In hydraulics there are really only two motions. Going around in a circle, or going back and forth. Let’s look at things that go back and forth.
In this lesson we will examine some of the external and internal structure of a linear actuator.
Or as they are more commonly known, a hydraulic cylinder.
Hydraulic cylinders come in many, many, many different shapes and sizes, but for the most part they all share some basic elements.
The two most obvious components are the rod and the barrel. The barrel is the part which the rod moves in and out from.
The end of the cylinder that the rod is sticking out of is called the rod end. The opposite end of the cylinder is called the blind end for some reason.
From the outside you can see the ports, where hydraulic fluid can enter or exit the barrel, as well as the type of mounting connections.
There's a lot going on inside the cylinder barrel!
The inside diameter of the cylinder barrel is called the bore. This is the space that the rod moves through.
The piston is attached to the rod. It's the component that nearly makes contact with the bore.
The piston contains two o-rings. The first prevents the high pressure oil from escaping between the clearance of the piston and the bore.
The second o-ring, usually called the stem seal, is inset into the piston and prevents high pressure oil from leaking between the piston and the rod stem.
The piston will also have what are known as backup rings.
A backup ring prevents a round seal (o-ring) from extruding through the clearance between the piston and the bore.
In this example there are two backup rings being used. One to backup the o-ring when high pressures are introduced from the right hand side and one to backup the o-ring when high pressures are introduced from the left hand side.
Two components keep the rod centered in the barrel.
One is the piston. But what is the other one?
In a simple and inexpensive cylinder, the gland is held in place by a retaining snap ring.
More complex cylinder designs can use a whole gland end assembly to hold things in place. More on this later.
The gland also has a couple of seals. One on the outside, the gland seal, prevents high pressure oil escaping between the clearance of the gland and the bore.
And a second, internal seal, which prevents high pressure oil from escaping between the clearance of the gland and the rod.
The rod seal has its own backup ring. It is similar in function to the piston backup rings.
Finally the gland also has a wiper seal. This is used to keep birds, rocks, dirt, and ice from getting into the cylinder barrel.
Finally there is the stroke.
The stroke is the distance that a piston travels in a barrel.
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There is a whole range of additional components that can be used to increase the durability and refine the functioning of a hydraulic cylinder.
In this example, each of the ends can be removed from the barrel. This is pretty common in the more expensive industrial type cylinders.
Each of these ends contains a fluid port, and may contain an adjustment for setting the speed when the rod reaches the end of its stroke in either direction.
This assembly is held together by four tie rods. These are long, threaded rods which can be screwed directly into an end, or pass through the end and be held in place with nuts.
It is important that the tie rods are torqued properly. Improperly torqued tie rods can pull the barrel out of alignment with the piston/rod, causing all sorts of damage.
A complex cylinder shares many of the same internal components as the simple cylinder; there are just more of them. Wear rings and seals, for example, multiply in a complex cylinder.
But wait, there's more!
These additions to the rod are called cushions and may be part of the actual rod or they may be separate parts added to the rod assembly.
A cushion is used to slow a fast moving rod as it approaches the end of its stroke. Slowing the rod down decreases the amount of impact force that the piston makes on the head at the end of stroke.
Cushions increase the cylinder's lifespan by decreasing vibration and reducing the operating noise.
(These impacts can be loud!)
As the rod reaches the end of stroke the cushion stem/sleeve enters the cushion bore.
This blocks the main flow path for the oil leaving the cylinder barrel.
With the main flow path blocked, oil is redirected through a flow control orifice.
It is common for this orifice to be adjustable, to allow the precise setting of the exiting flow rate.
The presence of the cushion stem in the cushion bore will now restrict flow as the cylinder begins to extend.
To get around this there is a check valve in the head end which allows oil in to the barrel.
Once the stem is clear it becomes easier for oil to pass through the through the bore than the check valve and the valve closes once more.
Here's the same cylinder, with a slightly different perspective on the ports.
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This is only scratching the surface of the subject of hydraulic cylinders.
Features like telescoping, double rod, and position sensors can add further complexity to the world of cylinders.
But this gives you a good base for understanding what you are seeing when examining or taking apart a cylinder.