Rotary Seals - Dilanda.it

Rotary Seal
Aggressive media:
Generally aggressive media (e.g. solvents) have poor
lubricating properties and therefore we recommend
Turcon ® Varilip ® or PDR seals. Turcon ® and other PTFE
materials solve the problem of chemical resistance and the
metal cage can be produced out of various Stainless Steel
materials. For further details on these products please refer
to our separate catalogue on “PTFE Rotary Shaft Seals“
Circumferential velocity
The trend in the market shows an important increase of
the circumferential velocity and therefore new heatresistant
materials need to be developed.
Velocity mainly influences the heat generation in the
sealing gap and thus limits the use of the seal. The
dissipation of the heat generated by the friction occurs via
the media and the shaft itself. Depending on the capability
of the fluid to be sealed to convey the heat away from the
sealing area, the circumferential velocity must be reduced
accordingly e.g. in dry running conditions the heat
generated in the sealing area can be 40 °C above the
fluid temperature. In such cases we recommend to limit the
use in maximum environment temperature by the above
value.
In addition to the heat generation, the important criteria
to be considered is the possible loss of contact of the
sealing lip due to centrifugal forces. This applies to rotating
seals with axial sealing lips e.g. V—Ring or GAMMA-seal.
Velocity limits are specified in each individual chapter.
Problems may also occur when radial lip seals are
assembled into housings rotating at high angular velocity.
Pressure
Rotary seals often operate without system pressure
applied. Nevertheless the relative movements within the
equipment or heat generation can create pressure peaks
generally limited to 0.05 MPa.
The pressure applied to the sealing lip increases the friction
force and consequently the heat generation. Therefore the
operating conditions need to be adapted/reduced
accordingly. See recommendations in the corresponding
chapters.
Pressures up to 1MPa require either special back-up rings or
special radial oil seal profiles. See selection Table II.
Rotary joints are mainly dedicated to convey various fluids
under high pressures up to 30 MPa and thus require
Turcon ® based sealing elements either Turcon ® Roto Glyd
Ring ® or Turcon ® Roto Variseal ® depending on the
circumferential velocity. First selection can be made from
the Table II.
Pressure influences heavily the lubricating film in the
contact area of the lip and consequently the heat
generation. Therefore a reduction in circumferential
velocity is required when pressure is applied to the
sealing lip.
Temperature
Temperature is the most critical criteria to be considered
when selecting a rotary seal.
The temperature limits indicated in the selection tables are
maximum operating temperatures of the sealing material
in fluids for which the material compatibility is secured
(good chemical resistance and controlled swelling/
shrinkage).
The above descriptions show that the temperature in the
sealing area is influenced by various parameters especially:
- The lubricating capability of the fluid and its ability to
dissipate the heat generated under the sealing lip.
- The circumferential velocity
- The pressure applied
The resulting temperature in the sealing area must be
considered to select the appropriate material. The initial
temperature of the fluid can increase by up to 50%
depending on the above operating parameters. For all
applications please refer to the recommendations in the
various chapters and don’t hesitate to contact your local
Busak+Shamban company if any doubt remains.
Hardware
In all chapters the hardware parameters are described in
relation to the profile and material types.
Nevertheless one general rule is valid for all rotary seals:
the mating surface (shaft surface) must be free of spiral
grinding marks as they may cause pumping effects and
leakage. Plunge grinding is to be the preferred machining
method for the shaft.
Wear on the shaft in the contact area of the sealing lip is
the most frequent failure mode the shaft seal users can
meet with. It is mainly the result of unexpected metal
particles carried by the fluid to the sealing lip. The particles
are embedded in elastomer materials that act as a
millstone and grind grooves into the shaft. To avoid such
failure modes either particles must be kept away from the
sealing lip or the surface roughness must be adjusted to be
able to accommodate them. Therefore it requires high
surface hardness. Busak+Shamban recommend a minimum
hardness of 55 HRC on minimum 0.3 mm depth. Other
material combinations are possible depending on the
contamination in the system. See recommendations in
the chapter “Environment“.
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Edition April 2006