6 Types of Cylinder Seals

Construction machinery relies heavily on hydraulic cylinders, which are integral components in transmitting force and motion. The efficiency and reliability of these cylinders are critically dependent on high-performance seals.

One essential type of seal used in hydraulic cylinders is the sealing ring, also referred to as an oil seal or hydraulic seal. These components play a crucial role in maintaining system integrity by preventing oil leakage, contamination ingress, and pressure loss. Properly selected and installed sealing rings ensure optimal cylinder performance, extended service life, and reduced maintenance costs.

In this comprehensive guide, we have compiled an extensive list of various types and configurations of sealing rings commonly employed in hydraulic cylinders for construction equipment. 

Fig. 1 Sealing rings.

The hydraulic cylinder employs a variety of specialized sealing components to ensure optimal performance and longevity. These essential sealing elements include:

1. Dust seals (or wipers): Located at the cylinder’s exterior, these seals prevent contaminants such as dust, dirt, and moisture from entering the system, thereby protecting internal components.
2. Rod seals: Positioned around the cylinder rod, these seals maintain hydraulic fluid within the cylinder while the rod extends and retracts, preventing leakage and maintaining system pressure.
3. Buffer seals: These seals absorb impact and reduce noise at the end of the cylinder stroke, enhancing operational smoothness and minimizing wear on internal components.
4. Guide rings (or wear rings): While not strictly seals, these components support the piston and rod, reducing metal-to-metal contact and ensuring proper alignment throughout the cylinder’s range of motion.
5. Static seals: These include end cap seals and other fixed seals that prevent leakage at stationary joints, such as between the cylinder barrel and end caps.
6. Piston seals: Fitted around the piston, these bi-directional seals maintain pressure differential between the cylinder’s chambers, enabling force transmission and ensuring efficient operation.

Fig. 2 The position of different seal rings.

Basics of Hydraulic Cylinder Seals

Hydraulic cylinder seals are critical components in hydraulic systems, playing a vital role in maintaining system integrity and efficiency. Their primary functions include preventing fluid leakage, maintaining pressure, and ensuring smooth operation of moving parts. Understanding the characteristics and applications of different seal types is essential for optimal hydraulic system performance. Seals are broadly categorized into two main types: static seals and dynamic seals, each with distinct properties and applications.

Static Seals

Static seals are employed where there is no relative motion between mating surfaces, such as in flange connections, access covers, or ports. These seals are designed to create a tight barrier against fluid leakage and maintain system pressure. Common types of static seals include:

1. O-rings: Widely used due to their versatility and ease of installation, O-rings are elastomeric seals that can be applied in various applications. Their simple design and effectiveness make them a popular choice for many static sealing requirements.
2. Gaskets: Typically flat and available in various materials (e.g., rubber, metal, or composites), gaskets are used to seal flanges and other static interfaces. Material selection depends on factors such as temperature, pressure, and chemical compatibility.
3. Metal seals: Utilized in high-temperature or high-pressure applications, metal seals are typically made of aluminum, copper, or steel. They offer excellent durability and resistance to extreme conditions.

Dynamic Seals

Dynamic seals are used in applications involving relative motion between mating surfaces, such as in pistons or rods. These seals must withstand constant movement and friction, requiring enhanced durability and wear resistance compared to static seals. Key types of dynamic seals include:

1. Piston seals: Designed to prevent fluid leakage between the piston and cylinder bore, piston seals are crucial for maintaining hydraulic pressure. The U-cup seal, featuring a U-shaped cross-section, is a common and effective piston seal design.
2. Rod seals: These seals prevent fluid leakage at the rod-cylinder interface while allowing free movement of the rod. Rod seals often incorporate wiper seals, which remove debris and contaminants from the rod surface, maintaining system cleanliness.
3. Rotary seals: Used in applications with rotating components (e.g., motors or pumps), rotary seals maintain sealing integrity during continuous rotation. They are available in both radial and axial configurations to suit specific application requirements.

Seal selection and design considerations are critical for optimal hydraulic system performance. Factors influencing seal choice include:

• Operating pressure and temperature ranges
• Fluid compatibility and chemical resistance
• Speed of motion (for dynamic seals)
• Environmental conditions and contaminants
• Required service life and maintenance intervals
• Material properties (e.g., hardness, elasticity, wear resistance)

Advancements in seal technology have led to the development of specialized materials and designs, such as PTFE-based seals for high-temperature applications or polyurethane seals for improved wear resistance. Additionally, innovations like energized seals and composite seals offer enhanced performance in challenging operating conditions.

