A pillow block bearing does more than support a rotating shaft. Its sealing arrangement also plays an important role in keeping the bearing clean, retaining lubricant, and maintaining reliable operation over time.
The choice of seal becomes particularly important when equipment operates in dusty workshops, wet production areas, or at elevated speeds. Two commonly used options are the Lip Seal and the Labyrinth Seal. Although both help protect the bearing from contamination, they work in different ways and have different operating characteristics.
This guide looks at how each design works, the materials commonly used, and the factors worth considering when selecting a seal for a pillow block bearing.
1. Lip Seals: A Direct Barrier Against Contamination
How a Lip Seal Works
A Lip Seal is a contact-type sealing component designed to operate against a rotating shaft. Its flexible sealing lip presses against the shaft surface, creating a narrow contact area that restricts the movement of lubricant and contaminants.
Many designs consist of a metal casing and an elastomeric sealing lip. Some versions also incorporate a garter spring to maintain more consistent contact pressure.
During operation, a thin lubricant film may form between the lip and the shaft. This film helps reduce friction while supporting the sealing function.
Because the lip makes physical contact with the shaft, this design can provide effective protection even when the shaft is stationary. However, the contact also introduces friction, which needs to be considered when evaluating speed, temperature, and service life.
Common Lip Seal Configurations
The number and arrangement of sealing lips affect the level of protection.
| Seal Type | Main Characteristics | Typical Application |
|---|---|---|
| Single Lip Seal | Simple construction and economical cost | Light-duty equipment, such as conveyor idlers |
| Double Lip Seal | A primary lip retains lubricant while a secondary lip provides additional contamination protection | Agricultural and general industrial machinery |
| Triple Lip Seal | Multiple barriers provide additional protection in heavily contaminated environments | Mining and other demanding applications |
The appropriate configuration depends on the amount and type of contamination the bearing will encounter. Adding more lips does not automatically make a seal suitable for every operating condition; friction, available space, and shaft speed must also be considered.
Materials Used in Lip Seals
Elastomer selection affects a seal's resistance to heat, chemicals, moisture, and aging. The following materials are commonly considered for industrial sealing applications.
Nitrile Rubber (NBR / Buna-N)
NBR is widely used because it offers a practical balance of cost and resistance to common industrial lubricants.
· Good resistance to petroleum-based oils, grease, water, and many fuels.
· Commonly used in general-purpose bearing seals.
· Typical continuous operating temperatures are approximately -40°C to +100°C, depending on the compound.
· Short-term exposure to higher temperatures may be possible, subject to the manufacturer's limits.
Fluoroelastomer (FKM / Viton)
FKM is often selected where higher temperatures or greater chemical resistance are required.
· Suitable for many demanding industrial environments.
· Offers good resistance to oils, fuels, and a range of chemicals.
· Standard compounds commonly support continuous operation at temperatures approaching +200°C, although the lower limit and maximum temperature vary by formulation.
Specialized low-temperature and high-temperature grades are available. Their performance should be checked against the actual operating environment rather than assumed from the material name alone.
Ethylene Propylene Diene Monomer (EPDM)
EPDM is known for its resistance to weathering, ozone, and ultraviolet exposure.
Useful for outdoor equipment.
Offers good resistance to water and atmospheric aging.
Generally unsuitable for applications involving many petroleum-based oils.
Other materials may be specified for particular conditions, including polyurethane, polyacrylate, silicone, neoprene, fluorosilicone, and hydrogenated nitrile rubber (HNBR).
Where Lip Seals Perform Well
For many standard industrial applications, Lip Seals offer a straightforward and economical way to protect a bearing.
Their main advantages include:
· Effective lubricant retention: The contact lip helps prevent grease or oil from escaping.
· Good contamination protection: Properly selected seals can restrict the entry of dust, moisture, and other foreign matter.
· Compact construction: Their relatively small axial footprint makes them suitable for space-limited assemblies.
· Simple installation and replacement: Their familiar design can make routine maintenance easier.
· Practical initial cost: They are often an economical choice for general-purpose machinery.
Lip Seals are commonly used in conveyors, agricultural equipment, material-handling systems, and other machinery operating at low to moderate speeds.
