The sealing methods of bearings can be mainly divided into two categories: contact seals and non-contact seals. The choice essentially involves a trade-off between sealing effect and allowable rotational speed.
Contact seals: “Contact” for sealing
In this type of seal, the sealing piece is in direct contact with the rotating shaft, and the tight fit is used to prevent leakage and pollutant intrusion. The advantage is good sealing effect, especially effective in dust and water prevention. The disadvantage is that due to the sliding friction, heat and wear will occur, limiting its applicable rotational speed.
Common contact seals include the following types:
Gasket seal: Pressing the felt onto the shaft to achieve sealing. The structure is simple, the cost is low, but the sealing effect is poor and it is prone to wear. It is suitable for low-speed (<4-5m/s) and low-temperature (<90℃) grease lubrication scenarios.
Cup/Lip seal: Using oil-resistant rubber cups or lip-shaped sealing rings to tightly embrace the shaft, often reinforced by springs. The sealing effect is better than the felt ring, but the rubber will age and wear. It is suitable for medium-low speed (<6-7m/s) grease or oil lubrication. When installing, pay attention to the direction: the lip should face inward for oil prevention and outward for dust prevention.
O-shaped/J-shaped/U-shaped sealing rings: Made of leather, plastic or oil-resistant rubber, relying on their own elasticity to seal. The O-shaped ring has a simple structure, while the J-shaped and U-shaped lip seals have better sealing effect. They can be used for grease and oil lubrication and have a wider applicable temperature and speed range than the felt ring.
Frame seal: An improved version of the cup seal, with a metal frame embedded inside for enhanced strength. It has better strength and durability, and is currently a widely used sealing form. It is often used in medium-low speed (<7m/s) scenarios, such as centrifugal pump bearing boxes.
Sealing ring seal: A metal ring with a notch, relying on elasticity to rest against the stationary part. The structure is more complex. The advantage is that it can withstand very high speeds (up to 100m/s), often used in high-temperature, high-speed, and heavy-load conditions.
Non-contact seals: “Gap” for rotational speed
In this type of seal, there is a very small gap between the sealing piece and the rotating shaft, without direct contact. The advantage is that it hardly generates friction and wear, especially suitable for high-speed and high-temperature conditions. The disadvantage is that the sealing effect is relatively weak, and there are certain limitations on waterproof and dust-proof capabilities.
Common non-contact seals include the following types:
Gap seal: Leaving a very narrow annular gap (radius usually 0.1-0.3mm) between the shaft and the bearing cover. The structure is the simplest, but the sealing effect is limited. By opening a ring-shaped groove in the gap and filling it with lubricating grease, the sealing effect can be improved.
Maze seal: Designing a complex “maze” type channel between the rotating part and the stationary part. Using a complex path to greatly increase the leakage resistance, the sealing effect is very good, and the effect is better as the speed increases. Suitable for dirty and humid environments.
Oil splash ring seal: Using the centrifugal force of the oil splash ring to splash the leaked oil back. The sealing is reliable, but the rotational speed of the shaft neck should be no less than 5-6m/s. It is often used in oil lubrication systems or for isolating grease and oil lubrication areas.
Dust shield (Shield, Z-type): Made of stamped metal plates, fixed on the outer ring, forming an annular gap with the inner ring. The friction torque is small, the high-speed performance is good, it can prevent large particles of dust, but it is basically not waterproof. Suitable for general dust-proof scenarios without waterproof requirements.
Non-contact sealing ring (Seal, RZ-type): Made of rubber with a frame, the sealing lip forms a very narrow gap with the inner ring. The friction torque is small, the high-speed performance is good, the dust-proof effect is better than the dust shield, but not suitable for environments with water or humidity.
In summary, when selecting, you can follow the following principles:
Seeking the ultimate sealing, not afraid of low speed: Choose contact seals, such as frame oil seals.
Seeking ultra-high speed, light load dust-proof: Choose non-contact seals, such as dust shield (Z-type) or non-contact rubber sealing ring (RZ-type). Poor environment, with much dust or humidity: Choose contact-type sealing (such as RS type) or labyrinth sealing.
Oil lubrication system to prevent oil leakage: Consider using oil discharge rings or lip seals.
In practical applications, it is also common to combine multiple sealing methods to achieve better results