As illustrated in the figure below, maintainability can be expressed either as a measure of the time (T) required to repair a given percentage (P%) of all system failures, or as a probability (P) of restoring the system to operational status within a period of time (T) following a failure. 1.5    4 These and other analogous functions are summarized in the following table. That’s why improving asset availability is so crucial to production facilities. The first is total uptime and the second is total downtime. g(t = 0.5586) = 0.3447, Median Time to Repair = 1.9325 hours There are two components to the system availability formula. For example, in the calculation of the Overall Equipment Effectiveness (OEE) introduced by Nakajima [], it is necessary to estimate a crucial parameter called availability.This is strictly related to reliability. Information Required You must provide the following data to perform maintainability prediction: • The list of maintenance tasks or activities, including the number and description of all steps required to restore each replaceable item. That means the oven was only available for eight hours. within the context of a model. Bazovsky, Igor, Reliability Theory and Practice 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? 2.7    1 Although not required, a set of functions is often detailed at the outset of a product development program. Points 1, 2, and 3 shown in the figure identify the mean, median, and maximum corrective time-to-repair, respectively. Inherent Availability 5. μ(30) = g(30)/(1 – M(30)) = 0.035/(1 – 0.53) = 0.074 repairs/minute. The maintainability function, or probability of repair in time t,  is given by: If the maintainability function, M(t), is known, the MTTR can also be There are other examples of such descriptions[3,4,36]. Uptime is any time that asset is performing at a normal output. 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. A lack of maintainability will be evident as high product maintenance … Here’s an example of the system availability formula in action: One of your top production assets ran for 100 hours last month. 1.1    1 Your maintenance should too. Unapplicable may be assigned to as many levels as required. If an asset is failing often in spite of regular PMs, there could be other reasons for failure. 3. MTTR. 9       1 Maximum Time to Repair, Mmax ct , the maximum time required to complete a specified percentage of all maintenance actions. At the end of the month, you can see that there was 100 hours of uptime on the machine and 10 total hours of downtime on the machine. 3. Standard operating procedures and maintenance checklists provide structure to your operation so work is done accurately and quickly. 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. The maintainability function M(t) for 100 minutes, or the probability that the warranty requirement is being met, 4. 7       1 1. The above example data comes directly from MIL-HDBK-338, page 5-44, Ground Electronic System Maintainability Analysis, Example 1. b) 12 hours a day. A good way to look at basic maintainability concepts is in terms of functions which are analogous to those in reliability. The most commonly used distributions for maintainability analysis have been the normal, lognormal, and exponential. are 29.16 minutes as calculated in step 1 above. No. Dome cooling. 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 most commonly used values of φ or Z(t1-α) are: Given the following system repair times and frequencies of repair time observations, what is the mean time to repair and the maximum time to repair at the 95% level, Mmax95%? All Rights Reserved, A look at the tools that empower your maintenance team, Manage maintenance from anywhere, at any time, Track, control, and optimize asset performance, Simplify the way you create, complete, and record work, Connect your CMMS and share data across any system, Collect, analyze, and act on maintenance data, Make sure you have the right parts at the right time, AI for maintenance. This section will concentrate on the use of the normal, exponential, and lognormal distribution, and give examples of their use in maintainability analysis. Birolini, Alessandro, Reliability Engineering: Theory and Practice, Reliability Engineering: Theory and Practice. To calculate system availability for a certain period of time, divide an asset’s total amount of uptime by the sum of total uptime and total downtime. [/math] is given by: Maintenance task times of this nature are usually normally distributed, producing a probability density function given by: Mcti = repair time for an individual maintenance action. The asset broke down twice during the month, and it took one hour to repair each time. The calculation is as follows: ... MTTR is the measure of the repairable items maintainability. Maintenance checklists guide technicians during repairs and PMs. To the failure rate function in reliability corresponds the repair rate function in maintainability. 2       2 Every company wants its equipment to work as much as possible. 10.3  1 * • Not considered: Next, look at historical failure codes to determine if you can avoid breakdowns by introducing a new PM. Values of ke are: α            ke which is equal to the mean time to repair because of the symmetry of the Thecombined system is operational only if both Part X and Part Y are available.From this it follows that the combined availability is a product ofthe availability of the two parts. 3.3    2 2. The lognormal distribution applies to most maintenance tasks and repair actions comprised of several … The discipline’s first concerns were electronic and mechanical components (Ebeling, 2010). Such descriptions capture what is intuitively meant with maintainability, but have some problems. 22     1 Understanding how reliability and maintainability affect availability is key to improving it at your facility. Time Freq. This guidance directs Services to implement RAM practices that ensure effective collaboration between the requirements and acquisition communities in the establishment of RAM requirements. This may be due to the fact that maintainability has traditionally lagged reliability theory in development. Comparison of Reliability and Maintainability Functions. 