In this article I would like to discuss advantages of pneumatic cylinders in comparison to other alternatives. Please be aware that this is only theoretical discussion because in real world nobody is forced to make such decisions based only on their sole choice. Usually in real life applications you are presented some solution and when picking actuator you need to adapt to whatever system exists already – whether it is electrical system, hydraulic system, or any other. Alternatives I will take into account in my little analysis are of course most popular ones – electrical cylinders and hydraulic actuators. I’m not considering any other alternatives to both of them because I have no knowledge of any, which simply means that if they do exists, they are probably extremely rare.
But before we proceed further, I would like to clarify what piece of engineering equipment we’re going to discuss. Cylinder is a product consisting of three basic elements – tube, rod and piston. Piston moves inside some kind of a tube, which is tightly closed, rod is connected to piston and it extends outside. Whenever piston moves inside the tube the rod extends and retracts accordingly. This kind of two directional, linear motion is common to all kinds of cylinders. Differences between them are either in their design, or force that powers them (which is the subject of this article). That said we can continue our main thought.
Pneumatic cylinders are, as the name suggests, powered by air, or exactly compressed air. Air (and most of other inert gases) are easily compressed, and can be compressed to amazing levels. Compressed air can be transported, and used to power various elements including pneumatic cylinders. In case of pneumatic cylinders compressed air flows into a chamber on one side of the piston, and therefore piston moves pushing the piston rod.
Method of return is similar, but can be caused by air stream on other side or (very popular solution) spring.
Hydraulic cylinders are based on very similar mechanism, but in this particular case oil or other fluid is used as a power source. Hydraulic cylinders compared to pneumatic ones can achieve much larger forces, but its the case for hydraulics in general. Since hydraulic cylinders are using oils (often flammable or mineral oils) they do tend to react with various kinds of seals, thus sooner or later are prone to leakages.
Electric cylinders on the other side are based on slightly different rule. Electric cylinders consist of a ball and a roller screw connected to electric motor. While turning screw moves a nut connected to rod or carriage, which is moving the load. Great advantage of pneumatic cylinders are ability for precise control over speed and position of the rod. In modern electric cylinders it is possible to achieve positioning precision up to ten thousandths of an mm.
Again in case of pneumatic actuators output forces are definitely smaller when compared to hydraulic alternatives, but are similar to electric ones. One great advantage of pneumatic compared to electric is that when there is power cut pneumatic cylinders can still work, since pressure is usually stored in pressure tanks. Another interesting thing about pneumatic cylinders is that speed and force can be controlled independently, which is very important factor. Force can be controlled by pressure which can be raised or lowered using pressure regulator, and speed can be controlled by flow control valve.
Now electric ones are more flexible and accurate, you can precisely decide how far to extend the rod which in general can increase efficiency of your system. Pneumatic cylinder cannot be stopped easily precisely at any position, you could achieve similar effect by using piston position sensor and additional valve, but its never going to be as accurate as electric ones. Pneumatic actuators, and in general pneumatic system are on the other hand very inexpensive even in long term.
To sum up pneumatic solutions are great whenever you are looking for some inexpensive choice, which doesn’t have to be extremely accurate, but have thrust and speed. Whenever you need total control over piston rod, or are using some advanced application where cylinder have to perform unusual tasks go for electric. But when you need enormous forces on your side consider hydraulic solutions. In general its worth to remember that each solution have its advantages and disadvantages and you should not make your choice without consulting the expert.