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 effective at keeping out liquids and fine particles, but the
speed at which they operate is constrained by the friction that 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 avoid
seals 'popping' with the consequent loss of lubricant.
A new generation of seals has been developed, which combine high levels
of sealing with speed capabilities similar to those of bearing shields,
which do not contact the bearing inner ring. NSK's V-Type seal, for
example, is a non-contact unit that is held in the outer ring of a
bearing by the elasticity of the rubber seal material - friction is thus
not a problem, so bearings using these seals can be run at the same high
speeds as those equipped with shields. Compared with shields, however,
V-Type seals are better at sealing in grease and are effective against
fine particle ingress.
Positioned somewhere between the standard seal and V-type is NSK's new DW
seal, a light contact device facilitating high speed operation with good
resistance to moisture and particulate ingress. The support for the main
seal lip is long and thin, ensuring low torque operation. indeed, the
starting torque is approximately half that of a standard seal, while the
running torque is also lower. Additionally, the main lip contacts the
bevelled portion of the inner ring seal groove where dust is ejected
outwards by centrifugal force, so dust resistance is exceptionally good.
The main ring also makes outward contact with the inner ring seal groove,
so internal pressure does not open the seal and allow grease to
contaminate the process or impair the long term operation of the bearing
itself.