CHECK VALVES, ACCUMULATORS, AND CYLINDERS

1
Chapter 8

• CHECK VALVES, ACCUMULATORS, AND CYLINDERS

2
Check Valves

• Consists of a body with inlet and outlet
• ports and a moveable member biased by
• a spring force.
• Movable member can be a flapper,
• plunger, ball, or poppet.
• Fluid flow passes through a check valve
• in only one direction.
• System pressure gets high enough to
• overcome the spring force biasing the
• poppet, so it gets pushed off its seat, allowing
  the
• flow through the valve. Known as the free flow
• direction of the valve.

3
Check Valves in a circuit

• Check valves are a combination of a directional
  valve and pressure valve.
• Its often used as a bypass valve, as it allows
  flows to get around components, like flow control
  valves that restrict flow in a reverse direction.
• Used to isolate sections of a system or
  components such as an accumulator to keep it
  from dumping its flow over a relief valve or
  through the pump.

4
Suspending a Load

• For loads that require suspension indefinitely a
  check valve is used , because is it practically
  zero leakage.
• It can do this because it is a one-way valve.
• For a load to move up and down the use of a pilot
  operated check valve is required.
• Allows free flow in one direction and flow
  through the opposite direction when the pilot
  pressure unseats the valves moveable member.

5
Hydraulic Accumulators

• Stores the hydraulic pressure so it can be
  converted from potential energy to working
  energy.
• Three classifications- identified by what means
  it maintains a force on the liquid stored.
• Weight-loaded
• Spring-loaded
• Hydro-pneumatic

6
Weight-loaded Accumulators

• Maintains a force on the liquid it stores by
  means of heavy weights acting on a piston or ram.
• Weights are usually made of iron, concrete and
  even water.
• Usually large ones holding hundreds of gallons,
  used to service several systems at one time.
• Stores fluid under a relatively constant pressure
  whether its full or empty.
• If discharged quickly, it can have shock
  generation due to the inertia of the weight which
  can cause excessive pressure surges in the system.

7
Spring-loaded Accumulators

• Applies a force by means of a spring acting on a
  piston.
• Much smaller only holding up to several gallons
  serves individual systems at low pressures.
• Pressure is determined by the compression rate of
  the spring.
• To avoid leakage, spring chamber is vented, and
  not back to the tank because foaming can occur.

8
Hydro-pneumatic Accumulator

• Applies force by using a compressed gas, usually
  nitrogen.
• Most commonly used type in industry today.
• COMPRESSED AIR MUST NEVER BE USED DUE TO DANGER
  OF AN AIR-OIL VAPOR EXPLOSION
• Three types named on the device separating the
  liquid from the gas.
• Piston type
• Diaphragm type
• Bladder type

9
Piston type

• Consists of a cylinder and moveable piston with
  resilient seals.
• Gas occupies the volume above the piston and is
  compressed as the cylinder fills with liquid.
• As the liquid discharges the gas decompresses and
  the piston covers the outlet keeping the air
  within the accumulator

10
Diaphragm type

• Has two metal spheres bolted together but are
  separated by a synthetic rubber.
• The upper chamber contains the gas which is
  compressed when the cylinder is filling.
• As the liquid discharges the gas pressure drops,
  decompresses. The synthetic rubber covers the
  outlet keeping the gas in the chamber.

11
Bladder Type

• Consists of a synthetic rubber bladder inside a
  metal shell with the bladder containing the gas.
• As fluid enters the gas is compressed inside the
  bladder.
• After complete discharge of the liquid the gas
  attempts to push the bladder through the outlet,
  but it reaches the poppet and the flow is
  automatically shut off.

