Air cylinders are used primarily for their quick rate of cycling. By cycling I mean the rate at which they can cycle back and forth. The nature of pneumatics is that air cylinders will accelerate quickly, and move at a high velocity. For a given cylinder and plumbing size air moves more efficiently than does a liquid. This is pretty common sense. Everybody knows it’s a lot easier to blow through a hole than it is to shoot a liquid through a hole. If you ever try sucking up through a milk shaker or just sucking straw through an air you know how common sense that is.
That’s really the primary advantage of pneumatic cylinders. They cycle back and forth quickly. They’re good at starting and accelerating and getting to the end of their stroke, in literally blinks of an eye in some cases.
Take a four inch bore, 10-in. stroke cylinder. Let’s just say it’s an NFPA medium duty and that it’s the same cylinder that we’re going to use for both an air and a hydraulic application.
In this case, you have a ½-in. port that’s on the cylinder. A one-half inch typically flows about 12 gpm without creating excessive back pressure. The back pressure created trying to move a liquid is a lot higher than the back pressure created trying to move air. You’re limited with hydraulics in that case-how much flow you can have through a particular size port.
If we cycle fluid in this case, at 12 gpm, it takes about 3 seconds to extend this whole cylinder-only 10 in. of stroke. That’s not an unreasonable amount of time. Three seconds is still pretty quick, especially if only 12 gpm, and by all means you’re welcome to oversize the ports on this, throw 40 gpm at it and get it to do that in less than a second-but that takes a lot of energy. If your push priority is rate of cycling, probably not the best choice, would be hydraulics especially if you don’t want to throw 200-hp units at it.
Now let’s consider a pneumatic application. Let’s take the same cylinder, but this time we’re going to downsize the port. Just say, the only thing available was a ¼-in. port. That’s fine, we can handle that. In this case we’re going to go with 100 psi and we’re going to go at 10 cfm, which is a moderate amount. It’s a little bit more than you might get for your compressor that you have in your garage, but it’s well more than doable if you have an industrial size compressor and some flow controls.
In this case, we’re going to get the cylinder to extend in less than 0.5 second with just 10 cfm and 100 psi. That is how quickly pneumatic cylinders can accelerate and how quickly they can go full stroke.
There is a caveat though. There is an important consideration for pneumatic applications: pressure differential. Pressure differential literally means that the difference in two pressures. In this case, what’s important for pneumatic applications is the differential pressure between upstream and downstream pressure-between the cap port of the cylinder and the rod side port of the cylinder.
If you have a meter-out flow control or you have back pressure based on some small lines that are in your rod side, you absolutely have to consider this difference. When it comes to steady state flow of a cylinder, it gets up to speed and wants to go that speed based on the input flow you have. When it comes to acceleration, the mathematics involved, you want it to have the least amount of back-pressure as possible.
Here we have a pressure differential-100 psi of upstream pressure and 40 psi of downstream back-pressure leaves us with only 60 psi of net pressure. As far as the cylinder is concerned, it only has 60 psi to work with. All that piston really sees is that 60 psi. Just like if you were to run an application that has 100 psi gauge pressure, that already discounts what you have for atmospheric pressure.
You know that 14.7 psi of atmospheric pressure has to be overcome in any given ambient pneumatic application. You don’t notice it because everybody talks about gauge pressure but you’re really using, in this case it would be 114.7 psi on one end and it would be 54.7 psi on the other end. These kind of cancel each other out but, these are important considerations.
Why is this important? If you’re running 10 cfm and this was a 1000-in. stroke cylinder, it would just accelerate and get up to that 10 cfm, it would flow relatively well. But, Sir Isaac Newton has some thoughts on that. When it comes to acceleration, how quickly that cylinder gets going, you have to consider force and mass. The more force you can provide, the quicker it will accelerate. If you take away pressure, you’re taking away force and you’re reducing acceleration. In the case of the cylinder application, you might see it cycle in just under a second with 60 psi because of the acceleration force it requires rather than 0.44 seconds.
