Selection of seal types
Seals for bearing arrangements should provide a minimum amount of friction and wear while providing maximum protection even under the most arduous conditions. Because bearing performance and service life are so closely tied to the effectiveness of the seal, the influence of contaminants on bearing life is a key design factor. For more information on the influence of contamination on bearing performance, please refer to the section Selection of bearing size.
Many factors have to be considered when selecting the most suitable seal type for a particular bearing arrangement
- the type of lubrication: oil or grease
- the peripheral (circumferential) speed at the sealing surface
- the shaft arrangement: horizontal or vertical
- possible shaft misalignment
- available space
- seal friction and the resulting temperature increase
- environmental influences
- justifiable cost.
Selecting the correct seal is of vital importance to the performance of a bearing. It is therefore necessary to accurately specify the sealing requirements and to accurately define the external conditions.
If little or no experience is available for a given application, SKF, also one of the largest seal manufacturers in the world, can assist in the selection process or make proposals for suitable seals.
Two types of external sealing devices are normally used with rolling bearings: non-contact and contact seals. The type chosen depends on the needs of the application.
The effectiveness of an external non-contact seal depends in principle on the sealing action of the narrow gap between the rotating and stationary components. The gap may be arranged radially, axially or in combination (fig 1). These seals can be as simple as a gap-type seal or more complex like a labyrinth seal. In either case, because there is no contact, these seals generate virtually no friction and do not wear. They are generally not easily damaged by solid contaminants and are particularly suitable for high speeds and high temperatures. To enhance their sealing efficiency grease can be pressed into the gap(s) formed by the labyrinth.
The effectiveness of a contact seal depends on the seal's ability to exert a minimum pressure on its counterface by a relatively narrow sealing lip or surface. This pressure (fig 2) may be produced either by
- the resilience of the seal, resulting from the elastic properties of the seal material (a)
- the designed interference between the seal and its counterface (b)
- a tangential force exerted by a garter spring incorporated in the seal (c).
Contact seals are generally very reliable, particularly when wear is kept to a minimum by producing an appropriate surface finish for the counterface and by lubricating the seal lip/counterface contact area. The friction of the seal on its counterface and the rise in temperature that this generates are a disadvantage and contact seals are therefore only useful for operation up to certain peripheral speeds depending mainly on the seal type and counterface roughness. They are also susceptible to mechanical damage, e.g. as a result of improper mounting, or by solid contaminants. To prevent damage by solid contaminants it is customary to place a non-contact seal in front of a contact seal in order to protect it.