Heavy Duty Face Seals - Reference Manual

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  1. Introduction
  2. Function of the Heavy Duty Face Seals
  3. Face Pressure and Gap Settings
  4. Lubrication
  5. Mechanical Limits
  6. Pressure Limitations
  7. Installation of Heavy Duty Face Seals
  8. Maintenance

Introduction

Heavy Duty Face Seals, also known as ©Duo-Cone, Cat, or Toric Seals, consist of two identical elements. Each element consists of a cast iron Seal Ring which acts as the primary seal and an elastomeric (rubber) O-ring, also known as a toric, which acts as an energizer as well as a secondary (static) seal. These types of seals are designed to be installed in specially machined housings and are normally bathed in oil.

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.
[Figure 1]
FIGURE 1		 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 operation.

The elastomeric O-rings serve three functions:

  1. They generate axial face pressure because of their elasticity.
  2. They act as a static seal between the outer diameter of the Seal Ring and the inner diameter of the housing.
  3. They transmit torque from the rotating half of the assembly through the faces to the static half.

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

[Figure 2]
FIGURE 2		 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 Rings.

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.


Figure 3 shows the load/deflection graph for a 150mm diameter seal with a 8.5mm diameter O-ring. The ideal working face pressure is about 5.5kp/cm2 - The standard Gap Setting is 3mm

[Figure 3]
FIGURE 3

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.

Lubrication

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 applications.

Mechanical Limits

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 always preferable.

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.

[Figure 4]
FIGURE 4

Pressure Limitations

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 the pressure.

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]
FIGURE 5

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.
[Figure 7]
FIGURE 7		 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.

 

Heavy Duty Face Seals with O-Rings

  1. Only remove from original wrapping immediately before installation.
  2. Thoroughly de-grease housing surfaces that will contact the rubber rings of the seal with a clean solvent. Wipe dry.
  3. Position installation tool, or installation spring. Take care not to damage seal face.
  4. Replace the rubber toric on the metal ring, making sure it has not twisted and is seated correctly.
  5. Place the seal half evenly on the housing and apply even pressure. (See Fig.1)
  6. The seal will slip into the housing.
  7. Remove the fitting tool.
  8. The seal face should now sit parallel with the housing. The rubber should not be sitting wave-like or bulged out of the bore. (See Fig. 2)
  9. Wipe clean the seal face with a lint free cloth and apply a thin film of clean oil (SAE 30-40) to sealing face. Oil must not wet any other surface.
[Figure 6.1]

 

Heavy Duty Face Seals with Trapezoidal Rings

  1. Only remove from original wrapping immediately before installation.
  2. Thoroughly clean and de-grease the seal and housing that will contact the rubbers with a clean solvent. Wipe dry.
  3. Check that the rubbers are flush against the shoulder on the metal seal ring.
  4. Install each seal half into the housing.
  5. Check that the rubber is seated evenly in the bottom of the bore. (See Fig. 3)
  6. Wipe both metal seal faces clean with lint free cloth and apply a thin film of clean oil (SAE 30-40). Oil must not wet any other surface.
[Figure 6.2]

Maintenance

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 “run-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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