Let’s look at the mechanical world. Since we are maintenance and reliability folks, let’s concentrate on the specific world inside of a fleet’s machines and other assets. Elements inside these assets mesh, clank, roll, brake, catch, sense, restrict, squeeze and drive other parts to do useful work. Even something as simple as truck air brakes have a dozen or more of these interactions to operate and stop the vehicle.
Reliability is when all the elements work the way they are intended. Lack of reliability comes in a few varieties, like when brakes do not affect the speed, slow the truck but not quickly enough or grab and won’t let go.
Maintenance is simply the activity done to keep a vehicle reliable or return it to reliability.
Defining asset failure
There are a finite number of ways an asset can fail to function. The description of the cause of asset failure is called the failure mode. Each failure mode is a single way a system can fail. In other words, each failure mode is the cause of an inoperable or non-functioning component or system. Any decently complicated machines have dozens or more failure modes.
Here is where it can go off the rails.
Maintenance is what is done to either postpone the failure mode, or to detect when it is happening. All the activities we do to detect failure modes are approximate. In other words, since we can’t directly see the failure mode, we look for something we can see and measure that. Then through engineering, past experience or wild guessing we try to correlate the thing we measure with the thing we can’t measure.
Investigating failure modes
To investigate, let’s shrink down to a molecular level and look at an area of a vehicle. Excessive vibration in a bearing (generally) is a bad thing. Frequency and intensity of the vibration can be readily measured. Simplified to make a point, inside the bearing, pieces are breaking off because the strain of the dynamic shock when the bearing bounces up and down exceeds the yield limit of the steel. With that, a bit of the steel brace breaks off.
We can’t easily read the strain which is the measure of the cause of the failure. But, we can measure and read accurate vibration. This accuracy leads us to feel sure about an outcome. However, the indication we are sensing is only an approximation of what is going on.
Almost all inspections, whether by human senses, sophisticated sensors or by an instrument, are typically measuring what is convenient or available, rather than what is the actual cause.
Determining PMs
When smart people make preventive maintenance (PM) task lists, they make up lists of possible failure modes. They know some are much more likely, some are rare and some of each have dire (safety or environmental) consequences. Then, they match up the failure modes with inspections that will detect the failure mode early enough to intervene before failure.
The other choice is to figure out a task that will postpone the failure mode. An example of postponing a failure mode might be tightening a nut when the failure mode is a loose bolt.
Since the task is most often looking at something that we hope is related to the failure mode, there is quite a bit of variation in the timing of the result. That is half of the problem.
To complicate the challenge, each asset has dozens or hundreds of failure modes. Each of which is in various states of decay. The measurements we are using on each is an approximation.
When that smart person chooses an interval for a PM, they may be considering the dozens of failure modes of different components of the vehicle. Each pattern is different. The length and speed of decay is different. Each one has different consequences of failure.
Conscientious PM designers plot the different failure modes, evaluate failure consequences and assign the PM frequency. Of necessity, the interval is average, some kind of weighted average or sometimes made up.
Given all of this, you can imagine why PMs don’t always “work” in our favor. You also get a glimpse at why there is so much art to the development of useful PMs.
Joel Levitt is the president of the Springfield Resources, a management consulting firm that services a variety of clients on a wide range of maintenance issues. Levitt has trained more than 17,000 maintenance leaders from more than 3,000 organizations in 38 countries. He is also the creator of Laser-Focused Training, a flexible training program that provides specific targeted training on your schedule, online to one to 250 people in maintenance management, asset management and reliability.
