PNEUMATIC WORKING ELEMENTS

 PNEUMATIC WORKING ELEMENTS

COMPONENTS OF A PNEUMATIC SYSTEM

A Pneumatic system can be broken down into a number of levels representing hardware and signal flow. The various levels form a control path for signal flow from the signal (input) side to the work (output) side.

Diagrammatic representation of pneumatic circuit diagrams

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WORKING ELEMENTS - (CYLINDERS OR ACTUATORS)

Cylinder or Actuator is an output device, which converts Pneumatic energy to mechanical energy. The work done by a pneumatic actuator can be linear or rotary. Linear motion is obtained by piston cylinders, reciprocating motion with an angle up to 2700 by vane or rack and pinion type actuators and continuous rotation by air motors.

Linear Cylinders

Pneumatic cylinders of varying designs are the most common power components used in pneumatic automation. There are two basic types from which special constructions are derived.

• Single acting cylinders with one air inlet to produce a power stroke in one direction.
• Double acting cylinders with two air inlets to produce extending retracting power strokes.

Single Acting Cylinder

A single acting cylinder develops thrust in one direction only The piston rod is returned by a return spring or by external force from the load or spring. It may b a puss or pull type.

         

Single acting cylinders are used for clamping, marking or ejecting. They have a somewhat lower air consumption compared with the equivalent size of double acting cylinders. How ever there is an reduction in thrust due to the opposing spring force, and so a larger bore required. Also accommodating the spring results in a longer overall length and limited stroke length.

Double Acting Cylinder

With this actuator, thrust is developed in both extending and retracting directions as air pressure is applied alternately to opposite sides of a piston. The thrust available on the retracting stroke is reduced due to the smaller effective piston area, but is only a consideration if the cylinder is to pull the same load in both the directions.

    

Cushioning

Pneumatic cylinders are capable of very high speed and considerable shock forces can be developed on the end of the stroke. Smaller cylinder often have fixed cushioning i.e. rubber buffers, to absorb the shock and prevent internal damage to the cylinder. On large cylinders, the impact effect can be absorbed by an air cushion that decelerates the piston over the last portion of the stroke. This cushion traps some of the exhausting air near the end of the stroke before allowing it to bleed off more slowly though an adjustable needle valve.

               

SPECIAL CYLINDERS

Double ended piston rod

A double ended piston rod makes a cylinder stronger against side load, as it has two bearings at the widest distance possible. This type of cylinder is often mounted with rods fixed and the cylinder itself moving to displace a part.

Tandem Cylinder

A Tandem cylinder is two double acting cylinders joined together with a common piston rod, to form a single unit.

By simultaneously pressurising both cylinder chambers the output force is almost double that of a standard cylinder of the same diameter. It offers a high force from a given diameter of a cylinder; therefore it can be used where installation space is restricted.

Multi position cylinder

The two end position of a standard cylinder provides two fixed positions. If more than two positions are required, a combination of two double acting cylinders may be used.

   

The above method allows four different positions. A combination with 3 cylinders of different stroke length gives 8 positions. But rather unusual structure is required and the movement, when cylinders run in the opposite directions.

Rotary Actuators

Rack and Pinion type

The output shaft has an integral pinion gear driven by a rack attached to a double piston. Standard angle of rotation are 900 or 1800.

             

Vane type

Air pressure acts on a vane which is attached to the output shaft. The vane is sealed against leakage by a fitted rubber seal or elastomer coating. A special three dimensional seal seals the stopper against the shaft and the housing. The size of the stopper defines the rotation angle of 900, 1800 or 2700. Adjustable stops may be provided to adjust any angle of rotation of the unit.

Rodless cylinder

A conventional cylinder of say 500mm stroke may have an overall out stroked dimension of 1100mm. A rodless cylinder of the same can be installed in a much shorter space of approximately 600mm. It has particular advantage when very long strokes are required.

                    

                       

The force available from a magnetically coupled type of rodless cylinder is limited by magnetic retaining force. It equals that of a normal rod cylinder, up to 7 bar working pressure, but the dynamic shocks, a separation of the carriage from the piston is possible. Vertical movements are therefore not recommended, unless a safety margin specified by the supplier is observed.

Guided types, rodless cylinder with Mechanical coupling

       

For lifting or moving heavier load, a slotted cylinder type excludes the risk of disconnection of the carrier from the piston under dynamic shocks, but it is not totally leak free unlike the magnetically coupled type.

Slide Units

The slide unit is a precision linear actuator of compact dimensions which can be used on robotic manufacturing and assembly machines.

Precisely machined work mounting surfaces and parallel piston rods ensure accurate straight line movement when built in as part of the construction of a transfer and position machine.

In one position the body can be fixed and the rods with end bars can move. Upside down, the end bars touch the mounting surface and the body can move. In both the cases the valve an be connected to the fixed part.

Hollow rod cylinder

This actuator is specifically designed for pick and place applications. The hollow rod provides a direct connection between a vacuum source and a vacuuam pad, attached to the rods working end.

The connecting tube at the rear of the cylinder remains static, while the rod extends and retracts.

Locking cylinder

A cylinder can be fitted with a locking head in place of the standard head cover. It will hold the piston rod in any position. The locking action is mechanical, so ensuring the piston rod is securely held, even in case of pressure breakdown.

   

Air chuck

An actuator designed to grip components in robotic type applications. The type shown has two opposing pistons, to open and close the jaws.

CYLINDER PERFORMANCE CHARACTERISTICS

Piston Force

The piston force exerted by a working element is dependant on the air pressure, the cylinder diameter, and the frictional resistance of the sealing components.

The theoretical piston force is calculated using the following formulae:

Fth  = A . p 

Fth    =    Theoretical piston force in N.

A = Useful piston area in cm².
p = Operation pressure in kPa.

 

In practice, the effective piston force is significant. In calculating the effective piston force, the frictional resistance must be taken into account. Under normal operating conditions (pressure range 400-800kPa / 4-8bar), the frictional forces may be assumed to be between 3-20% of the calculated force.

Single acting cylinders

    Fn = A. p – (FR+FF)

Double acting cylinders (forward stroke)        Double acting cylinders (return stroke) 

 Fn = A. p – FR                        Fn = A’. p – FR 

Where,
Fn = Effective piston force in Newton (N)

A = useful piston area in cm² = D² x π/4 

A’ = useful piston ring area in cm² = (D² – d²) x π/4 

p = operation pressure (kPa, 105 N/m², bar)

FR = frictional force (3-20%) in Newton (N)

FF = force of return spring in Newton (N)
D = cylinder diameter in cm