Industrial seal self study guide - SKF.com

BASIC SEAL TYPES (cont.)
As was earlier stated, SKF’s standard nitrite compound provides good
service in most sealing applications from -40˚F to 250˚F (-40˚C to
121˚C). However, silicone, polyacrylate, Longlife or PTFE may be more
suitable for and should be considered for temperatures outside this
range.
For more information on the temperature compatibility and abrasion
resistance of various seal lip materials, turn to Chapter 4.
When a seal’s basic
function is to retain,
face the seal lip toward
lubricant (fig. 2g).
When the seal’s basic function
is to exclude, face the seal lip
toward contaminants (fig. 2h).
For retention/exclusion applications,
a combination of seals in a unitized
assembly can be the best solution
(fig. 2i).
Most applications for shaft seals involve the need to retain or separate
lubricants from some form of external contaminant or abrasion. But
many times it must be determined which is more important, retention
of lubricant or exclusion of foreign matter.
Retention
When the seal’s basic function is to retain lubrication, pressure, or both,
the lip of the seal (generally the spring side) must face toward the
lubricant or the pressure being retained and generally be spring
assisted (fig. 2g).
Exclusion
Most bearings fail from the entrance of foreign material and from the
loss or degradation of lubricant. Dirt, abrasives, water and other liquids
can interfere with the film of lubricant required to support the moving
parts of a bearing in a sealed system. Reliable excluders generally
include V-Rings and non-spring loaded seals.
Therefore, it’s vitally important for the seal to keep those materials
from entering the bearing cavity. When the seal’s basic function is to
exclude, the lip of the seal should face toward the contaminants
instead of toward the bearing (fig. 2h). However in this case, only
grease lubrication should be used since oil loss could be excessive.
Retention/Exclusion
Some extreme applications require the seal to perform both the
retention and exclusion functions at the same time. For example, the
seal may need to confine a lubricant while excluding dust or cleaning
solutions. In this case, a special type of protection is necessary either
a combination of seals back-to-back or dual sealing elements within
one unitized assembly (fig. 2i).
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