When back pressure is involved, make sure you do all your math, make sure that all the sizes of your lines are sized appropriately. Make sure that they’re producing a meter-out flow control. That you consider things like, how big that port is and how much back pressure you’re adding by trying to control it on meter out. I know a lot of time meter out is desirable, especially to help produce things like chatter and vibrations. It also can be very wasteful as well.
You’re really just trying to fight mother nature and you wouldn’t go for a run and have somebody standing in front of you pushing on you the whole time. Consider these things in your applications and consider how it affects your cylinder’s performance.
Now that we understand why they’re used for the quick snappy motion, there are some other cases that they’re used. I’ll discuss them and where are hydraulic cylinders used.
Let’s consider fluid power used in carwash equipment. When there are leaks, you don’t want a leaking cylinder on one of these soft cloth booms, blowing out a rod seal and spraying hydraulic all over a customer’s car. Hydraulic oil can sometimes be nasty especially in an environment where it mixes with water all the time such as a car wash.
Not only is it important to be used in something like a carwash, they’re easy to repair, easy to maintain and it’s easier to run lines of air in demanding application like this than hydraulic lines. To be honest, you don’t need that much strength and power. You have a spinning brush right here that is being energized by pneumatic cylinders. Worst case scenario, if one of these booms gets stuck on a car, the car will just push the book out of the way because it’s stronger that the force of the cylinder trying to push it against the car.
However, there are hydraulics on these machines. In some places, it can be on some of the cycling sprayers but also the track. Almost every single one of these tracks is powered by hydraulics.
In packing equipment, in machinery, you often see pneumatic cylinders and actuators. Packing can be a very fast process. There’s folding of boxes, there’s shuffling of material and boxes back and forth. The rapid cycling of pneumatics is perfect for packaging equipment.
Once again, you want that fast motion but not a lot of force is needed. You want to avoid the contamination possibility that’s related to hydraulic. You’d hate to have hydraulic oil blow out and you ruin thousands of printed boxes. Especially if it’s a food based packaging system.
On to food and beverage equipment. In most cases, nearly exclusively, they use pneumatic actuation, due to contamination potential. Machinery like this will have all stainless steel so it’s easier to clean. Certain types of stainless steel are also very contamination resistant. Bacteria does not like to nest itself within the pores of stainless steel surfaces. But, because of contamination, the volume and the speed at which food and beverages, especially if you’ve seen any bottling equipment. These factories are bottling thousands of bottles per hour and things move at a blinding speed. That’s perfect for pneumatics.
If you were to have a failure, a leak in an airline, you’re just blowing out air, you’re not going to be blowing out hydraulic oil. Even if you were using a bio oil that is safe on the product, it’s still messy to clean up, especially when it comes to these clean environment.
Automation. Once again, the speed of pneumatic applications is so fast that to automate a process and take away the human effort in doing something in the fraction of the time person or even hydraulic machine can do with the same input energy.
Pick and place is another one; they often use cylinder slides. They might be on a rodless cylinder and there are a few different cylinders on here and an actuator. One of them is a gripper but a few of these cylinders go up and down and side to side. It works in what’s called pick and place. There’s objects that are being automated and picked coming down a conveyor and this drops down and picks up and moves it to a new spot.
Some of these can use photo eye, some of them can have sensors in them to position accurately. But, obviously, very rapid and very accurate especially when you go full stroke which is another advantage to pneumatic cylinders. If you have a 10-in. stroke cylinder and the objects you’re moving are exactly 10 in. apart, this thing blasts back and forth all day at those exact stroke lengths.
Pneumatic presses. Let’s not downplay the force capability of pneumatic cylinder. Nothing beats the power density of hydraulics, but in this case you have what’s called the multi power cylinder. There are different lines that are connected midway through the cylinder. Those are actually more than one piston that are attached to the same rod. In this case there is four pistons that are pushing down on one rod. For any given pressure, you can get four times the force. This takes up a small amount of space and allows you to have a huge amount of force. Even with four pistons, you can still cycle fairly rapidly.