Page 184 - Tribology in Machine Design

170 Tribology in machine design

used in the industry, Brinell hardness or its equivalent value, should be used
for calculating K. At the present time the seal industry has not utilized the
wear coefficient, but as is readily seen it can be obtained, without further
testing and can be established from existing PV data, or immediately be
part of the PV evaluation itself, without the necessity of running an
additional separate test.

4.15.9. Parameters defining performance limits
The operating parameters for a seal face material combination are
established by a series of PV tests. A minimum of four tests, usually of 100
hours each, are performed and the wear rate at each level is measured. The
PV value and the wear rate are recorded and used to define the operating
PV for a uniform wear rate corresponding to a typical life span of about two
years. Contrary to most other industrial applications that allow us to
specify the most desirable lubricant to suppress the wear process of rubbing
materials, seal face materials are required to seal a great variety of fluids and
these become the lubricant for the sliding ring pairs in most cases. Water,
known to be a poor lubricant, is used for the PV tests and for most practical
applications reliable guidelines are achieved by using it.

4.15.10. Material aspects of seal design
In the majority of practical applications about twelve materials are used,
although hundreds of seal face materials exist and have been tested. Carbon
has good wear characteristics and corrosion resistance and is therefore used
in over 90 per cent of industrial applications. Again, over hundreds of
grades are available, but by a process of careful screening and testing, only
the best grades are selected for actual usage. Resin-filled carbons are the
most popular. Resin impregnation renders them impervious and often the
resin that fills the voids enhances the wear resistance. Of the metal-filled
carbons, the bronze or copper -lead grades are excellent for high-pressure
service. The metal filler gives the carbon more resistance to distortion by
virtue of its higher elastic modulus. Babbitt-filled carbons are quite popular
for water-based services, because the babbitt provides good bearing and
wear characteristics at moderate temperatures. However, the development
of excellent resin-impregnated grades over recent years is gradually
replacing the babbitt-filled carbons. Counterface materials that slide
against the carbon can be as simple as cast-iron and ceramic or as
sophisticated as the carbides. The PV capability can be enhanced by a
factor of 5 by simply changing the counterface material from ceramic to
carbide. For frequently used seal face materials, the typical physical
properties are given in Table 4.4.