SUBJECT: Stationary cartridge seals. How to solve the problem of seal movement 11-10
Everyone agrees that cartridge seals are the only way to go. The good news about cartridges includes:
But then there is the bad news, and in this instance the bad news is "really bad"!
In another paper I discussed the advantages of using stationary seals and you will recall that their advantage rested upon the seal designer's ability to position the rotating face perpendicular or square to the shaft.
As long as the rotating face is square, or perpendicular to the rotating shaft there will be no "back and forth" axial movement of the stationary face.
Please take a look at the following sketch. Here you can see that the rotating face is positioned square to the shaft because of the clamping arrangement of the rotating face. The clamp is manufactured on a mandrel and the faces are machined perpendicular to the mandrel, making them in turn, perpendicular to the rotating shaft.
If the rotating face had been set screwed to the shaft it would have "cocked" the rotating face relative to the shaft. This would cause the stationary face to move "back and forth" twice per revolution of the shaft, causing the same problems we experience with the rotating version of a mechanical seal.
The seal movement problem starts when we try to put this rotating face on a cartridge sleeve. Take a look at the next line drawing:
The drawing is exaggerated to emphasize the point. As you tighten the sleeve set screws to the shaft the sleeve will "cock or tilt" and, although the rotating face stays square to the sleeve, it is no longer square to the rotating shaft. This will cause the stationary seal to act like a rotating seal and you lose all of the advantages you gained with a stationary design.
If we look at the detail of the stationary face on the cocked sleeve we will see:
There are at least four ways to solve this "tilting" problem and prevent the "back and forth" axial movement described above:
1. The A.P.I. (American Petroleum Institute) recommends a tight tolerance fit between the pump shaft and the seal sleeve to prevent the sleeve from cocking when the set screws are tightened.
2. The next line drawing describes 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.
3. The following sketch describes the double O-ring method for keeping the rotating face square to the shaft.
4. The next drawing 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.
Now that you know at least four techniques to position the rotating seal face square to the shaft, the question becomes which of any of them is the best?
#1 The A.P.I. (American Petroleum Institute) version:
#2 Two spring loading both faces:
#3 The double O-ring system:
#4 The three point contact method:
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 you are approached by a seal salesman with some other method to insure squareness, have them attach their design to a shaft and measure the squareness with a dial indicator. It will either be square or it will not, you can tell quickly.
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