# Pneumatic Cylinders

Figure 1: Pneumatic cylinder

A pneumatic cylinder is a mechanical device that converts compressed air energy into a reciprocating linear motion. A double-acting cylinder uses compressed air to move a piston in and out, while a single-acting cylinder uses compressed air for one-way movement and a return spring for the other. They have numerous accessories, like sensors to detect the position of the piston and different mounting accessories to mount the cylinder or add components to the end of the piston. A wide range of industries requiring linear motion use pneumatic cylinders since they are simple to use and are a cost-efficient solution. They are also referred to as air cylinders.

## Pneumatic cylinder parts

Figure 2 shows the main components of a double-acting pneumatic cylinder. They are cap-end port (A), tie rod (B), rod-end port (C), piston (D), barrel (E), and piston rod (F). A single-acting cylinder will only have either a cap-end port (A) or a rod-end port (C) and utilize a mechanical spring for the secondary motion. The pneumatic cylinder barrel (E) is sealed on both ends with a head cap and end cap. The compressed air (or spring) moves the piston (D) and subsequently the piston rod (F). The stroke length of a pneumatic cylinder is how far the piston rod can extend.

Figure 2: Double-acting pneumatic cylinder parts

## Pneumatic cylinder working principle

### Double-acting pneumatic cylinder

Double-acting pneumatic cylinders are the most common type since they give the user complete control of the piston movement. Figure 3 shows how the piston and piston rod move when compressed air enters the cap-end port and the rod-end port. A negative position is when the piston rod is retracted, and a positive position is when the piston rod is extended. When compressed air enters the cap-end port, it pushes the piston forward (positively), extending the piston rod (shown in Figure 3 A). Air is forced out of the rod-end port. To retract the piston rod, compressed air enters the rod-end port, forcing air out of the cap-end port, and forcing the piston to retract to the negative position (shown in Figure 3 B).

Figure 3: Double-acting cylinder working principle with air going in (blue arrow) and air coming out (grey arrow). The left image shows positive movement (A). The right image shows negative movement (B).

Double-acting pneumatic cylinders allow the user full control, longer piston stroke length, and a constant output force through the entire stroke. They can also operate at higher cycling rates. However, a double-acting cylinder should not be used if the application requires a base position during fail-safe scenarios encase there is a loss in compressed air. Since they use compressed air for both directions, they also use more energy

Figure 4: Double-acting cylinders can be used in a wide variety of ways, here we see a vacuum pick and place application that uses a pneumatic cylinder to move the position of the suction cup.

### Single-acting pneumatic cylinder

A single-acting pneumatic cylinder only uses compressed air to drive the piston in one direction. A mechanical spring moves the piston in the opposite direction. Figure 4 shows the two design possibilities. Either the spring extends (Figure 4 A) or retracts (Figure 4 B) the piston. Single-acting cylinders are often used for fail-safe applications where it is required that the piston is in a certain position upon compressed air loss. Therefore, single-acting pneumatic cylinders have a "base" position.

Due to the mechanical spring, single-acting pneumatic cylinders do not provide a consistent output force throughout the full piston stroke length due to the opposing spring force. Furthermore, the stroke of single-acting cylinders is limited due to the space of the compressed spring. Therefore, the construction length of single-acting cylinders is longer than the actual stroke length.

Figure 5: Single-acting pneumatic cylinder working principle. Compressed air is used to move the piston in one direction, and a spring either extends the piston (A) or retracts it (B).

## Standards

Pneumatic cylinder designs typically adhere to ISO standards, allowing them to be interchangeable with products of different manufacturers. Therefore, the mounting dimensions, cylinder bore, stroke, piston rod characteristics, and air ports depend on the type/standard and use. However, there are still numerous non-standard cylinders for special applications.

### Round ISO 6432 (8-25 mm)

ISO 6432 is a metric ISO standard applicable to single rod pneumatic cylinders with bores from 8 mm to 25 mm and a maximum working pressure of up to 10 bars (1000 k Pa). They are commonly referred to as mini air cylinders or round cylinders. This standard establishes a metric series of mounting dimensions required for the interchangeability of the cylinders. This pneumatic cylinder standard does not have manual damping adjustment. The ISO 6432 is a perfect compact cylinder line suitable for automation systems in diagnostic instrumentation, bottling, automotive and commercial kitchen, and laundry equipment. View our online selection of ISO 6432 pneumatic cylinders.

### Profile ISO 15552 (32-320 mm)

ISO 15552 establishes metric mounting dimensions, bore sizes, mounting styles, piston rod characteristics, strokes for single or double rod pneumatic cylinders with a maximum working pressure 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. The ISO 15552 pneumatic cylinder series have adjustable cushioning, which helps to achieve perfect dampening. Therefore, ISO 15552 cylinders are suitable for the efficient movement of large loads. They are generally used in general automation systems in machine and systems constructions, the food and beverage industry, etc. ISO 15552 has replaced the older standards of ISO 6431 and VDMA 24562. View our online selection of ISO 15552 pneumatic cylinders.

### Compact ISO 21287 (20-100 mm)

ISO 21287 applies to single rod compact pneumatic cylinders with a maximum working pressure 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. However, there are rubber bumpers at both ends for cushioning. The ISO 21287 pneumatic cylinder series are compact and lightweight, thus desirable for applications that have space limitations. View our online selection of ISO 21287 pneumatic cylinders.

