Help

Anatomy of a Cylinder

Objectives

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.

The Simple Cylinder

Hydraulic cylinders come in many, many, many different shapes and sizes, but for the most part they all share some basic elements.

Barrel
Rod

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.

Blind End Port
Rod End Port
Cross Tube Mount
Cross Tube Mount

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.

The Piston

There's a lot going on inside the cylinder barrel!

Bore

The inside diameter of the cylinder barrel is called the bore. This is the space that the rod moves through.

Piston

The piston is attached to the rod. It's the component that nearly makes contact with the bore.

O-Ring

The piston contains two o-rings. The first prevents the high pressure oil from escaping between the clearance of the piston and the bore.

Stem Seal
Stem

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.

Backup Rings

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.

The Gland

Two components keep the rod centered in the barrel.
One is the piston. But what is the other one?

The gland!

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.

Retaining
Snap Ring
Gland

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.

Gland Seal

And a second, internal seal, which prevents high pressure oil from escaping between the clearance of the gland and the rod.

Rod Seal

The rod seal has its own backup ring. It is similar in function to the piston backup rings.

Backup Ring

Finally the gland also has a wiper seal. This is used to keep birds, rocks, dirt, and ice from getting into the cylinder barrel.

Wiper Seal

Stroke

Finally there is the stroke.

The stroke is the distance that a piston travels in a barrel.

Review

Click and drag each label to the correct box.

Gland Seal
Gland Seal
Backup Ring
Backup Ring
Piston Seal
Piston Seal
Barrel
Barrel
Rod
Rod
Wiper
Wiper
Backup Ring
Backup Ring
Rod Seal
Rod Seal
Retaining Snap Ring
Retaining Snap Ring
Stem
Stem

The Complex Cylinder

There is a whole range of additional components that can be used to increase the durability and refine the functioning of a hydraulic cylinder.

Gland End Head
Blind End Head
Barrel

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.

Cushioning

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.

Barrel Seal
Cushion Seal
Piston Seal
Piston Seal
Wear Ring
Barrel Seal
Cushion Seal
Gland Seal
Piston
Gland
Stem Seal
Wear Ring
Rod Seal
Wiper Seal

But wait, there's more!

Cushion
Cushion

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.

Retracting

A cushion is used to slow a fast moving rod as it approches 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!)

Cushion
Bore
Port

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.

Flow
Control
Orifice

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.

Extending

The presence of the cushion stem in the cushion bore will now restrict flow as the cylinder begins to extend.

Check
Valve

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.

Review

Click and drag each label to the correct box.

Cushion Seal
Cushion Seal
Piston Seal
Piston Seal
Gland
Gland
Cushion
Cushion
Cushion
Cushion
Check Valve
Check Valve
End Head
End Head
End Head
End Head
Orifice
Orifice
Wear Ring
Wear Ring
Rod Seal
Rod Seal
Rod
Rod
Barrel
Barrel
Wiper Ring
Wiper Ring
Port
Port
Bore
Bore

Recap

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.

We hope you enjoyed Anatomy of a Cylinder

Loading
Vacuum Pressure
Drain Pressure
Low Pressure
Medium Pressure
High Pressure
Ground/Common
Lowest Voltage
Medium Voltage
Highest Voltage
Magnetic Field
Check Your Console