what is the difference?

Hydraulic cylinders are an essential component in many industries, including most of Australia’s critical sectors. Hydraulic cylinders and rams all provide the power a hydraulic system needs to function, but within the category of ‘hydraulic cylinders’, there are a few key types to understand.

The major way of dividing hydraulic cylinders is between ‘single acting cylinders’ and ‘double acting cylinders’. While fairly similar in function, there are differences between single and double acting hydraulic cylinders, meaning they aren’t interchangeable.

The main difference between single and double acting hydraulic cylinders is that a double acting hydraulic cylinder uses hydraulic pressure to move a piston in two directions, while a single acting hydraulic cylinder relies on a spring or gravity to return to its original position.

To help outline the differences in these fairly similar hydraulic systems, we have produced this handy guide. This article outlines the key differences between single and double acting hydraulic cylinders, and when is the best time to use each one.

Whether you’re choosing the right hydraulics components or need to repair hydraulic cylinders, you’ll need to know what type of cylinder or ram you’re working with. That’s why we’ve also covered how to identify single vs double acting cylinders and whether you can convert them.

Ready to learn more about single and double acting hydraulic cylinders? Read on!

Single Acting Hydraulic Cylinders

What Is a Single Acting Hydraulic Cylinder?

A single acting hydraulic cylinder is a hydraulic cylinder in which the fluid acts only on one side of the piston. It relies on the springs, gravity, load, other cylinders, or separate reservoirs to push the piston back into the opposite direction. 

The fluid used a type of oil (usually mineral oil), which allows the fluid to push a load without being compressed back down by the weight. This type of hydraulic cylinder is best for straightforward applications, where the goal is to have something move in one direction.

There are two types of single acting cylinders:

• Push Type Hydraulic Cylinder – Fluid enters to push the piston out of the cylinder. This is also referred to as a hydraulic cylinder with a ‘sprung in’ position, where the piston is fully retracted at rest.
• Pull Type Hydraulic Cylinder – Fluid enters to pull the piston inside the cylinder. This is also referred to as a hydraulic cylinder with a ‘sprung out’ position, where the piston rod is fully extended at rest.

How Does a Single Acting Hydraulic Cylinder Work?

In a single acting hydraulic cylinder, a plunger extends when the cylinder is pumped full of pressurised fluid. When it is retracted, the plunger can retract using a return spring, by the load or gravity. 

Single acting hydraulic cylinders have only one port, where pressurised fluid enters. These are fitted with either a spring, or a load. When the fluid is compressed, it enters through the single port which forces the piston to extend in one direction, consequently compressing the spring. The spring then retracts the piston back into its previous position after releasing the fluid from through the same port it originally flowed from.

What Are the Applications of a Single Acting Hydraulic Cylinder?

The typical applications of single acting hydraulic cylinders are simple lifting jobs, light industrial and commercial applications, and any other application where fast and consistent retraction is not essential. 

Single acting hydraulic cylinders are ideal for machines that:

• Clamp – Through this application, a workpiece can be held in position by jaws that are operated by the cylinder. A process such as drilling can then be done, before the spring retracts the jaws.
• Position And Line Up Objects – Through this application, a workpiece or product is either pulled or pushed into place, or into a specific machine such as a conveyor belt, before the spring acts and prepares the hydraulic cylinder for the next workpiece.
• Punch Or Mark Objects – In this application, thrust from a single acting hydraulic cylinder is used to either punch or mark an object. The spring then retracts the punch, ready for the next object.

This makes single acting hydraulic cylinders especially useful in the manufacturing industry and other fixed applications.

Keep in mind that double acting hydraulic cylinders are able to do all of the above, albeit without the cost savings of a single acting hydraulic cylinder. When it comes to hydraulic cylinder parts, single-acting cylinders are simpler and therefore cheaper to purchase.

What Are the Pros of a Single Acting Hydraulic Cylinder?

The key advantages of a single acting hydraulic cylinder include:

• Simplicity – Its straightforward design allows for much easier installation and control. It’s also simpler to disassemble and repair these cylinders.
• Lower cost – The initial cost of single acting hydraulic cylinders is much lower than double acting ones. This makes it ideal for situations where a double acting hydraulic cylinder may be too much, and a single acting hydraulic cylinder will work fine. In addition, its single port and small housing leads to reduced valve and hose cost compared to double acting cylinders.
• Easier maintenance – Since force is only applied to one direction, there are fewer areas or segments which have to be maintained. Single acting hydraulic cylinders are easier to maintain and repair, especially if you know how to maintain hydraulic cylinders.
• Safety benefits – Single-acting hydraulic cylinders have a specific rest state, which can be easily identified in the event of a power outage. Knowing the cylinder’s resting state will prevent surprise and injury once power is restored, if the cylinder is halfway through an operating cycle.

