The Reliability Formula Gives The First Element In The Definition Of Reliability

Everything we do in a business, from business support functions such as HR to frontline core business functions such as production and reliability centered maintenance, is a process. Every process has five (5) key elements: 1. a process requires UNDERSTANDING; 2. when you understand a process you will realised that a process has VARIABLES; 3. these process variables must be PROPERLY CONTROLLED; 4. every process has a CAPACITY; and 5. that process capacity requires CONTINUOUS IMPROVEMENT.

Once we understand a particular process and especially in this case, the definition of reliability and what variables are involved that interact to define it, then we can work on this areas with clarity and assurance to continuously improving the reliability of a system/equipment/component.

There are four (4) key fundamental elements in the definition of reliability. Reliability is:

1. A probability (i.e. any number between 0 and 1) that a system/equipment/component will perform;

2. A specific function over;

3. A specific period of time; and

4. Under specific support conditions (i.e. care and maintenance).

Element no.1 in the definition of reliability is given by the reliability formula.

Element no.2 encompasses the philosophy of Reliability Centered Maintenance (RCM) which is geared toward the ‘Preservation Of Function’. There is no failure unless the failure parameters are defined and results in the defeat of the ‘Function’ or ‘Functional Failure’.

And if you notice, element no.3 in the definition of reliability is a key variable in the reliability formula. Whenever you state the reliability of a system/equipment/component, you must reference this over a time period. Otherwise, your reliability percentage is practically meaningless.

Although reliability is inherent in design, element no.4 influences the 'Weibull Shape Factor/Parameter' as well as the 'Characteristic Life' in the reliability formula.

This reliability formula was invented by Waloddi Weibull in 1937. This formula could describe the different shaped graphs in each of the three zones of the Bathtub Curve. The three zones are: high probability of failure when new (burn-in); steady state probability of failure (random or constant); and increase probability of failure when old (wear-out).

The shape factor tells you the location of the failure mode on the Bathtub Curve and the maintenance strategies to apply. The failure is 'burn-in' if the shape factor is less than 1 and is often the result of improper installations/rebuild and precision maintenance is the strategy to mitigate this failure. The failure is random (or constant) when the shape factor is equal to 1 and a run-to-fail strategy could be applied if the risk of failure is acceptable, but if the risk is not acceptable, then on-line condition monitoring can be the maintenance strategy to be applied. The failure is slow aging or wearing at constant rate if the shape factor is equal to 2. If the shape factor is greater than 2 and less than 4, the failure mode is wear-out (i.e. aging) and condition-based maintenance will be the strategy. If the shape factor is 5 and above, the failure is very predictable and a time based schedule changeout should be the maintenance strategy to mitigate the risk of this failure mode in the maintainable item.

The bottom line is that, for RCM Practitioners, if you know the practical application of this formula, you can read a lot from it (i.e. where it is on the Bathtub Curve and the appropriate maintenance strategy to apply to preserve the function of the system/equipment/component) when you see the formula with the functional variables and take quick decisive pragmatic actions knowing what you know.

As Deming said "It is not enough to do [our] best, [we] must know what to do and then do [our] best."