Definitions

service lives

Service life

A product's service life is its expected lifetime, or the acceptable period of use in service. It is the time that any manufactured item can be expected to be 'serviceable' or supported by its originating manufacturer.

Expected service life consists of business policy, using tools and calculations from maintainability and reliability analysis. Service life is a unique commitment made by the item's manufacturer and is usually specified as a median. Actual service life is the maximal recorded life of a product.

Service life is different from a predicted life, or MTTF/MTBF (Mean Time to Failure/Mean Time Between Failures)/MFOP(Maintenance-free operating period). Predicted life is useful such that a manufacturer may estimate, by hypothetical modeling and calculation, a general rule for which it will honor warranty claims, or planning for mission fulfillment. The difference between service life and predicted life is most clear when considering mission time and reliability in comparison to MTBF and service life.

For example: A missile system can have a mission time of less than one minute, a service life of 20 years, active MTBF of 20 minutes, dormant MTBF of 50 years and a reliability of .999999.

A consumer item will have different expectations about service and longevity based upon factors such as use, cost, and quality.

Product strategy

Manufacturers will commit to very conservative service life, usually 2 to 5 years for most commercial and consumer products (for example computer peripherals and components). However, for large and expensive 'durable' items (airliner, hydroelectric power plants) the items are not 'consumable', and service lives and maintenance activity will factor large in the service life. Again, an airliner might have a mission time of 11 hours, a predicted active MTBF of 10,000 hours with maintenance (or 15,000 hours without maintenance), a reliability of .99999 and a service life of 40 years.

The most common model for item lifetime behavior, in relative failure terms follows a bathtub curve. On the abcissa of this curve is the term 'lambda' or failure rate, which is the inverse of MTBF. The ordinate axis is time. Initially, the bathub shows early life failures, generally not witnessed by the consumer and usually termed as factory defects. The flat middle portion of the bathtub, or 'useful life', is a slightly inclined, exponentially increasing, constant failure rate period where the consumer enjoys the benefit conferred by the item. As the bathtub reaches its far right terminus, it is exponentially increasing, modeling untoward physical effects related to Arrhenius rate effects.

For an individual product, there may be several bathtub curves, related to different aspects of the product. For instance, a tire will have a service life partitioning related to the tread and the casing.

Service life examples

For maintainable items, those wear-out items that are determined by logistical analysis to be provisioned for sparing and replacement will assure a longer service life than manufactured items without such planning. A simple example is automotive tires - failure to plan for this wear out item would limit automotive service life to the extent of a single set of tires.

An individual tire's life follows the bathtub curve, to boot. After installation, there is a not-small probability of failure which may be related to material or workmanship or even to the process for mounting the tire which may introduce some small damage. After the initial period, the tire will perform, given no defect introducing event such as encountering a road hazard (a nail or a pothole), for a long duration relative to its expected service life which is a function of several variables (design, material, process). After a period, the failure probability will rise; for some tires, this will occur after the tread is worn out. Then, a secondary market for tires puts a retread on the tire thereby extending the service life. It is not uncommon for an 80,000-mile tire to perform well beyond that limit.

It may be difficult to obtain reliable longevity data about many consumer products as, in general, efforts at actuarial analysis are not taken to the same extent as found with that needed to support insurance decisions. However, some attempts to provide this type of information have been made. An example is the collection of estimates for household components provided by the Old House Web which gathers data from the Appliance Statistical Review and various institutes involved with the homebuilding trade.

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References

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