Fluid power has the highest power density of all conventional power-transmission technologies. Learn the benefits and limitations of fluid power, how to analyze fluid power components and circuits, and how to design and simulate fluid power circuits for applications.
Introducing the Cylinder
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Do curso por University of Minnesota
Fundamentos da Energia Hidráulica
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Week 1: Fundamentals of Fluid Power
This week: An overview of the course, introduction to hydraulics and pneumatics, and introduction to fundamental concepts of fluid power through the cylinder.
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James D. Van De Ven, PhDAssistant Professor
Mechanical Engineering
Will Durfee, PhDMorse Alumni Distinguished Teaching Professor
Mechanical Engineering
We're here at the Ziegler Cat yard in Bloomington, Minnesota.
And I'm standing in front of a Caterpillar 349E Excavator.
And I wanted to show you some of the cylinders that run the machine.
There's three main sets of cylinders.
The one I'm standing right next to is a stick cylinder.
This is the biggest one, and it runs the stick that goes back here.
Back behind me, you can see the two cylinders that run the boom.
This is the boom part of the excavator.
Those cylinders are a little smaller, but
there's two of them, so they can generate more force.
And then the third one is tucked underneath, which is the bucket cylinder,
that runs the bucket back.
Now that you've seen a cylinder in action, let's take a deeper dive into cylinders.
Here on the table I have three examples of cylinders.
This is a small one which is a specialty cylinder with an unusual property that
I'll explain in just a few minutes.
Then here is a typical pneumatic cylinder, an air cylinder rated at 17 bar or
about 250 pounds per square inch.
And then here we've got a industrial tie rod cylinder.
This one is a 7.6 centimeter or 3 inch bore, 15 centimeter or
6 inch stroke and has a maximum pressure of about 172 bar,
which is about 250,000 pounds per square inch.
This means that the extension force of this cylinder is about 79,000 Newtons or
18,000 pounds.
Cylinders come in all sizes.
Some as small as your fingers, and some as large as a bore of one and a half meters.
Here's a cartoon showing the insides of a cylinder, and
here are some labels that describes some of the important features.
The actual casing is the barrel, and then the piston,
you can think of it as a thick disk that slides in and out in the barrel.
There are ports on both sides of the barrel to allow fluid to enter and exit.
And then the rod that connects to the piston is how the force is
transferred from the piston out to the surrounding world.
There are two sides of that piston.
The full surface side is called the cap side, and
the side that the rod is on is called the rod side.
This is important because the force is proportional to the area, and you can
see that on the rod side, you don't have the full surface area of the piston for
the fluid to act against because of the area of the rod.
When you put energized fluid into one side, so in this case,
energized fluid is on the, the cap side, it pushes the piston against the pressure.
And the rod, in this case, will extend out of the cylinder.
The key parameters in an ideal cylinder are the two pressures,
the pressure on the cap side and the pressure on the rod side,
and the two areas, which is the area on the cap side and the area on the rod side.
And here's the equation that describes the force in that rod which is
the pressure times the area.
So what's inside a reel cylinder?
Here's an image of an tie rod cylinder,
which is pretty much like the one that we have up here on the table.
It was called the tire rod cylinder because you can see those
shiny metal bars.
There are four of them around the outside, and they're,
they're the tension tie rods that hold the whole cylinder together.
One advantage of this type of cylinder is that you can take it apart to clean out
the insides.
Another type of cylinder is a welded steel cylinder, and they're,
it's a little bit lower cost, but everything is welded shut, so
you can't get at the inside to do maintenance.
So let's take a look at a cut away view of a tie rod cylinder and
look at some of the important parts.
So here is the barrel, that same barrel that's on the cartoons, so
typically, the barrels are made out of stainless steel or a steel.
And then, here's the piston that's riding inside the barrel.
And then you can see if you look closely,
there are seals that around, around the piston.
This is a critical part because without seals,
then the fluid would simply shoot right by and not press against the piston.
