Different environments put different requirements on bearing shields and seals. For cleaner operating environments and for those where high speed is necessary, seals and shields which do not contact or slide along the bearing inner ring have been developed. They are called “noncontact” shields and seals. For more contaminated environments, seals which have a lip contacting and sliding along the inner ring of the bearing are used. They are “contact” seals. Each type has its own advantages and limitations. Note that, in common usage, a “shield” is always a noncontact device.
A number of factors can limit bearing speed, not just seal type. They include, among others:
• Type of bearing.
• Raceway finish.
• Type of retainer.
• Fit and clearance.
• Operating temperature.
The use of noncontact shields and seals does not limit bearing speed. Because both components are fixed to the bearing outer ring and do not contact the inner ring, there is no sliding friction affecting the bearing.
Contact seals, conversely, have a limiting effect on bearing speed. The lip of this type of seal slides along the bearing inner ring seal groove during rotation. Therefore, a bearing with a contact seal is limited by friction and by the resultant seal-lip heating.
Shielded bearings are meant to run over a wide temperature range and have a speed capability equal to that of an open-type greased bearing. Because they are noncontact devices, there is no sliding friction to generate heat, increase torque, or decrease bearing speed capability. Although shields are primarily installed to retain grease, they also protect bearings from large contaminants. However, they cannot keep out small particles. Bearing shields are usually constructed of steel with a corrosion- resistant coating.
Some noncontact seals keep smaller particles from entering the bearing by means of a rubber lip with close-running clearance to create a labyrinth seal. By design, noncontact seals do not increase torque, and they let bearings maintain higher speed capability than that of contact-type sealed bearings. The most common noncontact seals are made from nitrile rubber with a steel insert. They can operate continuously up to about 110 to 120 C. If the temperature exceeds this range on the seal plate, the rubber will harden, losing elasticity. This will reduce the ability to seal out contaminants and retain grease, leading to premature bearing failure.
The contact-type seal bearing provides maximum protection against all contaminants. It gives the best protection against dust, water, and other contaminants because the seal lip contacts the inner ring, separating and protecting the rolling elements from the external environment. The pressure of the rubber lip against the seal groove wall maintains seal effectiveness. However, this also reduces bearing speed capacity and increases torque. It also results in sliding friction between the seal lip and inner ring, which generates heat.
To prevent overheating, premature wear, and seal-lip seizure, it is important to keep an oil film between the seal lip and groove. A film is developed in the groove when the bearing is initially put into service and grease is pushed against the seal by the rotation of the rolling elements and retainer. Because contact seals are also usually made from nitrile rubber with steel inserts, operating temperature limits speed capacity. If the seal’s maximum temperature is exceeded, the seal material will lose elasticity and harden, causing it to wear quickly, which reduces its sealing capability. The maximum continuous operating temperature recommended for a typical nitrile rubber contact seal is 100 C.
Sliding friction between the seal lip and inner ring also generates heat and increases with increasing rotary speed. Experience shows that a sliding speed of 8 to 10 m/sec, depending on bearing size, should be considered standard to keep sliding friction from generating damaging heat. However, because bearing temperature is influenced by several other factors as well, such as speed, load, heattransfer conditions, and grease quantity, it is impossible to determine the exact bearing speed limit by circumferential velocity of the seal lip alone. Therefore, use the seal lip sliding speed of 8 to 10 m/sec only as a reference.
Manufacturers have developed many other seal materials besides nitrile rubber. They operate at higher temperatures while staying compatible with various chemicals and lubricants. You should check with the manufacturers for temperature limits, susceptibility to various environments, and so forth. However, regardless of composition, each type of seal has its own speed and temperature limits. You must consider them.
Raymond J. Salazar is Branch Engineer, NSK Corp., Atlanta office. He has also served at NSK headquarters in Ann Arbor, Mich.