Inside of your hydraulic cylinders, there are a number of seals that perform important functions, keeping your cylinders working efficiently while protecting internal components from being damaged by contaminants, corrosion, and other factors. These often-overlooked components are one of the most important parts of your hydraulic system — in fact, without those seals, your systems wouldn’t work at all.

While you’ll almost never see these hydraulic cylinder seals during operation, it’s important to be familiar with the different types of seals that your machines use. When you know the function of each seal, as well as the differences between the materials used to fabricate hydraulic cylinder seals, you’ll ensure that your equipment will not only be best suited for its intended function but that it will also stand the test of time.

Types of Hydraulic Cylinder Seals

Hydraulic cylinder seals can be divided into two large categories: Static and dynamic. Static seals are used in situations in which the component parts aren’t designed to experience relative movement. These seals include O-rings, D-rings, backup seals, and other seals that are intended to remain in a fixed relative location.

One kind of static seal is usually found where the piston and the rod join. A static piston seal sits within the piston with the rod running through it. The pressure from the rod creates an impermeable seal that prevents hydraulic fluid from being pushed through to the other side of the piston. This is especially important in single-action pistons, which only have hydraulic fluid on one side of the piston.

Dynamic hydraulic cylinder seals, on the other hand, are intended for areas in which there is relative motion between two components. Because there are a number of different functions performed by dynamic seals within your hydraulic cylinder, it’s probably best to look at some of the specific seals you’re likely to come across in greater detail.

Piston Seals

Piston seals provide the power for your hydraulic cylinder by preventing hydraulic fluid from flowing past your piston. This makes piston seals one of the most important hydraulic cylinder seals in your system.

The seal formed by a piston seal keeps contact between the piston and the cylinder bore, helping to control the motion of your cylinder and keeping your cylinder at the same position when at rest — even when it’s under a load.

Piston seals can be either single- or double-action. As mentioned earlier, a single-action piston has hydraulic fluid on only one side of the piston, which means that the piston seal must remain extremely close to the bore, otherwise fluid can seep through and lessen the effectiveness of your cylinder.

Double-acting cylinders, as the name implies, have hydraulic fluid on both sides of the piston. Because of this, the shape of a double-acting piston seal is symmetrical, possessing the same sealing properties in both directions.

These seals can also allow for more clearance between the piston and the cylinder bore, as they are not designed to prevent fluid leakage.

Rod Seals and Buffer Seals

Rod seals perform two seemingly contradictory functions. First, they keep hydraulic fluid from leaking outside of the cylinder during operation. However, these seals are also responsible for ensuring that there is a thin layer of lubrication on the piston rod. This not only prevents corrosion but also lubricates other important parts of your cylinder.

Depending on the duty level of your cylinder, you may or may not also have a buffer seal, which is set further within the cylinder, past the rod seal. These hydraulic cylinder seals are intended to smooth out the pressure within your cylinder, preventing pressure peaks resulting from external factors. They also prevent any contaminants within the cylinder, such as metal flakes, from causing damage to your rod seals.

Wiper Seals

Sometimes referred to as dust seals, excluders, or scrapers, wiper seals protect your equipment by preventing contaminants from entering your cylinder as the piston rod retracts.

Since the piston rod is coated with a thin layer of fluid to prevent corrosion, it can also pick up dirt and other contaminants, which could quickly damage the interior components of your cylinder. Wiper seals keep these contaminants outside of your system.

Guide Rings

These seals are intended to maintain moving parts in the proper position throughout the process of relative motion. In addition to keeping the important components properly aligned, guide rings prevent any metal-on-metal abrasion that might occur during operation, preserving the life of your equipment.

These seals also add another level of protection against contaminants by keeping your more sensitive seals safe from added wear.