Types of Hydraulic Cylinder Seals

1. Dust ring

The dust seal is located on the exterior of the hydraulic cylinder’s end cap and serves to prevent external debris from entering the cylinder.

Based on the method of installation, dust seals can be classified into two types: snap-in and press-in.

Fig. 3 Basic form of snap-in dust ring.

The most commonly used dust ring is the snap-in type. As the name implies, it snaps into the groove on the inner wall of the end cover and is used in less harsh environmental conditions. The material used for the snap-in dust ring is usually polyurethane and it comes in different structures, such as the double-lip structure type H and K-sections. Despite the variations, their basic design remains the same.

Fig. 4 Some variants of snap-in dust ring.

The press-in type dust ring is used in harsh and high-stress environments. Unlike the snap-in type, it is not inserted into the groove but is reinforced with a metal layer to enhance its strength and then inserted into the hydraulic cylinder’s end cover. There are different types of press-in dust rings, such as single-lip and double-lip.

Fig. 5 Press in dust ring and some variants.

2. Cylinder Rod Sealing Ring

The cylinder rod sealing ring, commonly referred to as a U-cup, serves as the primary sealing element for the piston rod. It is strategically installed within the end cover of the hydraulic cylinder to effectively prevent hydraulic oil leakage. These sealing rings are predominantly manufactured from high-performance materials such as polyurethane or nitrile rubber, chosen for their excellent wear resistance, low friction properties, and compatibility with hydraulic fluids.

In high-pressure applications or when dealing with larger cylinder bores, the sealing ring is often used in conjunction with a support ring (also known as a back-up ring or anti-extrusion ring). This additional component prevents the sealing ring from undergoing extrusion deformation under intense pressure, thereby extending the seal’s lifespan and maintaining its integrity. The support ring is typically made of harder materials like PTFE or high-density polyethylene.

The design of cylinder rod sealing rings has evolved to address various operational requirements and environmental conditions. Manufacturers offer a wide range of variations, including:

1. Single-acting vs. double-acting designs
2. Different lip configurations for optimized sealing and lubrication
3. Specialized materials for extreme temperature or chemical resistance
4. Reinforced designs for high-pressure systems
5. Low-friction coatings for improved efficiency and reduced stick-slip

Selecting the appropriate cylinder rod sealing ring involves considering factors such as operating pressure, temperature range, fluid compatibility, surface finish of the rod, and reciprocating speed to ensure optimal performance and longevity of the hydraulic system.

Fig. 6 Common types of cylinder rod seal ring.

Fig. 7 Some variants of cylinder rod sealing rings.

3. Buffer seal ring

The buffer seal ring, a critical auxiliary component in piston rod sealing systems, plays a vital role in safeguarding the piston rod against sudden pressure spikes and fluctuations. This dynamic seal not only protects the primary sealing elements but also contributes to the overall system longevity and reliability. There are three primary configurations of buffer seals, each designed to meet specific operational requirements:

Type A: A single-piece, monolithic seal fabricated from high-performance polyurethane. This design offers excellent wear resistance, good elasticity, and moderate pressure capabilities. It’s particularly suitable for applications with moderate pressure fluctuations and where simplicity of installation is desired.

Type B and Type C: These are advanced two-piece seal assemblies engineered to prevent extrusion and withstand significantly higher pressure differentials. The two-piece construction typically consists of a resilient sealing element paired with an anti-extrusion ring:

• Type B often utilizes a combination of polyurethane for the dynamic sealing element and a rigid thermoplastic or fiber-reinforced composite for the anti-extrusion component.
• Type C may employ a PTFE-based sealing element coupled with a high-strength backup ring, offering superior chemical resistance and low friction properties.

Both Type B and Type C configurations provide enhanced pressure resistance, improved stability under dynamic conditions, and better protection against seal failure in high-pressure environments. The choice between these types depends on factors such as maximum system pressure, temperature range, fluid compatibility, and cycle frequency.

Fig. 8 Common types of the buffer seal ring.

