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The Effect of Pressure
on Closed System Lubricators
The purpose of this technical information
sheet is to make end-users aware of the effect of pressure
on a closed system lubricator on a sealed bearing housing
that is not equipped with a Trico Expansion Chamber.
Trico offers two closed system lubricators,
the Closed System Opto-Matic® and the WATCHDOG®.
These lubricators should be used in environments that are
contaminated with high humidity and/or airborne particulate,
but may also be used to ensure maximum oil purity in indoor
or clean environments. They help answer the question, "How
can I improve the purity of my equipment's lubrication?".
Both of Trico's offerings operate on the constant level
lubrication principle. The most important difference between
these products and vented constant level lubricators is
that the air required to cause feeding when the oil level
drops, comes from the pressure balancing line connected
to the bearing housing.
Testing conducted at Trico has shown that
both the Closed System Opto-Matic® and the WATCHDOG®
will feed oil when the pressure differential between the
air trapped in the top of the oil reservoir and the air
in the bearing housing is greater or equal to ½ psi.
This feeding is cyclic and will eventually stabilize, but
the overall effect will be a higher oil level in the bearing
housing than was originally intended. The dynamics of this
cyclic feeding is as follows:
Figure 1
Figure2
Figure 3
As the bearing housing goes from ambient (starting)
temperature to steady state running temperature, a pressure
rise will occur as the air molecules move faster. As the
pressure rises, the air trapped in the top of the oil reservoir
begins to compress and you will see a slight rise in the
oil level in the reservoir (Fig. 1). As the pressure approaches
½ psi, the oil in the housing has been lowered enough
for air to be introduced through the control point of the
lubricator to cause a small amount of oil to feed. The control
point is where the spout of the oil reservoir contacts the
column of oil. The vertical location of this point dictates
the oil level in the bearing housing (Fig. 2). After feeding
has occurred, the oil level in the reservoir is at its starting
level. This cyclic feeding will continue as the pressure
rise in the bearing housing continues. When the equipment
has been shut down and begins to cool, the pressure in the
bearing housing will decrease and the compressed air in
the top of the lubricator's reservoir will force more oil
into the bearing housing (Fig. 3).
There are three styles of shaft-to-housing
closures that make up the vast majority of bearing housing
seals. They are as follows:
Lip Seals - Lip seals are typically
made of an elastomer and have a circular spring that keeps
the elastomer pressed against the shaft to prevent the entry
of dirt and moisture. Testing conducted at Trico has shown
that some lip seals will hold pressure less than or equal
to 0.4 psi when the shaft is not rotating. The pressure
holding ability of a lip seal should be expected to deteriorate
as the elastomer of the seal wears from contact friction
with the shaft. Consult the manufacturer for the pressure
holding ability of a specific seal.
Labyrinth Seals - Labyrinth seals
consist of two mating parts that form an intricate structure
of interconnecting passes through which it is difficult
for dirt and moisture to find its way to the bearing housing.
This is only true while equipment is rotating due to the
centrifugal forces created by the shaft. While static however,
a labyrinth seal may permit dirt and moisture to enter the
bearing housing. Consult the manufacturer for the pressure
holding ability of a specific seal.
Lip/Labyrinth Hybrids - Lip/Labyrinth
hybrids have gained in popularity. These seals combine the
benefits of both the lip and labyrinth design. The lip seals
upon the stator, in both static and dynamic applications,
hermetically seal the bearing housing, thus preventing
contamination from entering into it. Consult the manufacturer
for the pressure holding ability of a specific seal.
The type of seal used on a bearing housing
will play an important role in determining what closed system
lubricator setup is right for your application. A properly
sized expansion chamber will eliminate the pressure differential
between the lubricator and the bearing housing. See Technical
Information Sheet “Selection of expansion chamber”
for further information.