Reliability Analysis

It is defined as the probability that a given system will perform it’s function adequately for it’s specified period of lifetime under specified operating conditions.

Common measures are :

1. Failure rate.
2. Mean time between failures(MTBF)
3. Survival percentage.

Failure Rate:

Rate which components of population fail.

R(t)=N_s(t)/N_f(t)

 

Where,
            N_s(t)- No. of components that survived during time ’t’
            N_f(t) – No. of failures that occurred during the same time.


Mean time between Failures(MTBF):

The reciprocal of the failure rate(1/λ).
Where
           λ —-Failure rate.

Failure rate = (No.of failure )/(Time period during all components were exposed to failure)

Reliability Analysis:
Failure Mode and Effect Analysis(FMEA)

Purpose :
1. To recommend design changes.
2. To identify design weakness.
3. To help in choosing alternatives.

Four Stages:
I^st Stage – System boundaries and the scope of the analysis is decided.
II^nd Stage – Data Collection
                      Ex: Specification,Operating Procedure,Working Conditions.
III^rd Stage- Preparing the component or parts list.

IV^th Stage – Failure frequency and the functions of the part identified,causes of       failures,Failure detection.

FMEA | Failure Mode And Effect Analysis

Failure Mode – A particular way in which an item fails, independent of the reason for failure.

 Failure Mode and Effects Analysis (FMEA) – A procedure by which each credible failure mode of each item from a low indenture level to the highest is analyzed to determine the effects on the system and to classify each potential failure mode in accordance with the severity of its effect.

Indenture Levels – The hierarchy of hardware levels from the part to the component to the subsystem to the system, etc.

Redundancy – More than one independent means of performing a function.  There are different kinds of redundancy, including:
            (1) Operational – Redundant items, all of which are energized during the operating cycle; includes load-sharing, wherein redundant items are connected in a manner such that upon failure of one item, the other will continue to perform the function.  It is not necessary to switch out the failed item or switch in the redundant one.

            (2) Standby – Items that are inoperative (have no power applied) until they are switched in upon failure of the primary item.

(3) Like Redundancy – Identical items performing the same function.

            (4) Unlike Redundancy – Non identical items performing the same function

THE FMEA PROCESS

• Define the system to be analyzed.  A complete system definition includes identification of internal and interface functions, expected performance at all indenture levels, system restraints, and failure definitions.  Also state systems and mission phases not analyzed giving rationale for the omissions.
• Indicate the depth of the analysis by identifying the indenture level at which the analysis is begun.
• Identify specific design requirements that are to be verified by the FMEA.
• Define ground rules and assumptions on which the analysis is based.  Identify mission phases to be analyzed and the status of equipment during each mission phase.
• Obtain or construct functional and reliability block diagrams indicating interrelationships of functional groups, system operation, independent data channels, and backup or workaround features of the system.
• Identify failure modes, effects, failure detection and workaround features and other pertinent information on the worksheet.
• Evaluate the severity of each failure effect in accordance with the prescribed severity categories.

FMEA Flow Diagram:

History:

The FMECA was originally developed by the National Aeronautics and Space Administration (NASA) to improve and verify the reliability of space program hardware.

FMECA Flow Diagram: ( Failure Mode, Effects and Criticality Analysis )

Criticality Analysis Flow:

Who is the Team ?

Areas to be represented are:

• Quality

• Logistics

• Engineering

• Purchasing

• Manufacturing

• Sales

• Tooling

• Marketing

• Customer

• Supplier