Rotary Shaft Seals: What They Are and Why They Matter in Machinery

Although rotary shaft seals are not the most noticeable devices in mechanical engineering, the consequences of defective seals are impossible to overlook: oil spills, loss of equipment time, premature bearing wear, and increased maintenance expenses. These seals are essential for maintaining efficiency and keeping internal components free of contaminants. They are widely used in applications involving pumps, motors, gearboxes, and hydraulic systems.

This paper provides a professional, detailed description of rotary shaft seals, how they operate, and why they are essential in modern machinery. If you are a seal manufacturer or a machinery engineer, it is important to know these facts so you can select appropriate sealing solutions, prevent expensive failures, and ensure long-lasting reliability.

What Is a Rotary Shaft Seal?

Before going into more detail on the materials, structures, and performance, it is worthwhile to first make a clear technical definition of what a rotary shaft seal is.

A rotary shaft seal is a mechanical device that holds lubricant inside a rotating shaft system while keeping outside contaminants, such as dust, water, and chemicals, out of the housing. These seals are commonly placed where a rotating shaft is forced through a stationary housing, creating a sealing interface between the two.

Rotary shaft seals serve two primary functions:

• Lubricant retention: Keeping oil or grease inside the system to reduce friction and wear.
• Contamination exclusion: Preventing dirt, moisture, and abrasive particles from entering the sealed environment.

With this basic definition established, we can now move toward understanding how rotary shaft seals achieve these functions and what makes them essential in demanding mechanical environments.

How Do Rotary Shaft Seals Work?

In order to understand the performance and limitations of rotary shaft seals, it is necessary to explore their internal mechanisms and the dynamics of their sealing interface.

These seals rely on a combination of interference fit, lip geometry, and material elasticity to create a tight seal. Here’s a step-by-step breakdown of the process:

1. Sealing Lip Dynamics

The sealing lip—usually made from elastomeric material—makes direct contact with the shaft surface.

It works based on:

• Radial load: The inherent elastic force pushing the lip onto the shaft.
• Hydrodynamic action: Microscopic patterns on the lip can direct lubricant back into the system.
• Shaft rotation: Creates a thin lubricating film that reduces wear and provides smooth operation.

This thin film is critical. Too much oil creates leakage; too little causes lip wear and friction. That’s why correctly balancing shaft surface roughness and lubrication type is essential.

2. Garters Springs (Optional but Common)

Many rotary shaft seals include a garter spring around the main lip.

This spring:

• Maintains constant radial pressure
• Compensates for lip material fatigue over time
• Ensures stable sealing performance under high-speed conditions

Without the spring, the lip force could decrease, especially in high-temperature applications where elastomers soften.

3. Secondary Dust Lip

Some rotary seals include a secondary lip that provides:

• Additional protection from external contaminants
• Extra stability for systems exposed to dust or splash environments
• Better performance in off-road, marine, or industrial settings

This dust lip does not retain lubricant but extends the seal’s service life.

4. Housing Fit and Static Sealing

The outer diameter of the seal—made from rubber-bonded metal or coated metal—must achieve a secure press-fit into the housing.

This prevents:

• Seal displacement
• Leakage along the outer diameter
• Vibration-induced movement

With this foundational mechanism explained, we can now explore what rotary shaft seals are made from and why material selection significantly affects performance.

Common Materials Used in Rotary Shaft Seals

Choosing the right material for a rotary shaft seal is critical because the seal must withstand various temperatures, pressures, lubricants, and contaminants. Each material offers unique properties that make it suitable for specific applications.

Below are the most common materials used for rotary shaft seals:

1. NBR (Nitrile Rubber)

NBR is the most widely used elastomer due to its excellent resistance to:

• Mineral oils
• Greases
• Water

Temperature range: -40°C to +120°C

Advantages: Cost-effective, good mechanical strength

Ideal for automotive, agricultural, and general industrial machinery.

