SELF ALIGNING SEAL FACES S032
We would like to have both the rotating and stationary seal faces square to, or perpendicular to the rotating shaft, but that is almost impossible:
- If we set screw one of the seal parts to the shaft it will “tilt” as the setscrew tightens.
- If the stationary face is positioned in a gland that is tightened against the face of the stuffing box it will cock because the stuffing box face is not square to the shaft.
- If you insured squareness of both faces by using a dial indicator and taking the time to do it correctly, they will still tilt when the pump comes up to temperature because of thermal expansion, pipe strain, etc.
In the following illustrations I will show a rotating face on a cartridge sleeve to explain the problem and show some possible solutions. If we look at the detail of the rotating face on the cocked sleeve we will see:
- The first is the API (American Petroleum Institute) method. They recommend a tolerance of 0.001″ to 0.003″ on the diameter between the pump shaft and the seal sleeve to prevent the sleeve from cocking when the setscrews are tightened. The metric numbers are 0.025 to 0.075 on the diameter.
|This line drawing describes the second method.
A design where both the stationary and rotating faces are spring loaded. In this design you are running a rotating seal against a stationary seal.
|This sketch describes the third method that uses a double O-ring for keeping the rotating face perpendicular or square to the shaft|
The fourth method is shown in the next drawing. It describes a three point contact similar to what you would find on a three jaw chuck used on a lathe or drill press. This arrangement is called a “cloverleaf” design by one of the major seal manufacturers.
- Three setscrews positioned at 120 degrees apart deform the sleeve to the shaft outside diameter to insure squareness of the rotating face.
- An additional three setscrews go through the sleeve and lock the sleeve to the shaft. These setscrews are positioned 120 degrees apart and are located between the setscrews that are centering the sleeve to the shaft.
The A.P.I. (American Petroleum Institute) version:
- The tight tolerances required to get the “slip fit” are expensive.
- There is wide variance in the tolerance used on the outside diameter of conventional pump shafts. If you adopted this method to get “squareness” you would have to rework or replace many of your existing shafts or shaft sleeves.
- Close fitting shaft sleeves are difficult to remove. The necessary heating and banging will almost guarantee a bearing replacement along with the new seal.
Spring loading both faces:
- Centrifugal force is working for you. The greater the centrifugal force, the stiffer or more stable the system.
- The centering of both faces is critical. If the hydraulic balance lines are not exact the faces could cock. This is a difficult problem to over come.
- Building two spring-loaded faces is expensive. You are actually running a stationary seal against a rotating seal
The double O-ring system:
- This design requires a lot of axial space. When ever possible you will want to get the seal faces as close as possible to the pump’s inside or radial bearing.
The three point contact method:
- At this writing this is the lowest cost of the four solutions.
- This design takes a very short axial length, making the cartridge assembly no longer than a conventional cartridge design.
- Replacing the seal components is low cost and easy with this design.
The cartridge mounted stationary seal doesn’t make any sense unless you are using one of the above solutions, or some other comparable design that corrects the problem of “cocking or tilting” the rotating seal face.
If a seal salesman approaches you with some other method to insure squareness, have him attach his design to a rotatable shaft and measure the rotating face squareness with a dial indicator. It will either be square or it will not, you can tell quickly.