4S Reliability McGraw-Hill/Irwin Copyright © 2007 by The McGraw-Hill Companies, Inc All Learning Objectives  Define reliability  Perform reliability computations  Explain the purpose of redundancy in a system 4S-2 Reliability  Reliability: The ability of a product, part, or system to perform its intended function under a prescribed set of conditions  Failure: Situation in which a product, part, or system does not perform as intended  Normal operating conditions: The set of conditions under which an item’s reliability is specified 4S-3 Reliability is a Probability  Probability that the product or system will:  Function when activated  Function for a given length of time  Independent events  Events whose occurrence or nonoccurrence not influence each other  Redundancy  The use of backup components to increase reliability 4S-4 Rule Lamp 90 Lamp 80 90 x 80 = 72 4S-5 Rule 80 Lamp (backup) 90 + (1-.90)*.80 = 98 90 Lamp 4S-6 Rule 70 80 90 Lamp (backup for Lamp 2) Lamp (backup for Lamp1) – P(all fail) 1-[(1-.90)*(1-.80)*(1-.70)] = 994 Lamp 4S-7 Example S-1 Reliability Determine the reliability of the system shown 98 90 92 90 95 4S-8 Example S-1 Solution The system can be reduced to a series of three components 98 90+.90(1-.90) 95+.92(1-.95) 98 x 99 x 996 = 966 4S-9 Failure Rate Failure Rate Figure 4S.1 Infant mortality Failures due Few (random) failures to wear-out Time, T 4S-10 Exponential Distribution Figure 4S.2 Reliability = e -T/MTBF 1- e -T/MTBF T Time 4S-11 Normal Distribution Figure 4S.3 Reliability z 4S-12 Availability  The fraction of time a piece of equipment is expected to be available for operation Availability  MTBF MTBF  MTR MTBF = mean time between failures MTR = mean time to repair 4S-13 Improving Reliability        Component design Production/assembly techniques Testing Redundancy/backups Preventive maintenance procedures User education System design 4S-14