SUBJECT: The rubber bellows seal
Available from a number of seal companies (the Crane #1 is
typical), and manufactured in a variety of materials, it looks like
the following illustration:
You will find this seal used in water pumps and oil pumps, but it
is seldom used with chemicals in the process industry. In recent
years one manufacturer is mounting the seal on a cartridge sleeve and
promoting it as a slurry seal.
To the casual observer the seal appears to
have several real advantages:
- Low cost. It is probably the largest mass produced seal in the
- No sliding elastomer or O-ring. Nothing to "hang up" on the
- Easy to install. There are seldom any measurements to make or
set screws to tighten to the shaft. Most designs are positioned
against a shaft shoulder, or attached to a shoulder cast into the
back of the impeller. Push it on the shoulder, and that is all
there is to it !
- Availability. You can purchase these seals from a variety of
manufacturers, distributors and bearing houses. Most distributors
have replacement charts that will reference your present seal and
recommend an equivalent design.
- No shaft or sleeve fretting. When installed properly the seal
cannot frett or damage a shaft or sleeve because there is no
relative movement between the rubber boot and shaft or
Let's look at each of these features in detail and see if they
really are an advantage:
- The seal is low cost only if you purchase it manufactured from
brass metal with a low grade carbon face and a Buna N rubber
- The Crane company advertises they use 76 different grades of
carbon in their seals and this is the seal that uses most of them.
The problem surfaces with replacement seals. No reputable
distributor can afford to stock 76 carbons so he often inventories
the seal with a better grade of carbon, 316 stainless steel parts
and possibly a fluroelastomer bellows. This makes the low cost
original equipment seal (O.E.M.) an expensive replacement
- Once the seal is cartridge mounted, and balanced versions of
this seal require it, the seal is no longer low cost.
- No sliding elastomer.
- The rubber boot must transmit the turning torque from the
shaft to the seal face and provide enough flexibility to
compensate for axial motion and carbon face wear. This means that
the bellows must stick to the shaft, so the lubricant you choose
to install the seal becomes critical.
- Buna N is one of the few rubber compounds that has a shelf
life because it is sensitive to ozone attack. The seal is packaged
with the rubber boot wrapped in waxed craft paper to retard ozone
attack. Once the package is opened you have about one year shelf
- O-ring seals offer a wide choice of elastomer materials that
are readily available at low cost. The rubber boot choice of
materials is very limited.
Easy to install
- Pushing the seal against a shoulder sounds very attractive,
but it a real installation problem. Looking again at the
illustration you can see that the rubber bellows location is
critical to the operation of the seal. The mechanic can position
the seal spring, but he has no control over the rubber bellows
which can be either extended or compressed with respect to the
- The lubricant used to install the seal must make the rubber
slippery enough to slide easily on the shaft and yet stick to the
shaft to provide the turning torque to the rotating seal face. A
lubricant that will attack the rubber bellows is selected for
this. It will make the rubber "slimy" for about fifteen minutes
and then cause the rubber to "swell up" and lock to the shaft.
This means you have about fifteen minutes to assemble the seal
into the stuffing box and tighten up the seal gland, which is just
about impossible on a double ended pump. That is the reason you
find so many of these seals sleeve mounted. Silicone grease is the
first choice for lubricating rubber parts in other seals, but it
must never be used in this design, because it will not attack the
- If the old seal was installed correctly it vulcanized to the
shaft. This means that the seal has to be physically scrapped off
the shaft or sleeve before a new seal can be installed. It is very
common for mechanics to polish the cleaned up shaft with crocus
cloth or fine emery paper, but this must be avoided because if the
shaft or sleeve is too smooth the new rubber boot will not stick.
You want a finish of no better than 40 rms, which is very
different than the requirement of at least 32 rms (0,8 microns)
used in other seal designs.
- Because of the great variety of materials used in this design,
and because many of the replacement seals are sold by non
professionals, it is easy to mix up the seal materials. The print
that came with the seal shows part numbers and not grades of
material. Call up a local distributor of these seals and he will
often ask you to bring one of the seals over so they can pick out
a seal that looks like it. Most distributors cannot identify the
seal materials because there are many different grades of
stainless steel, carbon/graphite/, ceramic, Buna N, neoprene,
- If you experience damage or fretting on the shaft or sleeve
under the rubber boot it means that the boot did not attach its
self to the shaft. The faces had stuck together and you were
sealing between the stationary rubber boot and the rotating shaft.
You had used the wrong lubricant when the boot was installed on
the shaft or sleeve and the rubber boot is acting like a "grease
or lip" seal.
Here are some additional things you
should be aware of:
- The carbon is often a loose fit in the metal holder. In many
designs the carbon can be installed backwards and often is. The
problem is caused by the seal packaging method. Many manufacturers
will install the carbon backwards in the holder to protect the
lapped face and then "bubble packed" the assembly to save
packaging costs. The never tell you to turn the carbon face around
at assembly. Mechanics usually install hardware the way it came
out of the box. There is nothing in their experience to tell them
to do it differently.
- The most common failure with this seal is to rupture the
rubber bellows and experience a sudden and massive bellows
failure. Well designed, balanced, O-ring seals tend to drip
excessively when they begin to fail. This bellows design leaks
massively at failure. Very scary!
- Buna N has a high temperature limit of about 210°F
(100°C) meaning that the seal can be injured if hot water or
steam is used to clean the lines. There are much better elastomers
on the market for a wide range of chemical compatibility and
is my personal opinion of this type of product ? It is probably the
best original equipment seal (O.E.M.) made and the worst replacement
seal ever designed. I do not like it for the following
- The normal failure mode is dangerous. A rubber bellows rupture
is a very severe seal failure.
- The carbon seal face is thermally insulated by the rubber
boot. This is never a good idea when you are trying to remove heat
between the seal faces.
- The special lubricant required to install the seal makes
installation different than other seals. Any time something is
different, errors occur.
- The rougher shaft finish requirement makes installation
- The Buna N boot is sensitive to ozone attack once the package
is opened, and people like to open packages. Shops often have high
levels of ozone caused by the sparking of electric motors.
- The carbon face can be put in backwards. If it can be, it will
- The seal is not usually hydraulically balanced, limiting its
pressure and speed capability.
- Unless the seal is on a cartridge you cannot make the initial
impeller setting and other impeller adjustments when using open
- The seal is limited to a replacement part, and with the great
variety of materials specified you will have too many replacement
seals in your inventory. With just a few exceptions, you should be
able to use he same seal in every pump of the same shaft size.
Imagine what a difference that would make in your inventory costs
and spare parts availability.
- Your spare seals should be acceptable for packing replacement
as well as a replacement part for an existing application. This
seal is limited to replacement only.
- The stainless steel spring is a major component of this seal.
Stainless steel springs are not recommended in mechanical seals
because of the possibility of chloride stress corrosion problems
that can break the spring.
If you are willing to invest in a high priced, balanced design,
cartridge mounted version of this seal; then there is something to be
said for its performance in slurry applications where you are trying
to cut down on water flushing.
Given the choice you will probably be better off with the
stationary version, but you will still have to contend with all of
the points mentioned above.
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