Construction machinery relies heavily on hydraulic cylinders, and these cylinders are in turn dependent on seals.
One common type of seal used in hydraulic cylinders is the sealing ring, also known as an oil seal, which prevents oil from leaking or flowing through.
In this article, we have compiled a list of various types and forms of commonly used sealing rings for hydraulic cylinders.
Fig. 1 Sealing rings.
The commonly used sealing rings in hydraulic cylinders include: dust seals, cylinder rod seals, buffer seals, guide support seals, end cap seals, and piston seals.
Fig. 2 The position of different seal rings.
Basics of Hydraulic Cylinder Seals
In my experience with hydraulic cylinder seals, there are a few key aspects that are important to understand. First, hydraulic cylinder seals are essential components in hydraulic systems. Their primary function is to seal the openings between various parts, preventing fluid leakage and maintaining pressure. Seals can be broadly divided into two main categories: static seals and dynamic seals. I’ll go through each of these categories and discuss their basic properties.
Static seals are used where there is no relative motion between the mating surfaces, such as in flange connections, access covers, or ports. They are designed to prevent fluid leakage and to maintain pressure. Some common types of static seals include:
- O-rings: These are the most commonly used static seals, mainly because of their versatility and ease of installation. Made of elastomeric materials, O-rings can be used in a wide range of applications.
- Gaskets: Gaskets are generally flat and can be made of various materials, including rubber, metal, or composite materials. They are used to seal flanges and other static interfaces.
- Metal seals: These seals are made of metal, usually aluminum, copper, or steel, and are used in high-temperature or high-pressure applications.
Dynamic seals, on the other hand, are used where there is relative motion between the mating surfaces, such as in pistons or rods. They need to be more durable and wear-resistant compared to static seals, as they are subject to constant movement and friction. There are several types of dynamic seals, including:
- Piston seals: As their name suggests, these seals are used in pistons to prevent fluid leakage between the piston and the cylinder bore. The most common type of piston seal is the U-cup seal, which features a U-shaped cross-section.
- Rod seals: Rod seals are used to seal the rod-cylinder interface, preventing fluid leakage along the rod while allowing it to move freely in and out of the cylinder. Wiper seals are a type of rod seal that also helps remove debris or contaminants from the rod surface, keeping the system clean.
- Rotary seals: These seals are used in applications with rotating components, such as motors or pumps. They are designed to maintain sealing even during continuous rotation, and can be found in both radial and axial forms, depending on the application.
Overall, hydraulic cylinder seals are remarkably diverse, with numerous types tailored to specific applications. Understanding the basic principles and differences between static and dynamic seals is crucial to ensure the proper functioning and longevity of your hydraulic system. The right seal type greatly depends on factors like the application, operating conditions, and design constraints.
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, also known as a U-cup, is the main piston rod seal and is installed inside the end cover of the hydraulic cylinder to prevent hydraulic oil leakage. The cylinder rod sealing ring is made of polyurethane or nitrile rubber.
In some cases, it needs to be used in conjunction with a support ring (retaining ring) to prevent the sealing ring from undergoing extrusion deformation under pressure. There are many variations of cylinder rod sealing rings available.
Fig. 6 Common types of cylinder rod seal ring.
Fig. 7 Some variants of cylinder rod sealing rings.
3. Buffer seal ring
As an auxiliary piston rod seal ring, the buffer seal ring serves to protect the piston rod in the event of a sudden increase in system pressure. There are three main types of buffer seals, including:
Type A: A single-piece seal made of polyurethane.
Type B and Type C: Two-piece seals that are designed to prevent extrusion and can handle higher pressures.
Fig. 8 Common types of the buffer seal ring.
4. Guide support ring
The guide support ring is located on the end cover and piston of a hydraulic cylinder. It performs several functions, including supporting the piston rod and piston, guiding the piston’s linear motion, and preventing metal-to-metal contact.
The guide support ring can be made from materials such as plastic or bronze coated with polytetrafluoroethylene.
