reliability engineering

Mechanical components suffer from interactions and degradations of overloads, strength deterioration, wear, corrosion, process variations during the fabrication process, effects of special processes where the procedures must be controlled as discovery of the end results would result in destruction of the component, and removal of safety factors by increasing loads.

Suppliers have two strategies for testing: 1) test for success and 2) test for failures. Reliability testing produces failures, particularly when the tests are accelerated with extra loads, and this may be troublesome to have in the records for future lawsuits. Thus it is often to everyone's advantage to perform reliability test under code names to protect against the broad rules of legal discovery.

Probability plots make sense of the chaos of failure data on an X-Y plot.

Most business decision have considerable uncertainty which implies at least two outcomes if you choose a course of action. Making decisions in the face of uncertainty requires the costs for taking action and the probability along with the cost for not taking action and the probability of the occurrence. In most cases the probabilities are not well known (maybe to one significant digit) and the costs are not well know (maybe to $10000). The quantitative assessment is called risk assessment. The issue is to take these not well identified issues and devise a strategy which can minimize exposure to risk for the business. The graphical representation of the methodology is called decision trees to reach the expected values for decision to take/not-take action.

The measure of the ability of an item to be retained in or restored to specified condition when maintenance is performed by personnel having specified skill levels, using prescribed procedures and resources.

Say what you want and want what you say to your vendors. Provide explanations of the objectives in contracts in terms the vendors will understand.

Reliability audits verify your reliability program is effective and find areas of weakness for corrective action. They are inquiries by factual examination of elements of the system with a written an objective criteria for performance beginning with an assessment of how management is involved and are they effective in building an productive reliability program.

The cost of unreliability is a big picture view of system failure costs, described in annual terms, for a manufacturing plant as if the key elements were reduced to a series block diagram for simplicity. It looks at the production system and reduces the complexity to a simple series system where failure of a single item/equipment/system/processing-complex causes the loss of productive output along with the total cost incurred for the failure. If the system IS sold out, then the cost of unreliability must include all appropriate business costs such as lost gross margin plus repair costs, scrap incurred, etc. If the system is NOT sold out, and make-up time is available in the financial year, then lost gross margin for the failure cannot be counted. The cost of unreliability is a management concern connected to management's two favorite metrics: time and money.

Life cycle cost (LCC) are all costs associated with the acquisition and ownership of a system over its full life. The usual figure of merit is net present value (NPV). Projects are considered most favorable for large positive NPVs. However for many cost individual cases, decisions are made for the least negative NPVs. In all cases, the default position for accounting is to know the NPV for making no change and this is usually the last alternative for most people associated with change.

For inexpensive components and inexpensive tests, simultaneous tests involve many components under test loads/conditions at the same time for the purpose of quickly acquiring data and producing test analysis as the failures occur. In simultaneous testing the suspensions (censored data) become important details for use in the statistical analysis. Most simultaneous tests are accelerated to generate the data in a short period of time although this carries the risk of introducing unexpected failure modes (but this can also be useful information for anticipating field failures).

Fault tree analysis (FTA) is a top down processes of defining the top level problems and through a deductive approach using parallel and series combinations of possible malfunctions to find the root of the problem and correct it before the failure occurs. The reliability tool can be used as qualitative or quantitative methods.

Failure rates, in the simplest form, are S(time in use)/S(number of failures) or the reciprocal of mean times to/between failure.