Materials Used for Hydraulic Cylinder Seals

Hydraulic cylinder seals can be manufactured from a number of different materials. Each material has different properties that make them better suited to different conditions, including high pressures and temperatures, as well as caustic chemicals. Here are the different materials you’re likely to encounter:

Rubber

Rubber seals are used in situations that require a high degree of flexibility and compressibility. Nitrile butadiene rubber (NBR), fluoroelastomer (FKM), and hydrogenated nitrile butadiene rubber (HNBR) are all commonly used to fabricate dynamic cylinder seals. They possess good resistance to grease, mineral oils, and other hydraulic fluids.

PTFE

Polytetrafluoroethylene (PTFE) offers superior durability under extreme conditions, including temperature, pressure, and caustic chemicals. PTFE also has a very low coefficient of friction, which means that it requires less lubrication than other seals and can actually function quite well in dry operation.

Rigid Plastics

Encompassing a wide range of materials, plastic hydraulic cylinder seals offer moderate resistance to temperature and pressure, as well as chemical reactivity. 

The durability of a plastic seal is largely dependent on the quality of the material itself, which can range from engineered plastics, which are superior to commodity plastic, to high-performance plastics. Because they have less flexibility than rubber or PTFE, these seals focus more on adding strength to your cylinder.

Thermoplastic Elastomers

Thermoplastic elastomers, or TPEs, combine the strength of plastic with the flexibility of rubber, which means that they stand up to high use while also providing superior elasticity. These hydraulic cylinder seals are highly resistant to abrasion and tearing and will maintain a constant pressure for longer than other materials can.

Choosing the Right Hydraulic Cylinder Seals

Choosing the right seal is about more than materials, however. There are a number of external factors that you need to consider when you’re designing your cylinders. Each of these factors can go a long way toward determining which hydraulic cylinder seals you’ll need to ensure that your system can work efficiently for the long haul.

Cylinder Application

One of the most important considerations to keep in mind as you’re designing your hydraulic cylinder is to know precisely what you will need your cylinder to do. In general, you can break your application down into three different categories:

• Light Duty:  Pressures up to 2,300 psi, temperature up to 70˚ C
• Medium Duty:  Pressure up to 3,625 psi, temperature up to 90˚ C
• Heavy Duty:  Pressure up to 5,800 psi or more, temperature over 90˚ C

Having realistic expectations about the duty level that you’ll require from your cylinder will ensure that you won’t end up having to replace your equipment prematurely. If you’re unsure where your cylinder falls, or if you think you’re right on the dividing line between two service levels, it’s usually a good idea to choose a heavier-duty model — just in case.

Temperature

You’ll need to know your average operating temperature, but also the temperature of your equipment when it’s at rest.

Speed

This is the speed at which your piston operates within your cylinder. Typically, the faster the cylinder operates, the hotter the cylinder will become.

Fluid Pressure

It’s important to know what kinds of pressure your cylinder will encounter, which is usually expressed as a range. You’ll also want to identify any pressure peaks, noting both intensity and frequency.

Hydraulic Fluid

There is a wide range of different hydraulic fluids available, and each fluid possesses a different viscosity, which will affect the hydraulic cylinder seals you’ll require.

Dimensions

The shape of your cylinder will also have an impact on which seals you’ll want to use in your cylinder. You’ll need to know the exact measurements of these parts:

• Rod diameter
• Bore diameter
• Stroke length
• Overall cylinder length

Each of these factors will play a part in determining other considerations, including pressure, speed, and power. These can go a long way in helping you choose the right hydraulic cylinder seals to keep your equipment working properly.

MAC Hydraulics

No matter what your hydraulic system needs, MAC Hydraulics is here for you. Our team of experts can answer all of your questions about hydraulic cylinder seals. And our highly trained technicians will make sure that you get the information you need.

We’re always happy to offer you on-site repairs for any problems you encounter, as well as preventative maintenance to ensure your hydraulic systems continue working at peak performance. 

We have extensive experience in a number of industries, including food and beverage, construction, manufacturing, and waste management. If you’re looking for a trusted partner to help you with hydraulic services, contact us today!