One of my little jokes about pumps and seals is that you only have to know two numbers:  1/8 inch and 0.002 inch.  Well, at least I think it’s funny anyway.  When I was younger and had better vision, I could not understand why certain limits were based on 0.002 inch when 0.001 inch increments could be seen so clearly.  Many years later, I can better understand the 0.002 inch criteria.

It turns out that the 0.002 inch criteria may actually have some basis from lab tests of long ago.  In the early 1950s, the question of seal reliability vs shaft deflection was a hotly debated topic.  The book “People, Products and Progress:  The Durametallic Story” by A. H. Miller relates how Durametallic addressed the question: 

“The Development Committee directed the Research Department to run a series of shaft deflection tests.  After much thought, a regular tester was modified and a shaft which extended 2-1/2 to 3 feet beyond the seal cavity was installed, allowing the shaft to be deflected several thousandths of an inch in any direction.

“… those tests did demonstrate that any amount of shaft deflection which exceeded 0.003 inch at the seal face decreased the effectiveness of the pump and seal.  The pump companies evidently accepted the results of those tests, because as they developed new pump lines, the amount of shaft support provided in them was increased.”

Although no details were given, this anecdote is perhaps the origin of the 0.002 inch maximum allowable deflection at the seal faces and probably made its way into other rules-of-thumb as well.  Just think about all the publications and training programs that emphasize checking runouts and wanting them to be less than 0.002 inch.