The most popular style of pneumatic actuator consists of a piston and rod moving inside a closed cylinder, designed to move in a straight line to deliver linear motion. This style of linear actuator is utilized in a variety of industrial sectors, factory automation and packaging, plus transportation and life science applications.
When looking to specify linear actuators there are two types of operating principle: single acting and double acting. We will explore these variations, typical applications plus advantages and disadvantages.
Single acting cylinders
A single acting cylinder is one where the thrust or output force is developed in only one direction.
The piston is returned by a fitted spring, or by some other external means such as a weight, mechanical movement, gravity or an external spring. They have a single port to allow compressed air to enter the cylinder to move the piston to the desired position.
There are two types of single acting cylinder:
‘Push’ type – where the application of air pressure produces a thrust, thus ‘pushing’ the piston.
‘Pull’ type – where the application of air pressure produces a thrust, thus ‘pulling’ the piston.
Single acting cylinders, such as Parker’s P1P Series, are typically used for applications where work is done only in one direction, such as clamping, positioning, marking, stroking and light assembly operations.
Advantages:
Simple design,
Compact size,
Reduction in valve and piping costs, and
Air consumption is halved compared with the equivalent sized double-acting cylinder.
Disadvantages:
Return spring side of the cylinder is vented to the atmosphere – may allow the ingress of foreign matter, which may lead to malfunctioning and reducing the life of the cylinder.
Spring operation with extended cylinder life can become inconsistent and provide uncertain end of stroke positions.
Bore size and stroke of the cylinder is restricted due to limitations of the spring size and force.
A slight reduction of thrust due to the opposing spring force.
Double-acting cylinders
A double-acting pneumatic cylinder is one where the thrust, or output force, is developed in both extending and retracting directions. Double-acting cylinders have a port at each end and move the piston forward and back by alternating the port that receives the high-pressure air, necessary when a load must be moved in both directions such as opening and closing a gate.
Air pressure is applied alternately to the opposite ends of the piston. Application of air pressure produces a thrust in the positive (push) stroke, and thrust in the negative (pull) stroke.
Double-acting cylinders are typically used in all applications where the thrusts and stroke lengths required are in excess of those available from single-acting cylinders. Small double-acting cylinders are also used for applications where positive end-of-stroke positions are required for both strokes.
Advantages:
Generally, ISO standards are based on the design of double-acting cylinders.
A more extensive range of double-acting cylinders than for single-acting cylinders, giving many more options of bore and stroke sizes.
Many variations are available on the basic double-acting cylinder design.
Disadvantages:
Cannot be simply held in a mid-position,
Air is a compressible medium – if a pneumatic cylinder is to be used as a feed cylinder, it has to be coupled to a hydraulic slave cylinder to give a constant feed, and
Long-stroke cylinders need adequate guiding of the piston rod.