In maintainability analysis, the exponential distribution applies to maintenance tasks and maintenance actions whose completion times are independent of previous maintenance experience (e.g., substitution methods of failure isolation where several equally likely alternatives are available and each alternative is exercised, one at a time, until the one which caused the failure is isolated), producing a probability density function given by: The fundamental maintainability parameter is repair rate, u(t), which is the reciprocal of Mct, the mean time to repair (MTTR). In this scenario, the availability would be 90%. For example, think about your car — reliability is having it start every time you put the key in the ignition. This section will concentrate on the use of the normal, exponential, and lognormal distribution, and give examples of their use in maintainability analysis. The following are illustrative examples. Your maintenance should too. Its maintainability [math]M\left( t \right)\,\! To the failure rate function in reliability corresponds the repair rate function in maintainability. and the average repair time for N observations is: The standard deviation of the distribution of repair times, based on N observations is. Failure codes tell you everything about why an asset failed so you can quickly understand how to make it right. The oven also breaks down for an hour. 2.7    1 1. Understanding how reliability and maintainability affect availability is key to improving it at your facility. 3. Improving system availability can have a ripple effect across your entire organization. 4.5    1 For example, if a system specification indicated Mmax ct(95%) = 1 hour, this means that 95% of all maintenance actions must be completed within one hour. The second curve shows the reliability at 30, which is represented by the green shaded area in the pdf plot.  We are interested in the pink shaded area, which is one minus this value, or 0.53. That means the oven was only available for eight hours. They do not in any way guide in how to estimate or measure maintainability. However, if prior knowledge exists, perhaps from a Maintainability estimation, then a sequential test can result in a significant reduction in sample size, otherwise a fixed sample size test would be required. The sum of corrective maintenance times divided by the total number of repairs of the item. Some of the commonly used maintainability engineering terms are shown on maintainability function, M(t), illustrated below. Click here to see the rest of the series. There is a 71.4 % chance of meeting the warranty requirement. ke is 3 for 95%, as shown in the list above. The M(t) curve is a cumulative density function that provides an estimate of the probability of completing an arbitrary system repair action in a given amount of time. Maintainability Measures • MTTR = A basic measure of maintainability. Cooling Plant. 4.7    1 System availability is a metric used to measure the percentage of time an asset can be used for production. The world has changed. It is a software metric that measures the logical complexity of the program code. Join over 14,000 maintenance professionals who get monthly CMMS tips, industry news, and updates. metric that measures the probability that a system is not failed or undergoing a repair action when it needs to be used The median time to repair is (0.69)(80) = 55.2 minutes. of uses per year. Instantaneous (or Point) Availability 2. Maintainability, then, is a measure of the ease and rapidity with which a system or equipment can be restored to operational status following a failure. Calculation of the state function of a series-parallel. Comparison of Reliability and Maintainability Functions. Asset availability is impacted by several factors. For example, if misaligned bearings cause consistent failure, you can make changes to your processes and tools to solve the problem. In reliability, one is concerned with designing an item to last as long as possible without failure; in maintainability, the emphasis is on designing an item so that a failure can be corrected as quickly as possible. This post outlines everything you need to know about system availability: What it is, how to calculate it, and why it’s important to your maintenance operation. Dome cooling. Next, look at historical failure codes to determine if you can avoid breakdowns by introducing a new PM. Such descriptions capture what is intuitively meant with maintainability, but have some problems. 95%       3.00 In fact, as the exponential distribution has been the one most widely used in reliability analysis of   equipment/systems, the lognormal distribution is the most commonly used for equipment/system maintainability analysis. Start by looking at failure rates to optimize PM intervals. 24.5  1. A warranty contract between the computer company and the customer calls for a penalty payment of any downtime exceeding 100 minutes. The time within which 95% of the maintenance actions can be completed, the repair rate, μ is 1/80 = 0.0125 repairs/minute, 2. 4       2 Cyclomatic Complexity may be defined as- 1. 5.5    1 which is equal to the mean time to repair because of the symmetry of the 1. Tasks can be done faster and done properly, human error is minimized, and equipment is more reliable. Reliability and maintainability achievement will be monitored during all development testing. That’s why improving asset availability is so crucial to production facilities. Here’s an example of the system availability formula in action: One of your top production assets ran for 100 hours last month. 8.8    1 Keep the design simple is difficult, and the payoff is fewer parts, fewer tools, less complexity, and organization needed to conduct maintenance (which screw goes where? 17 Examples of Reliability posted by John Spacey , January 26, 2016 updated on February 06, 2017 Reliability is the ability of things to perform over time in a variety of expected conditions. Maintainability is a probability measure that a product will be maintained or restored to the specified function for a specified period of … These and other analogous functions are summarized in the following table. Everyone desires products that offer more features, provide higher value, cost less and last longer. Looking down the fourth column, we see 0.9501 at a time of 12.17 hours, indicating that 95% of the repairs will take 12.17 hours, or less.  (Note, the exact time for 0.9500, as calculated by the tool, is 12.08 hours.). The tool calculates the following system maintainability parameters: Mode Time to Repair = 0.5586 hours The combination of high reliability and high maintainability results in high system availability. To the probability of system failure, or system unreliability, corresponds the probability of successful system maintenance, or system maintainability. 22     1 [/math] is given by: As an example, consider the maintainability equation for a system in which the repair times are distributed exponentially. That maintainability performance which is expected at some future time, postulated on analysis of models, test, and past experience. Levelling up with improved system availability. Breakdowns are fixed, but they’re also investigated so the root cause is dealt with. The asset was also shut down for eight hours in a month for routine inspections. This section will concentrate on the use of the normal, exponential, and lognormal distribution, and give examples of their use in maintainability analysis. If failure rates are low, revisit the frequency of PMs on that asset. Availability = 0.909 They may be derived in a way identical to that done for reliability, by merely substituting t (time-to-restore) for t (time-to-failure), μ (repair rate) for λ (failure rate), and M(t) probability of successfully completing a repair action in time t, or P(T ≤ t) for F(t) probability of failing by age t. In other words, the following correspondences prevail in maintainability and reliability engineering functions. From lines-of-code measures, McCabe measures and Halstead complexity measures a ripple effect across your entire organization probability system... 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