When the individual pieces of equipment have been identified for preventive maintenance, there must be a procedure for identifying the order in which they are to be done. Not everything can be done first. First In–First Out (FIFO) is one
way of scheduling demand. Using FIFO means that the next preventive task picked off the work request list, or the next card pulled from the file, is the next preventive maintenance work order. The problem with this ‘‘first come, first served’’ method is that the more desirable work in friendly locations tends to get done while other equipment somehow never gets its preventive maintenance. The improved method is

Priority = Need Urgency X Customer Rank X Equipment Criticality.

The acronym NUCREC will help in remembering the crucial factors. NUCREC improves on the Ranking Index for Maintenance Expenditures (RIME) in several ways:
1. The customer rank is added.
2. The most important item is given the number-one rating.
3. The number of ratings in the scale may be varied according to the needs of the particular organization.
4. Part essentiality may be considered.

A rating system of numbers 1 through 4 is recommended. Since most humans think of number 1 as the first priority to get done, the NUCREC system does number 1 first.

Need urgency ratings include
1. Emergency; safety hazard with potential further damage if not corrected immediately; call back for unsatisfactory prior work
2. Downtime; facility or equipment is not producing revenue
3. Routine and preventive maintenance
4. As convenient, cosmetic.

The customer ranks are usually as follows:
1. Top management
2. Production line with direct revenue implications
3. Middle management, research and development facilities, frequent customers
4. All others.

The equipment criticality ratings are as follows:
1. Utilities and safety systems with large area effect
2. Key equipment or facility with no backup
3. Most impact on morale and productivity
4. Low, little use or effect on output.

The product of the ratings gives the total priority. That number will range from 1 (which is 1x1x1) to 64 (4x4x4). The lowest number work will be first priority. A ‘‘1’’ priority is a first-class emergency. When several work requests have the same priority, labor and materials availability, locations, and scheduling fit may guide which is to be done first.

The priorities should be set in a formal meeting of production and maintenance management at which the equipment criticality number is assigned to every piece of equipment. Similarly, a rank number should be applied to every customer and the need urgency should be agreed on. With these predetermined evaluations, it is easy to establish the priority for a work order either manually by taking the numbers from the equipment card and the customer list and multiplying them by the urgency or by having the computer do so automatically. Naturally, there may be a few situations in which the planner’s judgment should override and establish a different number, usually a lower number so that the work gets done faster.

Ratings may rise with time. A good way to ensure that preventive maintenance gets done is to increase the need urgency every week. In a computer system that starts with preventive maintenance at 3, a preventive task that is to be done every
month or less frequently can be elevated after one week to a 2, and finally to a 1 rating. Those increases should ensure that the preventive task is done within a reasonable amount of time. If preventive maintenance is required more often, the incrementing could be done more rapidly. Dispatch of the preventive maintenance work orders should be based on the
demand ordered by priority, consistent with availability of labor and materials.

As discussed earlier, predictive maintenance provides a good buffer activity in service work, since time within a few days is not normally critical. The NUCREC priority system helps ensure that the most important items are done first.

Some pressure will be encountered from production people who want a particular work request filled right away instead of at the proper time in the priority sequence. It can be helpful to limit the ‘‘criticality 1’’ equipment and ‘‘rank 1’’ customers to 10%, since, according to Pareto’s Principle of the Critical Few, they will probably account for the majority of activity. If rank 2 is the next 20%, rank 3 is 30%, and the balance is 40% for rank 4, the workload should be reasonably balanced. If temporary work needs exist for selected equipment or a customer needs to be given a higher priority, then equipment should be moved to a lower criticality for each equipment that is moved higher. After all, one objective of prioritization is to ensure that work gets done in proper sequence. A preventive maintenance action that is done on time should ensure that equipment keeps operating and that emergency work is not necessary.

***Excerpts from Maintenance Fundamentals (Second Edition) by R Keith Mobley, published by Elsevier Butterworth-Heinemann 2004***