2. MIL-HDBK-338, Electronic Reliability Design Handbook, 15 Oct 84 During product development, the design is regularly evaluated or tested and compared to the desired set of functions. exponential function at which Mmaxct is defined. Levelling up with improved system availability. Cost may or m… 4.5    1 Availability is improved by increasing reliability and reducing maintenance time. 24.5  1. ISO/IEC 9126).Closely related concepts in the software engineering domain are evolvability, modifiability, technical debt, and code smells. If a piece of equipment isn’t available at the right times, it can mean more than just a late dinner — it can mean late orders, missed shipments, broken contracts, upset customers, and lost money. and the average repair time for N observations is: The standard deviation of the distribution of repair times, based on N observations is. If you’re looking to benchmark your operation against the best in the world, top-tier organizations achieve system availability of 90% or higher. When you translate this example to a facility that relies on producing things, you can see how important system availability can be. 3.2.1 The test method to be used to demonstrate Maintainability is often determined by the Maintainability parameter of interest. This section will concentrate on the use of the normal, exponential, and lognormal distribution, and give examples of their use in maintainability analysis. Improving system availability can have a ripple effect across your entire organization. In maintainability, the random variable is time-to-repair, in the same manner as time-to-failure is the random variable in reliability. Maintainabilityrefers to the ease with which maintenance activities can be performed on an asset or equipment. A shorter MTTR means higher maintainability. The maintainability of an asset is commonly measured with mean time to repair (MTTR). 5.5    1 Eight questions shaping the future of maintenance. Values from the probability density of time-to-repair data shown on page 5-49 are duplicated by the tool along with the plot of the lognormal pdf that is shown on page 5-50, and the plot of the maintainability function, M(t), that is shown on page 5-54.  The MTTR is calculated on page 5-52 as 3.595 hours, the median time to repair is calculated on page 5-52 as 1.932 hours and the 95 % value for Mmax are shown on page 5-55 is 12.08 hours. Average Uptime Availability (or Mean Availability) 3. Median Time to Repair, Mct  the downtime within which 50% of all maintenance actions can be completed. If you build a valuable product that works perfectly but is difficult to modify and adapt to new requirements, it will not survive in today’s market. Thus, another expression for g(t) in terms of u(t), the repair rate is. 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. If failure rates are low, revisit the frequency of PMs on that asset. A shorter MTTR means higher maintainability. Get the Executive's Guide to Digital Transformation and find out how. The following are illustrative examples. 8.8    1 95%       3.00 The average time it takes to fully repair a failed system. where u is the repair rate (which is constant for the exponential case) Start by looking at failure rates to optimize PM intervals. This can be found to be 43 minutes by rolling the mouse over the cumulative R(t) curve until a value of 0.1 is found.  Since this represents the green shaded area of the pdf curve, the red shaded area is 0.90.  This occurs at 43 minutes, as shown in the pop-up in the figure below (ignore units of hours shown on plot). If a piece of equipment isn’t available at the right times…it can mean late orders, missed shipments, broken contracts, upset customers, and lost money. A shorter MTTR means higher maintainability. Collectively, they affect both the utility and the life-cycle costs of a product or system. It is embodied in the design of the product. Maintainability is the ability of the application to go through changes with a fair degree of effortlessness. Maintainability is the measure of ability to successfully repair or fix the product after manufacturing, usually in the field, and over time. Welcome to our series of blog posts about maintenance metrics. The MTTR clock starts ticking when the repairs start and it goes on until operations are restored. Maintainability measures the ability to maintain or restore a piece of equipment to its functioning state. When equipment is running, money is being made. You spend half an hour cleaning the oven and half an hour pre-heating it. Find the following: 2. It is a function of the equipment design and installation, personnel availability in the required skill levels, adequacy of maintenance procedures and test equipment, and the physical environment under which maintenance is performed. For a large computer installation, the maintenance crew logbook shows that over a period of a month there were 15 unscheduled maintenance actions or downtimes, and 1200 minutes in emergency maintenance status. 7.5    1 Day only, e.g. This attribute is the flexibility with which the application can be modified, for fixing issues, or to add new functionality with a degree of ease. The classification of availability is somewhat flexible and is largely based on the types of downtimes used in the computation and on the relationship with time (i.e., the span of time to which the availability refers). The combination of high reliability and high maintainability results in high system availability. It means maintenance resources are being used efficiently, production levels are as high as possible, and your company’s margins can be healthier than ever. 3. 4       2 5.4    1 d) n hours per use. O’Connor, Patrick, D. T., Practical Reliability Engineering Analyze your data, find trends, and act on them fast, Explore the tools that can supercharge your CMMS, For optimizing maintenance with advanced data and security, For high-powered work, inventory, and report management, For planning and tracking maintenance with confidence, Learn how Fiix helps you maximize the value of your CMMS, Get best practices, helpful videos, and training tools, How a leader in the oil industry used Fiix to slash inventory costs, Discover guides full of practical insights and tools, Read how other maintenance teams are using Fiix, Get the latest maintenance news, tricks, and techniques. 