And we'll come back and
look at those seals in a little more detail in just a minute or two.
But one of the things to mention about the seals is because they rub against that
cylinder wall or that barrel, it is going to make that piston inefficient.
So now the fluid pressure has to push against the piston, but
it also has to overcome the friction against the side.
You want that seal to be tight,
because otherwise, the fluid is going to leak right by it.
So typically, in an industrial cylinder like this,
the seals are very tight with almost no leakage, but
there is substantial friction, as we'll see in just a minute.
So here's the rod, it's a shiny stainless steel shaft typically, and one of
the things about the rod is that there's this trade-off between when it's pushing,
there can be substantial forces in it, so you want to make sure that rod doesn't
buckle, you can imagine a flexible ruler in your hands, push in on either end, and
that ruler buckling.
So that rod has to be big enough that it doesn't buckle, but then the bigger you
make it, then the less area you have on the rod side of the piston, so
there's always that trade-off with the rod.
Then up at the nose end,
you can find the rod seal, this one is particularly important because this is
the seal that keeps the oil that's on the inside from escaping out.
So you can imagine that for
an agricultural machine that's got a cylinder like this on it,
you certainly wouldn't want that toxic oil dripping all over your crops.
So that seal is really important.
Then on the front side, we also have the rod bearing,
because that rod wants to run true and not have a lot of side load, so
there's a rod bearing this at the front side.
Then there's the two ports.
I've labeled the nose end port.
And this is how fluid gets into and out of the cylinder.
Let's go back and take a look at that seal that's on the piston.
There's a wide variety of types of seals.
You might be familiar with a regular O-ring type of seal.
The seals that are used in pistons are a little bit more complex.
And what we have here is a cartoon version of a cup seal.
And it's in the shape of a cup.
It goes all the way around the piston.
And it's got a particular property that when it's pressurized, and
that's what the red arrows are indicating, you can see that it
pushes that cup seal up against the barrel, forming a tighter seal.
So it's got the properties that the higher the pressure,
then the tighter the seal is,
which is just what you want to keep that highly pressurized fluid from leaking by.
The design of these seals is quite complex, and
here I've indicated a seal backing because one of the things you don't want to
have happen is as that seal is pressurized,
that the whole seal just jumps right out of the groove on the piston.
So there's a seal backing or sometimes a collection of seal backings and
then often, you will have two cup seals back to back, so
that it can handle pressures in, in both directions.
Let's go back to the pistons that I have here on the table and look at those three.
In general for a fluid power piston the seals are tight enough
that the that the, the air or the hydraulic fluid doesn't leak by.
So here's that air cylinder that I was talking about,
and I can move it back and forth.
I can feel some friction, and
it's tight enough that the cylinder comes out just a little bit.
It doesn't fall right out.
This specialty cylinder is really interesting.
This is called an airpel cylinder.
And on the inside, it's got a precision piston that's ground out of a graphite.
And then the cylinder use a type of glass with a very high precision internal bore.
So it's a graphite cylinder inside a glass tube,
and it's interesting because it has almost no friction.
So if I hold it upside-down, it falls right out.
And so, while it's not rated at very high pressure, if you ever need
a very low friction application, there are cylinders like this.
Then let's look at our industrial hydraulic cylinder.
And let's see if we can pull it out.
Boy, and I can't pull it out.
And the, the reason why I can't pull it out is on the inside, that seal is so
tight against the walls you recall that before we talked about the pressure
in this being about 2,500 PSI, and it takes about 500,
3 to 500 pounds to overcome the friction of the seal, and
so I just can't pull that out.
And that immediately leads you to think about the efficiencies of these.
Because when the pressurized fluid is pushing against the cap side, it's gotta
push against that 300 pounds of friction before you get any force out of the rods.
And later in the course, we'll talk more about how you,
how you model the efficiency and
think about efficiency because that's a very important part of hydraulic systems.
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