Failure and Failure Modes
A failure is the disability of an item to carry out any of its functions. Therefore, if the previous step is successfully made (the identification of the specific and general functions) it is very easy to determinate the failures. Thus, a failure is the anti-function, the lack of achieving the technical specification or the general functions.
In turn, these failures can be into four different types. They can be total failures, where the function is completely lost (the item doesn’t work at all) o partial failures, where the studied item works but doesn’t reach it specification. The importance of distinguish between one case or another lies in: when analysing failure modes or causes, and when analysing the severity of these failures, there could be differences between them the failure been total or partial.
Failure are consequences. What matters in RCM is not identifying the consequence, that is the failure itself, but it cause, in order to analyse the severity of this consequence subsequently and its easiness to stop it, and in accordance with that, adopt preventive measures in order to avoid the causes that provoke the failures. Unfortunately, methodology used the word ‘failure mode’ for referring to the causes of the failures, making the word, which would be perfectly intuitive if using the word ‘cause’, generate doubts about its meaning.
The failure modes are nothing but the diverse causes that generate failures. A specification is associated with only one function, and one function is related to one, two or at most, three failures. But each one of the failures could have multiply causes, even more than 200, what implies the application of RCM3 methodology. But this hassle has nothing to do with methodology: it is the cause of the failures that can be multiple.
RCM3 comes from a very simple concept: only identifying every possible potential cause of a failure and taking accurate preventive measures, a failure gets avoided.
The central part of RCM3 consists in identifying failures mode, as first step for the final objective: adopting preventive measures which avoid the causes to appear and to get materialized, in event of failure.
A failure may have multiple causes, though these may be classified in the following big groups:
- Causes linked with design.
- Causes linked to the assembly.
- Causes linked to the way of operating the equipment.
- Causes linked to the maintenances made on it.
- Causes linked to the supplies it needs.
- Causes linked to the internal components.
- Causes linked to environmental factors.
- Causes linked to other equipment, which provoke a consequential failures.
Itemised every possible failure, there can be more than 250 different causes which can provoke a failure in a complex equipment. The tedious work consists in, once the failures are identified (easy step if the specifications have been properly described and the general functions of the items have been properly defined), to every failure it has to be verified which of those more than 250 failures can be behind the failure occurred. Failure by failure it has to be verified if this wide list of more than 250 failures could have been related to the failure or not.
It gets a little bit more complicated, every failure mode could be considered a failure itself, and therefore, it could be provoked by different causes. For each one of the failure modes, it has to be verify which are the causes that provoke it, defining what is known as failure modes at second level. In other words, failure modes that provoke failure modes. And it can continue further on, even to failure modes at fifth level. The problem appears when a level in the determination of failure modes gets step down, when it multiplies the work. Hence, lots of organizations only analyse one of the failure modes at first level and, failure modes at second level at most. Trying to descend is practically impossible for mostly every organization, except for the ones which have great resources. These organization belong to sectors where security is an issue, and also they have substantial resources: aeronautical or nuclear field are clear examples, they can afford reaching fifth level without any problem.
No one ever said that the application of RCM3 methodology would be easy or quick. The main obstacle and what makes its application take some time is precisely the determination of failure modes, and being this the principle blockage, it is exactly what gives potential to this methodology: it allows the identification of the potential failure modes of an installation.
This article is an excerpt of Guide 4th: Guide for the implantation of RCM3 on installations.