Limitations to Consider
The same contact that makes a Lip Seal effective also creates several operating constraints.
Friction and heat generation
Continuous contact with the shaft produces friction. As rotational speed increases, the resulting heat and wear can become more significant. Inadequate lubrication or abrasive contamination may accelerate deterioration.
Dependence on shaft condition
The sealing lip must run against a suitable shaft surface. Excessive runout, misalignment, poor surface finish, or insufficient shaft hardness can reduce sealing performance.
Over time, the lip may also create a wear track on the shaft, particularly if lubrication is inadequate.
Temperature and chemical compatibility
Excessive heat can cause an elastomer to harden, crack, or lose its elasticity. Chemical exposure can have similar effects if the material is not compatible with the lubricant or surrounding fluids.
Limited pressure capability
Conventional Lip Seals are generally intended for low-pressure sealing duties. A large pressure differential can deform the lip and cause leakage. Applications involving substantial pressure require a seal specifically designed for those conditions.
2. Labyrinth Seals: Protection Without Continuous Contact
What Is a Labyrinth Seal?
A Labyrinth Seal protects the bearing through a series of narrow clearances and changes in direction rather than by pressing against the shaft.
The arrangement typically uses two components that form a winding passage. Contaminants must travel through this restricted path before reaching the bearing.
Because the sealing components do not normally touch during operation, a Labyrinth Seal produces very little friction at the sealing interface. This makes it particularly useful in applications where speed, heat generation, and wear are important considerations.
How the Labyrinth Design Restricts Contaminants
The seal's performance comes from its geometry and the operating conditions around it.
1. A tortuous passage
The winding path forces particles and other contaminants to change direction repeatedly. This makes it more difficult for them to travel directly into the bearing.
2. Restricted clearances
Narrow gaps limit the space available for contaminants to pass through. The effectiveness of this arrangement depends on maintaining the specified clearances.
3. Centrifugal Force
In rotating designs, centrifugal force can help move particles and liquid away from the shaft or sealing region. The extent of this effect depends on the seal geometry, rotational speed, and the properties of the contaminants.
Labyrinth Seals should therefore be understood as flow-restricting barriers, not as completely impermeable closures.
Materials for Labyrinth Seals
The material is selected according to the operating temperature, chemical exposure, mechanical requirements, and cost.
| Material | Common Considerations |
|---|---|
| Bronze | Used in certain industrial sealing designs where its mechanical properties are suitable |
| Stainless Steel | Offers corrosion resistance for demanding environments |
| Aluminum Alloy, such as 6061 | Provides a lightweight option for suitable applications |
| Inconel 718 | Used in specialized high-temperature environments |
| Engineering Thermoplastics, including PTFE and PEEK | Can provide low-friction and chemical-resistance characteristics, depending on the grade |
Not every material is available for every pillow block bearing design. Compatibility with the housing, shaft, and operating environment should be confirmed with the supplier.
Common Labyrinth Seal Designs
Labyrinth Seals are available in several configurations. Their geometry, mounting arrangement, and intended environment vary by manufacturer.
| Design | Typical Construction | Main Characteristics |
|---|---|---|
| CF | Steel or aluminum, depending on design | A complex sealing path intended for demanding contamination conditions |
| L and M | Commonly metal construction | Sawtooth-style paths; some M designs include external drainage grooves |
| S and SA | Often made from engineering plastics | Conical or sawtooth-style passages; some SA designs include drainage features |
Installation requirements also differ. Some designs fit directly against the bearing, while others are mounted with their own clearances and tolerances. Always follow the manufacturer's instructions for the specific seal series.
Advantages of Labyrinth Seals
No continuous contact
The absence of rubbing contact minimizes friction and eliminates normal contact wear at the sealing interface.
Suitability for high rotational speeds
With little friction generated by the seal itself, Labyrinth Seals can be suitable for high-speed machinery, provided the complete bearing assembly is rated for the intended speed.
Reduced maintenance associated with seal wear
Since there is no continuously rubbing lip, the seal does not experience the same type of contact wear. This can reduce the need for seal replacement caused by normal rubbing.
No contact-induced shaft grooving
A correctly installed non-contact design does not wear the shaft through continuous seal contact.
