LESSON 7: CHECK VALVES, CYLINDERS AND MOTORS

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LESSON 7 CHECK VALVES, CYLINDERS AND
MOTORS
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Check Valves

• Air flows through the valve in only one
  direction.
• Often used as bypass valves.

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Cylinders

• Convert fluid power energy into straight-line
  mechanical
• Consists of body, a movable piston and a piston
  rod attached to the piston.
• Piston rod is guided by bushing and seal assembly
  (rod gland).
• Side rod protrudes on is called the head
• Side without the rod is called the cap

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Seals and Stroke Adjusters

• A positive seal must exist across the piston and
  the rod gland
• Usually use a resilient seal which will not leak
  under normal conditions, typically o-rings or lip
  seals are used.
• A rod gland seal typically has a lip seal to give
  a positive seal and a wiper seal that wipes the
  rod clean during retraction and deposits a thin
  film on the return stroke.
• Stroke must be adjusted to specific limits and
  usually done with a threaded rod which serves as
  a stop for the piston.

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Mounting, Motions, and Loads

• Mounts are typically flange, trunnion, side lug
  and side tapped, clevis, tie rod, and bolt
  mounting.
• Cylinders can provide varieties of non-linear
  motions depending on how their attached to
  mechanical linkages.
• Load pushed by a cylinder rod called thrust or
  compressive load.
• Load pulled by a cylinder rod called tension load.

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Types of Cylinders

• Single acting air pressure applied in only one
  direction
• Spring return spring returns the piston
  assembly
• Ram the movable element is the piston rod
• Double acting air applied alternately to piston
  to drive movable element in either direction
• Single rod a piston rod extending from one end
• Double rod single piston and piston rod
  extending from each end, rods may be different
  diameters

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CLYINDER TYPES
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Sizing a Cylinder

• First total load evaluation must be made this
  must include-
• Basic load
• Force and friction needed to accelerate load
• Force needed to exhaust air from other end of
  system
• Any other force that must be overcome
• Pressure must be assumed, but must be the
  pressure seen at the piston when in motion.
• Now using Pascals law (F P x A) the size of
  the cylinder can be determined.

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Stop Tube

• A metal collar that fits over the
• piston rod which keeps the
• piston and rod gland bushing
• separated when the long stroke
• cylinder is fully extended.
• This reduces the load on the bushing.
• Selection depends on the basic length and maximum
  thrust.
• Basic length stroke x stroke factor
• Function of actual stroke of the cylinder and
  type of cylinder mount.
• Stroke factor is determined from charts
  increases as cylinder becomes less rigid.

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BUCKLING

• Cylinders fail by buckling
• when the load induces rod stresses
• far less than the elastic
• limit of the material
• Reasons
• Axial loading resulting thrust forces on rod
• Bending moment placed on the rod by the load
  due to eccentric loading on torque
• Type of end conditions existing on the cylinder

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CUSHIONS

• When pistons stop abruptly,
• the excess shock damages the
  cylinder. To stop this, cushions
  are used
• They absorb the kinetic energy by slowing down
  the piston movement just before reaching the end
  of its stroke.
• Major consideration is that the maximum pressure
  developed by the cushion doesnt exceed the
  maximum cushion pressure of the cylinder.

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CUSHIONS

• When determining whether the cylinder is large
  enough to absorb the energy of the load to be
  cushioned certain things must be known
• Total weight to be moved
• Maximum piston speed
• Distance available for deceleration
• Direction of load horizontal or vertical,
  thrust or tension
• Load friction
• Whether back pressure exists in the cushion
  cavity before cushioning takes place

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Flow Rate into Cylinder

• Amount of air needed to move the piston load and
  to force out exhaust air from the other side of
  the cylinder at a specific speed
• Determine volume of cylinder
• V A (in2) x S (in) (A is the area S is the
  stroke)
• Determine compression ratio
• Pressure at cylinder Absolute Pressure
• Absolute Pressure at sight

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Flow Rate into Cylinder

• Next calculate the flow rate which is measured in
  Cubic Feet per minute
• CFM V x compression ratio
• Time to fill cylinder x 28.8

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Pneumatic Tools

• Turn pneumatic energy into linear or rotary
  energy
• Two types
• Percussion such as the riveting hammer,
  scalars, chipping hammers
• Rotary - turbines

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Percussion Tools

• Works off the reciprocating action of a piston
  within a cylinder, i.e. progressively loading and
  unloading a piston
• Power obtained by varying the mass and velocity
  of the piston and the number of blows per second.
• They can be medium or long stroke with light,
  medium, or heavy pistons.

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Rotary Tools

• They are compact light weight units allowing wide
  variable control of torque and speed.
• Work as a result of an imbalance of pressure
  across a moving member resulting in generation of
  torque and rotation.
• Torque is a force present at a distance from the
  motor shaft and is measured in lb. in.
• Torque Force (lb) x Perpendicular distance
  from shaft (in)

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Rotary Motors

• Hydrostatic
• Piston motor low power, low speed, with a high
  price to power ratio
• Vane motor develops an output torque at its
  shaft by allowing compressed air to act on vanes.
  Most common in industrial systems.

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Rotary Motors

• Hydrokinetic
• Turbine motor compressed air is allowed to pass
  through a nozzle so that air can expand and its
  speed can be increased. Rotates at speeds near
  20,000 RPM

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Selection of Air Motor

• Things to Consider
• Horsepower
• Starting and running speed
• Torque
• Motor should deliver the horsepower at about 65
  of the available line pressure
• Type of motor is often determined by the required
  application.