1. Core selection principle
Sealing vs. Friction and Temperature Rise
Core contradiction: The better the sealing performance (such as contact seals), the greater the friction with the bearing, which will lead to an increase in bearing temperature, restricted rotational speed, and a slight increase in energy consumption. A balance needs to be struck between "absolute sealing" and "low-friction operation".
Solution: Select the most suitable type based on the specific operating conditions of the pump.
Medium Type
Water: Is it fresh water, sewage or seawater? In fresh water, the main consideration is waterproofing; for sewage and seawater, the resistance of the sealing material to chemical corrosion and particulate impurities also needs to be taken into account.
Other: Have you come into contact with chemicals, oils, etc.?
Work Environment
Pressure: Is there any pressure inside the pump bearing chamber? The seal needs to be able to withstand a certain difference in pressure between the inside and outside.
Speed: In high-speed applications, consideration must be given to the friction and temperature rise of the seals.
Temperature: The temperature resistance range of the sealing material must cover the operating temperature of the pump.
Impurities: Are there any dust, sand or other hard particles in the environment? These particles can accelerate the wear of contact seals.
II. Common Types of Sealing Rings and Their Applicability
The sealing of bearings can be mainly classified into two types: contact type and non-contact type.
Contact seal - High protection, medium to low speed
This type of seal has its lip directly in contact with the inner or outer ring of the bearing, providing the best sealing effect.
Rubber gasket
Common designations: 2RS, RS, RSH, RZ, etc.
Material: Usually it is nitrile rubber or fluorine rubber.
Nitrile rubber: Low cost, excellent oil resistance, good water resistance, suitable for most conventional water pumps (temperature range approximately -40°C to 100°C).
Fluorine rubber: It has a high price, but it possesses excellent heat resistance (approximately -20°C to 200°C), oil resistance, and chemical resistance. It is suitable for use in demanding environments such as high-temperature water pumps and chemical pumps.
Common forms:
RS Type (Low Friction): The lip seal has a slight contact, resulting in relatively low friction and temperature rise, and it has excellent dust and water resistance. It is a common choice for pump bearings.
2RS Type (Double Sealed): Both sides of the bearing are equipped with RS seals, providing comprehensive protection. It is the most commonly used and standard option.
ZZ Type (Metal Dust Cover): Strictly speaking, it is of the non-contact type. There is a small gap between it and the bearing ring, mainly for dust prevention and larger foreign objects. It is basically not waterproof. It is absolutely not recommended for use in water pumps in damp or environments where water may splash.
2. Non-contact Sealing - Low Friction, High Speed
This type of sealing does not come into contact with the bearing ring. It uses a labyrinth-like series of curved gaps to prevent contaminants.
Features: Zero friction, zero temperature rise, infinite lifespan, suitable for high-speed applications.
Disadvantage: The static sealing effect is poor and it cannot completely prevent the intrusion of pressurized water or steam.
Application: It is commonly used in high-speed scenarios where oil lubrication or special protection is required internally. It is less often used as the main seal in ordinary grease-lubricated water pumps.
3. Combined Seal - Optimal Balance (Preferred for Water Pumps)
This is currently the best and recommended choice for bearings in mid-to-high-end water pumps. It combines the advantages of both contact and non-contact types.
Structure: Usually consists of a non-contact maze groove and a contact-type rubber lip.
Working principle:
The first line of defense (the maze): It blocks most impurities and splashing water, and uses centrifugal force to dislodge the small amount of water or impurities that have entered.
The second line of defense (lip seal): Provides the ultimate seal to ensure absolute cleanliness within the bearing chamber.
Advantages: While providing a level of protection similar to that of a contact seal, the friction and temperature rise are much lower than those of a pure contact seal, resulting in extremely high reliability.
Common models: Each bearing brand has its own patented design, such as SKF's Energy Efficient (E2.) series seals, NSK'sLLU seals, etc.
IV. Summary of Practical Suggestions
Default Option: If unsure, choosing 2RS (double-sided nitrile rubber seal) for the pump bearings is a safe, cost-effective, and versatile starting point.
Upgrade option: For critical equipment or in harsh conditions (such as submersible pumps, sewage pumps, etc.), it is an extremely wise investment to spend a little more to choose bearings with "combined seals", which can significantly extend the lifespan of the bearings and the entire pump.
Focus on the brand: Well-known bearing brands (such as SKF, FAG, NSK, etc.) usually have more reliable sealing technologies, and their material formulations and mold precision are higher.
Correct Installation: The seal is a precision component. During installation, avoid using sharp tools to damage the lip seal. Ensure that the direction of the seal lip is correct (usually the lip faces the outside of the bearing to prevent external contaminants from entering).
Synergistic consideration: The selection of seals must be considered in conjunction with the grease and the internal clearance of the bearings. For instance, after using high-performance seals, a lubricant with a wider temperature range can be selected; contact seals may generate more heat, and sometimes a bearing with a C3 clearance is chosen to compensate for the expansion caused by the temperature rise.
Ultimately, the most reliable approach is still to refer to the original specifications provided by the pump manufacturer. However, if you need to make your own selection or replacement, following the above guidelines will help you make the right decision.
