Pneumatic cylinders are similar to hydraulic cylinders in terms of the linear motion of the piston. But instead of using hydraulic fluids, a pneumatic cylinder uses compressed air to create the forces. It is one of the simplest and most cost-efficient solutions across the industries.
There are many kinds of pneumatic cylinders in the market today, the most popular of which is the rotary air model, often referred to as the NC rotary air cylinder. The convenience of this type of cylinder stems from the control systems that are lightweight and smaller in size.
Before diving into the working principles behind these cylinders, it is important to know what makes up a pneumatic cylinder. In reference to the image below, the main components of a standard pneumatic include a piston, a piston rod, a cylinder barrel, a wiper seal, a cylinder cap, and a cushion sleeve. With a single push of button, the air moves the piston rod outward and the operation commences.
The required motion, such as clamping, is realized as the compressed air acts on the piston inside the cylinder. The basic (single-acting) pneumatic cylinder has one port for the air to enter and pushes the piston towards one direction. The piston then returns to its initial position by a spring.
Since pneumatic cylinders are designed with limited stroke length, they are not ideal for faster operations. But the less air requirement results in better efficiency and lower operating cost. Generally, just like hydraulic cylinders, the larger the cylinder bore, the greater the forces generated.
Here is a video demonstrating how a pneumatic cylinder operates:
Pneumatic cylinders can either be single-acting or double acting. The single-acting cylinders receive air from one side of the piston and generate enough output to move the piston to only one direction. The simplest way to distinguish a single-acting and double acting cylinder is to see how many ports it has. Single-acting cylinders will only have one port to carry out the mechanism we’ve just described. In terms of applications, single-acting cylinders are typically used for clamping, positioning, marketing, stroking, and other light-duty assembly operations. Its single-direction nature is not suitable for complex operations.
The primary benefits of the single-acting cylinder are compact size and simple design. And since its air consumption is halved compared with a double-acting cylinder, a single-acting cylinder is more energy-saving. Valve and piping costs are relatively lower as well.
The downside of a single-acting cylinder is the inconsistent output force and a slight reduction of thrust due to the opposing spring force. The extent of stroke and bore size is also limited because of the spring, which may come in a variety of lengths.
A double-acting pneumatic cylinder applies output forces toward both extending and retracting directions. It has a port on each end of the piston. Both ports alternate to receive air, and compress the air into load so that the piston moves back and forth. When one end of the piston intakes air, the force produces a thrust in the push (positive) stroke, and a thrust in the pull (negative) stroke. Double-acting cylinders are used in applications where the required thrusts and stroke lengths are significantly in excess of a single-acting cylinder. They actually account for about 95% of all cylinder applications in pneumatic control circuits.
One advantage of double-acting cylinders is that the manufacturing is typically in compliance with ISO standards. This means that in most cases, a double-acting cylinder is less likely to malfunction and has longer life compared with the single-acting counterpart. Also, double-acting cylinders come in a wider selection of bore and stroke sizes, allowing users to utilize one on a broader scope.
The drawback of a double-acting pneumatic cylinder is that it is generally more expensive than a single-acting cylinder. And for long-stroke double-acting models, adequate guiding of the piston rod and bigger housing are required, which incurs additional costs.
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