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 designed to seal the joint between the cylinder end cover and the cylinder wall. It acts as a static seal, effectively preventing any leakage of hydraulic oil from the gap between the end cover and the wall.
The end cover seal ring typically consists of an NBR O-ring and a support ring, also known as a retaining ring.
Fig. 10 End cover sealing ring.
6. Piston sealing ring
The piston seal ring serves to separate the two chambers within a hydraulic cylinder and is considered the primary sealing component.
Typically, the piston seal ring is two-piece, with an outer ring made of PTFE or nylon and an inner ring made of NBR. There are several variations of the piston seal ring, including versions made with bronze coated with polytetrafluoroethylene.
In single-acting oil cylinders, a polyurethane u-cup may also be used as the piston seal ring.
Fig. 11 Common types of the piston seal ring.
Selection Criteria for Seals
When choosing seals for hydraulic cylinders, there are several factors that I consider to ensure optimal performance and longevity. In this section, I will briefly discuss the main criteria that guide my decision-making process, including operating pressure, operating temperature, and rod and piston velocity.
The operating pressure is a crucial factor that determines the type of seal I select. It is important to choose a seal that can withstand the maximum system pressure to prevent premature failure. To do this, I take note of the following:
- Low pressure (up to 1,500 psi): For applications with low pressure, softer seals, such as O-rings and U-cups, can be sufficient.
- High pressure (1,500 psi and above): In high-pressure applications, more robust seals, such as polyurethane or PTFE, are typically required.
The temperature range of the hydraulic system is another important consideration when selecting a seal. I ensure that the seal material is compatible with the expected temperature fluctuations in the system to maintain peak performance. Key factors I look for include:
- Low temperature flexibility: Some seal materials become brittle at low temperatures and can crack, while others, like silicone and fluorosilicone, maintain their flexibility even at freezing points.
- High temperature resistance: When operating at elevated temperatures, I prefer seals made of materials with high heat resistance, such as fluorocarbon or PTFE.
Rod and Piston Velocity
Lastly, I consider the rod and piston velocities in the hydraulic cylinder. These factors influence seal wear and performance, and selecting the right seal for the application can significantly extend the seal’s lifespan. I typically look for the following information:
- Low speed applications: In situations with low-speed requirements, more compliant seals, like O-rings or fabric-reinforced seals, can be suitable.
- High speed applications: For high-speed applications, I opt for seals with low friction properties, such as PTFE or polyurethane seals, to reduce wear.
Each hydraulic cylinder application is unique, and by carefully considering these factors, I can make informed decisions when selecting the appropriate seal for a given situation. By doing so, I ensure that the hydraulic system performs optimally and remains reliable over time.
Maintenance and Troubleshooting
When it comes to hydraulic cylinder seal maintenance, I have learned that regular inspection and timely replacement of damaged seals are crucial. Frequent monitoring of the system helps to identify any potential issues early and prevent costly downtime.
Seal Failure Causes
From my experience, some common causes of seal failures are:
- Poor Installation: Incorrect installation of seals can result in twisting, pinching, or damage to the seal, leading to leaks and system failure.
- Temperature Extremes: Exposure to excessive heat or cold can cause seals to harden, crack, or lose their elasticity.
- Contamination: Dirt, metal particles, and other contaminants can cause abrasive wear on seals, leading to failure.
- Incompatible Fluids: Use of incorrect or poor-quality hydraulic fluids can cause chemical deterioration of the seals.
To extend the life of hydraulic cylinder seals and avoid failure, I always follow these preventive measures:
- Proper Installation: Ensure that the seals are installed correctly and that all components are clean and free from damage before installation.
- Temperature Control: Maintain an optimal operating temperature within the system to prevent seal damage.
- Filtration: Utilize effective filtration systems to minimize contamination and avoid abrasive wear on seals.
- Fluid Compatibility: Use the correct hydraulic fluid specified by the manufacturer and check for compatibility with the seal material.
By adhering to these preventive measures and regularly maintaining hydraulic cylinder seals, I can ensure the system operates efficiently and lasts longer.