What Are the Cons of a Single Acting Hydraulic Cylinder?

Some of the main negatives associated with single acting hydraulic cylinders include:

• Spring Inconsistency – The spring’s extended operation can become inconsistent over time and provide uncertain end of stroke positions.
• One-directional – These cylinders can only push or pull into one direction, which does not provide much control during the operation.
• International Standards – Compared to double acting hydraulic cylinders, there aren’t many single acting hydraulic cylinders that are compliant with ISO standards.

Double Acting Hydraulic Cylinders 

What Is a Double Acting Hydraulic Cylinder?

A double acting hydraulic cylinder is a hydraulic cylinder in which the fluid alternately acts on both sides of its piston. It has a port on each end, filled with hydraulic fluid for the pistons’ retraction and extension. 

This type of hydraulic cylinder is ideal for applications that require fast and predictable retractions, or applications where consistent accuracy is necessary.

How Do Double Acting Hydraulic Cylinders Work?

In a double acting hydraulic cylinder, the plunger retracts when the hydraulic fluid, which has now been pressurised, is injected into the top port, which forces the plunger back to its original position. It is possible for this process to be done either very quickly, or very gradually with precise control. 

Double acting hydraulic cylinders have two ports where pressurised fluid can flow in and out. Fluid flows through one port, which moves the piston forward, which then applies pressurised fluid into the second port which then retracts the piston back into the cylinder.

What Are the Applications of a Double Acting Hydraulic Cylinder?

The standard applications for double acting hydraulic cylinders are those that require repetitive presses and situations that require both pushing and pulling forces. Elevators and forklifts are good examples of double acting cylinder applications.

Double acting hydraulic cylinders can do anything a single acting cylinder can. However, these are especially good for industrial and robotics industries, tasks such as opening and closing doors, and lifting and moving merchandise off conveyor belts. Other uses include medical applications, earthmoving and construction equipment, and even in space programs.

What Are the Pros of a Double Acting Hydraulic Cylinder?

Double acting hydraulic cylinders have a lot of benefits over single acting cylinders, including:

• Manoeuvrability – more control over movement since pressurised fluid moves back and forth.
• Efficiency – Double acting hydraulic cylinders are often more resilient and can work more efficiently while consuming less energy.
• Variety – Double acting cylinders offer more design variations in stroke sizes and even the sizes of bores.
• ISO Compliance – Double acting cylinders are often far more in line with the standards laid out by the International Organisation of Standardisation.

What Are the Cons of a Double Acting Hydraulic Cylinder?

Some of the more important disadvantages of double acting hydraulic cylinders to note are:

• Complexity – Double acting hydraulic cylinders are built slightly different to their single acting counterparts. The parts of a double acting cylinder are more complex. They generally have more pistons, and more seals to accommodate the flow of fluid on both ends for a push and pull control.
• Costly – Double acting cylinders are much more expensive than their single acting counterparts. They require larger housing, for example, if it’s used as a feed cylinder and requires coupling with another machine system. Because the system is larger, fluid is often consumed at a faster rate than in single acting hydraulic cylinders.

Single Acting vs Double Acting Hydraulic Cylinders: How Do They Compare?

Single and double acting hydraulic cylinders both serve similar functions, with the double acting hydraulic cylinder being able to do any job a single acting cylinder can, at the cost of being more expensive and requiring more dedicated maintenance. 

Single acting hydraulic cylinders are simple, inexpensive, have one job they can do and do it well. These cylinders are exceptionally useful in fabrication, where one repetitive movement is required, such as on a factory line. Manufacturing hydraulics systems strongly rely on low-cost single acting cylinders.

Double acting hydraulic cylinders, in comparison, are far more sophisticated and powerful. Not only can a double acting cylinder perform any job a single acting hydraulic cylinder could, but by having the piston pressured to move in two opposing directions, it’s essential to the way modern pneumatic machinery works today. Things like elevators and forklifts could not exist without double acting hydraulic cylinders.

If an application only needs motion in one direction, a single acting hydraulic cylinder is the safest and most economical choice. However, any situation requiring greater control and power requires a double acting hydraulic cylinder, and the ISO standards compliance associated with them.