What is maintenance and why is it performed? Past and current maintenance practices in both the rivate and government sectors would imply that maintenance is the actions associated with quipment repair after it is broken. The dictionary defines maintenance as follows: “the work of keeping something in proper condition; upkeep.” This would imply that maintenance should be actions taken to prevent a device or component from failing or to repair normal equipment degradation experienced with the operation of the device to keep it in proper working order. Unfortunately, data obtained in many studies over the past decade indicates that most private and government facilities do not expend the necessary resources to maintain equipment in proper working order. Rather, they wait for equipment failure to occur and then take whatever actions are necessary to repair or replace the equipment. Nothing lasts forever and all equipment has associated with it some predefined life expectancy or operational life. For example, equipment may be designed to operate at full design load for 5,000 hours and may be designed to go through 15,000 start and stop cycles.
The need for maintenance is predicated on actual or impending failure – ideally, maintenance is performed to keep equipment and systems running efficiently for at least design life of the component(s). As such, the practical operation of a component is time-based function. If one were to graph the failure rate a component population versus time, it is likely the graph would take the “bathtub” shape shown in Figure 5.1.1. In the figure the Y axis represents the failure rate and the X axis is time. From its shape, the curve can be divided into three distinct: infant mortality, useful life, and wear-out periods.
The initial infant mortality period of bathtub curve is characterized by high failure rate followed by a period of decreasing failure. Many of the failures associated with this region are linked to poor design, poor installation, or misapplication. The infant mortality period is followed by a nearly constant failure rate period known as useful life. There are many theories on why components fail in
this region, most acknowledge that poor O&M often plays significant role. It is also generally agreed
that exceptional maintenance practices encompassing preventive and predictive
elements can extend
this period. The wear-out period is characterized by a rapid increasing failure
rate with time. In most
cases this period encompasses the normal distribution of design life failures.
The design life of most equipment requires periodic maintenance. Belts need
adjustment,
alignment needs to be maintained, proper lubrication on rotating equipment is
required, and so on.
In some cases, certain components need replacement, (e.g., a wheel bearing on a
motor vehicle)
to ensure the main piece of equipment (in this case a car) last for its design
life. Anytime we fail
to perform maintenance activities intended by the equipment’s designer, we
shorten the operating
life of the equipment. But what options do we have? Over the last 30 years,
different approaches
to how maintenance can be performed to ensure equipment reaches or exceeds its
design life have
been developed in the United States. In addition to waiting for a piece of
equipment to fail (reactive
maintenance), we can utilize preventive maintenance, predictive maintenance, or
reliability centered
maintenance.
