Pneumatic Cylinders

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Pneumatic Cylinders

• Chapter 3

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Basic Operation

• A cylinder is a device used to produce linear
  motion.
• It is composed of 2 basic assemblies, the barrel
  and head assembly and the piston and piston head
  assembly.
• As compressed air is applied to the rear of the
  piston and the chamber in front of the piston is
  open to exhaust.

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Basic Operation

• To retract the cylinder, the air on the rear of
  the piston is exhausted to atmosphere and
  compressed air is applied to the front of the
  piston.
• The pneumatic cylinder is composed of several
  precision machined parts.
• Components common to most cylinders include the
  barrel, the front and rear head, the piston, the
  piston rod and a number of seals.

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Cylinder Components

• The barrel
• Is a carefully machined cylindrical tube.
• Usually made of aluminum, brass, or stainless
  steal.
• It forms a chamber which contains the air and
  provides a surface for the piston packing to seal
  against.
• It is closed at each end by heads.
• Each head may also contain provisions for
  mounting the cylinder

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Cylinder Components

• They are available in two styles round head and
  square head.
• The rod bearing
• These give support to the piston rod and guides
  it during its stroke.
• These are made of bronze, nylon, or other bearing
  material and may be pressed into the head or
  placed in a replaceable cartridge.

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Cylinder Components

• The rod seal
• Prevents air from escaping between the piston rod
  and the front head.
• They are usually of o-ring or u-cup design and
  are usually made of buna-N rubber.
• The rod wiper
• Is designed to prevent dirt and foreign material
  from entering the cylinder and damaging the rod
  bearing and seal

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Cylinder Components

• The piston seal is designed to prevent air from
  passing between the piston and barrel.
• O-ring seals are the most popular and are usually
  standard in round line cylinders
• They come in a variety of materials and provide a
  simple, economical means of sealing the piston to
  the barrel.

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Cylinder Mounts

• Cylinder mounting styles are
• Centerline, foot, or pivot.
• Centerline Mounts
• Centerline mounts provide the most secure means
  of mounting a cylinder.
• With this mount the thrust is on the same plane
  as the mount thereby removing compound forces
  from the mounting bolts

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Cylinder Mounts

• This type of mount is very strong but requires
  accurate alignment
• Foot mounting
• Foot mounted cylinders are rigidly mounted, but
  the mounting plane does not run through the
  centerline of the cylinder.
• This places compound forces on the mounting bolts
  and creates higher stresses than centerline.

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Cylinder Mounts

• Pivot Mounting
• Some applications, such as moving a lever,
  require a cylinder to move through an arc as it
  cycles.
• To move a cylinder through an arc it is necessary
  to pivot mount the cylinder.
• The 2 types of pivot mounts are clevis and the
  trunnion.

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Cylinder Mounts

• Clevis Mount
• Is attached to or is an integral part of the
  cylinder.
• This gives a pivot point at the rear of the
  cylinder.
• Normally a rod clevis is used with a clevis
  mount to provide a pivot point at the rod end.

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Cylinder Mounts

• Trunnion Mount
• A trunnion can be attached at any point along the
  cylinder.
• This allows pivoting from the front or middle of
  the cylinder which reduces some of the side load
  inherent in pivot mounting.
• A rod clevis is recommended for use with trunnion
  mounting.

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Cylinder Types

• Cylinders are available in a variety of types to
  meet the requirements of the various situations
  in which they are applied.
• They fall into 2 major categories
• Single acting and double acting.

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Cylinder Types

• Single acting cylinders
• A single acting cylinder is any cylinder which
  uses air on only one stroke.
• It uses some other means, usually a spring to
  move the piston on the other stroke.
• The stroke using air is the working stroke.
• They are usually classified as push type and pull
  type.

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Cylinder Types

• Push type cylinder
• Push type cylinders use air to extend the piston.
• When this air is exhausted the spring returns the
  piston to its retracted position.
• Pull type cylinder
• A pull type cylinder uses air to retract the
  piston and the spring extends the piston when the
  air is removed.

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Cylinder Types

• Single acting cylinders are often used for
  clamping applications and for those applications
  where the cylinder must move to a certain
  position whenever the air is removed.
• A typical application would be to open and close
  gates in a grain elevator.
• If air pressure is lost the cylinder would close
  the gates to prevent loss of grain.

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Cylinder Types

• Double Acting Cylinders
• Is a cylinder designed to use air to extend and
  retract the piston.
• Double acting cylinders are far more popular than
  single acting.
• They are available in single rod end and double
  rod end, cushioned and non-cushioned.

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Cylinder Types

• Although double acting cylinders are designed to
  use air on both strokes.
• They may be used as single acting cylinder with
  load weight, counter weights or external springs
  to return the cylinder.

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Cylinder Types

• Single Rod End
• The single rod end, double acting cylinder are by
  far the most popular cylinder in industry.
• It is used to move parts and levers, position
  tools, control product flow, assemble products,
  and package goods.
• They are available in cushioned or non cushioned.

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Cylinder Types

• Double Rod End
• The double rod end cylinder is a specialized
  cylinder which has a piston rod extending from
  the piston through each end of the cylinder.
• It can be used in applications where 2 work
  stations are being used alternately.
• This allows one work station to be loaded and
  unloaded while the other is being worked.

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Cylinder Types

• The second common application is where one end of
  the cylinder is used to do the intended work and
  the other end is used to actuate limit valves
  which control the work.
• Cushioned Cylinders
• Are designed to reduce the speed of the piston
  during the last fraction of an inch of the
  stroke.
• This reduction in speed, when properly applied,
  reduces shock and vibration which means less
  noise, wear, maintenance.

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Cylinder Types

• However they have little effect when applied to a
  massive load moving at a rapid speed.
• In such applications an external means of
  cushioning should be used.
• Non Rotating Rod Cylinders
• Some applications call for cylinders in which the
  rod will not rotate inside the cylinder.

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Cylinder Types

• Rodless Cylinders
• These cylinders are available for applications
  which require long strokes and must operate in
  limited spaces.
• A conventional cylinder takes up at least twice
  its stroke in linear space.
• A rodless cylinder occupies little more than its
  stroke length.
• The 2 types of rodless cylinders available are
  cable and the magnetic band cylinder.

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Cylinder Types

• Cable Cylinders
• These double acting cylinder consist of a piston
  which reciprocates within the barrel and a
  flexible cable which is attached to both sides of
  the piston and runs out each end of the cylinder.
  
• The load is attached to this bracket which
  reciprocates opposite piston.

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Cylinder Types

• Magnetic Band
• This cylinder has a lug mounted to the side of
  the piston which projects through a slot in the
  extruded aluminum barrel.
• The slot is sealed by two stainless steel bands,
  one inside and one outside, held in place with
  permanent magnets.
• As the piston moves, the jug separates the bands
  between the piston and recloses them after it has
  passed through.

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Cylinder Types

• This cylinder has all the features of the cable
  cylinder and has eliminated most of the problems.
• They are available in lengths up to 30 feet and
  generate a force up to approximately 440 pounds.
• Tandem Cylinders