Page 44 - Root Cause Failure Analysis

Root Cause Failure Analysis Methodology 35

Measurement Devices
Most machines and systems include measurement devices that provide a clear indica-
tion of the operating condition. A visual inspection of these devices confirms many of
the failure modes that cause process deviations and catastrophic failures.
For example, pressure gauges are a primary tool used to evaluate operating dynamics.
A steady pressure reading is a good indication that the equipment (e.g., pump, fan,
compressor) is operating normally and within its designed operating envelope. A fluc-
tuating gauge is a good indication that something is wrong. Turbulent flow, cycling or
variation in load or demand, and a variety of other process deviations result in a radi-
cal fluctuation of the pressure-gauge reading.

Failed Machine Components
Visual inspection of failed machine components can be an effective diagnostic tool.
For example, visual inspection of a failed bearing can determine the load zone, wear
pattern, and in many cases the probable cause of the failure. The same is true of gears.
V-belts, and a variety of other common machine components.

In many cases, visual inspection provides positive clues that point the investigator
toward the probable failure mode or root cause. For example, the wear pattern of a
rolling-element bearing or gear set provides visual indications of abnormal loading OJ
behavior.

A number of reference books discuss and illustrate the common failure modes of
machine components that can be detected using visual observation as the primary
diagnostics tool. Most of these books use photographs to clearly display the visual
image of various failure modes.

Figure 3-6 illustrates a damaged rolling-element bearing. From the wear pattern, it is
clear that the load zone is abnormal. Rather than being centered in the races, the wear
pattern is offset. This visual inspection confirms that some forcing function within the
machine or its system has caused an abnormal loading of the bearing. Failure modes
(e.g., misalignment, aerodynamic or hydraulic instability, and changes in load or
speed) are primary sources of this type of abnormal wear pattern.

Figure 3-7 illustrates an abnormal wear pattern on a spur gear caused by too little
backlash. The vertical score marks are caused by sliding wear resulting from too little
clearance between the gear set.

Wear Particles
Wear-particle analysis is a diagnostic technique that uses visual analysis aided by an
electron microscope to evaluate the wear patterns of the metallic solids contained in a