Figure 6: ISO 21287 pneumatic cylinders

### Rodless cylinders

A rodless cylinder is similar to pneumatic cylinders in that they use compressed air to move a load in a linear path. However, a rodless cylinder moves the load alongside the piston rather than pushing or pulling the load. Therefore, there is no piston rod buckling, same force in both directions, and they are more compact for the same stroke length. Rodless cylinders are commonly used for material handling, loading, lifting, web cutting, etc. Read our rodless cylinder technical article for more information. View our online selection of rodless cylinders.

## Cushioning

The movement of the piston in a pneumatic cylinder can be very fast as the compressed air enters the cylinder. This fast movement can create a hard shock when the piston hits the head or end cap. This shock imposes stress on the air cylinder components, makes a noise, and transmits vibration to the machine structure. To prevent this, cushioning at the caps is used to decelerate the piston. Cushioning can also prevent the piston from rebounding (bouncing) off the end position. Most pneumatic cylinders have end-of-stroke cushioning in one of the following ways:

### Flexible shock absorbers

For smaller air 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 best suited for slow operating speeds, low loads, and shorter strokes.

For larger pneumatic cylinders with higher piston speeds or stronger forces, shock absorption is achieved by trapping a certain air volume in the end position. At the end of the stroke, the air is compressed, generating a braking effect. For this purpose, throttling non-return valves are installed directly on the end ports of the cylinder. This allows for the free inflow of pressurized air and exhaust port to be adjustable with a 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 are adjusted for ideal cushioning. Too much cushioning results in slow strokes and too little cushioning increases the end-of-stroke shock.

In this cushioning method, the exhaust air escapes through longitudinal slots on the inside of the cylinder. The cross-section of this exhaust depends on the stroke. This will allow the cushioning to automatically adjust to different energy levels by changing loads and speeds. Therefore, allow for optimal cushioning with no manual intervention.

## Accessories

### Piston position feedback

Pneumatic cylinder sensors are used to provide piston position feedback to a control system in automated machinery and equipment. It is typically standard that the piston has a magnet inside the cylinders body. Then, a sensor can be mounted onto the pneumatic cylinders body, like Figure 6 shows, to provide the pistons position. Depending on where the sensor is mounted, it can detect extension, retraction, or individual positions along the cylinder body. If multiple position feedback is needed, multiple sensors can be mounted to the cylinders body.

Reed sensors are the most common type of sensor as they have a long life cycle (over 10 million) and are typically not the first point of failure for high shock or vibration applications. Read our pneumatic cylinder sensor article to learn more about how they work. View our online selection of pneumatic cylinder sensors..

Figure 7: A pneumatic cylinder with a sensor (C) mounted via set screw (B) with a screw driver (A)

### Pneumatic grippers

A pneumatic gripper is a pick-and-place device that uses compressed air to operate gripper jaws, also called fingers. They typically have two or three fingers and have an internal pneumatic cylinder to operate and control them. They are mostly used in automated manufacturing processes to grip a workpiece. Read our pneumatic gripper article to learn more about how they work. View our online selection of pneumatic grippers.

Figure 8: Pneumatic grippers

### Mounting accessories

Mounting accessories are used to mount the pneumatic cylinder or for coupling the piston rod to a load. They are typically designed based on the ISO standard of the pneumatic cylinder. Mounting accessories will affect system performance, reliability, and overall design. Flanges, food mounted, pivots, angle brackets, spherical eye, etc., are just some of the different mounting accessories. Read our pneumatic cylinder mounting accessories article to learn more about the mounting options. View our online selection of pneumatic cylinder mounting accessories..

## Symbols

ISO has developed well-defined symbols for pneumatic cylinders to distinguish their function in schematics. These symbols are independent of pneumatic cylinder ISO standards, diameter, or stroke.

 Double-acting cylinder Double-acting cylinder with magnetic piston Double-acting cylinder with adjustable cushioning Double-acting cylinder with adjustable cushioning and magnetic piston Double-acting cylinder with through piston rod, adjustable cushioning and magnetic piston Single-acting cylinder (minus) Single-acting cylinder (plus)

## Selection Criteria

1. Cylinder Diameter: The diameter of the pneumatic cylinder is directly proportional to the amount of force it can generate from an input air pressure. For single-acting pneumatic cylinders, the opposing spring force should also be taken into consideration.

F = (P x A) - f

• F: Cylinder’s Force (N)
• P: Air Pressure (MPa)
• A: Piston Area (mm2)
• f: Friction Drag (N)
2. Stroke Length: Ensure that the piston rod can extend or retract the required length. It is also important to note that longer piston rods puts higher stress on the rod and requires a longer pneumatic cylinder.
3. Mounting Style
4. Position Feedback
5. Cushioning
6. Operating pressure
7. Connection size

### What does a pneumatic cylinder do?

A pneumatic cylinder is a mechanical device that converts compressed air energy into a reciprocating linear motion.

### Can pneumatic cylinders stop mid-stroke?

Double-acting cylinders are capable of stopping mid-stroke. For high accuracy applications, special locking cylinders and position feedback should be used.

### How do pneumatic cylinders work?

A double-acting pneumatic cylinder uses compressed air to move a piston in and out, while a single-acting pneumatic cylinder uses compressed air for one-way movement and a return spring for the other.

### What does stroke mean for pneumatic cylinders?

The stroke is the total distance that the piston rod is capable of moving in one direction.