Air cylinders are pneumatic linear actuators driven by pressure differential in the cylinder chambers. Compressed gas (typically air) acts as the power to impart a force and cause actuation. The compressed gas is allowed to expand due to the pressure gradient, and forces a piston to move in the desired direction. One side of a piston flange can be pressurized to provide force and motion with a spring (providing return force after pressure is released), or both sides can be alternately pressurized for bi-directional powered motion.

Air cylinders are used in applications which push, pull, and lift objects, close doors, or hold, remove, and position parts. They are commonly used in materials handling and processing. Air cylinders are also important in fail safe systems.Image Credit --SMC Controls               

Air Cylinder Styles

Air cylinders are used in many positioning, actuating, and clamping applications. Choices for air cylinder style include: 

Tie rod- Tie rod cylinders are held together by exterior tie rods, and are usually in a rectangular bolt pattern. They can be used for all types of cylinders and for various sized loads. Tie rods can be used to protect the cylinder from damage, however the rods must be tested under high torque to ensure the rods don't elongate under high pressure.

Smooth body- In a smooth-body air cylinder, the cylinder's body is smooth and typically encases the piston.

Pancake- Pancake air cylinders have a shorter length and larger diameter than other air cylinder styles. This gives the cylinder a "pancake" appearance. The piston rod may not be entirely encased in the frame.

Miniature- A miniature cylinder operates in the same manner as a regular sized cylinder except that its profile is smaller and the body may be threaded for mounting purposes. They may be as small as 2.5mm and are mainly single acting cylinders for use in light duty miniature assemblies and manufacturing.

Ram- A ram is a device in which the cross-sectional area of the piston rod is more than one-half the cross-sectional area of the moving component. Rams are primarily used to push rather than pull, and are most commonly used in high pressure applications.

Welded- A smooth hydraulic cylinder that uses a heavy-duty welded cylinder housing to provide stability 

Rectangular- A rectangular air cylinder is encased in a rectangular, box-shaped frame.

Rotating- Rotating actuators use air pressure to cause rotary motion. This type of actuator typically provides angular rotation up to 360°. Rotary actuators can be a vane design or a rack and pinion design.

Multiple bore- In a multiple-bore cylinder, two or more bores and pistons are combined or stacked in same air cylinder.   

Image Credit: Taiwan Chelic Corp.

Rodless- Rodless actuators use a mechanical or magnetic coupling to impart force to a carriage that moves along the length of the cylinder body.

Air Cylinder Configuration

Simple configurations consist of a single rod that only travels the length of its rod and back.

Telescoping air cylinders have a number of stages that can extend out the length of the cylinder and retract fully into the base, flush with the front cap. This configuration is used in a variety of applications that require the use of a long cylinder in a space-constrained environment.

Air Cylinder Specifications

Important operating specifications to consider when selecting an air cylinders includes cylinder stroke and operating pressure range.

Stroke- Stroke is the distance between fully extended and fully retracted rod positions.

Operating pressure- The operating pressure range specifies the full-required range of operating pressure.

Bore size- Bore size is also important to consider.

Body material- Common body material choices include aluminum, steel, stainless steel, and plastic.

Rod diameter- The rod diameter determines how much load the piston is able to handle before it buckles. The table below shows the minimum rod diameter under various load conditions. The exposed length of the piston, listed at the top of the table, is typically longer than the stroke length of the cylinder. The vertical scale is in English tons (1 ton = 2000lbs.)

Figures in body of chart are suggested minimum rod diameters. Image Credit: Mead

Force- Force is related to the diameter of the piston, but they are not directly proportional. Force ratings can differ somewhat in opposite directions. The instroke (pull) force is less than the outstroke (thrust) force when powered pneumatically by the same supply of compressed gas, due the effective cross sectional area reduced by the area of the piston rod. The relationship between force on outstroke, pressure, and radius is as follows:

F_τ = P(∏r²)

Where:

F_τ is the resultant force

P is the pressure distributed load on the surface

∏ is pi, approximately equal to 3.14159

r is the radius of the piston

∏r² represents the effective area of the piston surface where the pressure is acting on.

Instroke uses a similar equation as outstroke force; however, the cross section area is less than the piston area so the relationship with the radius is different.  The relationship between force exerted for instroke, pressure, radius of the piston, and radius of the piston rod is as follows:

F_τ = P∏(r₁² - r₂² )

Where:

F_τ is the resultant force

P is the pressure distributed load on the surface

∏ is pi, approximately equal to 3.14159

r₁ is the radius of the piston

r₂ is the radius of the piston rod

Fail safe mechanisms- Fail safe mechanisms are in place to prevent brief or permanent system failure. If an error is detected, such as a loss of air pressure or supply, the fail safe mechanism will engage to prevent damage to the device or system.

Size- The size of an air cylinder can range from 2.5 mm (able to lift a small transistor or electronic component) to 400mm (able to lift a car). Very large air cylinders may be used in place of hydraulic actuators in applications where leaking hydraulic oil could impose an extreme hazard. To determine the size of the cylinder, calculate (in pounds) the force needed for the application, add 25% for friction and to provide enough power for a reasonable rate of speed. Then determine the air pressure which will be used and maintained, and select a power factor from the table below. The power factor should, when multiplied by the air pressure, equal the force calculated. The power factor is the mount of square inches for the cylinder bore, and the bore diameter need will be found direct above power factor selected.

