MadSci Network: Engineering

Re: How does steel purity affect the fatigue life of a bearing?

Date: Mon May 1 16:29:32 2000
Posted By: Bob Gibilisco, Staff, Process Engineering, Retired
Area of science: Engineering
ID: 955124756.Eg

I can appreciate your dilemma because I have "been there, done that". By
that I mean dealing with the issue of variation. Two adages come to mind:
"If you cannot find anything wrong with something, you are not looking close
enough" and  "One cannot measure quality, only the lack thereof".
In this discussion, purity and cleanliness are synonymous.

The lack of cleanliness is defined as the level (distribution and size) of
foreign material within the steel measured by a standard method of
detection. For your application, the presence of foreign material in the
steel certainly affects fatigue life and several other mechanical
characteristics. However, there are other properties that can affect
premature failure. Among a few: hardness, grain size and chemistry.
In order to determine the cleanliness of steel, one must know the
specification the steel must meet to be acceptable. For example, a bearing
made for an aircraft application must comply with a higher standard of
cleanliness compared to a bearing made for a packaging machine. This will
involve extensive melting and re-melting operations to remove impurities and
also more extensive testing. It is no surprise that a bearing for the 
aircraft specification will cost more. When you perform your product
function deployment, economics is a factor that should be evaluated.

Lets talk about testing for a moment. There are two types of tests that are
performed to evaluate cleanliness:
Destructive - Samples are removed from the steel bar (Normally the ends of
the bar), polished and examined with a microscope. The variable involved is
the magnification used.
Non-Destructive - One method is to scan the bar using an ultrasonic flaw
detector. The variables involved are inspection procedure, transducer
frequency, scan type and scan speed.

The criteria for testing are documented in the purchase contract. The
standards can include SAE, AISI, ASTM, NDT, customer requirements, internal
manufacturing requirements or in the case of aircraft applications NASA
standards. Material that does not meet the testing requirements is diverted
or scrapped.

Variation in the purity of the steel is affected mostly by the process
performance during melting and re-melting operations. Process performance
can be measured using statistical methods. A measure of confidence can be
ascertained by knowing if the steel supplier maintains registration with
ISO-9000. During an ISO audit, compliance to documented standards and
procedures is evaluated. Evidence of product characteristic measurements,
calibration of measuring equipment, training of manufacturing personnel are
just some of the areas that are evaluated.

So Jon, it becomes necessary to know something about the steel supplier to
rate the quality of the raw material going into the bearing. This also
applies to the bearing supplier too since purity is only one factor that
affects reliability.

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