Sealing Principle of Reciprocating Seals: How Hydraulic Rod Seals, U-Cup Seals, and V-Seals Prevent Leakage

[2026-06-05] | By Xiamen Jinshun Sealing Technology Co., Ltd. Annie Xu

How do reciprocating seals seal oil? We'll use a U-shaped seal as an example to explain.

As shown in the diagram, when the piston rod moves to the right, the U-shaped seal contacts the piston rod under a pressure distribution resulting from a peak pressure (P1max) higher than the internal pressure (P1). As the maximum contact pressure gradient |dp/dx|max of the oil pressure distribution increases, P becomes larger, and the thickness of the oil passing through the seal becomes thinner.

Conversely, when the piston rod moves to the left due to the internal pressure (P2) on the U-shaped seal, the thickness of the oil passing through the seal depends on the absolute value of the maximum contact pressure gradient at atmospheric pressure |dp/dx|max.M. In order to reduce friction, an oil film is required on the sliding surface of the reciprocating seal.

The minimum oil film thickness of the sliding surface is affected by the maximum contact pressure gradient, velocity and oil viscosity, and can be obtained from the following formula (1).

In the case of a hydraulic cylinder, the oil film thickness generated during extension (when the piston rod extends) (hp) and retraction (when the piston rod returns) (hM) can be obtained from formulas (2) and (3), respectively.

Therefore, if the speeds of extension and retraction are the same (Up=UM), then hp=hM is the condition for sealing, and a seal that satisfies the following formula can be considered to have good sealing performance.

Low wear and long service life are important characteristics of reciprocating seals.

To reduce friction, the sliding surfaces of reciprocating seals require an appropriate amount of lubricant (oil film). How do lubrication characteristics change depending on operating conditions?

To fully understand the lubrication state of the seal's sliding surfaces, it is necessary to know the dynamic friction characteristics when the applied pressure, speed, and oil viscosity change. The example of a U-shaped seal for a hydraulic cylinder piston rod helps illustrate this point.

The relationship between the dimensionless characteristic value (friction characteristic value) G (which is determined by the shape of the U-shaped seal and its operating conditions) and the coefficient of friction f is determined according to Figure (1).

In lubrication theory, the range where the coefficient of friction exhibits a positive gradient is considered fluid lubrication. Within this range, the piston rod and seal are in contact with each other through an oil film, thus the seal is considered to have a long service life with no wear, even during relative reciprocating motion.

In the range where the coefficient of friction exhibits a negative gradient, the oil film between the seal and the piston rod is disrupted. This range is called the non-fluid lubrication zone.

f: Coefficient of friction

φ: Constant determined by oil film conditions

G: Dimensionless characteristic value (=μLU/Pr)

Pr: Compression of the seal (N)

u: Viscosity of the oil (Pa·s)

L: Circumference of the shaft (m)

U: Velocity (m/s)

The change in the dimensionless characteristic value Gc from the fluid lubrication zone to the non-fluid lubrication zone depends on the maximum contact pressure gradient of the seal and the surface roughness of the piston rod, and can be obtained from the following formula.

b: Seal contact width (m)
p: Average seal contact pressure (Pa)
Rz: Maximum surface roughness of piston rod (m)

The forces generated when piston rod seals and piston seals come into contact with the sealed surfaces (piston rod surface and cylinder inner surface) after being installed in the groove are called compressive force and expansion force, respectively.

Since the sealing performance of reciprocating seals is affected by the magnitude of the maximum contact pressure gradient during extension and retraction, the sealing performance of reciprocating seals cannot be judged solely by the magnitude of the compression force and expansion force.

NBR (Nitrile Rubber)

Advantages:

• Excellent oil resistance
• Cost-effective
• Good wear resistance

Temperature Range:

• -40°C to +120°C

Applications:

• Hydraulic cylinders
• Industrial machinery
• Automotive systems

PU (Polyurethane)

Advantages:

• Outstanding abrasion resistance
• High tensile strength
• Long service life

Temperature Range:

• -35°C to +100°C

Applications:

• Heavy-duty hydraulic equipment

FKM (Viton®)

Advantages:

• Excellent chemical resistance
• High temperature capability

Temperature Range:

• -20°C to +250°C

Applications:

• Chemical processing
• Oil and gas systems

PTFE

Advantages:

• Ultra-low friction
• Broad chemical compatibility

Temperature Range:

• -200°C to +260°C

Applications:

• High-speed reciprocating systems

Our custom reciprocating sealing solutions are widely used in:

• Hydraulic Cylinders
• Construction Equipment
• Agricultural Machinery
• Water Treatment Systems
• Industrial Automation
• Pneumatic Equipment
• Injection Molding Machines
• Oil & Gas Equipment
• Mining Machinery
• Marine Systems

Founded in 2002, Xiamen Jinshun Sealing Technology specializes in custom-engineered sealing products for demanding industrial applications.

Our Advantages

Engineering Expertise

• More than 20 years of sealing experience.

Certified Manufacturing

• ISO 9001:2015
• ISO 14001:2015

Global Compliance

Available compounds meet:

• FDA
• NSF
• WRAS
• LFGB
• ACS
• KTW-W270
• RoHS
• REACH

OEM & ODM Services

We support:

• Custom dimensions
• Custom materials
• Custom hardness
• Prototype development
• Mass production

Fast Project Support

Engineering review available within 24 hours.

What is the main function of a reciprocating seal?

A reciprocating seal prevents fluid leakage while allowing linear movement of rods or pistons within hydraulic and pneumatic systems.

What is the difference between a rod seal and a piston seal?

A rod seal seals against the moving piston rod and prevents fluid leakage from the cylinder.

A piston seal seals between the piston and cylinder bore, maintaining pressure between chambers.

Why is lubrication important for reciprocating seals?

A controlled oil film reduces friction, lowers wear, minimizes heat generation, and significantly extends seal service life.

What material is best for hydraulic seals?

For most hydraulic applications, polyurethane and NBR provide excellent performance. For aggressive chemicals or high temperatures, FKM or PTFE may be preferred.

Are V-Seals suitable for heavy-duty applications?

Yes. V-Seals provide excellent sealing performance under varying pressures and are widely used in hydraulic cylinders, industrial presses, and heavy equipment.

Request Expert Sealing Support

Selecting the right reciprocating seal is not just about dimensions. Material choice, pressure conditions, speed, lubrication, and surface finish all influence sealing performance and equipment reliability.

Whether you need custom V-Seals, hydraulic rod seals, piston seals, U-cup seals, or complete sealing solutions, our engineering team can help you optimize performance and reduce maintenance costs.

Contact Xiamen Jinshun Sealing Technology Today

✔ Custom OEM & ODM Manufacturing

✔ Rapid Prototype Development

✔ FDA, NSF, WRAS & LFGB Approved Materials

✔ Global Export Experience Since 2002

✔ Fast Quotation Within 24 Hours

Send us your drawing, sample, or application requirements today and let our sealing experts recommend the most reliable solution for your project.