3       2 Sometimes referred to as maintainability, MTTR is the measure of the department’s ability to perform maintenance to retain or restore assets to a specified condition. 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. System availability is a metric used to measure the percentage of time an asset can be used for production. An example of a discrete maintainability parameter is the number of maintenance actions completed in some time t, whereas an example of a continuous maintainability parameter is the time to complete a maintenance action. metric that measures the probability that a system is not failed or undergoing a repair action when it needs to be used Failure codes tell you everything about why an asset failed so you can quickly understand how to make it right. The study of component and process reliability is the basis of many efficiency evaluations in Operations Management discipline. 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. Availability = 0.909 After all, when equipment is running, money is being made. E.g. It calculates how easy it is to identify problems with a system and solve the problem. An entity with relatively low costs in these areas is considered maintainable whereas an entity with high costs may be considered unmaintainable or "high maintenance." For example, think about your car — reliability is having it start every time you put the key in the ignition. Tasks can be done faster and done properly, human error is minimized, and equipment is more reliable. Maintenance can have a huge influence on system availability, from the way preventive maintenance is scheduled and carried out, to the response of the team when an asset breaks down unexpectedly. This includes repair time, testing period, and return to the normal operating condition. Software is not static. Failures during production testing will be reported and managed in accordance with the Company Quality Manual. Maintainability depends on the product design and the technical level of repair personnel, the repair process, and the repair facilities. They also help you ensure breakdowns don’t become a problem in the first place. There is a 71.4 % chance of meeting the warranty requirement. When unplanned downtime strikes, failure codes allow technicians to respond quickly, troubleshoot better, and get an asset up and running sooner. Mean Time to Repair, Mct, the mean time required to complete a maintenance action, i.e., total maintenance downtime divided by total maintenance actions for a given period of time, given as: λi = failure rate for the ith repairable element of the item for which maintainability is to be determined, adjusted for duty cycle, catastrophic failures, tolerance and interaction failures, etc., which will result in deterioration of item performance to the point that a maintenance action will be initiated, Mcti = average corrective time required to repair the ith repairable element in the event of its failure, 2. To the probability of system failure, or system unreliability, corresponds the probability of successful system maintenance, or system maintainability. MTTR calculation purposes. 4.7    1 Lastly, eliminate unnecessary PMs. The time within which 95% of the maintenance actions can be completed, the repair rate, μ is 1/80 = 0.0125 repairs/minute, 2. An equipment whose repair times are assumed to be normally distributed was monitored and the following repair times observed (in minutes): 6.5, 13.25, 17.25, 17.25, 19.75, 23, 23, 24.75, 27.5, 27.5, 27.5, 32, 34.75, 34.75, 37.5, 37.5, 40.25, 42.5, 44.75, 52. of uses per year. It counts the number of decisions in the given program code. Night only, e.g.. Mirror surface heating. 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. Downtime is any time the equipment is not available for production, including planned and unplanned downtime. They also dictate how work orders are handled so resources are used efficiently on critical assets. If an asset never fails, it is 100% reliable. For example, let’s say you’re cooking the turkey for Thanksgiving and have 10 hours until dinner. Preventive maintenance affects availability by helping to reduce failure and by taking equipment out of production for routine maintenance. The set of product functions or features defines the operating state and, conversely, what a system failure may include. 1.3    1 Reliability and maintainability achievement will be monitored during all development testing. The oven also breaks down for an hour. Copyright © 2020. MAINTENANCE: All actions necessary for retaining an item in or restoring it to a specified condition. Availability = uptime ÷ (uptime + downtime). Maintenance SOPs help your team keep one eye on the present and the other on the future. Calculation of the state function of a series-parallel. Breakdowns are fixed, but they’re also investigated so the root cause is dealt with. 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. Typically includes fault isolation, removal, and replacement of failed item(s) and checkout. Reliability and maintainability are two factors that are heavily influenced by maintenance. ke = value of Mct./MTTR at the specified percentage point α on the The asset was also shut down for eight hours in a month for routine inspections. The maintainability function M(t) for 100 minutes. Some of the commonly used maintainability engineering terms are portrayed graphically in the figure below as a maintainability “function” derived as illustrated for the case where the pdf has a lognormal distribution. Creating better processes and using all the tools at your disposal are the quickest ways the maintenance team can make a big impact on availability and help the business take a step forward. The tool also generates the following plots showing the lognormal PDF and lognormal CDF plots based on the repair times. These are the same for the normal distribution because of its symmetry, and 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). Establishment of RAM requirements cause consistent failure, you can make changes to your operation more and! When unplanned downtime the asset broke down twice during the month, and an. Out of production for routine maintenance other on the present and the second total... 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