4. Guide support ring

The guide support ring, a critical component in hydraulic cylinders, is strategically positioned on both the end cover and piston. This multifunctional element plays a pivotal role in ensuring the cylinder’s optimal performance and longevity. Its primary functions include:

1. Supporting the piston rod and piston: The ring provides crucial structural support, distributing loads evenly and minimizing wear.
2. Guiding the piston’s linear motion: It ensures precise, smooth, and consistent linear movement of the piston, enhancing overall system efficiency.
3. Preventing metal-to-metal contact: By acting as a buffer between metal components, the ring significantly reduces friction, wear, and potential damage.

Guide support rings are typically manufactured from high-performance materials carefully selected for their specific properties. Common materials include:

• Engineering plastics: Such as PTFE-filled polyacetal (POM) or polyamide (PA), offering excellent wear resistance and low friction.
• Bronze with PTFE coating: Combining the strength of bronze with the low-friction properties of polytetrafluoroethylene (PTFE).
• Fiber-reinforced composites: For applications requiring higher load-bearing capacity and thermal stability.

The choice of material depends on factors like operating pressure, temperature, fluid compatibility, and expected service life. Advanced manufacturing techniques, such as precision CNC machining or injection molding, ensure tight tolerances and superior surface finish, critical for the ring’s performance.

Fig. 9 Guide support ring used on the end cover of the hydraulic cylinder and the piston.

5. End cover sealing ring

The end cover seal ring is a critical component engineered to provide a hermetic seal at the interface between the cylinder end cover and the cylinder wall. Functioning as a static seal, it plays a crucial role in maintaining system integrity by preventing hydraulic fluid leakage from the juncture of these two primary components.

Typically, the end cover seal ring assembly comprises two key elements: a resilient NBR (Nitrile Butadiene Rubber) O-ring and a rigid support ring, often referred to as a backup ring or anti-extrusion ring. This dual-component design offers several advantages:

1. The NBR O-ring, known for its excellent oil resistance and elasticity, provides the primary sealing function. It deforms under compression, filling microscopic surface irregularities to create an effective barrier against fluid escape.
2. The support ring, usually made of a harder material such as PTFE (Polytetrafluoroethylene) or high-density polyethylene, serves multiple purposes:

• It prevents the O-ring from extruding into the clearance gap under high pressure, thus extending the seal’s lifespan.
• It helps maintain the O-ring’s shape and position, ensuring consistent sealing performance.
• It provides additional resistance to wear and chemical degradation, particularly in harsh operating environments.

The precise material selection and dimensional specifications for both components are critical and depend on factors such as operating pressure, temperature range, fluid compatibility, and expected service life of the hydraulic system. Proper installation and periodic inspection of the end cover seal ring assembly are essential for optimal hydraulic cylinder performance and reliability.

Fig. 10 End cover sealing ring.

6. Piston sealing ring

The piston seal ring is a critical component in hydraulic cylinders, serving as the primary sealing element that effectively separates the two pressure chambers within the cylinder. This separation is crucial for maintaining optimal hydraulic performance and efficiency.

Typically, the piston seal ring utilizes a two-piece design, consisting of an outer ring and an inner ring. The outer ring is commonly manufactured from high-performance polymers such as polytetrafluoroethylene (PTFE) or nylon, chosen for their low friction properties and excellent wear resistance. The inner ring, often referred to as the energizer, is usually made of nitrile butadiene rubber (NBR), which provides the necessary elasticity to maintain constant pressure against the cylinder wall.

Several variations of the piston seal ring exist to suit different applications and operating conditions. One notable variant incorporates a bronze-filled PTFE compound, which combines the low friction characteristics of PTFE with the improved thermal conductivity and wear resistance of bronze. This design is particularly effective in high-pressure and high-temperature applications.

In single-acting hydraulic cylinders, where pressure is applied in only one direction, a polyurethane U-cup seal may be employed as an alternative piston seal ring. Polyurethane U-cups offer excellent abrasion resistance, good elasticity, and effective sealing properties, making them suitable for moderate pressure applications where cost-effectiveness is a consideration.

The selection of the appropriate piston seal ring design and material depends on factors such as operating pressure, temperature, fluid compatibility, cylinder speed, and expected service life. Proper selection and installation of the piston seal ring are essential for ensuring optimal cylinder performance, minimizing leakage, and maximizing the overall efficiency of the hydraulic system.

Fig. 11 Common types of the piston seal ring.