Function of the Heavy Duty Face Seals
The two halves of the seal are mounted in separate housings
facing one another. The housing's bore, which locates the
O-ring is tapered with the taper increasing with the depth
of the bore. The cast iron Seal Ring also has taper on its
outside diameter to locate the O-ring. This area is known
as the ramp.
When a Seal Ring and an O-ring are axially compressed in the
housing the O-ring deforms between the two tapers producing
a radial force as well as an axial one. The axial component
of this force can be calculated and it is this force, known
as "Face Pressure" that is most important for proper
The elastomeric O-rings serve three functions:
It is important to understand that only one of the seal halves rotates.
The interface between the rotating and non-rotating half are the finely lapped faces of the seal. The Seal Ring should never rotate relative to the O-ring nor should the O-ring rotate relative to the housing.
In order to obtain a "leak free" seal the contact band of the faces are lapped with a surface finish of R max 1.8 micrometers. The tapered portion of the face forms a 1-3mm tapered gap at the ID of the seal set.
The purpose of this tapered gap is to allow lubricant to feed the sealing faces. It also enables new faces to be continu ously formed as wear takes place. This ability to form new faces considerably enhances the service life of the seal. The life of a seal is determined by the time it takes the sealing band to move from the OD to the ID of the face. Special care should be taken not to damage the face area of the seal set.
Face Pressure and Gap Setting
The angle of the ramp on the Seal Ring is always greater
than the angle of the taper in the housing bore. This
prevents the O-ring from being squeezed to the back of the
housing and exerting excessive face pressure on the Seal
Face pressure is extremely important to the proper function of the seal and is directly related to the "Gap Setting". Just as in compressing a spring, the required force increases in proportion to the amount of compression. As well, the force begins to increase more rapidly as maximum compression is reached.
The elastic quality of the O-ring insures that
small changes in the Gap Setting result in even smaller
changes in face pressure. The minimum face pressure is
approximately 2.0 kp/cm2 which in this example corresponds to
a Gap Setting of approximately 6.4mm. This is how the tolerance
of the Gap Setting is determined. Reducing the Gap
Setting to less than 3mm is not desirable, as the face
pressure will begin to rise rapidly.
A constant supply of lubricant must be supplied to the
faces of the seals in order for them to function properly.
The lubrication serves two extremely important functions.
It reduces the friction between the seal faces to allow for
free rotation and it serves as a cooling agent for the seal
faces. In fact, the lubricant serves to cool the entire
seal/housing area and it is often for this specific
reason that engineers choose Heavy Duty Face Seals
over conventional sealing systems for continuous load
Heavy Duty Face Seals can be run at peripheral speeds
of up to 10m/sec. Some alteration to the Gap Setting
may be necessary at higher speeds. It is necessary to
use oil lubrication at speeds in excess of 3m/sec.
Below this speed grease can be used, although oil is
Lubrication by grease guns should be avoided because excessive internal pressures may lead to misalignment of either the seal face or the O-ring leading to rapid failure.
Figure 4 shows the limiting speeds below which grease lubrication may be used, and above which oil lubrication must be used.
The standard Heavy Duty Face Seal can withstand a
positive hydraulic pressure of up to 42.5 psi (3 bar)
dynamic and 85 psi (6 bar) static. If necessary,
higher pressures can be achieved but this normally
means resorting to special seal designs to equalize
Installation of Heavy Duty Face Seals
Heavy Duty Face Seals are normally considered to be a
set, but actually they are 4 separate parts which
will, if installed correctly, move independently to
each other. To properly install the seal set always
follow these instructions.
Care should be taken, at all times, when handling these seals. The Seal Rings are made of an extremely hard white iron alloy engineered specifically for wearability and corrosion resistance. As a result the rings are very brittle and need to be handled with care. They should be regarded as precision elements and the lapped faces protected at all times. They can also be very sharp on the diameters of the lapped surfaces, so gloves should be worn when handling.
The housing profile forms a "lip" on the installation radius that is necessary to hold the seal half in position prior to drawing the faces together during assembly of the unit. This lip resents an interference between the outer diameter of the O-ring and the installation radius on the housing.
The natural reaction is to simply push on the metal ring in an attempt to force the O-ring past this lip. Because of the taper on the outside diameter of the Seal Ring the O-ring rolls up this ramp, further increasing the interference, until it becomes impossible to install. At this point fitters generally reach for the oil can to lubricate the O-ring and a screw driver to force the rubber into the housing around the lip. WARNING: Use of oils or sharp instruments can cause permanent damage to the seal which can lead to rapid failure!
Figure 5 shows what can happen to the seal if it is installed in this way. Most often the O-ring will be "looped" on the Seal Ring causing uneven face pressure and premature failure. Occasionally the O-ring will be pushed to the very back of the housing or off the lip of the Seal Ring. In this case, when the seal set is assembled and face load is applied, the result would be that the Seal Ring's lip would cut into the O-ring causing failure.
If the O-ring is assembled using oils, the O-ring may revolve relative to the housing or the Seal Ring causing rapid wear to take place on the rubber.
In the above examples the assembled units may pass leak tests but would fail very quickly once the machine is working.
To avoid these problems- an installation tool or spring must
be used. The installation tool consists of a collar made
from an impact resistant material which is profiled to suit
a particular seal. The tool "clips" over the seal face,
protecting the lapped surface and resting on the O-ring.
When force is applied to the tool it bears only on the
O-ring which deforms and slips into the housing carrying the
seal ring with it. After installation, the tool is removed.
The only limitations to using a fitting tool is when the tool cannot be withdrawn after installation, or because the seal is a special design. In this situation an installation spring can be used. The spring is a metal, closed loop spring which is placed between the O-ring and the rear of the face flange. When force is applied to the Seal Ring during installation the spring "backs up" the O-ring and doesn't allow it to roll up the ramp. After installation, the spring is easily removed.
After the two seal halves are in the housings, the faces should be thoroughly cleaned and a thin film of oil (SAE 30-40) applied to only the faces. The seal set is now ready to be drawn together in the assembled unit.
For trouble free function of the seal, adequate lubrication
of the sealing faces is necessary at all times. Most seals
will "weep" a little oil during their first hour of
operation until the faces have ôrun-in". A small amount of
oil should be visible on the outer diameter of the seal at
all times when it is working. This is not a leak, but only
the seal faces being properly lubricated. If a seal is
removed or the faces are separated after the run-in period, it
should normally be replaced with a new set.
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