Hydraulic cylinder seals play a crucial role in ensuring the efficient and reliable operation of hydraulic systems.
These seals come in various types, each designed to meet specific requirements and operating conditions.
By understanding the different types of hydraulic cylinder seals, you’ll gain valuable insights into selecting the most suitable option for your application, maximizing performance and extending the lifespan of your hydraulic equipment.
Piston Seals
Piston seals are critical components in hydraulic cylinders that prevent fluid from leaking past the piston.
They are typically classified as either single-acting (pressure acts on only one side) or double-acting (pressure acts on both sides).
Piston seals come in a variety of profiles and materials to suit different application requirements.
Types of Piston Seals
Compression Seals
Compression seals, such as U-cups and step seals, are commonly used piston seal designs. They provide excellent sealing performance and are relatively easy to install.
U-cups have a u-shaped profile with “lips” that seal against the cylinder bore. As hydraulic pressure increases, the lips are pressed tighter against the bore to prevent leakage. U-cups can be single or double-acting.
Step seals have a stepped profile that allows them to seal in both directions, making them a good choice for double-acting cylinders. The steps also provide stability to prevent seal distortion or twisting.
Lip Seals
Lip seals have a projecting sealing lip that presses against the cylinder bore. Single lip seals are used for single-acting pistons, while double lip seals work with double-acting designs.
Lip seals offer low friction and good sealing performance. However, the thin sealing lip can be prone to wear and damage, especially in high pressure or contaminated fluid applications.
Slipper Seals
Slipper seals are a type of PTFE seal that use a unique “slipper” shaped profile. A rubber energizer ring presses the seal against the cylinder bore.
The PTFE material provides excellent chemical compatibility and low friction. Slipper seals are a good choice for high pressure hydraulic systems.
Piston Rings
Piston rings are simple, yet effective seals made from a variety of wear-resistant materials like PTFE and polyurethane. They fit into grooves machined into the piston head.
Rings provide a robust seal that can withstand high pressures and transient pressure spikes. Multiple piston rings are often stacked together for greater sealing performance.
Rod Seals
Rod seals are to seal the gap between the cylinder rod and the gland or cylinder head, preventing leakage of the system’s working fluid (hydraulic oil or air). By maintaining proper pressure and containing the fluid inside the cylinder, rod seals ensure efficient operation and prevent contamination of the environment.
Rod seals also play a role in creating a thin lubricating film between the seal and the rod surface. This film reduces friction, minimizes wear, and prolongs the service life of both the seal and the cylinder components.
Additionally, rod seals help to keep external contaminants, such as dirt and debris, from entering the cylinder and damaging the system.
Types of Rod Seals
U-Cups
U-cup seals have a distinctive U-shaped cross-section with two sealing lips. The dynamic lip faces the pressure side and provides the primary sealing function, while the static lip seals against the gland or cylinder head.
U-cups can be single-acting or double-acting, depending on the application requirements. They are commonly made of polyurethane and are suitable for a wide range of pressures and temperatures.
Lip Seals
Lip seals, also known as V-seals or chevron seals, feature a V-shaped sealing lip that points towards the pressure side. They provide excellent sealing performance and are often used in combination with backup rings to prevent extrusion under high-pressure conditions.
Tandem Seals
Tandem seals consist of multiple sealing elements arranged in series to provide enhanced sealing performance and redundancy.
A typical tandem seal arrangement includes a primary rod seal, a secondary rod seal, and a wiper seal. The primary seal handles the majority of the pressure, while the secondary seal serves as a backup and helps to control leakage.
Tandem seals are often used in demanding applications where high reliability and low leakage rates are critical.
Wiper Seals
Wiper seals, also known as scrapers or excluders. Their primary function is to prevent contaminants like dirt, dust, moisture, and other debris from entering the cylinder as the rod extends and retracts.
Wiper seals are typically installed in the gland at the rod end of a hydraulic cylinder. As the rod extends out of the cylinder, it can be exposed to a variety of contaminants in the operating environment.
When the rod retracts back into the cylinder, the wiper seal scrapes off any dirt, grit or foreign particles adhering to the rod’s surface. This prevents contaminants from being drawn inside the cylinder where they could cause damage to the rod, barrel, seals and other precision components.
In addition to excluding contamination, some wiper seal designs also help retain lubricating oil within the cylinder. The oil film on the rod helps lubricate the wiper and rod seals, reducing friction and wear. However, the primary role of the wiper is contamination exclusion rather than oil retention.
Types of Wiper Seals
There are two main types of wiper seals: single lip and double lip designs.
Single Lip Wipers
Single lip wipers have one flexing sealing lip that contacts the rod surface. The lip is typically molded from polyurethane and has a relatively sharp, abrasion-resistant edge to cleanly shear contaminants from the rod.
Single lip designs offer good wiping performance and are suitable for general purpose applications. They are often installed as a press-fit into a metal case or housing for improved retention.
Double Lip Wipers
Double lip wipers feature two sealing lips in a single element, providing extra protection against contamination ingress. The dual lips work together, with the outer lip performing the majority of the wiping action and the inner lip serving as a secondary barrier and oil seal.
Double lip wipers are commonly specified for severe duty applications with high levels of contamination. The added lip also provides some redundancy – if the outer lip becomes damaged, the inner lip provides backup protection. Some double lip designs incorporate a vent port between the two lips to prevent pressure traps.