12
Accumulators in a circuit

• Maintain system pressure accumulator supplies
  pressure in one leg of a circuit while an
  electric motor is delivering flow to another
  compensates for pressure loss due to leakage and
  for increases due to thermal fluid expansion and
  external mechanical forces on a cylinder.
• Supplement pump flow the accumulator has stored
  energy that is used to develop flow when system
  demand is greater then the pump alone can supply.
• Absorbing shock shock is caused from the inertia
  of the load attached fluid inertia when sudden
  flow is suddenly blocked or changes direction.
  Hydro-pneumatic types are used to absorb the
  shock so that it will not be transmitted fully
  throughout the system, though they are difficult
  to design into systems.

13
Isothermal and Adiabatic

• Charging
• Isothermal the operation of an accumulator as
  the gas is maintained at a constant temp. The
  gas is being compressed slowly enough for the
  heat of compression to dissipate
• Adiabatically the operation of an accumulator
  as the gas temp changes. The gas is being
  compressed rapidly so that all heat of
  compression is maintained.
• A heated gas occupies more space then gases at
  lower temps, therefore an accumulator operated
  isothermally will hold more liquid then if
  operated adiabatically.

14
Isothermal and Adiabatic

• Discharging
• Isothermal Discharge occurs slowly as gas
  expands and is capable of acquiring heat from the
  ambient through the walls or from the fluid.
• Adiabatically Discharge occurs rapidly with no
  heat gain as gas expands it cools.
• Discharges until lower pressure is reached, more
  liquid will discharge if done so isothermally
  rather then adiabatically.
• Isothermally is the ideal situation but
  generally it is done adiabatically.

15
Precharge

• The gas pressure present in a hydro-pneumatic
  accumulator when it is drained of hydraulic
  fluid.
• This affects the accumulators usable volume and
  operation as a shock absorber.

16
Effect on Usable Volume

• Hydro-pneumatics operate between a minimum and
  maximum pressure. The usable volume is determined
  by these pressures.
• The pre-charge is what determines these
  pressures.
• To develop flow, a certain volume of fluid must
  be discharged between two pressures. To maintain
  pressure, sufficient enough volume must still be
  present to compensate for leakage. Hence the
  importance of usable volume.
• Usable volume should be discharged at a
  controlled rate. So accumulators are often
  equipped with flow control and bypass check
  valves at their inlet and outlet ports.

17
Effect on Shock absorber ability

• Shock is a pressure rising caused from mechanical
  forces acting on the cylinder.
• Accumulators reduce the shock by absorbing or
  limiting this excess pressure from being
  displaced throughout the system.
• It does this by absorbing the liquid the shock
  attempts to displace or compress.
• The pre-charge value must be sufficient enough
  that it allows the displaced fluid to be
  absorbed. Pre-charges are usually slightly above
  the maximum working pressure.

18
Pre-charge

• Precharges can be lost. Usually do to the fault
  of a bad seal in a gas relief valve or with a
  piston type. With a bladder or diaphragm, it is
  usually a rupture in the synthetic membrane.
• Precharges should be checked periodically by the
  use of precharge and gauging assembly made up of
  a gas chuck, bleeder valve, and pressure gage.

19
Pump Unloading

• When accumulator is filled and work is not
  required the flow of the electric motor is
  returned to the tank with least possible
  pressure. This method is only good for a few
  seconds.
• To keep electric motor fully unloaded an electric
  pressure switch is used. Which works by sensing
  the pressure of the accumulator and when there
  needs to be an electrical signal sent to the
  electric motor / pump.
• With this method a relief valve must also be
  installed to get fully unloading of an electric
  pump.

20
Differential Unloading Relief Valve

• This can be used instead of the
• combination of the two valves.
• It consists of a check valve, pilot
• operated relief valve and
• differential piston all in one body.
• The differential piston is the key to keeping the
  pump unloaded, because it allows flow through the
  valve until the accumulator pressure drops low
  enough again calling for the valve to resume flow
  to it.

21
Hydraulic cylinders

• Consists of a body, movable piston
• and a piston rod attached to the
• piston.
• End caps attached by threads, keeper rings, tie
  rods, or a weld.
• Rod is guided in and out and supported by a
  removable bushing called rod gland.
• The rod protrudes through the end called the
  head. The other end without the rod is called
  the cap.
• The inlet and outlet ports are located at the
  head and cap ends.