Useful in demanding operating environments
Appropriately designed labyrinth arrangements can provide durable protection against airborne particles and splashing contaminants, particularly when combined with suitable housing and drainage features.
Limitations of Labyrinth Seals
Although Labyrinth Seals offer several advantages, they are not suitable for every application.
Limited sealing when stationary
Their performance relies on the restricted passage and, in some designs, the movement of the rotating components. They generally cannot provide the same static sealing capability as a contact seal.
Limited resistance to pressure and standing liquids
A labyrinth passage restricts flow but does not form a fully closed barrier. Significant pressure differences or pooled liquids may allow leakage or ingress.
Clearance requirements
The gaps between components must remain within the specified limits. Manufacturing tolerances, thermal expansion, shaft runout, and misalignment can all affect performance.
Potential for blockage
Heavy, sticky, or compacted contaminants may accumulate in the narrow passages. This can reduce effectiveness and, in certain designs, interfere with operation.
Potentially higher initial cost
Some labyrinth designs require more precise machining and assembly. The additional manufacturing complexity can increase the initial cost compared with a conventional Lip Seal.
3. Lip Seal vs. Labyrinth Seal: Key Differences
The two designs address different sealing priorities. A Lip Seal relies on physical contact to form a barrier, while a Labyrinth Seal uses a non-contact passage to restrict contaminant movement.
The table below summarizes their general characteristics.
| Feature | Lip Seal | Labyrinth Seal |
|---|---|---|
| Sealing Principle | Contact between the lip and shaft | Non-contact, restricted passage |
| Friction | Present | Very low at the sealing interface |
| Wear | Lip wear may occur | No normal rubbing wear |
| Lubricant Retention | Generally effective when correctly selected | Depends on design; not a fully closed barrier |
| Static Sealing | Generally effective within its design limits | Limited |
| High-Speed Suitability | Restricted by friction, heat, and material | Often suitable when clearances and bearing ratings permit |
| Shaft Surface Requirements | Important for sealing performance and service life | Clearance and alignment are key considerations |
| Resistance to Splashing Water | Depends on seal design and condition | Depends on passage design, orientation, and drainage |
| Maintenance | May require periodic inspection and replacement | Often lower contact-wear maintenance |
| Initial Cost | Often lower | May be higher |
| Typical Applications | General industrial machinery and moderate-speed equipment | High-speed equipment and applications where contact friction should be minimized |
What About Speed and Temperature Ratings?
There is no single speed or temperature limit that applies to every Lip Seal or Labyrinth Seal.
For example, the operating temperature of a Lip Seal depends heavily on its elastomer, lubrication, shaft speed, and surrounding conditions. A Labyrinth Seal may tolerate a different temperature range depending on whether it is made from steel, aluminum, or an engineering polymer.
Likewise, a seal's maximum speed should not be confused with the maximum speed of the bearing assembly. The bearing, lubricant, shaft, housing, and sealing arrangement must all be suitable for the intended operating conditions.
For this reason, generic figures such as a fixed rpm limit or a guaranteed service life should be treated as indicative only. Use the manufacturer's data for the exact product and application.
4. How to Select a Seal for Your Pillow Block Bearing
Before specifying a seal, consider the machine's operating environment and maintenance requirements.
Start With the Contamination Type
Identify what the seal needs to keep out.
Fine dust and airborne particles: Both designs may be suitable, depending on the particle size, seal construction, and operating conditions.
Water spray or frequent washdown: Check the seal's water-exclusion capability. A conventional labyrinth design may not be sufficient where liquid exposure is severe.
Heavy abrasive contamination: Consider a purpose-designed multi-stage seal or another heavy-duty sealing arrangement.
Oil or grease retention: A contact seal may be appropriate where effective lubricant retention is a priority.
Check Shaft Speed and Operating Temperature
At higher speeds, friction and heat from a contact seal can become important limitations. A non-contact Labyrinth Seal may be suitable where reducing these effects is a priority.
For high-temperature operation, check both the seal material and the bearing lubricant. Selecting a temperature-resistant seal alone does not ensure that the complete bearing unit can operate at the same temperature.
