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In the development of new
and improved grades of car- bon for sealing rings not only
the required
material proper – ties, but
also the question of cost had to be taken into account,
particularly for sealing rings in low- priced mass produced seals.
carbon/graphite and
electro-graphite grades, carbon/graphite and electro
graphite grades with various synthetic resin and metal
impregnations to high strength electro graphite grades with
special impregnations to improve the oxidation resistance or
the dry running capacity.
The properties of synthetic
resin- bonded carbon grades have been improved considerably compared to synthetic resin
molding compounds
containing carbon.
These grades are
particularly suitable for the pressing-to-size of rings,
even in fairly complicate-
Ed designs, for
mass-produced seals.
In addition, a range of
carbon/graphite grades with the above mentioned impreg-nations
are available which are also suitable for the
pres-sing-to-size or partially ores-sing-to-size for
mass-produced seals.
Mechanical seals are mainly
used for sealing between liquids and gases.
It should be noted that,
with carbon seal rings, even liquids with low hydrodynamic
lubricating capacity provide sufficient lubricating
effect.
The sealing of gases and the
dry running that arises from this is possible at low sliding
speeds with carbon sealing rings e.g. in agitator seals,
provided that the wear rate is sufficiently low.
For the sealing of gases at
high running speeds the use of carbon seal rings in so –
called gas seals is also common, provided that the design
of the seal ensures that contact between the sliding
During normal running, the
gas pressure ensures contact-free operation of the sliding
surfaces.
Sealing between gases is
carried out otherwise with double-acting mechanical seals
and a sealing liquid, the sealing as a lubricant for the
sliding faces and for the dissipation of the frictional
heat.
1
- Grade selection
It must be said
that it is impossible to cover all service conditions with
one carbon/graphite material.
2-
Application limits
Running
speed: 70 m/sec max.
Pressure
difference: 160 bar max.
Sliding
pressure: 10-200 N/cm2
Generally<50 N/cm2
Product of
pressure
And
speed: pxv
max
= 1250 N/cm2 x m/sec
3-
Pressure tightness of carbon sealing rings
Sliding rings made of redensified or impregnated
carbon/graphite material are impervious to liquids and gas.
the pressure tightness of sliding ging is
inspected by means of a hydraulic pressure test, from the
inside of the ring with water, or under water using
nitrogen.
Large series are inspected on a statistical basis.
Usual test pressures: 3, 5 or 10 bar
For special applications test pressures up to 20
bar are possible.
The same applies to ores-in and shrink-in fist.
Here, the following criteria are important
Here it is important to maintain tight dimensional
tolerances particularly shape tolerances such as concentricity
and conicity of both the bore and outside diameter.
Press in fits: H7/s6
Shrink in fits: H7/x6
– x7
H7/z6
– z7
Because of the changes in shape that occur during
shrinking in, the flatness of the sliding surfaces
can only be achieved by refashioning after
shrinking in.
Due to the lower shrinking stress at working
temperature, compared to room temperature, the sliding
surface is no longer as flat over its whole width at working
temperature as it is at room temperature,
Resulting in a certain leakage until the running
in of the sliding surfaces.
4-
Installation of carbon sealing rings
Usually, carbon
sliding rings are installed in a push-fit seating over
O-rings and in rubber
Or plastic
sleeves, ant rotation locking being provided for in each
case.
Adhesive bonding
is customary for installation in metal holders or metal
bellows.
The adhesive must
be suited to the chemical and thermal require-ments.
Special attention
must be given to the pressure tight-ness of the joint.
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