The Importance of Piston Seals in Hydraulic Systems

Every component in a system – any system – is essential. The harmony of every part doing its job makes for a well-working machine. Hydraulic pumps are no different. 

In this piece, we look at piston seals in hydraulic cylinder systems. Why, in the hierarchy of hydraulic cylinder parts, are piston seals important? What are the different designs of hydraulic seals in pistons? How do you select the proper piston seal for a given application?

How Hydraulic Systems Operate

A hydraulic system consists of a machined cylinder or bore with a piston. Fluid is introduced under pressure into one side of the cylinder, forcing the piston down the cylinder. A rod connected to the piston transforms this fluid pressure into mechanical force. This mechanical force fills a variety of functions, from a simple task such as an adjustable office chair to more complex, heavy-duty applications like heavy manufacturing machinery.

Single-Acting vs. Doubling-Acting Seals

A hydraulic cylinder like the one just explained creates force in one direction. There are also double-acting cylinders that create force in both directions. In the case of more complex machinery, such as the cylinder of a mobile excavator, pressure from one side of the cylinder is used to move the blade in one direction. When it is necessary to move the blade in the opposite direction, fluid is introduced into the other side of the cylinder, and the mechanical force goes in the opposite direction. Many industrial processes require greater control and precision. For this reason, double-acting hydraulic cylinders are common in industrial applications. 

In a double-acting hydraulic cylinder, the piston seal is designed so that it prohibits fluid and pressure from escaping either way. This type of seal is referred to as a double-acting seal. In a single-acting hydraulic cylinder, the piston seal needs to prevent fluid and pressure from escaping only one way- but the piston seal still needs to be double-acting so that the piston’s return to its original position is smooth and effortless.

Wear Rings

As the piston travels up and down in the cylinder bore, a set of wear rings mounted in machined grooves in the piston (referred to as glands) prevent direct metal-to-metal contact between the piston and the cylinder bore. These wear rings also center the piston and control lateral movement as the rod transfers energy. While these wear rings operate in very close tolerances, they do not keep pressure and hydraulic fluid from escaping the pressurized side of the piston. That is the job of the piston seal. 

The Role of The Piston Seal

The piston seal sits in a separate gland between the wear rings. This seal, often installed under pressure, protrudes up and over the gland and presses against the cylinder bore, preventing hydraulic fluid from escaping the pressurized side of the bore and into the low-pressure chamber. This leakage would compromise pressure in the hydraulic cylinder and not allow it to do its job.

What Makes A Piston Seal?

Piston seals are often elastomeric, meaning they consist of elastic materials. They can be compressed and twisted, and when released, they will return to their original shape. Elastomers include any number of rubber and rubber-like compounds, including polyurethane, nitrile, and similar materials.

Piston seals that serve more critical roles in high-pressure applications might consist of harder materials such as PTFE (also known as Teflon). PTFE is not considered an elastomer, however it still displays many similar properties that make it an excellent material choice for an industrial-grade piston seal.

How Pressure Affects a Piston Seal

As it sits in the groove, a piston seal is subject to two types of pressure: mechanical pressure and system pressure. 

Mechanical Pressure

Mechanical pressure is the pressure on a seal that exists by virtue of the seal design, the way it compresses into the groove, and how the piston fits into the bore during assembly. Tolerances are very close, and these tolerances hold the seal in place and press it tightly against the cylinder bore. Mechanical pressure is most dominant when the hydraulic cylinder is at rest or in low-pressure applications. When subjected to mechanical pressure, the seal tends to retain its shape. 

System Pressure

During operation, the motion of the hydraulic system generates not just pressure but also heat. This system pressure is both good news and bad news. In many cases, a well-designed seal and the other hydraulic cylinder parts do their job even after long periods of operation and subsequent wear. Under increasing amounts of system pressure, piston seals lose their original shape- but conversely, they tend to perform better because as the seal distorts, it conforms to the surfaces being sealed. The system pressure of the hydraulic fluid further compresses the piston seal in the groove, pushing it up against the far wall of the cylinder bore. This distortion also pushes the seal up and out against the cylinder bore, creating a tighter seal against the pressure of the hydraulic fluid. Thus, pressure is maintained.

