Lecture 1

INTRODUCTION, IMPORTANCE & DEFINITION

Reliability is an engineering discipline for applying scientific know-how to a component, assembly, plant, or process so it will perform its intended function, without failure, for the required time duration when installed and operated correctly in a specified environment.

Reliability is "quality changing over time" or A motion picture instead of a snapshot

Reliability is a measure of the result of the quality of the product over the long run. Reliability terminates with a failure—i.e, unreliability occurs. Business enterprises observe the high cost of unreliability. The high cost of unreliability motivates an engineering solution to control and reduce costs.

MIL-STD-721C Definitions of Terms for Reliability and Maintainability gives the following definition for reliability:

Reliability is the probability than an item can perform its intended function without failure for a specified interval under stated conditions.

This definition provides the following four aspects of reliability:

  1. Reliability is a probability based concept; the numerical value of the reliability is between 0 and 1
  2. The functional performance of the product has to meet certain stipulations and a functional definition of failure is needed. For example, a failure means different things to the user and to the repair person.
  3. It implies successful operation over a certain period of time
  4. Operating or environmental conditions under which product use takes place are specified

Cost of Unreliability

Cost improvement efforts are more productive when motivated from the top-down rather than bottom-up because it is a top management driven effort for improving costs. Finding the cost of unreliability (COUR) starts with a big-picture view and helps direct cost improvement programs by identifying:

  1. Where is the cost problem--what sections of the plant,
  2. What magnitude is the problem--all business loss costs are included in the calculation, and
  3. What major types of problems occur

Cost of unreliability programs study plants as links in a chain for a reliability system, and the costs Incurred when the plant, or a series of plants, fail to produce the desired result.

Cost of unreliability begins with the big picture of failures to produce the desired business results driven by failures of the process or it's equipment. Elements of the process are considered as a series reliability model comprising links in a chain of events that deliver success or failure. Logical block diagrams of major steps or systems are identified. Failure costs are calculated by category expecting that history tends to repeat in a string of chance events unless the problems have been permanently removed and success demonstrated by objective measures.  

DESIGNING FOR RELIABILITY

Reliability does not just happen. It requires that the following three key elements be in place

  1. A commitment from top management to ensuring reliability
  2. A reliability policy (that goes hand-in-hand with a Quality Policy)
  3. A philosophy that designs reliability in at an early stage

  WAYS TO IMPROVE RELIABILITY

  • Use proven designs
  • Use the simplest possible designs
  • Use proven components that have undergone reliability component testing
  • Use redundant parts in high risk areas. Placing two components in parallel will reduce the overall probability of failure
  • Always design fail safe
  • Specify and use proven manufacturing methods

MEASURES OF RELIABILITY

Reliability is the probability that a system will still be functioning at time t.

This can be expressed as “the cumulative distribution of failure”

These two measures are the mirror image of each other (Refer Figure below). The reliability will start at 1 and decay to approach 0 over time. The cumulative distribution of failure will start at 0 (no failures) and approach 1 as all the items fail over time. The slope of the reliability curve at any time t is the failure rate at that point in time. These measures give the overall reliability or failure at time t

Probability density function

We wish to have an idea of the probability of an item failing in a given unit time period. This is termed the “probability density function” and is given by

The failure or hazard rate gives the failure density over a period of time as with the “probability density function”, but is based on the current population. This gives a much better indication of the changing reliability of a system over time.

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