Review the Shaft and Housing Arrangement
A Lip Seal requires a suitable running surface and correct contact with the shaft. A Labyrinth Seal requires the specified clearances and alignment.
Check the shaft diameter, runout, housing arrangement, available installation space, and any expected thermal movement before confirming the design.
Consider Maintenance and Total Operating Cost
Initial purchase price is only one part of the decision.
A lower-cost contact seal may be suitable when replacement is straightforward and operating conditions are moderate. In equipment where access is difficult or seal friction is a concern, a non-contact design may offer different long-term maintenance benefits.
The choice should reflect the expected service conditions, replacement costs, downtime, and the consequences of bearing contamination.
5. Understanding Lip Seal Designations
Seal markings vary among bearing manufacturers. Similar-looking codes do not always indicate identical construction or sealing performance.
The following examples show designations used by selected manufacturers. Confirm the meaning in the relevant manufacturer's catalogue before specifying a replacement.
| Designation | Manufacturer | General Description |
|---|---|---|
| 2RSR | FAG | Two standard contact seals |
| 2RS1 | SKF | Two standard contact seals, commonly using NBR |
| 2RSL | SKF | Two low-friction contact seals |
| 2BRS | FAG | Two low-friction contact seals |
| DDU | NSK | Two contact seals |
| DDG | NSK | Two high-performance contact seals |
| DDW | NSK | Two high-performance seals with enhanced water resistance |
| LLU | NTN / Enduro Bearings | Contact-seal designation; exact construction varies by manufacturer |
| LLB | NTN / Enduro Bearings | Light-contact seal designation; verify the manufacturer's definition |
| LLH | NTN | Low-torque synthetic rubber seals |
| LLE | NTN | Water-resistant synthetic rubber seals |
| 2RU | Koyo | Two contact seals |
| RS suffix | Timken | Rubber-seal designation; confirm the specific product details |
These codes are not universal standards. When replacing an existing bearing or ordering a pillow block unit, verify the manufacturer's part number, seal arrangement, dimensions, and material.
FAQ
1. Is a Lip Seal or a Labyrinth Seal better for a pillow block bearing?
The choice depends on the operating conditions. A Lip Seal provides a contact barrier and is commonly used for lubricant retention and general contamination protection. A Labyrinth Seal avoids continuous contact and can be useful where friction and wear need to be minimized.
2. Can a Labyrinth Seal prevent water from entering a bearing?
It can restrict the entry of splashing water, depending on its design and installation. However, it is not a completely watertight barrier. Applications involving direct water jets, standing water, or frequent washdown may require a different sealing arrangement.
3. Do Labyrinth Seals require lubrication?
The seal itself generally does not require lubrication at a contact interface because it operates without continuous rubbing. The bearing still requires the lubrication specified for its design and operating conditions.
4. Do Lip Seals cause shaft wear?
They can. Since the sealing lip runs directly against the shaft, friction may gradually produce a wear track. Correct installation, a suitable shaft finish, and adequate lubrication help reduce this risk.
5. Can I replace a Lip Seal with a Labyrinth Seal?
Not necessarily. The two designs may differ in dimensions, mounting arrangement, clearances, and sealing capability. Before making a substitution, confirm that the replacement is compatible with the bearing housing, shaft, operating speed, and contamination conditions.
6. Which seal is suitable for high-speed equipment?
A Labyrinth Seal is often considered when minimizing friction is important. However, the allowable speed depends on the complete assembly, including the bearing, lubricant, shaft, and seal design. Check the manufacturer's rated operating conditions.
Final Thoughts
Lip Seals and Labyrinth Seals protect pillow block bearings through two different approaches. Lip Seals create a direct contact barrier, making them useful for general industrial applications where lubricant retention and contamination exclusion are priorities. Labyrinth Seals rely on a non-contact passage, reducing friction and contact wear in suitable operating conditions.
When choosing between them, consider contamination, shaft speed, temperature, lubricant retention, installation requirements, and maintenance access together. Matching the seal to the actual operating environment helps maintain bearing reliability and avoid unnecessary replacement costs.
For a specific pillow block bearing, consult the manufacturer's technical data to confirm seal compatibility, operating limits, and installation requirements.
