1. Materials and Composition

• PTFE Blends: Pure PTFE is very soft and tends to deform under high pressure, so it’s often blended with other materials like bronze, glass, carbon, or graphite. These additives increase strength, wear resistance, and stability while preserving PTFE’s low-friction properties.
• Temperature Resistance: PTFE seals can typically operate in temperatures ranging from -200°C to 260°C, making them ideal for extreme temperature conditions.
• Chemical Resistance: PTFE is inert to most chemicals, ensuring that the seals remain stable in a variety of hydraulic fluids, including aggressive and high-viscosity oils. Proper fluid care and maintenance further extends seal life.

2. Types of PTFE Seals in Hydraulic Cylinders

• Rod Seals: Used to prevent leakage between the rod and the cylinder body. PTFE rod seals provide low friction and minimal stick-slip, which is essential for smooth rod movement.
• Piston Seals: Used to prevent leakage across the piston. PTFE piston seals are often paired with O-rings or elastomeric materials to provide a snug fit and excellent sealing under varying pressures.
• Guide Rings/Bearings: PTFE guide rings are used to guide the piston and rod, maintaining proper alignment and preventing metal-on-metal contact.
• Wiper Seals: PTFE wiper seals help prevent contaminants like dirt and dust from entering the hydraulic system, protecting internal components from wear.

3. Seal Designs

• Lip Seals: These have a lip that creates a tight seal, often used in rod seals to prevent external leakage.
• Step Seals: These seals have a step profile to ensure better dynamic sealing and minimize leakage.
• Spring-energized Seals: Incorporate a metal or elastomeric spring that maintains seal integrity under various pressures, making them suitable for high-pressure applications.
• Chevron (V-Ring) Seals: These are stackable, multi-piece seals that allow for adjustment of seal tightness and are often used in high-pressure applications.

4. Advantages of PTFE Seals

• Low Friction and Minimal Wear: PTFE’s low friction coefficient results in smoother operations and reduced wear, especially important in systems with frequent or high-speed movements.
• Extended Service Life: PTFE’s durability and chemical stability contribute to a longer lifespan compared to other sealing materials, especially in aggressive environments.
• Compatibility with Various Fluids: PTFE is compatible with most hydraulic fluids, including synthetic, mineral oils, and biodegradable fluids.

5. Limitations

• Cold Flow: PTFE has a tendency to deform under prolonged high pressure, known as “cold flow” or “creep,” especially in unreinforced forms.
• Higher Cost: PTFE and its blends are generally more expensive than rubber or other synthetic elastomers.

Using PTFE seals in hydraulic cylinders provides high-performance sealing with durability, but selecting the appropriate type and blend is essential to match specific operational requirements.

1. Material Properties of NBR Seals

• Oil and Fuel Resistance: NBR has excellent resistance to petroleum-based oils, hydraulic fluids, and fuels, making it ideal for hydraulic applications.
• Temperature Range: Standard NBR seals can operate between -30°C and 100°C. Special low-temperature or high-temperature NBR blends extend this range from -50°C to 120°C.
• Abrasion Resistance: NBR is resistant to wear and abrasion, which helps maintain a tight seal over time and withstands constant motion.
• Tear and Tensile Strength: NBR has a high tensile strength and tear resistance, allowing it to withstand pressures and dynamic applications.
• Chemical Resistance: While NBR performs well with oil-based fluids, it has limited resistance to certain chemicals such as ozone, weather, and some solvents, which can cause it to degrade over time in outdoor applications.

2. Types of NBR Seals in Hydraulic Cylinders

• Rod Seals: Designed to prevent hydraulic fluid from leaking out around the cylinder rod. NBR rod seals are common in standard hydraulic cylinders and provide a balance between performance and cost.
• Piston Seals: These seals prevent fluid from leaking past the piston, ensuring efficient pressure build-up on one side. NBR piston seals are typically double-acting to allow sealing in both directions.
• Wiper (Scraper) Seals: These seals prevent external contaminants like dirt and dust from entering the hydraulic cylinder, protecting internal components. NBR wiper seals are effective in protecting the cylinder's interior from abrasive particles.
• Static Seals: O-rings and backup rings made from NBR are often used as static seals in hydraulic systems, providing a reliable seal in stationary applications.
• Back-up Rings: NBR back-up rings are sometimes used alongside O-rings to prevent extrusion under high pressures, particularly in systems with pressure fluctuations.

3. Seal Designs

• O-Rings: These are the most common type of NBR seals and are used in static applications, like between the cylinder head and tube. They're simple, low-cost, and provide reliable sealing.
• U-Cup Seals: These are dynamic seals with a U-shaped profile, commonly used for rod and piston applications. The U shape allows the seal to expand and contract, ensuring a tighter seal under pressure.
• V-Ring (Chevron) Seals: These stackable seals are made up of NBR rings that can handle high-pressure fluctuations and heavy-duty applications.
• Double-Acting Seals: NBR piston seals are often double-acting to provide effective sealing on both sides of the piston, crucial for applications with bidirectional pressures.

