The term ‘failure’ here will be discussed in the context of replacement decisions. There are two types of failure:
(1) Gradual failure, and
(2) Sudden failure.
Gradual failure is progressive in nature. That is, as the life of an item increases, its operational efficiency also deteriorates resulting in
Increased running (maintenance and operating) costs.
Decrease in its productivity.
Decrease in the resale or salvage value.
Mechanical items like pistons, rings, bearings, etc. and automobile tyres fall under this category.
This type of failure occurs in items after some period of giving desired service rather than deterioration while in service. The period of giving desired service is not constant but follows some frequency distribution which may be progressive, retrogressive or random in nature.
Progressive failure: - If the probability of failure of an item increases with the increase in its life, then such failure is called progressive failure. For example, light bulbs and tubes fail progressively.
Retrogressive failure: - If the probability of failure in the beginning of the life of an item is more but as time passes the chances of its failure become less, then such failure is said to be retrogressive.
Random failure: - In this type of failure, the constant probability of failure is associated with items that fail from random causes such as physical shocks, not related to age. For example, vacuum tubes in air-born equipment have been found to fail at a rate independent of the age of the tubes.