Page 272 - Intro Predictive Maintenance

Visual Inspection 263

12.1.3 Spectrometric Oil Analysis

The spectrometric oil analysis (SOA) process is useful for any mechanical moving
device that uses oil for lubrication. It tests for presence of metals, water, glycol, fuel
dilution, viscosity, and solid particles. Automotive engines, compressors, and turbines
all benefit from oil analysis. Most major oil companies provide this service if you
purchase lubricants from them. Experience indicates that the typical result is that less
oil is used and costs are reduced from what they were before using SOA.

The major advantage of SOA is early detection of component wear. Not only does it
evaluate when oil is no longer lubricating properly and should be replaced, but it also
identifies and measures small quantities of metals that are wearing from the moving
surfaces. The metallic elements found, and their quantity, can indicate what compo-
nents are wearing and to what degree so that maintenance and overhaul can be care-
fully planned. For example, presence of chrome would indicate cylinder head wear;
phosphor bronze would probably be from the main bearings; and stainless steel would
point toward lifters. Experience with particular equipment naturally leads to improved
diagnosis.

The Air Force and commercial airlines have been refining these techniques on jet
aircraft for many years. They find that SOA, together with bore scopes to look inside
an engine and vibration analysis, enables them to do a very good job of predicting
when maintenance should be done. The aircraft maintenance techniques that required
complete teardown of propeller-driven aircraft every 1,000 hours, whether they needed
it or not, are rapidly vanishing in that industry. Many manufacturing plants can gain
improvements through the same maintenance techniques.

12.2 THRESHOLDS
Now that instrumentation is becoming available to measure equipment performance,
it is still necessary to determine when that performance is “go” and when it is “no-
go.” A human must establish the threshold point, which can then be controlled by
manual, semiautomatic, or automatic means. First, let’s decide how the threshold is
set and then discuss how to control it.

To set the threshold, one must gather information on what measurements can exist
while equipment is running safely and what the measurements were just before or at
the time of failure. Equipment manufacturers, and especially their experienced field
representatives, are a good starting source of information. Most manufacturers will
run equipment until failure in their laboratories as part of their tests to evaluate quality,
reliability, maintainability, and maintenance procedures. Such data are necessary to
determine how much stress can be put on a device under actual operating conditions
before it will break. Many devices, such as nuclear reactors and airplanes, should not
be taken to the breaking point under operating conditions, but they can be made to
fail under secure test conditions so that knowledge can be used to keep them safe
during actual use.