SUBJECT : Reading seal face flatness 6-3

There is often confusion between the terms "Seal face flatness" and "Seal face surface finish".

Seal face surface finish addresses the subject of roughness, and is measured in terms of "rms" ( root mean square) or CLA (center line average). There are a couple of ways to make the measurement:

Flatness is a different term that describes a level surface that has no elevations or depressions. We use terms like waviness, or concave and convex surfaces to describe the condition when we refer to mechanical seal faces. It is this flatness that is of the most concern to us because testing has shown that if the faces are separated by a space of about two microns or more, the seal faces will show visible leakage, and depending upon the separation, let solids penetrate that might score or in some way injure these lapped faces..

There are several ways you could measure flatness :

To understand this last method of measurement you only have to know that it is a characteristic of light that when two lights of the same wave length interfere with each other, the light disappears and the reflecting piece goes black. When you discuss visible light, color and wave length mean the same thing, so to make the measurement we use :

The optical flat is placed on the piece to be measured. The monochromatic light is aimed at the piece and this light reflects off of the piece back through the optical flat causing interference light bands. If the distance between the optical flat and the piece we are measuring is one half the wave length of helium, or an even multiple of the number, the band will show black. This is referred to as a helium light band and because it is one half the wave length of helium it measures 0,3 microns or 0.0000116 inches.

To understand this measurement I might mention that the smallest object that can be seen with the human eye is forty (40) microns. Another way to understand this measurement is to know that the average coffee filter is in the range of ten to fifteen (10 to 15) microns. Sophisticated seal people know that this means that solids cannot penetrate between the seal faces unless they open.

We check the flatness of our seal face by comparing the pattern we see to a chart that is supplied by the measuring equipment manufacturer. You can find a copy of these patterns in the chart section of this web page. The paper is labeled Seal face flatness readings

These charts were supplied by:

Surface Finishes Co. Inc.

39 Official Rd. Addison, Illinois, 60101-4592 U.S.A.

There are some things that you should know about flatness readings :

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