22
Seals

• Positive seal must exist across a cylinders
  piston as well as the rod gland.
• Cylinder piston usually done with cast iron
  piston rings, lipseals, or a single bidirectional
  sealing element.
• Piston rings are durable but might leak under
  normal conditions.
• Lipseals and bidirectional seals offer more
  positive seals but are not as durable.
• Rod glands seals are usually V or cup shaped
  used in combination with a wiper seal which
  prevents foreign material from being drawn into
  the cylinder.
• Usually made of leather, viton or polyurethane.
  The material should be verified for compatibility
  with the fluid and operating conditions.

LIP SEALS
ROD GLAND SEALS
23
Gland Drain

• Any fluid that collects between primary and wiper
  seals is drawn back into the cylinder during
  retraction.
• Where there is an excess of fluid it collects in
  an area between the two seals. If this is the
  case it must be equipped with a gland drain so
  that it can be externally drained.

24
Cushions

• To protect against shock, cushions are installed.
• They work by slowing down the cylinder speed just
  before it reaches the end of its stroke, thus
  reducing the impact of a piston running into a
  dead end.

25
Stroke adjustors

• Sometimes the piston stroke has to be adjusted
  externally.
• This is accomplished through a threaded rod which
  can be screwed in or out of the cylinder cap.

26
Cylinder Mounting styles

• Cylinders can be mounted in many ways.
• Flange, trunnion, side lug and side tapped,
  clevis, tie rod and bolt mounting.
• Centerline mounts are the best choice to minimize
  leakage due to cylinder movement.

27
Mechanical motions

• Cylinders convert working energy into straight
  line, or linear mechanical motion.
• Depending on how they are attached to mechanical
  linkages, cylinders can provide different
  mechanical motions.
• Straight line motion in two directions
• Horizontal parallel motion
• Practical continuous rotary motion

28
Types of loads

• Load which is pushed by a cylinder rod is known
  as a thrust load.
• Load which is pulled by a cylinder rod is known
  as a tension load.

29
Stop tubes

• Is a solid, metal collar
• which fits over a piston
• rod that keeps the piston
• rod and rod gland bushing
• separated when a long-stroke cylinder is fully
  extended.
• Rod gland bushing is a fulcrum for the piston
  rod, the stop tube reduces the bearing load when
  rod is at full extension.
• Not necessary for most cylinders, just on long
  stroke cylinders.

30
Cylinder types

• Single rod- piston rod extending from one end
• Double rod single piston and a piston rod
  extending from both ends.
• Double acting fluid is applied alternately to
  both sides of cylinder piston to offer extension
  and retraction of a rod

31
Cylinder types

• Telescoping nested multiple tubular rod
  segments with provide a long working stroke in a
  short retracted envelope.
• Tandem two or more cylinder bodies mounted in
  line with their piston rods connected to form a
  common piston rod rod seals are installed
  between cylinder bodies to permit double acting
  operating of each
• Duplex cylinder consisting of at least two
  cylinder bodies to permit double acting operation
  of each

32
Double acting single rod

• Concerned with accepting gpm and psi and
  converting it into mechanical force and piston
  rod motion.
• Rod speed is determined by gpm.
• Mechanical force is effected by psi.
• Both rod speed and mechanical force are affected
  by the piston area.

33
Piston and effective area

• Piston major area the area exposed to pressure
  at cylinder cap end.
• Effective minor area the area exposed to
  pressure at the cylinder rod side.
• Effective minor are always less then major area
  because the rod covers a portion of the piston at
  this point.