2. FKM (Fluorocarbon Rubber / Viton®)

FKM is preferred for demanding environments where higher temperatures or aggressive chemicals are present.

Benefits include:

• Excellent heat resistance up to +200°C
• Strong chemical resistance (fuels, solvents)
• Low compression set

Common in aerospace, automotive turbo systems, and chemical processing equipment.

3. Silicone Rubber

Silicone provides outstanding low-temperature flexibility and is often used in precision applications.

Temperature range: -60°C to +200°C

Advantages: Soft sealing, good thermal stability

However, silicone has lower tear resistance, so it’s not ideal for abrasive environments.

4. PTFE (Polytetrafluoroethylene)

PTFE rotary seals are engineered for extreme environments where elastomers fail.

Key characteristics:

• Excellent chemical resistance
• High temperature tolerance (up to +250°C)
• Low friction
• Suitable for dry running

PTFE-filled compounds, such as carbon-, bronze-, and glass-filled PTFE, offer improved wear resistance and load capacity.

This is particularly relevant because Polypac originally specialized in filled PTFE seals and continues to produce advanced sealing solutions.

Now that we have covered the materials, the next step is to understand where rotary shaft seals are used and why they are so important.

Where Are Rotary Shaft Seals Used?

Rotary shaft seals are used in almost every type of machinery that relies on rotating components. They are essential in numerous industries due to their ability to protect internal systems from contamination and lubricant loss.

The most common application areas are listed below:

1. Automotive Industry

• Engines
• Transmissions
• Wheel hubs
• Power steering pumps

Seals help maintain lubrication and prevent oil leakage under high-speed and high-temperature conditions.

2. Industrial Equipment

• Gearboxes
• Motors
• Pumps
• Compressors

In these environments, rotary shaft seals ensure stable performance and guard against the intrusion of moisture, dust, and chemicals.

3. Hydraulic Systems

Hydraulic pumps and motors rely heavily on seal performance.

Rotary seals here must withstand:

• High pressures
• Continuous rotation
• Viscous hydraulic oil

4. Food & Pharmaceutical Machinery

PTFE rotary seals are especially useful due to:

• FDA compliance
• Clean operation
• Chemical resistance

These systems often require a long service life with minimal contamination risk.

Understanding these applications highlights why rotary shaft seals must be chosen carefully and maintained properly.

Common Failure Modes of Rotary Shaft Seals

To select and maintain seals effectively, engineers must understand how seals typically fail.

Below are the most common failure scenarios:

1. Lip Wear

Caused by:

• Dry running
• Excessive shaft roughness
• Misalignment
• Insufficient lubrication

2. Thermal Degradation

Elastomers harden or crack when exposed to temperatures beyond their rating.

3. Chemical Attack

Incompatible lubricants or chemicals can cause swelling, softening, or corrosion.

4. Shaft Damage

Grooves worn into the shaft disrupt sealing and cause leakage.

5. Improper Installation

A seal installed at an angle or without lubrication can fail prematurely.

Recognizing these failure modes helps engineers design longer-lasting systems and choose materials more effectively.

Conclusion

Rotary shaft seals are essential for ensuring that machinery is not contaminated or poorly lubricated and that it remains reliable in the long term. The operation of these seals directly influences the lifespan of bearings, shafts, and entire mechanical systems. Understanding the functionality of rotary seals, the materials they are made of, and the factors that lead to failure enables engineers and purchasers to make informed decisions that minimize downtime and enhance functionality.

Polypac is a reliable supplier of rotary shaft seals, custom seals, and high-performance seals. If you need high-performance seals or sealing solutions, Polypac can help you develop sealing materials, create PTFE-filled seals, and establish high-quality elastomer components. With more than 10,000 square meters of manufacturing space and long-term partnerships with world-class research institutes, Polypac offers stable, engineered sealing products for challenging applications.

Would you like to improve your machinery's performance? Learn more about Polypac's sealing solutions.