Another common algorithm seeks to find the optimal task interval by determining the minimum overall cost of maintenance. It uses the formula below:

Ct=Cf +Cpa +Csa

Where

• Ct is the product of the cost of actual failure and the probability of failure
• Cpa is the cost of the primary maintenance action multiplied by the analysis period divided by the frequency of the primary maintenance action
• Csa is the cost of the secondary action multiplied by the analysis period divided by the frequency of primary action multiplied by the probability of failure divided by the period of analysis

The Impracticality of Statistical Methods

The body of data required to generate accurate numbers for the above equations rarely exists in industry. For example:

• The value of T itself can only be determined by running equipment to absolute failure sufficient times to gain a statistically significant sample - a practice that is rarely justifiable.
• Determining the probability of failure or MTBF is precluded for similar reasons to determining T.
• There are no simple and reliable methods for determining the task effectiveness S.
• All the cost factors in the formulae are subject to the vagueness of the accounting and cost allocation methods employed.

Whilst the formulae for optimisation may be mathematically correct they are practically useless for all but a few applications where the variables can be determined with some certainty. Furthermore, they require the employment of people skilled in mathematics and statistics and hence such approaches, when applied in industry can easily develop into a back office pursuit of statistical perfection and lead to a program completely out of touch with reality.

Such programs have also been responsible for the relentless pursuit of accurate data that in reality takes decades of consistent operation for it to be of any realistic use as a tool for deriving maintenance task intervals˛.

˛  The author understands that failure history is essential for defect elimination however its use as a determinant for maintenance task interval is widely overstated in industry.

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