CARBON SEAL FACE IN A METAL HOLDER
Carbon Graphite is probably the best seal face you can use in most
of your water and chemical applications. Its corrosion resistance and
natural lubricity provides two of the features we need in a good seal
face material. The question sometimes comes up about how to insert
the carbon into the mechanical seal. Should it be captured into some
type of a metal holder or would you be better off using a
"monolithic" or solid version?
And to make it even more confusing, the top seal companies use
both versions interchangeably. Let's see if we can make any sense out
of the subject
Here is a picture of a carbon/graphite seal face
sometimes called a "monolithic" face
This is a picture of a carbon/graphite face inserted into
a metal holder. This design is often called a
Although monolithic faces have become popular in recent years,
there are some real advantages in using a carbon/metal holder
composite over a solid carbon/graphite face:
- You can use a smaller cross section carbon that, when
impregnated, tends to be more homogenous eliminating unwanted air
pockets and making it a better conductor of heat.
- Carbon is stronger in compression than it is in tension. The
metal holder keeps the carbon face in compression
- The metal holder makes a good "heat sink' to carry unwanted
heat away from the seal face.
- Dynamic O-rings and other elastomer shapes slide on a metal
holder easier than they do on carbon. When the shaft is not
turning the elastomer tends to creep into the porosity of the
carbon causing a higher "breakaway friction" at start up that can
translate into the lapped seal faces opening.
- Metal anti-rotation pins and slots work best when metal is
The are some disadvantages to a carbon/metal composite:
- At elevated temperatures the differences in thermal expansion
between the two materials can cause the carbon to loosen in the
holder and leak or spin. Low expansion metals such as Carpenter 42
and Invar 36 are available for those applications.
- There are problems keeping the carbon face flat after it has
been installed in the metal holder. Differential expansion is just
one of these problems. The stresses caused by inserting the carbon
into the metal holder can cause it to go "out of flat".
- Sometimes a filler material is inserted behind the carbon face
between it and the metal holder to prevent pressure from
penetrating in vacuum and reverse pressure applications. You have
to be careful of the corrosion resistance of this filler material.
Also be aware that many of these filler materials are poor
conductors of heat.
There are at least three ways to insert the carbon into the metal
- Glue it in.
- Shrink it in.
- Press it in.
Let's look at each of the methods:
- Gluing is never acceptable. The fluid you are sealing could
attack the glue and the glue will act as an insulator that will
prevent the conduction of heat generated at the seal faces, back
to the metal holder.
- Heating the metal holder, dropping in the carbon and letting
the metal holder grab the carbon as it cools, causes all kinds of
stress problems that will allow the lapped carbon face to go "out
of flat". The out of roundness of the carbon outside diameter does
not match the out of roundness of the metal holder. This means
that the metal holder will engage the carbon at high spots and
introduce stress into the lapped face. This insertion method is
still used by some seal manufacturers causing them serious
- Pressing the carbon into a metal holder with an arbor press
causes the carbon to shear and conform to the "out of roundness"
of the metal part. The stress is greatly reduced and this is
without question the best method if you intend to use a carbon /
There does not appear to be a straightforward answer to this
composite / monolithic subject so seal designers choose one or the
other to simplify or lower the cost the of their particular seal
design. Metal bellows designs almost exclusively use the composite
version because of problems attaching a monolithic face to a welded
Now you know about the advantages and disadvantages of both
designs for those few times you get to make a choice.
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