34
Rod Speed While Extending

• Rod speed is determined by how quickly the volume
  behind a piston can be filled with liquid.
• The more flow a cylinder receives, the more
  quickly it will fill with liquid and the faster
  it will extend.
• Rod speed gpm x 19.25
• (ft/min) Piston area (in2)

35
Discharge Flow While Extending

• Flow entering the cap end of a double-acting,
  single rod cylinder determines the rate at which
  a cylinder piston rod will extend.
• While extending, discharge from a single rod
  cylinder is always less than the flow rate
  entering the cap end.
• gpm rod speed (ft/min) x minor piston area
  (in2)
• 19.25

36
Rod Speed While Retracting

• During retraction, when full pump flow is
  directed to rod side of a single rod cylinder, a
  piston rod will retract faster than it extended.
• With constant flow to a double acting, single rod
  cylinder it will retract faster than it
  extended.
• Retraction gpm x 19.25
• Rod speed Minor piston area (in2)
• (ft/min)

37
Discharge Flow While Retracting

• During retraction, when full pump flow is
  directed to the rod side of a single rod
  cylinder, discharge flow will be greater than
  incoming flow.
• Because of this pump flow is not necessarily the
  maximum flow rate in a system.
• During system design of a return side, the
  discharge flow from retracting single rod
  cylinders is considered. This is the reasoning
  why piping, valving and filters at the return are
  sized larger then their counterparts in the main
  system.
• gpm rod speed (ft/min) x major piston area
  (in2)
• 19.25

38
Cylinder Force While Extending

• The mechanical force developed by a cylinder is
  the result of hydraulic pressure acting on the
  cap end of the cylinder piston.
• A back pressure which is effected on the minor
  area is usually present and this additional force
  must be accounted for in determining the force
  needed to move a load.
• Anytime a load is to be extended by a cylinder
  through a distance at a certain speed, pressure
  required at a cylinder piston is used to equal
  load resistance as well as liquid resistance
  flowing back to the tank.
• Pressure (psi) Force load (lbs.) Backpressure
  Force on minor area (lbs.)
• Major piston Area (in2)

39
Cylinder Force While Retracting

• The mechanical force developed by a cylinder is
  the result of pressure acting on the effective
  area.
• The back pressure that is present on the major
  area (cap end) must be considered just as the
  minor area was during extending. This back
  pressure is usually higher than while extending
  because the discharge flow during retraction is
  usually more than during extension.
• Pressure (psi) Force load (lbs.)
  Backpressure Force on major
  area (lbs.)
• Minor piston Area (in2)

40
Affecting Cylinder Force

• The force generated by a cylinder is a function
  of fluid pressure acting on the cylinder piston
  area.
• If more output force is required, then the fluid
  pressure is increased to the appropriate level.
• Pressure generated by electric motor is limited
  to low value, so to increase output force
  cylinder size can be increased.
• Sometimes there are dimension restrictions and
  other options must be used such as tandem
  cylinders.

41
Tandem cylinder circuit

• Consists of two or more
• cylinder bodies mounted in
• line. Piston rods connected to
• form common rod, and rod
• seals placed between cylinders to permit double
  acting operation of each.
• Gives increased output force when cylinder bore
  size and maximum working pressure is limited but
  its overall length is not.
• Calculating force and rod speed is done by
  summing the appropriate minor and/or major areas
  of all the pistons.

42
Duplex Cylinder circuit

• Sometimes its desirable to stop a load in an
  intermediate position, this is accomplished
  through use of a duplex cylinder.
• Made up of two or more cylinder bodies with
  different stroke lengths.
• Bodies are connected but rods are not.
• Seals connected between the bodies to permit
  double acting of each.

6 inches
10 inches
43
Duplex Cylinder circuit

• Using for example the 6 inch and 10 inch rod
  lengths.
• Three positions available
• Both piston rods retracted load doesnt move,
  or is at start position
• Extending the shorter rod to full length moves
  the longer one that same length. load moves 6
  inches
• When the longer rod is extended the rest of the
  way. load moves 10 inches.

