February 1, 2017
Maintenance Strategies: Part Two
By Mac MacCormack
Preventive Maintenance, also known as planned maintenance, scheduled maintenance, calendar based maintenance, historical maintenance, etc. has its own set of pro’s and con’s.
The theory is machinery has a life cycle. The probability of failure increases as the machine ages so maintenance needs to be performed prior to failure to extend its life and avoid failure.
The following are examples of what has come to be known as “bathtub curves”. They are primarily used to estimate the life of a machine which is difficult when taking infant mortality into consideration. In other words, the probability of failure for a new machine does exist and does happen. This includes an overhaul triggered by a preventive maintenance plan.
This information I gleaned from Mobius Institute.
This study shows one of the cons of doing calendar based maintenance. Because machines are still failing (89% of failure modes are random) the plant is still in reactive mode. They also have a huge amount of preventive maintenance scheduled and probably won’t be able to handle all, therefore there will be stress, maybe some resentment towards planned maintenance tasks that seem unnecessary and planned maintenance tasks are costly in terms of manpower, spare parts and downtime, even though it is planned.
Preventive Maintenance still has its place.
So when would PM be a pro?
- When a machine or component wears or degrades at a known frequency
- The risk of infant mortality is less than the risk of failure
- The Cost of PM is less than the cost of failure
- Condition monitoring is not available
Next month we will touch on Predictive Maintenance. I just want to note that there are in-depth studies on the four basic maintenance strategies I will or have touched on available, the internet is a good place to start if you are curious to know more.