Presentation of the reliability information


The following data relating to the number of failures and time of failure of a given product or part or component or machine etc are calculated to illustrate or project the reliability information in the form of charts or curves:

1. Number of failures during a given period as a proportion of the total is calculated. It is called the failure probability rate. Suppose out of 200 time bombs, 2 bombs fail. The rate of failure i.e. the failure probability here is 0.01 or say 1%.

2. Total number of failures up to a given period, expressed as a proportion of the total is calculated. Here cumulative total is considered. For example, if during the first, second and third period, the number of bombs failed per 100 is 2, 4 and 3 respectively, the cumulative total of bombs failed comes to 9 and the proportion of failures comes to

2+3+4 / 100+100+100 = 0.03 or say 3%

3. Total number of survivors up to a given time expressed as a proportion to the total is calculated. It is called the cumulative survival probability rate.

4. Sometimes, the total number of failures during a given period is expressed as a proportion of the total number of survivors. This rate is called the conditional failure rate.

If a graph is plotted with the time on X-axis and failure probability rate on Y-axis and the plotted curve is shooting upwards then the reliability of a product or a part decreases as the time goes on. On the other hand, if the curve starts at the higher probability rate and comes down towards the lower end of the probability as the time elapses then the reliability of the product or a part increases as the time goes on.

How can it be improved?

Reliability is measured to see to what extent a product or a part or a machine or a component or a process is reliable. If the reliability shows deceasing trend, corrective actions to improve the reliability are taken.

Reliability of a product depends on the reliability of parts or material used in manufacturing that product and also on the reliability of tools and equipments, machines, methods or production processes etc. used for the manufacture of that product. Therefore to improve the reliability of a certain product:

1. More perfect parts should be used to make the product

2. Standard quality materials, purchased from the reliable suppliers should only be used

3. Plant and machinery, tools and equipment must be repaired and maintained properly. A car, if checked after a journey of every 500 km, and if properly maintained, may not put the owner in difficulty in the middle of the way

4. Standard and suitable tools and equipment should only be used

5. Parts or tools or machines should be replaced before they become unreliable. For example, if a bulb with an estimated useful life of 1,000 hours is replaced at the interval of every 800 to 900 hours, there will be no possibility of its failure. This is already in vogue for various critical items used in a plant or institution and is called “Preventive Maintenance�.

Use of Computers in Quality Control:

These days there is software available for Quality Control (QC). There is a provision for controlling the process on-line. The software packages are programmed to take care of all critical aspects of functioning of components and can provide a feedback to the concerned managers at the desired frequency. These software programs of QC make the QC work much more comfortable than it previously ever was and allow sufficient time for taking corrective actions.