Guidelines for selecting pneumatic cylinders

Pneumatic cylinders are widely used to generate force and motion on a vast range of OEM equipment. They can move products directly or indirectly by pushing, pulling, lifting, lowering, or rotating, and can keep them from moving by clamping them in place.

Wide acceptance comes in large part because cylinders are simple, economical, durable, and easy to install. They can produce thousands of pounds of force over a broad range of velocities; cycle at high speeds without overheating; and stall without internal damage. And they readily tolerate tough conditions such as high humidity, dusty environments, and repetitive high-pressure wash downs.

Pneumatic actuators come in literally thousands of styles, sizes, and configurations. This variety makes more innovative-equipment possible, but sorting out the best cylinder for an application can be a bit overwhelming. Here are some key considerations.

Cylinder design
The basic, rod-style industrial cylinder consists of a tube sealed by end caps. A rod attached to an internal piston extends through a sealed opening in one of the ends. The cylinder mounts to a machine and the piston rod acts upon the load.

A port at one end of the cylinder supplies compressed air to one side of the piston, causing it (and the piston rod) to move. The port at the other end lets air on the opposite side of the piston escape — usually to atmosphere. Reversing the roles of the two ports makes the piston and rod stroke in the opposite direction. Rod-style cylinders function in two ways:

Double-acting cylinders use compressed air to power both the extend and retract strokes, moving the rod back and forth. This arrangement makes them ideal for pushing and pulling loads. Controlling the rate at which air exhausts determines rod speed.

Single-acting cylinders have compressed air supplied to only one side of the piston; the other side vents to atmosphere. Depending on whether air is routed to the cap or rod end determines whether the rod extends or retracts. The most common type is pressure-extended, with an internal spring returning the piston to its original position when air exhausts. In other designs, gravity or an external spring powers the return stroke.

Rod-style cylinders come in various designs:

Repairable cylinders can be disassembled to replace seals and other internal components. This extends a cylinder’s life. These durable cylinders are generally used in rugged, heavy-duty applications.

Sealed-for-life or “disposable” cylinders have end caps mechanically crimped to the tube. Internal components are prelubed prior to assembly. Although they are less expensive to manufacture than comparable repairable cylinders, they cannot be taken apart to repair without destroying the housing. These cylinders are usually used in lighter-duty applications and must be replaced when they fail.

Compact cylinders fit into smaller spaces where only a short stroke is required. They are used in lighter-duty applications due to the small bearing surface on which the rod slides. They mainly come in single-acting versions, but double-acting styles also are available.

Guided cylinders have guide rods and guide blocks mounted parallel to the piston rod, or dual piston rods. They prevent the piston from rotating and provide precise, controlled linear motion — especially when the unit is subject to high side loads. In such cases, the guides reduce rod and piston bending and uneven seal wear. They are recommended in applications with sizeable offset loads or require that the load be guided, for example, down a conveyor.

Rack-and-pinion units convert a cylinder’s linear motion to angular rotation that can exceed 360°. The rotary actuators — with the rack mounted on the rod — are often used in process industries to operate quarter-turn valves.