In many industries, creation of a linear motion during an operation sequence is often required. A pneumatic cylinder, which is also referred to as an air cylinder, is one of the simplest, most cost-efficient solutions. Pneumatic cylinders are mechanical devices that convert the energy of compressed air to a linear motion.
The main components of a typical pneumatic cylinder are: cap-end port (A), tie rod (B), rod-end port (C), piston (D), barrel (E), and piston rod (F). Figure 1 and the function described below is for a double-acting pneumatic cylinder.
Figure 1: Standard components of a pneumatic cylinder
As shown in Figure 1, the cylinder barrel is sealed on both ends with a head cap and an end cap. Inside this cylinder, a piston drives the rod in a linear manner. When compressed air enters through the cap-end port, the piston moves away from the end cap and pushes the rod out. This movement is called the positive/plus movement and the chamber associated with this movement is called the plus chamber. The minus chamber is located on the opposite side. When compressed air enters the rod-end port the rod is pushed back to the negative position.
Figure 2: The movement of the piston and rod with air going in (blue arrow) and air coming out (grey arrow). Left image shows positive movement with a plus chamber (A), while the right image shows negative movement and a minus chamber (B).
The cylinder diameter is the inside diameter of the cylinder or the diameter of the piston. The stroke length identifies how far the piston/piston rod can travel. The diameter and stroke of a pneumatic cylinder are two important attributes by which it is identified.
Pneumatic cylinders can be either single-acting or double-acting.
In a single-acting cylinder (SAC), air is only supplied to one side of the piston and is responsible for the movement of the piston in only one direction. The movement of the piston in the opposite direction is performed by a mechanical spring. A single-acting cylinder can be designed to be with base position minus (spring return) or base position plus (spring extend) depending on whether the compressed air performs the out stroke or the in-stroke respectively. In case of pressure or power loss, a single-acting cylinder has the advantage of returning the piston to a base position.
A downside of single-acting cylinders is the inconsistent output force through a full stroke due to the opposing spring force. Furthermore, the stroke of a single-acting cylinder can be limited due to the space the compressed spring takes up and spring length availabilities. The construction length of a single-acting cylinder is therefore longer than its actual stroke.
In a double-acting cylinder (DAC), air is supplied to chambers on both sides of the piston. Higher air pressure on one side can drive the piston to the other side. Double-acting cylinders are the most common type, as they give the user full control.
The advantages of double-acting cylinders are their longer strokes (up to several meters) and constant output force through a full stroke. These cylinders provide relatively better control and operate at higher cycling rates. The drawbacks of double-acting cylinders are their need for compressed air for movement in both directions and a lack of a defined position in case of a power or pressure failure.
Figure 3: A pneumatic cylinder with an installed position sensor
To detect the position of the piston, the piston can be equipped with a magnet. Body mounted sensors can then receive information from the magnetic field created and therefore recognize the position of the piston in the cylinder. Reed switches and hall effect sensors are the most commonly used sensor types.
The movement of the piston in a pneumatic cylinder can be very fast as the compressed air enters the cylinder. This can create a hard shock when the piston hits the head or end cap. This imposes stress on cylinder components, makes a noise, and transmits vibration to the machine structure. To prevent this, the piston can be decelerated at the vicinity of the caps by cushioning. Cushioning can also prevent the piston from rebounding (bouncing) off the end position. Most cylinders are equipped with end-of-stroke cushioning in one of the following ways:
In smaller cylinders where the impact is not that high, a flexible material is used at the cap end/head. This material is often made from elastomers and comes in the form of a ring. These bumpers are either integrated as a part of the piston or at the head and end caps. This type of cushioning is most suitable for slow operating speeds, low loads and shorter strokes.
