Pneumatic Cylinder Working Principle

Pneumatic cylinders are used as actuators. They work on the principle of pressure differential, meaning that one side of the cylinder is pressurized and the other is not, resulting in a linear movement when a piston is pulled in one direction.

Using pneumatic cylinders to transfer power makes them very simple and efficient over electromechanical or hydraulic counterparts. They also do not need maintenance since the operation does not depend on a motor but rather just a compressor and its fluid supply.

Pneumatic cylinders are also active for a very long time, and as long as the cylinder and its piston are not removed from the cylinder block, it can always keep pushing loads. For this reason, pneumatic cylinders are often found in industrial applications where only a constant force is required.

Pneumatic cylinders work on a pressure differential from one side to the other; when air or gas is pressurized on one side of the piston, it moves in that direction due to pressure that acts on both sides of the piston. When it does so, it will have an opposite equal force pushing against it from the other side.

If we take a cylinder of diameter (d1) and length (L), air or gas fills it in on one side of the piston, which can be seen as the pressurized source; next to it, there will also be an equal amount of gas/air filling that cylinder on the other side, effectively serving as a depressurized return. The opposite will happen if we put a piston in this cylinder and move it in either direction.

To make these cylinders work, we must develop the fluid principle. This is easier than it sounds because we are using a fluid. The fluid (pneumatic or hydraulic) has zero pressure at rest and when stationary but has a certain force (pressure increase) when moved from one point to the other.

Pneumatic cylinder working principle

To make a pneumatic cylinder work, we must move the gas/air from one side of the piston to the other, which is done by differentiating and applying alternating pressure. The first step is to ensure a constant flow of air/gas from both sides of the cylinder. If not, we will not be able to fill it up enough on both sides, and it will not work properly.

The second step is to create a pressure difference between one side and another, which allows the piston(s) to move. This can be done either by pressuring one side and depressurizing the other or by creating an elastic medium (a spring) between them.

Filling the cylinder with constant air pressure is simple and can be achieved with a compressor. If we have only a single pneumatic cylinder, it will be filled with air or gas on both sides of the piston. We must depressurize one side and pressurize the other to create a pressure difference.

The simplest way to achieve this is to use an elastic medium known as a spring. In this case, when we want to push the piston in one direction, it will have compression on one side and stretch on another; if we pull it in another direction, it will have a stretch on one side and compression on the other.

The above image shows a schematic of how an elastic/pneumatic spring works in a pneumatic cylinder. The spring can be an external or internal one, which is discussed below:

External Pneumatic Spring

External springs are the ones that are installed inside the cylinder itself. On the other hand, internal springs exist outside and around the cylinder. External springs will produce more pressure since they take up less space than internal ones. However, since they require a bigger surface area for installation and support, this can become an issue for mounting reasons.

Internal Pneumatic Spring

Internal springs are those that go around the cylinder. This is much easier to do since it does not require external support and is much more efficient; the pressure lost when it goes around a cylinder is negligible. However, in this case, the pneumatic spring will have an elasticity that will reduce its compression ratio and produce less pressure than an external one of equivalent size.

The essential difference between these two types of the pneumatic piston is that one absorbs more space for installation purposes (external), while the other does not (internal).

This image shows internal pneumatic springs installed in a pneumatic piston.


Now that we know how to make a cylinder work, it is important to understand how the fluid behaves when moving. A constant flow of air/gas on both sides of the piston will produce a pressure difference between one side and the other. If we do not have any air/gas on one side and only have it on another, there will not be any pressure difference between them (assuming they only have each other as a pressure source).

In a continuous flow system, there will always be air/gas on both sides of the piston so that pressure difference will exist. However, if we need more air/gas on one side and only on the other (as in a pneumatic cylinder), this will not produce a pressure difference since both sides are at the same pressure.

A pneumatic cylinder is an example of a continuous flow system that does not require any input source to keep working. Hence it can remain active for as long as its internal environment remains constant.

Compressible Fluid

To see the difference between a continuous flow system and a pneumatic cylinder that is not continuous, we must understand what causes pressure differences in a compressible fluid.

In point B of the piston, air or gas fills the cylinder on both sides of the piston (point A). If we are trying to compress it, there will be an increase in pressure at point B due to compression. This means if we can pressurize by increasing pressure on one side and reduce on the other, this will cause a pressure difference (compressibility) between these two points.

If one side of the cylinder does not have air or gas, there will be no pressure difference between points B and A. This means that if there is an area where air or gas doesn't exist (vacuum), any pressure difference will be reduced to zero. This is due to a principle called Pascal's principle, which states that the higher the pressure difference, the higher the force of pressure; also, if we have a vacuum, there is no force since there is no air/gas.

Pascal's principle states that "the pressure applied on a confined fluid is transmitted undiminished to every part of the containing vessel."

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