Surface finish


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 R.M.S. (root mean square) or C.L.A. (center line average). There are a couple of ways to make the measurement:

  • We can do it manually by comparing our sample to metal standards that have been polished to different degrees of roughness.
  • You can place the sample in piece of equipment that projects a blown up version of the sample on a screen and measures the roughness by a built in scale. This equipment is called a profilometer.
  • You can use an instrument that drags a sensitive probe across the piece and measures finish in that manner.

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:

  • You could place a straight edge on the surface and look for daylight between the straight edge and the sample. As you would guess this method is not accurate enough for our purposes.
  • You could place “machinist’s bluing dye” on a known flat, rub the sample piece against it and look for transfer of the dye. Again this method would not be accurate enough for our purposes.
  • You could read the flatness by using an optical flat and a monochromatic light source, and this is the method that is used by all of us in the sealing industry.

Optical flat, O014

Monochromatic light, M027


  • On February 17, 2018