In larger cylinders with higher piston speeds or stronger forces, the shock absorption can be achieved by trapping a certain volume of air in the end position. At the end of the stroke the air will be compressed to generate a breaking effect. For this purpose, throttling non-return valves are installed directly on the end ports of the cylinder. This allows the free inflow of pressurized air while allowing the adjustment of the area of the exhaust port with an adjusting screw. This method of cushioning is wear-free and offers optimal cushioning performance. Depending on the operating pressure and the cylinder force, the screw settings on the cylinder need to be adjusted for ideal cushioning. Too much cushioning results in slow strokes and too little cushioning increases the end-of-stroke shock.
In this method, the exhaust air escapes through slots in a cushioning boss. The cross-section of this exhaust depends on the stroke. This will allow for the cushioning to automatically adjust to different levels of energy by changing loads and speeds.
Pneumatic cylinders have been standardized to be interchangeable with products of different manufacturers. Therefore, in a standard cylinder, the mounting dimensions, cylinder bore and stroke, piston rod characteristics and air ports depend on the type/standard. However, there are still numerous non-standard cylinders for special applications.
ISO 6432 is a metric ISO standard applicable to single rod pneumatic cylinders with bores from 8 mm to 25 mm and maximum working pressures of up to 10 bars (1000 k Pa). This standard establishes a metric series of mounting dimensions required for the interchangeability of the cylinders. The ISO 6432 is a perfect compact cylinder line suitable for automations systems in analytics, diagnostic instrumentation, bottling, automotive and commercial kitchen and laundry equipment.
Figure 4: Mindman ISO 6432 pneumatic cylinders
ISO 15552, which has replaced ISO 6431, establishes metric mounting dimensions, bore sizes, mounting styles, piston rod characteristics and strokes for single or double rod pneumatic cylinders with maximum working pressures of up to 10 bar (1000 k Pa) and bore sizes from 32 mm to 320 mm. This standard applies to cylinders with detachable mountings. VDMA 24562 is common in Germany and is for profile and tie-rod cylinders.
Figure 5: Mindman ISO 15552 pneumatic cylinders
ISO 21287 applies to single rod compact pneumatic cylinders with maximum working pressures of up to 10 bar (1000 k Pa) and bore sizes from 20 mm to 100 mm. This pneumatic cylinder series is not equipped with adjustable cushioning.
Figure 6: Mindman ISO 21287 pneumatic cylinders
ISO has developed well-defined symbols for pneumatic cylinders to distinguish their function in simple schemes. These symbols are independent of Cylinder ISO standard, diameter or stroke.
|Double-acting cylinder||Standard design|
|Double-acting cylinder with magnetic piston||the piston is different from figure 1, indicating the magnetic piston|
|Double-acting cylinder with adjustable cushioning||Cushioning symbolized by two rectangular objects; adjustable symbolized by an arrow|
|Double-acting cylinder with adjustable cushioning and magnetic piston||Combination of figure 2 and 3|
|Double-acting cylinder with through piston rod, adjustable cushioning and magnetic piston||The through piston rod is added|
|Single-acting cylinder (minus)||Single-acting cylinder with spring in minus chamber|
|Single-acting cylinder (plus)||Single-acting cylinder with spring in plus chamber|
The diameter of a cylinder is directly proportional to the amount of force it can generate from an input air pressure:
F = (P x A) - f
F: Cylinder’s Force (N)
P: Air Pressure (MPa)
A: Piston Area (mm2)
f: Friction Drag (N)
A longer stroke requires a longer piston rod, which translates into higher stress on the rod. In these applications, often larger cylinders that can accommodate larger piston rod diameters are recommended.
In single-acting cylinders some of the work is lost due to the opposing spring force. It is important to account for the reduction of output force in sizing calculations of these cylinders.
When cylinder diameter and stroke are determined, cylinder mounting arrangements and configuration must be decided. There are various mounting options offered by manufacturers.
Among other selection criteria are end-of-stroke cushioning options and position detectors.
Here is a list of characteristics to consider when selecting a pneumatic cylinder: