SKEDSOFT

Introduction:

Replacement model is the part of operation research mostly used in the industries when a purchased items like machinery, buildings efficiency is reduced or wear out due to much usage.

failure mechanism of items:

The word failure has got a wider meaning in industrial maintenance than what it has in our daily life.

We can categorize the failure in two classes. They are (i) Gradual failure and (ii) Sudden failure.

Once again the sudden failure may be classified as: (a) Progressive failure, (b) Retrogressive failure and (c) Random failure.

Gradual failure:

In this class as the life of the machine increases or due continuous usage, due to wear and tear of components of the facility, its efficiency deteriorates due to which the management can experience:

(a) Progressive Increase in maintenance expenditure or operating costs, (b) Decreased productivity of the equipment and (c) decrease in the value of the equipment i.e. resale value of the equipment/facility decreases.

Examples of this category are: Automobiles, Machine tools, etc.

 Sudden failure:

In this case, the items ultimately fail after a period of time. The life of the equipment cannot be  predicted and is some sort of random variable. The period between installation and failure is not constant=for any particular type of equipment but will follow some frequency distribution, which may be:

(a) Progressive failure: In this case probability of failure increases with the increase in life of an item. The best example is electrical bulbs and computer components.

(b) Retrogressive failure: Some items will have higher probability of failure in the beginning of their life, and as the time passes chances of failure becomes less. That is the ability of the item to survive in the initial period of life increases its expected life. The examples are newly installed machines in production systems, new vehicles, and infant baby (The probability of survival is very less in infant age, but once the baby get accustomed to nature, the probability of failure decreases).

(c) Random failure: In this class, constant probability of failure is associated with items that fail from random causes such as physical shocks, not related to age. In such cases all items fail before aging has any effect.