Intervention and preventive maintenance

Failures are commonly the result of a cascade of events. This cascade will be slightly or grossly different for different failures. Preventive maintenance (PM) developed for failures, where there is an intervention, can interrupt the cascade.

PM is not a pancea and does not address all types of failures. Failures that are random cannot be predicted or even mitigated by PM.

Consider tires, for instance. While tire life can be impacted by a PM program that keeps tires  filled with air and replaces them when required and matches duals, it has no, or little, impact on road hazards. 

Bearings

Bearings serve as a good example of failures due to a cascade of events, because the details of their deterioration can be more easily seen than some other vehicle components.

As a bearing deteriorates, energy efficiency (increased friction) and performance (some horsepower tied up in driving a powertrain producing heat instead of rotation) also deteriorate. Other symptoms might be roughness, noises and heat.

The following is an example of a cascade of events caused by a bearing from a piece of agriculture equipment used on a farm in Saudi Arabia. There, sand is ever-present and the summer temperatures top out above 120 degrees Fahrenheit.

Here is the cascade for the bearing failure:

1. Dirt enters the bearing.

2. Dirt mixes with the grease.

3. The grease becomes less slippery and hones the balls and races.

4. Vibration is caused from the less than smooth surface.

5. Heat is generated.

6. The heat cooks the dirty grease, further degrading the grease. (In some cases, the heat may become hot enough to start a fire).

7. The bearing steel changes (maybe softens).

8. The bearing goes out of round or allows greater movement between races and balls.

9. The greater movement makes vibration, accelerates deterioration and loosens fasteners, producing a rough ride, noise, poor tracking, etc.

10. Eventually, the bearing fails. 

In RCA (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 that. 

The Goal of PM

There are two main objectives of preventive maintenance:

1. Stop the initiating conditions that lead to a failure. In the case of the bearing example, this might include cleaning out the dirt before it becomes a problem, replacing or supplementing the lubricant, tightening the bolts loosened by vibration, etc.

2. Notice when the cause tree has started. This 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.

In the bearing example, if you are slightly late in doing this, you still will have a corrective bearing replacement but no failure, and more importantly, no failure consequences.

PM is an organized attack on the cause tree.