Power factor table. Image Credit: Mead

Speed - Cylinders are typically allowed to run at their maximum natural speed. This maximum speed is determined by cylinder size, port size, air pressure, bore and length of the hoses, and the load against which the cylinder is working. From this natural speed, the user can increase speed or more often, reduce it. Smaller valves have slower cylinder movement, however the maximum natural speed of these cylinders can often be achieved with a valve that is one or two sizes smaller then the cylinder port size.

Cylinder Action

The cylinder can be single or double action.

In a single acting air cylinder, the cylinder contains pneumatic porting for drive in only one direction. Single-action air cylinders frequently incorporate a return spring to the unpowered position.

In double acting air cylinders, both sides of the piston can be pressurized for reversible motion (instroke and outstroke). Double acting air cylinders can be used as a multistage compressor.

For more information please see GlobalSpec's Valve Actuators page.

Cylinder Mounting

There are several mounting options for air cylinders. The mounting option should be selected based on the design of the cylinder, load specifications of the mount, and the environment (i.e. surrounding space and conditions) the system will be placed in.

Clevis mount. Trunnion mount. Lug mount
Image Credit: UHAUL | FBValve | Garvin

• Clevis or eye attachment connects the cylinder to the extended end of the piston via threads.
• Double end mounts have a nose and rear which contain threaded bosses for nut attachment.
• Flange mounts are brackets placed on the cylinder
• Floating mounts are for more convenient installation of the cylinder.
• Foot brackets are flanges that rest underneath the cylinder.
• Lugs are short blocks with holes that attach to the side of the cylinder and allow mounting to another surface.
• Face mount have threaded holes on the front face for attachment.
• Nose mounts are threaded for mounting through a hole with a nut on the other side.
• Rear mount have tapped holes or mounting flanges on the rear.
• Threaded mounts are designed to mount to bolts via threaded holes in the cylinder head or cap.
• Trunnion mounts are specially designed mounting blocks which can be located at the cap, end, or an intermediate location along the cylinder. 

Pneumatic Cylinder Mounting Options. Video Credit: FrightProps / CC BY-SA 4.0

NFPA Mounting

National Fluid Power Association (NFPA) mounting geometry conforms to standards of the National Fluid Power Association (NFPA). Several standard configurations fall under the NFPA designation. The most common NPT thread sizes are 1/8" NPT, 1/4" NPT, 3/8" NPT, 1/2" NPT, 3/4" NPT, 1" NPT, 1 1/2" NPT, 2" NPT, British Standard Pipe Thread, and Metric Pipe Thread.

Mount Location

When selecting the mount location on the cylinder it is important to consider the area around the actuator, and the surrounding environment.

• Cap- The cap is also referred to as the base or rear of the cylinder.

• Head - The head is also referred to as the front or face of the cylinder.

• Intermediate - Intermediate mounting is when the mount is located at a particular point along the cylinder body.

Features

Many air cylinders come with features for better usability and performance.

Adjustable stroke- Adjustable stroke allows for the end points or the total stroke length to be adjusted.

Air/oil tandem- An Air/oil tandem provides smooth, hydraulic-like motion. This combination is only used in pneumatic cylinder.

Bumpers- Bumpbers, also known as cushions, slow the piston before it comes to a stop. Non-cushioned cylinders have the piston stop by hitting the end cover. The energy and noise from the piston and the rod must then dissipate throughout the cylinder. This is often very loud and will eventually cause fatigue to the piston and end cover material. Cushioning can be adjustable in the case of a large piston or static for smaller, lighter duties.

Closed loop control- Also known as servo control, the cylinders have external devices that send back a signal to the pump control giving it position information.

Holding brakes- Holding brakes work in conjunction with the self-locking feature to increase holding force.

Shock absorbers- Shock absorbers are used in pneumatic of hydraulic fluid absorption of shock.

Double-ended rods- Double end rods extend from both ends of the cylinder with attachment features such as threads on both ends.

Multi-position end-plate- A multi-position end plate can be actuated to different positions along its stroke, not just the endpoint.

Integrated overload slip clutch or torque limiter- An integral flow control incorporates a flow control valve that limits the amount of air or fluid that enters the cylinder.

Protective boot- Protective boots are a cover that protects moving parts against environment damage.

Self-locking- Self-locking actuators lock in the current position when there is a loss of signal.

Integral sensors- Integral sensors are equipped within the cylinder to monitor position and proximity.

Integral flow- Integral flow control incorporates a flow control valve or device that limits the amount of air of fluid that enters the cylinder.

Non-rotating- Non-rotating denotes multiple rods to prevent the plunger from rotating.

Magnetic switches- Magnetic switches such as Hall Effect sensors indicate if the thruster is in the retracted or extended position.

Thermal overload protection- Thermal overload protection is used to trip a switch when a preset temperature is exceeded.

Intrinsically safe- Intrinsically safe electric linear actuators can be used in hazardous environments such as chemical processing facilities.

Environmental protection

Water resistant- Devices are sealed to prevent corrosion due to water or liquid entering the housing.

Thermal overload protection- Thermal overload protection is used to trip a switch when a preset temperature is exceeded.

Shock absorbers- The pneumatic of hydraulic fluid allows for the absorption of shock from the system.

Motor Features

• Home, limit, and position switches limit the travel of the piston rod.  
• Integral breaks will hold the position of the actuator.
• Position feedback provides a continuous output of position via analog or digital presentation.  Both motor encoder and linear position feedback options are available with select air cylinders.

Resources

What is the pneumatic cylinder?

The Norgren Guide to Specifying Pneumatic Actuators 

Pneumatic Application and Reference Handbook

Image credit:

SMC Controls | AutomationDirect |  Fabco  | Automation Direct