directional valve spool
  is centered and the flow returns to the tank at
  low pressure and the actuator is free to move.
• Disadvantage is that no other actuator can be
  operated while valve is centered and a cylinder
  load cannot be held in mid-stroke position.

28
Closed Center

• A directional valve with a closed center has all
  passages blocked.
• Stops the motion of an actuator and allows each
  actuator to operate independently from one power
  source.
• Disadvantages
• The pump flow cannot unload to tank through the
  directional valve during actuator idle time.
• The spool leaks just as in any spool valve, so if
  the spool is subjected to system pressure for any
  longer then a few minutes, pressure will build
  in the passages causing them to leak.

29
Tandem Center position

• A directional valve with tandem center has P and
  T passages connected and the actuator passages
  blocked.
• Stops actuator motion but allows pump to return
  to the tank while system is idling.
• Many times one valves T port is connected to
  another valves P port allowing for actuators to
  be operated individually or together, and
  emptying of the pump to the tank through the
  valve during idle times.
• The P and T passages are connected together in
  the same valve body through a passage in the
  spool shaft.
• Disadvantages
• The flow rating for the valve is reduced.
• If used as a replacement for valve with different
  configuration, the actuator controlled by the
  directional valve will operate backwards if all
  other things change the same.

30
Float Center

• Directional valve with float center has P passage
  blocked and the other three positions connected
  in the center position.
• Allows independent operation of actuators tied to
  the same power source and allows free movement of
  each actuator.
• Does not create a buildup in pressure in the P
  passage controlling rod drift.
• Disadvantage is that load cant be stopped or held
  in place. If this is necessary then use in
  conjunction with a pilot operated check valve.

31
Crossover conditions

• The crossover conditions are what the valve see
  for a fraction of a second while the spool is
  shifting from one extreme to another.
• Closed crossover does not allow a pump/electric
  motor pressure to drop during a spool shift.
• An open crossover allows actuator lines to bleed
  slightly before reversal takes place this
  bleeding helps eliminate shock as a load changes
  direction.

32
Solenoid controlled pilot operated directional
valve

• Directional control valves are often shifted via
  a solenoid. But the larger ones, those greater
  then 40 gpm require too much force for direct
  solenoid shifting.
• This are done by piggy backing a solenoid
  operated directional control valve with the
  larger valve.
• Pilot pressure moves the main valve spool but the
  solenoid controls the pilot.

33
Spool Centering in Pilot operated valve

• Its held in the center position by fluid or
  springs.
• Springs are most common there is a spring
  located at each end of the spool, a spring is
  compressed when the spool moves into an extreme
  position with the spring then moving it back to
  the center position. Spring centering have
  trouble handling flows beyond their intended
  capacity.
• Hydraulically centering ensures that the spool
  will center even with excessive flow rate. To
  center a valve this way, fluid is just pumped to
  both ends of the valve spool simultaneously.

34
Choke control

• When the main spool of the pilot operated valve
  is shifted shock can be developed as large fluid
  controls are forced to change direction quickly.
  A choke control slows the spool shift reducing
  this shock, but not totally eliminating.
• Choke control is a sandwich valve fitting
  between the main valve body and the pilot valve.
• Sandwich consists of two needle valves and two
  bypass check valves.
• As the spool shifts in one direction a needle
  valve meters flow out of the pilot spring
  chamber.
• As the spool reverses the other needle valve
  meters flow out of the other pilot spring
  chamber.
• The more the needle valves are restricted for
  flow the slower the spool shift, thus reducing
  the shock.

35
Stroke Adjustment

• Stroke adjustor is a screw adjustment which
  limits spool travel in the main valve of a
  solenoid controlled, pilot operated directional
  valve.
• This is not recommended for direct AC solenoid
  valve.
• Partially limiting the spool travel in one
  direction means that a port is not completely
  uncovered, causing a restriction through the
  valve when the spool is shifted in one direction.
• If stroke adjustor is screwed in completely on
  extreme position is blocked out essentially
  turning a 3-position valve into a 2-positon valve
  with positions in one extreme and the center.

36
Directional Valve drain

• Pilot valve tank passage of a solenoid
  controlled, pilot operated directional valve is
  the drain.
• The tank port of the pilot valve is connected by
  means of an internal passage to the tank passage
  of the main valve.

