How to Choose a Pneumatic Cylinder
Micro Pneumatic Cylinder come with a whole range of variable attributes, from size to load and speed capacity as well as options on port locations and mountings. There are also considerations around the design and type of pneumatic cylinder you want in relation to the application and its working environment. Below, we unfold some of the choices to be made when choosing cylinders, and provide a little guidance on which ones are suitable for which application.
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.
Determining the stroke length will also narrow down the type of cylinder required. Stroke length depends on the job, and overlaps may occur, but general classifications are:
short stroke, for compact cylinders, as little as 1/16"
intermediate stroke, for light duty automation, up to 3 feet
long stroke, for (e.g.) automatic doors, 40-99"
speciality stroke, with cable-and-clamp pulling a piston, 15-25 feet or more
Cable cylinders can be situated remotely, as the cable can be any length that suits the application and mounting requirements.
Speed affects how well your load can be controlled, and the longevity and productivity of the cylinder. The stroke speed of a pneumatic cylinder can be calculated using the formula:
s = 28.8q/A, where
s = speed, in inches per second
q = airflow in standard cubic feet per minute
A = piston area, in square inches
Speed can also be affected by other factors, including the size of ports, hoses or tubing, and the rate of inlet and exhaust flow via control valve . Bottlenecks can sometimes happen, restricting the air flowing into or out of the cylinder, and restricted air pressure will slow the cylinder down.
Rod style cylinders can be designed in two ways – single, or double acting. Single acting cylinders supply compressed air to only one side of the piston, thus creating force and motion only in one direction. Double acting cylinders supply compressed air to create enough force to extend as well as retract strokes. Single acting cylinders have limited extension due to the presence of a compressed spring in the design. The stroke length for double acting cylinders is unlimited, but the piston rod can bend or buckle after consistent use over a period of time.
Two primary types of pneumatic cylinders are compact, and guided. Compact cylinders are designed for light duty applications with less space and small stroke requirements. Guided cylinders provide precise motion solutions, and are used in heavy duty applications, where a guided, or large offset load is required.
The amount of force that a cylinder creates is also a guideline for selecting pneumatic cylinders. This can be established by examining the cylinder’s bore size, and air pressure. A general rule of thumb used here is, the force generated by the cylinder should twice as much as the load.
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