LESSON 9: FLOW CONTROL VALVES, SILENCERS, QUICK EXHAUST

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LESSON 9 FLOW CONTROL VALVES, SILENCERS,
QUICK EXHAUST
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Flow Control Valves

• It reduces the flow rate in a leg of a circuit
  and consequently slows the actuator speed by
  acting as a restriction.
• Orifices are the main way of controlling flow,
  these are affected by
• Size of orifice
• Length of restriction
• Pressure differential across the orifice
• Temperature of the fluid
• Orifices can be of fixed size or adjustable

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Orifices

• Fixed reduced opening of
• non-adjustable size. Example-
• pipe plug or check valve with
• hole drilled through center.
• Adjustable Reduced openings
• that can be adjusted to match the
• needs of the circuit
• Ball valve the size is changed by turning the
• handle which positions a ball with a cross
  drilled
• hole across the air path.
• Globe valve Fluid must bend several times as
• it passes through an opening, which is the seat
• of the globe. The size is changed by the
  positioning of
• the globe
• Needle valve Fluid must turn several times to
  pass through an opening, which is the seat for a
  rod with a cone-shaped tip. Size is changed by
  positioning of the cone to its seat.

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FLOW CONTROL

• Most common type is needle valve with a bypass
  check valve because it can be changed very
  gradually due to the fine threads on the stem and
  the shape of the cone.
• Sandwich flow controls to be inserted between
  a valve and its sub-plate adding integral
  capability to a directional control valve for
  controlling cylinder speed, extension and
  retraction. Improved speed is achieved when it
  is mounted closer to the cylinder, monitors the
  air flowing out of a cylinder

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Accuracy of Flow Control

• meter out measures the air coming out of a
  cylinder (Sandwich controls), for accuracy the
  load must remain essentially constant.
• meter in control flow into an actuator used
  in conjunction with meter-out when jump occurs in
  some circuit conditions.
• Uniformity of the applied load
• Friction forces
• Acceleration forces
• All forces that affect the force balance of the
  cylinder

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Flow control placements

• Single Acting Application
• Air exhausting from the cylinder must pass
  through the variable restriction (flow control)
  and the retraction rate will be controlled

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Flow control placements

• Double acting four ported four way valve
• The ratio of flow into the cylinder is controlled
  by how fast air is allowed to exhaust through the
  flow control
• Flow control valves are placed between the
  directional valve and the cylinder, as well as
  close to the cylinder as possible

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Pneumatic flow control problems

• jump or rapid partial stroking of the cylinder
  may occur.
• May occur when starting the cylinder in any
  position.
• Will continue until the cylinder is balanced
  again.
• Can occur if the directional control valve is
  shifted before the cylinder completes it stroke
  or if the valve is shifted too soon after stroke
  is completed.

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Quick Exhaust Valves

• Consists of resilient disc in a
• body that shuttles side to side
• changing the free flow paths through
• ports in the body.
• Used in conjunction with a three or
• four-way directional control valve to
• increase the exhaust flow and thus rod speed of
  the cylinder
• Should be connected to the cylinder port to give
  rapid cylinder velocities.
• Can be used as a shuttle valve which are
  automatic flow path selectors which allow the
  higher of two pressures to be directed into a
  flow path

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Silencers

• Exhausting air to the atmosphere can be loud and
  can cause damage to ears after long exposure and
  block out warning signals. To solve this problem
  mufflers are necessary
• Designs can range from a serious of baffles to a
  restricting device such as porous plastic or
  bronze
• Selection should be made so that OSHA noise
  requirements are met
• Typically maximum flow is given a particular
  sound pressure level.
• Good one should
• Have a low resistance to flow
• Provide sound attenuation
• Be corrosion resistant