6 inches
10 inches
44
Duplex Cylinder circuit

• Using for example the 6 inch and 10 inch rod
  lengths.
• Cylinders can also be attached at the cap ends
  one piston rod is attached to a machine member
  which remains stationary, but the cylinder body
  still moves making the requirement for the use
  of hoses as fluid conductors.
• Four positions available.
• Both piston rods retracted load at start
  position
• Short rod extended load is moved 6 inches
• Both rods extended load is moved 16 inches
• Short rod retracted/long rod extended load is
  moved 8 inches

6 inches
10 inches
45
Double rod cylinder circuit

• When required that extension and retraction occur
  at same speed these are used.
• To achieve equal speed the piston rod diameter
  and areas exposed to the system flow are equal.

46
Regeneration

• A cylinders speed is increased by
• taking the discharge flow from the rod
• end of a cylinder and adding its flow to
• the cap end.
• This is done by using a 21 cylinder which has a
  rod cross-sectional area equal to one half of the
  piston area.
• Flow and pressure are directed to both sides of
  the piston at the same time.
• The difference in piston areas being exposed to
  pressure results in a larger force being
  generated on the major piston are to extend the
  rod.
• The pump flow only has to fill an area equal to
  the cross-sectional area of the rod.

47
Regeneration

• With 21 cylinder, it extends at twice as
• fast as normal with pump flow remaining
• the same.
• Regeneration only occurs during rod extension.
• Rod speeds, extension and retraction, are
  basically the same with a 21 cylinder.
• Regeneration in a circuit gives faster extension
  speed but sacrifices with reduced output force.
• When determining the speed and force in
  regeneration the cross sectional area of the rod
  is used not the major piston area.
• Usually done in circuits with the center position
  of a directional valve or with an unloading
  valve.

48
Synchronizing Two Cylinders

• Most difficult thing to do in designing hydraulic
  systems.
• Typical values of synchronization is 1/8 to
  1/16 even with the most sophisticated control
  valves.
• Synchronization is usually short lived due to
  different wear characteristics of the cylinders
  and different reactions of the flow control
  valves to the same sets of conditions.
• Control can be better achieved by using the
  discharge flow from one cylinder and using it as
  input flow to the other. These type of circuits
  are usually equipped with replenish lines for
  piping between the cylinders.
• When two or more cylinders must stroke together
  it is recommended that their piston rods are
  mechanically connected together with a rigid
  structure.

49
Piston Seal leakage

• Rod seal leakage results in housekeeping problem,
  making it easily detectable. Piston seal
• leakage is not easily detectable.
• Wear and the leakage that results
• causes cylinder rod speed to
• decrease even though full pump
• flow enters the cylinder
• Work will be done over longer period of time but
  operating temperature will increase.
• They can leak in only portions of their strokes.
  Rod speed would be reduced through the
  contaminate area, but then would increase once it
  got past that portion of the stroke.

50
Piston Seal leakage

• Leakage can cause pressure intensification.
• Checking for leakage is accomplished by seeing
  the effect of bypass flow on rod speed.
• A needle or shutoff valve is piped into the rod
  side cylinder line, valve closed and piston
  bottomed, the cap end subjected to full system
  pressure.
• Crack the valve open moving piston short stroke,
  then close. Full system pressure is acting on
  the major area, resulting in intensified pressure
  at the rod side.
• Checks are performed at regular intervals along
  the cylinder.
• The rate at which the rod drifts determines the
  reduction in rod speed as the cylinder operates
  in a system.

51
Intensification at Cylinder rod side

• Flow control valve positioned at the rod side
  would restrict flow from the cylinder, not
  allowing the cylinder to run away from pump flow.
  (Meter-out circuit)
• Single rod cylinder pushing out a load, the
  pressure acting on the piston major area and its
  resultant force is more than is required to equal
  the load. Excessive force develops a
  backpressure on the piston effective area.
• This backpressure causes the intensification
  because of the metered out restrictions.
• Cushions are also metered out restrictions so
  they will always result in intensification when
  single rod is extending into a cushion.

52
Terms

• Back pressure check a check valve used to cause
  the generation of a system pressure level
  required for the operation of other valves.
• Load lock valve two pilot operated check valves
  in one valve body.
• P O check pilot operated check valve.