The planned-preventive maintenance and repair system (MRO) is a complex of organizational and technical measures of a precautionary nature that are carried out in a planned manner in order to ensure constant technical readiness of the equipment for operation and prevent increased wear of its components and parts.
The planned maintenance system is called because the equipment is put on maintenance in a planned (scheduled) manner after a certain period of time or after a certain operating time (in hours or in the number of operations performed).
The caution is that the maintenance and periodic repairs of the equipment are carried out before a failure occurs. Accordingly, such repairs are called planning-preventive (PPR).
When planning an outage, it is important to correctly determine at what point it is necessary to carry out the maintenance and repair to avoid equipment failure. If it is carried out too often, maintenance becomes prohibitively expensive, if unjustifiably rare – the equipment can fail, which can also lead to serious financial losses. For a long time it was believed that the probability of failure increases as the equipment operates due to its wear and aging. Accordingly, the maintenance and repair was performed after the equipment produced a certain number of hours or performed a certain number of operations (for example, after the production of every thousand stamped parts). Upon completion of maintenance or repair, the equipment was again put into operation, the operating time (in hours or number of operations) was again accumulated and the maintenance and repair was repeated. Accordingly, the terms “repair cycle” and “turnaround time” appeared.
For many years, scheduled preventive maintenance was the main method of maintaining the equipment in most industrial enterprises. An alternative to this method was reactive maintenance, or event-based service, in which equipment repair was performed after its failure. However, the results of studies showed that the use of such MRO is justified only when the cost of repairs is relatively low, and the production of products resulting from equipment breakdown is low and will not affect the fulfillment of obligations to customers. For complex equipment, the repair of which is expensive and time consuming, reactive maintenance, taking into account losses from idle time, can cost five to eight times more expensive than an outage maintenance and repair.
However, in the period from 1968 to 1982, large-scale studies of failure mechanisms were carried out, as a result of which it turned out that only 8% to 23% of failures correspond to the assumption adopted in the practice of carrying out CPD that the failure rate increases with equipment life. Such failures are characteristic of relatively simple objects. As for technically complex objects, it was found that there is little or no correlation between the probability of their failure and the service life. For these objects, the probability of failure has a constant value that does not increase with the operating time.
Therefore, in the 1970s, the world’s leading enterprises began to abandon CPD, and today the most progressive and effective approach for any manufacturing enterprise is considered to be maintenance based on reliability (RCM, reliability-centered maintenance).
Does this mean that the preventive maintenance system, as outdated, should be handed over to the museum? No, it does not mean. Indeed, in accordance with the “most progressive and effective” RCM method, at the first stage of its implementation, all equipment involved in the production process is classified according to the criterion criterion. After analyzing the equipment fleet and determining the most “critical” units, for each of them the most efficient type of service is selected:
Reactive – mean time to failure without maintenance. It is used when the equipment is not critical, is easily replaced, its repair is accompanied by relatively low costs, or it is aging equipment that is practically beyond repair and is being finalized until the final failure.
Preventive – similar to the system of scheduled preventive maintenance (CPD). Prescribed to equipment, the “criticality” of which is low; This is determined by such factors as low cost of downtime for production, the ability to quickly replace parts on a site or assembly unit.
For equipment, the degree of use in the production chain is assessed as high, and predictive (predictive) and proactive types of services are applied.
Thus, the preventive maintenance system is still widely used, but not for all equipment not subject to reactive maintenance, but only for that which has not yet been transferred to more effective predictive or proactive maintenance, but also for that and is not subject to such a transfer.