37
Directional Valve pilot pressure

• Pilot pressure is what is used to shift the main
  valve spool.
• The P port of the pilot valve is directly
  connected to the P passage of the main valve by
  means of an internal passage, making an internal
  pilot connection. Pilot pressure is the same
  value as system pressure in this case.
• Externally piloting is necessary when internal
  pilot pressure is too low or too high.
• Desirable if the system pressure is too high
  also.
• To change from internal to external an internal
  pipe plug must be added.
• Sometimes required when back pressure check valve
  is positioned into the pressure line ahead of the
  valve.

38
Backpressure Check Valve

• With tandem and open center directional valves it
  is often the case that the back pressure and
  pressure required to shift the main valve spool
  is greater then the system pressure being
  supplied.
• This can be overcome with the use of a
  backpressure check valve.
• It has a stiffer spring, which requires more
  pressure to be present at the valve inlet to push
  the poppet off its seat, thus allowing for extra
  pressure to be supplied to the pilot valve.
• Can be positioned in pressure or tank lines and
  can be located in the valve or externally to the
  valve.
• If used in a tank line, the valve must be
  externally drained.

39
Electrohydraulic Directional flow control valves

• Meet the use for higher response, stiffer
  systems, better flow characteristics with and
  efficient way to interface electronic control
  systems with hydraulic recovery systems.
• Two types are proportional and servo.

40
Proportional vs. Servo

• Response
• Proportional is the time required for the valve
  to achieve maximum rated flow due to an
  electrical step input command signal.
• Servo is the frequency response which is
  expressed in hertz with the frequency of a small
  sine wave command signal with flow amplitude of
  about -3db.
• Servo 10-300 Hz vs. Proportional 2-10 Hz.
• Spool center condition
• Servo are critically lapped by carefully matching
  both the width and the position of the spool
  lands to the metering recesses within the valve.
  These require line-line contact by hand fitting
  increasing costs.
• Proportional designed so that the spools and
  valve bodies are interchangeable resulting in a
  crossover overlap on the order 10-30 of the
  total stroke. This causes dead banding which
  results in instability.

41
Proportional vs. Servo

• Hysteresis the measurement of the difference in
  electrical command signal as percentage of rated
  electrical command for a given flow output.
• Repeatability an indication of the ability of
  the valve to repeat a given flow when a given
  electrical command signal is repeatedly applied
  to the valve.
• Threshold the smallest discreet change in
  electrical command signal that will produce a
  corresponding change in output flow.
• Open loop proportional exhibit levels as high as
  10 while closed loop for both are 3 or less.
• Filtration requirements Servos require 3
  micrometers or less because of their close
  tolerances proportional only require about 10
  micrometers.

42
Proportional Valve

• Consists of the torque motor pilot valve, adapter
  block, wire mesh pilot filter, internal pilot
  pressure regulator, main spool and body, and LVDT
  (linear variable differential transducer),
• How it works
• Main spool is in the center condition and all
  ports are blocked by the lands of the spool
• One solenoid is energized moving the main spool
  in that direction
• The LVDT is attached directly to the main spool
  which measures the precise movement of the spool
  and feeds this back to the electronics so that
  the correct solenoid can be activated.

43
Torque Motor Pilot

• This is a pressure differential type proportional
  directional valve operates in two stages
• 1st stage the pilot valve consists of coil,
  armature, suspension member, diverter plate, and
  blade.
• 2nd stage main valve consists of a spool,
  return springs and an LVDT, similar to that of a
  standard directional valve.

44
Servo Valve

• Consists of first stage pilot valve and second
  stage spool valve with a mechanical feedback
  spool position to the torque motor assembly.
• 1st stage has three possible designs a flapper
  nozzle, jet pipe and jet diverter. They have
  varying nozzle sizes but typical 0.010, 0.008 and
  0.020 minimums respectively. The larger the
  orifice the more contamination tolerant the valve
  is.
• 2nd stage spool land at the center position can
  and does vary depending on the requirements of
  the system. Most require them to be line to line
  if possible.

45
Servo Land configurations

• Line to line results in an ideal flow gain plot
  where the output flow to the cylinder ports is
  zero with the spool in the center position and
  increases immediately with spool travel.
• Under lapped has more clearance between the
  spool land edges and metering notches or ports in
  the spool sleeve or valve body. Results in
  higher leakage flow.
• Overlapped Reduce leakage flow to a minimum and
  no cylinder port flow will occur until the spool
  has traveled through the overlap. Creates
  deadband (a zone of valve movement in either
  direction from center where no actuator response
  to signal occurs).