What is Reliability Growth?
2010-09-27 9:42:52
What is Reliability Growth?
In general, the first prototypes produced during the development of a new complex system will contain design, manufacturing and/or engineering deficiencies. Because of these deficiencies, the initial reliability of the prototypes may be below the system's reliability goal or requirement. In order to identify and correct these deficiencies, the prototypes are often subjected to a rigorous testing program. During testing, problem areas are identified and appropriate corrective actions (or redesigns) are taken. Reliability growth is the improvement in the reliability of a product (component, subsystem or system) over a period of time due to changes in the product's design and/or the manufacturing process. A reliability growth program is a well-structured process of finding reliability problems by testing, incorporating corrective actions and monitoring the increase of the product's reliability throughout the test phases. The term growth is used since it is assumed that the reliability of the product will increase over time as design changes and fixes are implemented. However, in practice, no growth or negative growth may occur.
Reliability goals are generally associated with a reliability growth program. A program may have more than one reliability goal. For example, there may be a reliability goal associated with failures resulting in unscheduled maintenance actions and a separate goal associated with those failures causing a mission abort or catastrophic failure. Other reliability goals may be associated with failure modes that are safety related. The monitoring of the increase of the product's reliability through successive phases in a reliability growth testing program is an important aspect of attaining these goals. Reliability growth analysis (RGA) concerns itself with the quantification and assessment of parameters (or metrics) relating to the product's reliability growth over time. Reliability growth management addresses the attainment of the reliability objectives through planning and controlling of the reliability growth process.
Reliability growth testing can take place at the system, major subsystem or lower unit level. For a comprehensive program, the testing may employ two general approaches: integrated and dedicated. Most development programs have considerable testing that takes place for reasons other than reliability. Integrated reliability growth utilizes this existing testing to uncover reliability problems and incorporate corrective actions. Dedicated reliability growth testing is a test program focused on uncovering reliability problems, incorporating corrective actions and typically, the achievement of a reliability goal. With lower level testing, the primary focus is to improve the reliability of a unit of the system, such as an engine, water pump, etc. Lower level testing, which may be dedicated or integrated, may take place, for example, during the early part of the design phase. Later, the system and subsystem prototypes may be subjected to dedicated reliability growth testing, integrated reliability growth testing or both. Modern applications of reliability growth extend these methods to early design and to in-service customer use. Reliability growth management concerns itself with the planning and management of an item's reliability growth as a function of time and resources.
Reliability growth occurs from corrective and/or preventive actions based on experience gained from failures and from analysis of the equipment, design, production and operation processes. The reliability growth "Test-Analyze-Fix" concept in design is applied by uncovering weaknesses during the testing stages and performing appropriate corrective actions before full-scale production. A corrective action takes place at the problem and root cause level. Therefore, a failure mode is a problem and root cause. Reliability growth addresses failure modes. For example, a problem such as a seal leak may have more than one cause. Each problem and cause constitutes a separate failure mode and, in some cases, requires separate corrective actions. Consequently, there may be several failure modes and design corrections corresponding to a seal leak problem. The formal procedures and manuals associated with the maintenance and support of the product are part of the system design and may require improvement. Reliability growth is due to permanent improvements in the reliability of a product that result from changes in the product design and/or the manufacturing process. Rework, repair and temporary fixes do not constitute reliability growth.
Screening addresses the reliability of an individual unit and not the inherent reliability of the design. If the population of devices is heterogeneous then the high failure rate items are naturally screened out through operational use or testing. Such screening can improve the mixture of a heterogeneous population, generating an apparent growth phenomenon when in fact the devices themselves are not improving. This is not considered reliability growth. Screening is a form of rework. Reliability growth is concerned with permanent corrective actions focused on prevention of problems.
Learning by operator and maintenance personnel also plays an important role in the improvement scenario. Through continued use of the equipment, operator and maintenance personnel become more familiar with it. This is called natural learning. Natural learning is a continuous process that improves reliability as fewer mistakes are made in operation and maintenance, since the equipment is being used more effectively. The learning rate will be increasing in early stages and then level off when familiarity is achieved. Natural learning can generate lessons learned and may be accompanied by revisions of technical manuals or even specialized training for improved operation and maintenance. Reliability improvement due to written and institutionalized formal procedures and manuals that are a permanent implementation to the system design is part of the reliability growth process. Natural learning is an individual characteristic and is not reliability growth.
The concept of reliability growth is not just theoretical or absolute. Reliability growth is related to factors such as the management strategy toward taking corrective actions, effectiveness of the fixes, reliability requirements, the initial reliability level, reliability funding and competitive factors. For example, one management team may take corrective actions for 90% of the failures seen during testing, while another management team with the same design and test information may take corrective actions on only 65% of the failures seen during testing. Different management strategies may attain different reliability values with the same basic design. The effectiveness of the corrective actions is also relative when compared to the initial reliability at the beginning of testing. If corrective actions give a 400% improvement in reliability for equipment that initially had one tenth of the reliability goal, this is not as significant as a 50% improvement in reliability if the system initially had one half the reliability goal.