Pneumatic cylinders or actuators provide linear motion and apply force on equipment for various industrial processes. They push, pull, rotate and clamp. They move and position products or machine components. Pneumatic cylinders are simple, cost-effective, easy-to-install devices. They can produce high force and a broad range of velocities. Their motion can be stalled without causing internal damage. Various cylinder materials may be used to tolerate adverse conditions such as high humidity, dry and dusty environments, and repetitive washdown with high-pressure hoses.
Selecting the right cylinder can ensure long-term success of an application, as well as the proper overall function of the machinery on which it is installed. Here are some things to consider when specifying a cylinder.
The application
Selecting the right cylinder begins with understanding how it will be used. Designers need to answer some basic questions.
What has to be accomplished? What is the desired end result of the operation?
- What work will the cylinder perform? For example, will it move or lift an object? Open or close a valve?
- What is the load? How powerful will the cylinder need to be?
- How far must it move?
- What is the required cycle time? How quickly must the cylinder strike and recover?
- How will the motion be stopped?
- In what environment will the cylinder operate? Will it be exposed to extreme temperatures or caustic materials?
Cylinder types
The most common industrial pneumatic cylinder is a rod-style cylinder. It consists of a tube or barrel closed by sealed end pieces to form the envelope. Inside is a sealed piston. A rod, attached to the piston, extends through a sealed opening in one of the ends. Some cylinders have multiple rods passing through the same end to prevent piston rotation. The cylinder is mounted and the load is connected to or contacted by the piston rod. A port at one end of the cylinder allows compressed air to act on one side of the piston, moving the rod. A port in the other end of the cylinder allows air from the opposite side of the piston to escape, usually to atmosphere.
Rod-style cylinders function in two ways, double-acting and single-acting, both of which come in a variety of types, including repairable, disposable, compact, guided or bellows.
There also are rodless pneumatic cylinders. Instead of a rod extending from the end of the tube, an external carriage carries loads back and forth on the tube’s surface. Rodless cylinders are used for long-stroke requirements and offer high-moment loading capabilities for a variety of applications. This design saves space because the stroke is contained within the overall envelope of the cylinder. Three of the most common carriage options are internally guided, externally guided and roller guided.
Force
Understanding the applications and knowing what the cylinder must do allows the designer to calculate how much force is needed, which, in turn, determines bore size. In general, the force required can be up to two times the load to be moved. In some rare cases additional force to compensate for friction may need to be calculated.
A designer who knows the force required and the air pressure available can solve the following equations to determine the diameters of the cylinder bore and the piston. A cylinder’s push force on extension or pull force on retraction is calculated by multiplying the effective area of the piston by the working pressure. The effective area for push force is the full area of the cylinder bore. The effective area for pull force is the full-bore area reduced by the cross-sectional area of the piston rod.
The theoretical push force is, F = π (D2/4) P
Where: F is force in pounds
- D is cylinder bore in inches
- P is pressure in pounds per square inch
The theoretical pull force is, F = π (D2/4 – d2/4) P
Where: F is force in pounds
- D is cylinder bore in inches
- d is piston rod diameter in inches
- P is pressure in pounds per square inch
Calculating the forces of single-acting cylinders with a spring is more complicated. The spring force opposing the push or pull will increase as the stroke progresses. In practice, manufacturers’ catalogs often list push and pull values for both double-acting and single-acting cylinders.
When estimating the relative force of cylinders with different bore sizes, remember that thrust increases with the square of the diameter. In other words, if the bore is doubled, the thrust is quadrupled.
Stroking speed
Speed impacts productivity, longevity and controllability. The stroking speed of a pneumatic cylinder can be calculated from
- s = 28.8 q / A
Where: s is speed in inches per second
- q is air flow in standard cubic feet per minute
- A is piston area in square inches
Other factors that will affect speed in an application are:
- Port size
- Inlet and exhaust flow through the control valve
- Air pressure
- Diameter and length of the hoses
- Load against which the cylinder is working
With any fixed combination of valve, cylinder, pressure and load, it is usually necessary to have adjustable control over the cylinder speed. Flow regulators at the cylinder ports can provide this control and tune the speed to the application.
For the majority of applications, the best controllability results from unidirectional flow regulators that are installed to restrict flow out of the cylinder and allow free flow in. The regulator fitted to the rod-end port controls the extension speed, and the one fitted to the cap-end port controls the retraction speed.
Air consumption
Calculating a cylinder’s air consumption may be needed for fast-cycling production equipment. Enough supply air must be available to meet the application’s requirements. There are two parts to the air consumption of a cylinder. One is the volume displaced by the piston; the other is the unswept volume, such as cavities in the end covers, the cylinder ports, connecting tubing and valve cavities. The unswept part is likely to be a small percentage and will vary with individual installations.
It is best to connect the pneumatic equipment to a compressor system with enough capacity to supply it with sufficient air during a worst-case scenario. Otherwise there can be air starvation at a critical time, and performance will suffer.
Sheila Campbell is an application project engineer at Norgren Inc., an IMI Precision Engineering Branded Company.
