Failures and Reliability Adam Adgar School of Computing

Failures and Reliability Adam Adgar School of Computing and Technology

Failure ► Definition: n Non-conformance to some defined performance criterion. ► Other n n n terms Defect Malfunction Failure Fault Reject

Failure Classifications ► Cause n n n ► Production related Stress related Misuse Inherent Wearout Maintenance induced Suddenness n n Immediate Gradual ► Degree n n n ► Definition n n ► Catastrophic Intermittent Partial Applicable to specification Not applicable Result n n n Critical Major Minor

Reliability ► Definition: n ► ► Probability that an item performs its intended function for a stated period of time under specified operating conditions Manufacturing processes, machinery, systems are becoming more complex Reliability of whole process is dependent on all of the facilities, machines, components involved Reliability depends on design, operations, maintenance Generally defined as the ability to perform as expected over time (process, machine, product, person…)

Reliability ► Reliability of systems depends on many components ► Important concept because it is considered in several important issues n n n Probability of failure System criticality Appropriate maintenance strategy

Maintainability Probability that a system can be retained in, or one that has failed can be restored to, operating condition in a specified amount of time. ► Maintainability is the totality of design factors allowing maintenance to be accomplished easily ► ► Design Issues n Access of parts for repair n Modular construction and standardization n Diagnostic repair procedures and expert systems

Reliability Measurement ► Failure rate ► Mean time to failure n FR (%) = number of systems failed number of systems tested ► MTTF Mean time between failures MTBF = FR (N) = number of systems failed number of operational hours ► operating time for system number of systems failed Mean time to repair n MTTR

200 300 Thirty Identical 6309 Deep Groove Ball Bearings Run to Fatigue Failure Under Test l Load Conditions 100 Millions of Revolutions MTTF Information 1 5 10 15 Bearing Number 20 25 30 From: Ball and Roller Bearings: Theory, Design, & Application, Eschmann, et al John Wiley & Sons. 1985

Example 1 ► Situation n n A company is interested in learning more about the reliability of one type of its production machines. The company tests 12 machines over a period of 1 week. During the test, one machine failed at 20 hours, another after 25 hours and another after 35 hours. The company operates day shift only (Mon-Fri) with a typical shift being 8 hours long. ► Required n Failure rate as a percentage w FR(%) n Number of failures during a period of time w FR(N) n Mean time between failures w MTBF n Estimated number of failures per shift given there are 100 production machines in total

Failure Rate Curve Infant Mortality Period Normal Life Period Wear. Out Period

Average Failure Rate

System in Series X Component 1 Component 2 Component n Y

System in Parallel Component 1 X Component 2 Component n Y

Simple Example ► Series X ► R 1 R 2 Y Parallel R 1 X Y R 2 R 1 = 99% R 2 = 95% Rs = 0. 99 x 0. 95 Rs = 0. 9405 Rs = 1 - (1 -0. 99) x (1 -0. 95) Rs = 1 – (0. 01 x 0. 05) Rs = 0. 9995 Rs = 94. 05 % Rs = 99. 95 %

Average Component Reliability Number of components 1 5 10 50 100 200 System Reliability % 99. 0 95. 1 90. 4 60. 5 36. 6 8. 11 0. 66 500

Consequences Failure ► Production n n or Mission Impact Quantity Quality ► Environmental ► Health ► Safety ► Life Cycle Cost ► Morale

Example 3 ► No. breakdowns No. per month Situation n A consulting company has a thermographic camera which it uses to perform machinery inspections for its clients. The company estimates the cost associated to lost time for each breakdown of the camera is £ 500. Based on data extracted from the CMMS over the last 2½ years the table opposite was compiled n ► 0 1 2 3 4 5 2 5 4 9 7 3 The camera manufacture has offered a monthly service contract for £ 900. Past data has shown this to reduce breakdowns to only one every two months. Required n Determine whether the company should purchase the service contract