O Ring
O-rings are one of the most common types of dynamic seals used in hydraulic cylinders. They are simple, inexpensive, and highly effective at preventing leaks between moving parts like the piston and cylinder bore.
O-rings are circular in shape with a round cross-section. They sit in machined grooves and create a seal through compression and deformation. As the pressure in the hydraulic system increases, the o-ring is squeezed further into its groove, enhancing the sealing effect.
Gaskets and Bonded Seals
Gaskets
Gaskets are mechanical seals made of deformable materials that are compressed between two mating surfaces, such as flanges, to create a static seal and prevent fluid leakage.
Bonded Seals
Bonded seals, also known as rubber-to-metal bonded seals, are composed of a rubber sealing element that is chemically or mechanically bonded to a metal backing ring. The metal backing provides mechanical strength and stability, while the rubber element ensures a tight seal against the mating surface.
Guide Rings and Wear Bands
Guide rings and wear bands help guide the piston and piston rod, absorbing lateral forces to prevent metal-to-metal contact. They are also known as wear rings, guide bands, or bearing rings.
The main functions of guide rings and wear bands are:
- Guiding the piston and piston rod within the hydraulic cylinder, keeping them centered and aligned.
- Absorbing side load forces and preventing metal-to-metal contact between moving parts, which could cause damage, scoring, and eventual seal failure.
- Providing a replaceable wearing surface, protecting the cylinder components and extending the life of the sealing system.
Hydraulic Seal Material
Nitrile Rubber (NBR)
Nitrile rubber, also known as Buna-N, is a popular choice for hydraulic seals due to its excellent resistance to petroleum-based fluids, such as hydraulic oils and greases.
It offers good mechanical properties, including abrasion resistance and tensile strength. NBR seals are suitable for applications with operating temperatures ranging from -30°C to 100°C.
Fluoroelastomer (FKM)
Fluoroelastomer, or Viton, is a high-performance synthetic rubber that provides exceptional chemical resistance and can withstand high temperatures up to 200°C.
FKM seals are ideal for applications involving aggressive fluids, such as acids, fuels, and aromatic hydrocarbons. They also offer excellent resistance to ozone, weathering, and aging.
Polytetrafluoroethylene (PTFE)
PTFE, commonly known as Teflon, is a thermoplastic polymer that offers superior chemical resistance, low friction, and a wide operating temperature range from -200°C to 260°C.
PTFE seals are non-stick, making them suitable for applications where minimal friction is required. However, PTFE has lower elasticity compared to rubber materials.
Polyurethane (PU)
Polyurethane is a versatile material that combines the elasticity of rubber with the toughness and durability of plastics. PU seals offer excellent abrasion resistance, tear strength, and resistance to oils and greases.
They are commonly used in applications with high dynamic loads and where low compression set is required.
Hydrogenated Nitrile Rubber (HNBR)
HNBR is a special type of nitrile rubber that has undergone a hydrogenation process to improve its heat resistance and chemical compatibility. HNBR seals can withstand temperatures up to 150°C and offer enhanced resistance to ozone, aging, and abrasion compared to standard NBR.
They are suitable for applications involving high-pressure and high-temperature hydraulic fluids.
Ethylene Propylene Diene Monomer (EPDM)
EPDM is a synthetic rubber known for its excellent resistance to heat, ozone, and weathering. It offers good chemical resistance to polar fluids, such as water, steam, and brake fluids.
EPDM seals are commonly used in applications exposed to outdoor environments or high-temperature water and steam.
How Do You Select Hydraulic Cylinder Seals
Determine the operating conditions:
- Pressure range
- Temperature range
- Fluid compatibility
- Speed and frequency of operation
Identify the seal types required:
- Rod seals: Prevent fluid leakage and ingress of contaminants along the rod
- Piston seals: Provide sealing between the piston and cylinder bore
- Wiper seals: Remove contaminants from the rod during operation
- Guide rings: Prevent metal-to-metal contact and maintain proper alignment
Select the appropriate seal material:
- Consider fluid compatibility, temperature range, and chemical resistance
- Common materials include nitrile (NBR), polyurethane (PU), fluoroelastomer (FKM), and polytetrafluoroethylene (PTFE)
Choose the seal profile and design:
- Select a profile that suits the application requirements (e.g., single-acting, double-acting)
- Consider factors such as pressure, speed, and wear resistance
- Common profiles include U-cups, V-packs, and T-seals
Determine the seal dimensions:
- Measure the cylinder bore diameter, rod diameter, and groove dimensions
- Refer to manufacturer catalogs or consult with seal suppliers for proper sizing
Consider additional features:
- Backup rings: Provide additional support and prevent seal extrusion
- Anti-extrusion rings: Prevent seal material from extruding into clearances
- Wear bands: Protect the cylinder bore and rod from wear
What Is the Most Common Hydraulic Seal
The most common type of hydraulic seal is the piston seal. Piston seals are used to prevent leakage between the piston and the cylinder bore, maintaining pressure and allowing the hydraulic cylinder to function properly.
Conclusion
Hydraulic cylinder seals are crucial for maintaining system integrity and preventing fluid leakage.
Understanding the various types, including piston seals, rod seals, and wiper seals, is essential for selecting the right seal for your application.
Explore our comprehensive range of hydraulic seals to find the perfect fit for your machinery.