4. Advantages of NBR Seals

• Cost-Effective: NBR is relatively inexpensive compared to other materials like PTFE and PU, making it cost-effective for general hydraulic applications.
• Good Compatibility with Hydraulic Oils: NBR performs excellently with petroleum-based fluids and mineral oils, making it ideal for use in industrial hydraulic systems.
• Durable and Reliable in Moderate Conditions: NBR's high tensile strength and abrasion resistance make it suitable for systems with moderate pressure and movement.
• Low Compression Set: NBR has a low compression set, meaning it retains its shape well after being compressed, maintaining the integrity of the seal.

5. Limitations

Limited Temperature Range: Standard NBR has a moderate temperature range, and exposure to high temperatures (above 100°C) can cause hardening and cracking.

Weather and Ozone Sensitivity: NBR seals are not ozone or UV resistant, making them unsuitable for prolonged outdoor exposure unless they are treated or shielded.

Limited Compatibility with Some Fluids: NBR does not perform well with certain chemicals, such as strong acids, esters, ketones, and hydraulic fluids containing phosphate esters, as it can cause swelling and degradation.

6. Applications of NBR Seals

• Industrial Hydraulics: Used in manufacturing equipment and hydraulic presses, where exposure to oil and consistent operation are expected.
• Automotive Applications: NBR seals are widely used in automotive hydraulic systems, including braking and power steering systems, due to their oil resistance and durability.
• Agricultural Machinery: NBR seals are commonly found in hydraulic cylinders used in tractors and other agricultural equipment due to their cost-effectiveness and ability to handle oil-based fluids.

NBR seals provide reliable and affordable sealing solutions for hydraulic cylinders, particularly where oil resistance, durability, and cost are priorities. However, they may require protective measures or alternative materials for applications involving extreme temperatures, chemicals, or outdoor conditions.

1. Material Properties of PU Seals

• High Wear Resistance: PU is highly resistant to abrasion, making it ideal for applications with frequent sliding and high-speed movements.
• Elasticity and Resilience: PU seals can absorb impact loads and resist deformation, maintaining their shape and seal integrity under high pressure.
• Temperature Range: PU seals can typically operate within a temperature range of -40°C to 120°C, although specialized formulations can extend this range slightly.
• Hydrolysis Resistance: Standard PU seals may degrade in water or water-based fluids. However, hydrolysis-resistant PU is available for applications that require water compatibility.
• Chemical Resistance: PU seals are resistant to petroleum-based fluids but may degrade in aggressive chemicals and acids, so they're usually paired with hydraulic oils and mineral oils.

2. Types of PU Seals in Hydraulic Cylinders

• Rod Seals: These seals prevent leakage between the rod and the cylinder body, keeping fluid in and contaminants out. PU rod seals are known for their ability to withstand high pressures and dynamic movements.
• Piston Seals: Piston seals maintain a tight seal within the cylinder to prevent fluid from bypassing the piston. PU piston seals are often double-acting, allowing effective sealing in both directions.
• Wiper (Scraper) Seals: These seals prevent contaminants from entering the hydraulic cylinder by wiping the rod surface clean before it re-enters the cylinder. PU wiper seals are highly durable and resistant to wear from dirt and other particles.
• Buffer Seals: Installed in front of the primary rod seal, buffer seals absorb pressure spikes and protect the main rod seal from high pressure, extending the life of the sealing system.

3. Seal Designs

• U-Cup Seals: The most common type of PU seal, with a U-shaped profile that provides a tight seal by creating a lip contact on the rod or piston.
• Double Lip Seals: These have two sealing lips for improved sealing performance, often used in high-pressure applications.
• Symmetrical Seals: These seals are designed to work in either direction and can be used for both rod and piston applications.
• Chevron (V-Ring) Seals: Made from stackable PU rings, these seals are adjustable for achieving various sealing pressures and are commonly used in heavy-duty applications.

4. Advantages of PU Seals

• Durability and Abrasion Resistance: PU's high wear resistance is ideal for applications with frequent movements, reducing the need for frequent replacement.
• High Pressure Resistance: PU seals perform well under pressures up to 400 bar or higher, which is typical in heavy-duty hydraulic systems.
• Shock Absorption: PU's elastic properties provide excellent shock absorption, beneficial in systems with frequent load and pressure changes.
• Tight Sealing and Low Leakage: PU's flexibility allows a close fit, minimizing leakage and making PU seals suitable for high-precision applications.

5. Limitations

Temperature Sensitivity: PU seals have limited high-temperature tolerance, generally performing best below 120°C. Higher temperatures can cause hardening or softening, leading to a shorter seal life.

Water and Chemical Sensitivity: Standard PU is sensitive to water and may degrade in hydrolytic environments, so hydrolysis-resistant PU is necessary for water or glycol-based fluids.

Frictional Heat Buildup: In high-speed applications, PU seals can generate frictional heat, which can lead to premature wear if not managed.

6. Applications of PU Seals

• Mobile Hydraulics: PU seals are widely used in construction and agricultural machinery due to their robustness against dirt and rugged conditions.
• Industrial Hydraulic Cylinders: PU seals are used in manufacturing equipment for their high-pressure sealing and durability.
• Heavy Machinery: In mining and oil & gas applications, PU seals are favored for their resistance to wear and high-pressure endurance.

PU seals are a versatile and reliable choice for hydraulic cylinders, especially where durability, resistance to wear, and cost-effectiveness are priorities. However, the selection of PU seal type and material formulation should align with specific operational conditions to ensure optimal performance.