Failures are commonly the result of a cascade of events.
This series of events can be slightly or grossly different for different failures.
Preventative maintenance developed for failures is basically an intervention that can interrupt the cascade.
But, preventative maintenance does not address all types of failures.
Failures that are random cannot be predicted or even mitigated by preventative maintenance.
While tire life can be impacted by a preventative maintenance program that keeps them filled with air, replaces them when required and matches duals, it has no or little impact on road hazards.
Bearings are frequently used for preventative maintenance examples because it is easy to see the details of the deterioration — at least easier than some other components.
As the bearing deteriorates energy efficiency and performance also deteriorate.
Increased friction inside the bearing ties up horsepower producing heat instead of rotation.
Other symptoms might be roughness, noises and excess heat.
Let’s say that this is the cascade for a bearing failure:
- Dirt enters the bearing.
- Dirt mixes with grease.
- Grease becomes less slippery and even hones the balls and race.
- Vibration from the less than smooth surface.
- Heat is generated.
- Heat cooks the grease/dirt degrading grease even more.
- Heat is generated, in some cases enough to start a fire.
- Bearing steel changes, maybe softens.
- Bearing goes out of round or allows greater movement between race and balls.
- Greater movement makes vibration and accelerates deterioration and loosens fasteners. Rough ride, noise, poor tracking, etc.
- Eventually bearing fails.
These events are causes of other events.
In root cause analysis parlance, the whole picture of these events is called a cause tree.
If you follow it through, one event causes the next event, and the next event causes the event after.
Preventative maintenance practices have two goals.
First, stop the initiating conditions that lead to failure.
Clean out the dirt before it becomes a problem, replace or supplement the lubricant and tighten bolts loosened by vibration.
Second, notice when the cause tree has started.
That includes all types of inspections and the use of technology to detect physical manifestations of the cause tree.
If you interrupt the cause tree at any time you will stop the natural deterioration leading to the failure.
If you are slightly late, you still will have a corrective bearing replacement but no failure and more importantly no failure consequences.
Simply put, preventative maintenance is an organized attack on the cause tree.
At each stage we are stopping or derailing the cascade of events that will lead to the failure.
Using preventative maintenance means there is some time before an operation faces the consequences, such as associated part failure, a road call, accidents, late delivery, waiting for parts, etc.
Joel Levitt, a maintenance trainer and consultant, is director international projects with Life Cycle Engineering. Contact Levitt at Jlevitt@LCE.com and learn more about asset management services at www.LCE.com.