For applications with space limitations or size restrictions, integrating a standard linear actuator to achieve the required force / thrust can sometimes be a challenge. However, utilising specialist constructions of linear actuators, such as Tandem and Duplex cylinders can provide a solution.
When designing for a pneumatic application, there are times when size restrictions can limit the space available for the installation of the required linear actuator. This can potentially limit the maximum force (thrust) available, as this is relative to the air pressure working on the piston area governed by the cylinder bore size.
One method to overcome this issue is to install a Tandem cylinder, as a smaller bore size can be used to generate a greater force ratio.
Tandem cylinders are 2 double acting cylinders, of the same bore size and stroke length, assembled in series to share a common piston rod.
Air is provided to the ports of each cylinder and, because this acts on both pistons, they produce nearly double the force (thrust) of a single cylinder.
This can be an advantage when space is restricted, as a smaller bore size can be used but does have the disadvantage of producing a longer cylinder relative to stroke length.
Duplex arrangements provide intermediate positioning options that are not possible with a single pneumatic cylinder. When connected back to back as a single unit, Duplex cylinders can double the available force where space is limited.
Typically, a pneumatic rod type cylinder provides 2 static positions of operation – fully retracted and fully extended, with the distance being governed by the cylinder stroke length.
Being pneumatically operated means the cylinder cannot be stopped mid-stroke to a predetermined position.
However, if required, pneumatic cylinders can achieve 3 or 4 positions. Duplex cylinders are constructed by using 2 double acting cylinders arranged back to back with separate piston rods, connected by common tie rods or a flange mounted back to back mounting.
Two cylinders installed back to back with the same stroke give a 3 position cylinder with a symmetrical centre position. Whereas different strokes give a 4 position cylinder where the two central positions can be calculated from different stroke lengths.
For a 4 position cylinder (cylinders with differing stroke lengths):
Position 1: Both cylinders A and B are in the retracted stroke positions.
Position 2: Cylinder A is in the extended stroke position and cylinder B remains in the retracted stroke position.
Position 3: Both cylinders A and B are in the extended stroke position.
Position 4: Cylinder A is in the retracted stroke position and cylinder B is in the extended stroke position.
The challenges for pneumatic cylinders are as numerous as the sectors in which they are needed to operate. Downtime costs money and lost production must be avoided for anything other than routine maintenance. Hence careful selection based on operating requirements and prevailing environmental factors must be given proper attention from the outset.
Article contributed by Kevin Hill, product manager for Actuators, Pneumatic Division Europe, Parker Hannifin Corporation.