A Guide to Hydraulic Cylinders

Hydraulic cylinders are an integral part of our day-to-day lives and a crucial component of hydraulic systems.

Cylinders can be found in almost any hydraulic machinery that requires a strong push or pull force, and are used in a multitude of industries including manufacturing, construction, mining and offshore.

Whether you’re new to the world of hydraulics or are looking to gain more knowledge on the components you’re working with, this article aims to provide an entry-level understanding of hydraulic cylinders.

How does a hydraulic cylinder work?

Let’s start with a basic overview of how hydraulic cylinders work.

A hydraulic cylinder is a mechanical actuator used to convert hydraulic energy into linear movement in order to perform the desired action of the machine such as lifting, pressing or moving.

The casing of a hydraulic cylinder consists of a barrel with separate ports for fluid inlet and outlet, and a piston inside which separates the tube into two chambers. The piston is connected to a rod which moves back and forth inside the cylinder when exposed to pressure.

The chamber is partially filled with hydraulic fluid, leaving enough space for the piston to operate. The fluid powers the cylinder, by transmitting a force that retracts or extends the piston. As the first chamber is filled with hydraulic fluid it acts on the piston forcing it to extend and expelling fluid from the second chamber. If the second chamber is, then filled the piston retracts and fluid is expelled from the first chamber.

This process generates push and pull movements, delivering the large linear force needed for a machine to perform the required operation.

As with all other hydraulic components and applications, hydraulic cylinders work based on Pascal’s law. The theory behind this is that as hydraulic fluids are incompressible, the force generated on the piston transmits an equal pressure throughout the cylinder. Therefore, the force applied internally will equal the specified output force.

There are many different types of hydraulic cylinders, all carrying unique advantages to suit an array of applications.

Most types of cylinders fall into two categories:

• Single-acting cylinders – In a single-acting cylinder, the fluid can only act on one side of the piston rod. To operate the cylinder from the opposite end, another force such as spring pressure or the weight of the load must be applied.
• Double-acting cylinders – A double-acting cylinder can exert force in two directions, enabling the rod to achieve both outward and return strokes under the force of the liquid from either side of the chamber.

Under these categories, there are many variations in construction to create different types of cylinders. The difference between them primarily depends on how the two end caps are attached to the barrel, alongside materials and wall thickness.

Some of the most common cylinder types include:

Tie rod cylinders

This type of cylinder uses high strength threaded steel rods to secure the two end caps (typically square or rectangular) to the ends of the barrel.

The number of tie rods will depend on the size of the bore. Large bore cylinders may have up to 20 tie rods to keep the unit secure under tremendous pressure, whereas small bore cylinders may just have 4. Tie rod cylinders are typically used in heavy-duty industrial machinery.

Welded cylinders

This type of cylinder has no tie rods connecting the end caps, instead, the two end caps are welded to the barrel.

Generally, welded cylinders are narrower in the body than tie rod cylinders, and are usually shorter in length which makes them suitable for fitting in narrow machine confinements. Welded cylinders are easily customisable and their design eliminates the risk of failure from tie rod stretch at high pressures.

Hydraulic cylinders comprise 7 main components no matter the size or type of cylinder. These are:

Cylinder tube

Also known as the cylinder barrel, this section is usually made of steel and is the main body of the component that houses the hydraulic fluid and holds the pressure. Cylinder tubes are designed to be durable and high strength, with high corrosion resistance and precision tolerance.

Barrels have a smooth surface finish and the type of steel used varies depending on the strength needed and the thickness of the cylinder walls required.

Cylinder head

This is the end cap of the cylinder which sits inside the barrel and through which the piston rod passes. Its purpose is to enclose the pressure chamber on one end of the cylinder.

Cylinder heads are attached to the body of the cylinder by bolts, tie rods or threading and typically have an integrated rod sealing arrangement or a connection to add a seal gland.

Cylinder cap

Also known as the cylinder base, this seals the chamber at one end to enclose the pressure, and can also be used as a cylinder mounting component.

The cylinder cap can be connected to the cylinder barrel via welding or other methods such as bolts, threading or tie rods.

Piston

This is located inside the cylinder and is used to separate the pressure zones inside the barrel to create individual chambers. Pressure is created on either side of the piston which causes it to move up and down the cylinder bore and results in the cylinder expanding or retracting.

The piston must be sealed to ensure no hydraulic fluid passes between the chambers.

Piston rod

The piston rod is attached to the piston and passes through the cylinder head and is usually made from hard chrome-plated steel. It provides the connection from the hydraulic cylinder to the machine component in operation.

Seal gland

A hydraulic cylinder gland sits at the cylinder head and houses a number of seals to prevent the hydraulic fluid from leaking beyond the cylinder rod and cylinder head.

The gland provides easy access for removing and replacing all of the seals when needed, and typically includes the primary seal, secondary seal and static seal.

Seals

There are many different types of seals that sit in the gland and are designed specifically for the hydraulic cylinder depending on a number of variables including working pressure, cylinder speed, operating temperature and application.

In order to prevent leakage and maintain optimal performance of the pressurised fluid, a combination of high-performance static seals and dynamic seals is used.

As a leading UK distributor of hydraulic components, Primary Fluid Power offers industry-leading brands in hydraulic cylinders, and the ability to manufacture bespoke cylinders in-house to meet your specifications.

Our team of technical engineers can help to size up your hydraulic cylinder needs and build the perfect solution for you. Please get in touch with us to discuss your requirements.