The bad news is that this pressure does have the potential to exert too much force on the seal, forcing it to twist and roll over in the gland. In this case the seal is still usable- so long as it maintains its shape, the system can be repaired. However, good hydraulic system design and suitable seal materials can usually prevent this from happening to begin with.

The Extrusion Problem

A common challenge in high-pressure situations is when that pressure deforms the piston seal. The seal material starts to extrude or creep into the minuscule space between the piston wall and the cylinder bore. This distorts the seal beyond its useful shape. Extrusion often leads to damage to the seal and accelerates wear. 

In applications where extrusion poses a problematic risk, hydraulic designers often incorporate secondary seals that fit into the gland alongside the main piston seal. Anti-extrusion seals, made from stiffer material, prevent the main seal from extruding. Some seals designed for high-pressure applications include anti-extrusion seals built into them.

Need a seal that won’t quit under pressure? CRC delivers precision-engineered seals built for peak performance in the toughest environments.

PTFE Hydraulic Piston Seals

As mentioned earlier, some piston seals are made from PTFE, or Teflon. These are generally higher-quality seals used in critical applications. PTFE seals, which are not elastomeric, are harder and less likely to deform. Under extreme pressure, however, the material in PTFE piston seals can experience creep, a problem similar to extrusion. Designers often build in anti-extrusion seals to address this issue, resulting in a seal that is just as reliable as it is robust.

How to Choose the Right Piston Seal for an Application

When engineers specify a piston seal design for an application, they consider several factors. These include:

• Pressure. How much pressure does the hydraulic cylinder need to generate and maintain to perform a specific task? 
• Temperature. What kind of temperature does the seal need to tolerate? Each seal material has temperature limitations, from sub-zero to as much as several hundred degrees Fahrenheit. 
• Friction. Engineers also have to evaluate friction. Stiffer materials with lower elasticity are unsuitable for piston seals in low-pressure applications.  They will cause too much friction and will not allow the piston to move freely, or they will cause excess heat.  
• Type of hydraulic fluid. It is important to ensure that the material used in the piston seal is compatible with the chemical composition of hydraulic fluid specified in the system.

You should always check the documentation accompanying your hydraulic system to verify that you order the correct replacement seals. If you have a custom hydraulic system or a discontinued model, a source such as CRC Distribution can help you determine the proper parameters for a new piston seal. 

Custom spec? Discontinued system? CRC has the materials, machining power, and know-how to craft the perfect seal for your build.

How Seals are Manufactured

Most high-quality piston seals are machined to critical tolerances for specific uses, using a CNC spindle to shape them for particular applications. Some common materials for machined seals include polyurethane, NBR, HNBR, Viton, bronze-filled Teflon, virgin Teflon, polyacetal, and nylon. Once machined and completed, the seals undergo a quality control process to ensure they meet specifications. 

Seals also have different profiles. There are piston seals with a bead profile, where the seal presses against the cylinder as a single unit. Other seals feature a symmetrical double lip profile, while other seals have an asymmetrical double lip profile. Each profile design is based on how the seal functions in the piston. There are also seals that have an insert incorporated into the seal to give it extra strength. Individual seal designs are dictated by the application and the kinds of forces that will be working on the seal. 

CRC Distribution Is The Place for Seals

CRC Distribution carries a complete line of hydraulic seals in various materials, along with complete seal replacement kits. CRC engineers are also capable of machining custom seals for virtually any application. More importantly, our personnel are experts in helping you determine the best standard or custom seal solution for your application.

Don’t settle for off-the-shelf. CRC’s sealing solutions are engineered to outperform and built to last.