A second and more dominant area of confusion occurs when a scheduled task reveals unacceptable equipment deterioration (like the problem above in the MO situation, except it was not unexpected since a PM task discovered its presence). So actions are taken to repair/restore the full functionality before an unexpected operational impact can occur. Is the repair/restore action preventive or corrective?
If you will recall that the purpose of the PM task is to perform actions that will retain functional capabilities, then the answer is essentially self evident — the repair/restore action is preventive. Why? Because a proper structuring of the PM task will always include not only the search for equipment condition, but also the requirement to do something about it if the search uncovers a problem.
This search includes PM tasks that require inspection, monitoring parameters that detect failure onset, discovery of hidden failures and even restoration of equipment that was deliberately allowed to run to failure. Unfortunately, though, many CMMS programs will not allow the user to create or code a new work order to cover the emergent work as PM. This additional PM work can only be coded as CM. This inflates the cost of CM, and can lead management to question why CM costs are increasing even when their PM program had been recently improved.
As a general rule, corrective maintenance is more costly than preventive maintenance. If anyone should doubt this, then just compare two similar plants or systems where one has a proactive maintenance program and the other a reactive maintenance program. Which one do you think has the lower overall maintenance cost and higher availability?
Why do preventive maintenance?
For the past 15 years, as part of our seminars and client training programs, we frequently ask the question "Why do preventive maintenance?" The answers that we consistently hear reflect the popular belief that PM is done for a rather narrowly defined reason and this, as such, leads to the exclusion of a number of golden opportunities for PM enhancement.
So why do you do preventive maintenance? The overwhelming majority of maintenance and plant engineering personnel will respond "To prevent equipment failures." Would that have been your response? If so, you are correct — but not complete in your viewpoint. Unfortunately, we are not yet smart enough to prevent all equipment failures. But that does not mean that our ability to perform meaningful preventive maintenance tasks must end there.
In fact, there are three additional and important options to consider. First, while we may not know how to prevent a failure, frequently we do know how to detect the onset of failure. And our knowledge of how to do this is increasing every day, and is creating a whole new discipline called predictive maintenance. Second, even though we may not be able to prevent or detect the onset of failure, we often can check to see if a failure has occurred before equipment is called into service. Various standby and special purpose equipments (whose operational state is often hidden from the operator's view until it is too late) are candidates for this area. Thus, discovery of hidden failures is yet another PM option available to us.
There are also situations in a well planned PM program where economics and/or technical limitations can dictate a decision to do nothing — he appropriately labeled Run-To-Failure (RTF) option. This RTF option is not to be confused with the more general situation of missing potentially useful PM actions due to oversight or lack of attention to PM planning.
To summarize, there are four basic factors behind the decisions to define and choose preventive maintenance actions:
1. Prevent (or mitigate) failure occurrence.
2. Detect onset of failure.
3. Discover a hidden failure.
4. Do nothing, because of valid limitations.
Source: Anthony M. Smith and Glenn R. Hinchcliffe (Plant Engineering - November 1, 2005)
If you will recall that the purpose of the PM task is to perform actions that will retain functional capabilities, then the answer is essentially self evident — the repair/restore action is preventive. Why? Because a proper structuring of the PM task will always include not only the search for equipment condition, but also the requirement to do something about it if the search uncovers a problem.
This search includes PM tasks that require inspection, monitoring parameters that detect failure onset, discovery of hidden failures and even restoration of equipment that was deliberately allowed to run to failure. Unfortunately, though, many CMMS programs will not allow the user to create or code a new work order to cover the emergent work as PM. This additional PM work can only be coded as CM. This inflates the cost of CM, and can lead management to question why CM costs are increasing even when their PM program had been recently improved.
As a general rule, corrective maintenance is more costly than preventive maintenance. If anyone should doubt this, then just compare two similar plants or systems where one has a proactive maintenance program and the other a reactive maintenance program. Which one do you think has the lower overall maintenance cost and higher availability?
Why do preventive maintenance?
For the past 15 years, as part of our seminars and client training programs, we frequently ask the question "Why do preventive maintenance?" The answers that we consistently hear reflect the popular belief that PM is done for a rather narrowly defined reason and this, as such, leads to the exclusion of a number of golden opportunities for PM enhancement.
So why do you do preventive maintenance? The overwhelming majority of maintenance and plant engineering personnel will respond "To prevent equipment failures." Would that have been your response? If so, you are correct — but not complete in your viewpoint. Unfortunately, we are not yet smart enough to prevent all equipment failures. But that does not mean that our ability to perform meaningful preventive maintenance tasks must end there.
In fact, there are three additional and important options to consider. First, while we may not know how to prevent a failure, frequently we do know how to detect the onset of failure. And our knowledge of how to do this is increasing every day, and is creating a whole new discipline called predictive maintenance. Second, even though we may not be able to prevent or detect the onset of failure, we often can check to see if a failure has occurred before equipment is called into service. Various standby and special purpose equipments (whose operational state is often hidden from the operator's view until it is too late) are candidates for this area. Thus, discovery of hidden failures is yet another PM option available to us.
There are also situations in a well planned PM program where economics and/or technical limitations can dictate a decision to do nothing — he appropriately labeled Run-To-Failure (RTF) option. This RTF option is not to be confused with the more general situation of missing potentially useful PM actions due to oversight or lack of attention to PM planning.
To summarize, there are four basic factors behind the decisions to define and choose preventive maintenance actions:
1. Prevent (or mitigate) failure occurrence.
2. Detect onset of failure.
3. Discover a hidden failure.
4. Do nothing, because of valid limitations.
Source: Anthony M. Smith and Glenn R. Hinchcliffe (Plant Engineering - November 1, 2005)
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