The challenge in recent years has been to develop a new generation of seals which combine high levels of sealing with speed capabilities similar to those of shields. NSK's V type seal was developed for this very purpose. In contrast to conventional seals, V types are non-contact units which are held in the outer ring of a bearing by the elasticity of the rubber seal material. With this design friction is not a problem. Consequently, bearings employing the seals can be used at the same high speeds as bearings equipped with shields. However, compared to shields, V type seals provide better grease sealing efficiency and resistance to fine dust particles (Plates 2 and 3).

Shields have no contact with the inner ring, of the bearing, hence their low torque qualities. However, this same advantage (i.e. no contact) means that shields are generally unsuitable in applications where moisture ingress may occur. On the other hand, shields are more robust in resisting damage from solid airborne contaminants – small flying stones, for example.

(2003), "The role of seals and shields in modem bearing technology", Industrial Lubrication and Tribology, Vol. 55 No. 3. https://doi.org/10.1108/ilt.2003.01855cad.002

Of course, while the seals and shields are providing their protective function the ball bearing is rotating, often at high speed. So if the operating life is to be maximised then the type of seal or shield to use must be chosen, not only with the conditions in mind, but also with the frictional effect the device will have upon the bearing starting and running torques.

In many instances literature concerning seals and shields tends to blur the differences between the two and the areas where one or another should be used. Essentially shields are non-contact metallic (carbon steel or stainless) devices which allow high speed operation with limited protection against moisture and dirt ingress. Seals can be either contact or non-contact types. The former offer better protection, but with reduced speed capabilities, whereas the latter have higher speed capabilities and levels of protection superior to shields.

The RHP Self-Lube range of bearings is widely used in tough operating environments so there is a choice of sealing systems to give optimum protection under different conditions. The standard seal, up to and including 40 mm bore, is the S type. Above that standard is the M type. In addition, there is the Flinger seal for more hostile environments. The Flinger seal provides increased levels of protection without any loss of rated bearing speed due to the non-contacting "Flinger Seal" lip. It provides a first line barrier against contamination and thus improves bearing life.

V type seals were originally developed for use on computer spindles, where high reliability and quiet operation are paramount. The range and applications have expanded, and in one crucial test the seals have ensured that a multiple world champion has not experienced a single bearing failure on his bikes for 4 years in one of the worlds most demanding motorcycle sports – Long Track Speedway Racing.

In the most arduous environments triple lip seals are fitted to protect the bearing and extend its life. The triple lip seal offers the highest levels of protection with three sealing lips which contact the bearing inner ring. There is a consequent reduction in rated speed as a result of this configuration, but in the applications where the seal is used – on agricultural machinery, for example – this is not usually a problem.

The most significant developments have recently taken place in the area of seals. Traditionally seals for ball bearings are contact types made of rubber, usually with a metal backing. On smaller bearing sizes the seal is often made of glass reinforced Teflon and is fitted using a snap ring. Both types are very effective at keeping out water, liquids and fine particles. However, the speed at which they operate is constrained by the friction which exists between the seals and the bearing inner ring. The precise fit of standard contact seals also means that a small vent hole is necessary to ensure that a build up of pressure within the bearing does not "pop" the seals and release the grease lubricant.

The main impetus towards sealed or shielded bearings comes from the growing use of seal for life bearings in everyday items such as white goods, cars and power tools. These bearings use grease lubrication so seals or shields are required to prevent the grease leaking out. This is particularly true in applications where grease leakage may spoil the product. In most operating environments though, the requirement is twofold: retaining the lubricant within the bearing while preventing the ingress of moisture, debris and contaminants.

Offering a solution somewhere between the standard contact seal and the V seal is NSK's newly developed DW seal. A light contact device, the DW seal facilitates high speed operation but not to the same extent as shields or V type seals. In overall sealing terms, though, it is much superior, both in respect of moisture and dirt ingress. The DW seal also offers advantages over the standard contact seal. Its innovative design means that the support for the main seal lip is long and thin, resulting in light contact and low torque operation. Starting torque is approximately one half that of the standard seals, and running torque is also lower. In addition, the main lip contacts the bevelled portion of the inner ring seal groove where, if there is centrifugal force, dust is moved outwards. Consequently, dust resistance with DW seals is excellent. Finally, the main ring has outward contact with the inner ring seal groove, so internal pressure does not open the seal and allow grease leakage to contaminate a process or impair the long term operation of the bearing itself.

Bearing seals and shields have been available almost as long as ball bearings themselves. However, the move towards more integrated bearing structures has meant that their importance has grown, moving them from occasional add-on items to intrinsic parts of the ball bearings which they protect. In special applications the seal or shield is increasingly becoming an integral part of the bearing design rather than an extra that is added later.

The move towards sealed life bearings has almost created a situation where the primary function of a seal is to provide a mechanism for retaining grease rather than preventing the ingress of contaminants. This is certainly the case in "clean" applications with white goods and integrated automotive parts. However, the primary function of the seal reverts to one of protection in adverse operating environments. Often in such conditions the protection provided by the standard seals does not suffice and specially developed types have to be employed.