Pneumatic cylinders

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A pneumatic cylinder is a component which carries out a movement using compressed air as the medium. 
 

Cylinder types

The range of pneumatic cylinders is divided into the following types.

  • Cylinders with piston rod
  • Rodless cylinders (linear drives)
  • Swivel cylinders
  • Tandem and multi-position cylinders
  • Stopper cylinders
  • Clamping cylinders
  • Drives with linear guide
  • Bellows and diaphragm cylinders 


 

 Cylinders with piston rod

Basically, cylinders with piston rods can be divided according to two different functions.

  • Single-acting cylinders
  • Double-acting cylinders


Single-acting cylinders

Zylinder einfachwirkend.png
                      Single-acting cylinders


These cylinders have only one compressed air connection. The incoming compressed air moves the piston in one direction, and the cylinder force is built up in this direction. If the piston needs to return to its initial position, the air is simply expelled from the cylinder.
The mechanical spring pushes the piston back to its initial position. This part has a ventilation/exhaust hole so that no excess or low pressure is generated through the piston movement in the second cylinder chamber.
Advantages:

  • Defined position in the event of a power failure
  • Reduced air consumption
  • Easy actuation via 3/2-way valve


Disadvantages:

  • Cylinder has a longer construction length
  • Spring-dependent stroke length limits the maximum stroke length
  • Force is only built up in one direction
  • Force is reduced by the spring force
  • No constant force (stroke-dependent)


Double-acting cylinders

Zylinder doppeltwirkend eingefahren.png               Zylinder doppeltwirkend ausgefahren.png


               Double-acting cylinder - retracted                                              Double-acting cylinder - advanced


The double-acting cylinder requires compressed air for every direction of movement. On this type of cylinder, the force both both the advancing and retracting direction is built up using compressed air. The simplest way of actuating a double-acting cylinder is by using a 5/2-way valve.
Advantages:

  • Force builds up in both directions of movement
  • Constant force (dependent on stroke)
  • Strokes of several metres are possible


Disadvantages:

  • Every movement uses compressed air
  • No defined position in the event of compressed air failure


Design of a cylinder with piston rod

A standard pneumatic cylinder consists of five modules/parts.


Zylinder Aufbau.png 


  • Cylinder barrel
  • Bearing cap
  • End cap
  • Piston
  • Piston rod

Of course, that's usually not everything that makes up a cylinder. There are also various smaller components such as seals, bearings, guiding band, permanent magnets, etc. But these parts are all included in the above-mentioned five parts which make up a standard cylinder (cylinder with single-ended piston rod).


Cylinder barrels

Originally, these really were "just" tubes. However, nowadays extruded profiles instead of a tube are used for most cylinders. The advantage is that a profile can also be used for additional functions.

  • Mounting the sensors
  • Mounting option for attachment parts
  • One-sided pressurisation of double-acting cylinders

Piston rods

The piston rod is the part which transmits the force and the movement of the cylinder to the outside. The tip of the piston rod generally has a thread so that other customer components can be attached to it.


 

Pistons

The piston, which is connected to the piston rod, carries out the actual movement in the cylinder.
However, the piston needs to do more than just carry out a movement. It forms a seal between the front and rear cylinder chamber. In addition, the piston has to convert the kinetic residual energy in the end position. The bearing and end caps also have their part to play.


Bearing caps

The bearing cap closes the cylinder (cylinder barrel) on one side and at the same time forms a bearing and sealing point for the piston rod. One of the air connections is generally located in the bearing cap.


End caps

The end cap closes the cylinder (cylinder barrel) on the other side. The second air connection is usually located in the end cap.


Rodless cylinders

"Rodless cylinders" are usually cylinders which have no piston rod and which carry out a linear motion.
Although pneumatic rotary drives don't have a piston rod either, and, strictly speaking, are also rodless cylinders, they are classified as rotary drives and will be looked at separately.
Rodless cylinders are also defined as linear drives.
Rodless cylinders can be found on the market in various designs, as simple drives and as drives with integrated guides.
Thanks to this additional, external guide (as a plain-bearing or roller bearing), it is possible to load the slide with lateral forces and torques.
Tools or other drives can be mounted directly on the slide.
This makes it relatively easy to build, for example, multi-axis systems for part handling.

Rodless cylinders have two different functional principles

  • Mechanically coupled slide 
  • Magnetically coupled slide


Mechanically coupled slides

 

Linearantrieb Schnitt de.png

The piston is moved in the cylinder barrel using compressed air. The cylinder barrel is open on one side across its entire length, so that a mechanical connection can be established between the piston and slide. A sealing strip is clamped over the entire length to seal this open side. It is fed through the top side of the piston to ensure that there is a tight seal between the piston and slide, despite the mechanical connection. To protect the sealing strip from mechanical influences and dirt, a thin metal cover band is clamped parallel to it on some cylinder types. After applying pressure, the sealing band is pressed against the housing so that it is completely tight.


Linearantrieb1 de.png                             Linearantrieb2 de.png
Direction of movement of the slide to the right                                                          Direction of movement of the slide to the left

The air is supplied from one side for both directions of movement. The air is also fed to the opposite side through a duct along the housing.


Magnetically coupled slides

Linearantrieb Magnet Schnitt de.png

In this design, the piston moves in a completely enclosed, thin steel pipe. That is also one of the main advantages of this design. After all, with this system, which has practically zero leakage, the drive is also suitable for use under clean room conditions.
The piston and slide are connected using permanent magnets.
They are partially integrated in the slide and partially in the piston. Their north/south alignment is parallel to the longitudinal axis of the drive. The magnetic coupling is also a force limiter. If the slide can no longer be moved by external forces, the piston can release itself from the magnetic field and continue to move by itself. Like other cylinders, this drive has built-in magnets for sensing the end positions using proximity sensors. These magnets are located in the slide, and not on the piston.


Cylinder force

What force can a cylinder exert?
The theoretical force of a pneumatic cylinder can be calculated using the following formula:
F [N] = p [bar] x A [cm²] x 10 
For example: a cylinder with a nominal diameter of 100 mm has an area of 78.5
cm². At an operating pressure of 6 bar, this area exerts a force of approx. 4700 N (78.5 x 6 x 10).


Links

Article in Wikipedia on pneumatics