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Issue No. 42 – October 2010 |
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Since establishing this website last May of 2007, I have made many efforts to improve this site and provide some useful insights about our common link which is all about improving our equipment reliability and the way we do maintenance. Finally should you be interested to contribute to our articles section or share any comment or feedback, you may email me at Click Here !!!. If for any reason you wish to unsubscribe from our newsletter, kindly send my a blank email with unsubscribe as the heading and we shall remove you from our mailing lists. Once again welcome to our October 2010 Edition of our Monthly Reliability Newsletter for this year 2010 and I hope that you enjoy reading and hope you find this article worth sharing with your people.
My Warm Regards,
Website : www.rsareliaiblity.com Inspired by Change . . . |
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RELIABILITY & MAINTENANCE CONSULTANCY FIRM
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MTBF should not be used as a baseline for determining the frequency of replacement in our Preventive Maintenance, since MTBF is only about averaging. Determining the correct frequency of replacement should be based upon the useful life of the part or component, and this will only be applicable for age-related failures |
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By Rolly Angeles
Mean Time Between Failure (MTBF) Explained
Before anyone can attempt to calculate and use MTBF, it is very important that they have a crystal clear understanding on what will eventually constitute a failure and breakdown in the first place. Therefore, let us answer these following cases from Part 1 of this series of neweletter if they will be included in the MTBF calculation.
Case 1 : In one of the plants, half of the machines running on the production floor stopped because the main compressor in the utilities/facilities section failed and most of these equipment are pneumatics and are operated by air. My question, was there a failure on the pneumatic equipment on the production floor? Will the amount of time the equipment was down be included in the MTBF calculation or not?
Answer: No, there was actually no failure or breakdown encountered on the production floor consisting of the pneumatic equipment but there was evidence of downtime on the pneumatic equipment caused by the compressor on the utilities or facilities side of the plant. In this case, this should not be included in the MTBF calculation, but if we are going to calculate the MTBF of the compressor then the breakdown will be included in the MTBF calculation but not for the pneumatic equipment.
Case 2 : If you have a pump and the pump has a standby mode in which if the duty pump failed, the standby mode will operate automatically, the question to raise is, if the duty pump actually failed and the standby pump was automatically activated, there is eventually no downtime and operations was not even affected, but there is evidence that the duty pump had failed. Will this be included in the MTBF calculation or not?
Answer: Yes, for the failure of the duty pump, it should be included in the MTBF calculation whether we are getting the MTBF for the 2 pumps or for the single pump that failed since the duty pump eventually failed and will have to be actually repair..
Case 3 : Some manufacturing equipment encounter what you call short stoppages or assists, TPM term this as chokotei or minor stoppages, where the equipment stops, the operator resets and the equipment runs again. Usually it takes seconds or minutes to address this kind of problem. Will minor stoppages or short stoppages be included in the MTBF calculation or not?
Answer: No, assist are different from breakdowns, some industries include them as breakdowns when waiting time exceeds six minutes and above. This should not be included in the MTBF calculation since what should be included in the MTBF calculation will only be downtime caused by breakdown and failures. If you insist to include them in the MTBF calculation, then might as well change its name from MTBF (Mean Time Between Failure) to MTBFAA or Mean Time Between Failure and Assists. Hence, assists or short stoppages should not be included in the MTBF calculation.
Case 4 : The Predictive Maintenance or CBM group spotted a spike or increase in vibration on one of the equipment which is a clear sign of a potential failure. Finally the group decided to schedule the equipment for overhaul and replacement which took 4 hrs of downtime. Will the downtime be included in the MTBF calculation or not?
Answer: Yes, if we follow the formula that MTBF is equal to operating time divided by the number of failures, operating time is equal to loading time minus machine downtime caused by breakdowns and failures. And loading time is equal to the available time minus planned downtime. Hence, the time the maintenance repair the equipment will be included in the planned downtime on the MTBF calculation and not on the machine downtime since no failure actually occurred yet as was predicted by the Predictive Maintenance group.
Case 5 : A safety officer audited one of your equipment to have a severe leak and was provided a non-conformance ticket for safety reasons. The machine was not allowed to be used for safety purposes. The machine was finally repaired for leaks which took the maintenance 3 hours to fix. Will this be included in the MTBF calculation or not?
Answer: This is quite debatable. In my opinion, Yes, in this case it should be included in the MTBF calculation, although there was actually no breakdown that occurred in the equipment, but this will be included as a planned downtime of the MTBF Calculation. This situation is actually the same as in Case number 4. Let me give you a concrete example so you can follow.
Given: - Time to fix the leak is 3 hours which is considered as a planned downtime - Available time in one day is 24 hrs - Actual breakdown in this situation is zero
MTBF = Operating Time / Breakdown Occurrences MTBF = (Loading Time - Machine Breakdown) / Breakdown Occurrences But Loading Time = Available Time - Planned Downtime = 24 - 3 = 21 hrs
MTBF = (21 - 0) hrs / 1 = 21 hrs (In this situation, we compute MTBF for that given day and in case where there is no actual breakdown, two options will be required in order to calculate the MTBF value, first, lengthen the time to get the MTBF until an actual breakdown is encountered or second, assume a denominator of 1 to obtain the numerator otherwise an answer of infinity will be achieved)
Case 6 : In some manufacturing plants, some equipment are used to process more than one product. If an equipment will be used to run a different product, some parts of the equipment will need to be replaced in order for the next product to run. This is called the conversion time or set-up time. If some of the tooling were not fitted correctly which cause a breakage on the equipment which occurred right after the conversion, will this be included in the MTBF calculation or not?
Answer: In this case an actual breakdown had occurred on the equipment as caused by poor set-up and conversion, therefore, this will be included in the actual MTBF calculation. Note that only the time the equipment was down caused by the breakdown will be included but the downtime caused by the conversion and set-up time will not be included in the MTBF calculation.
What is important before attempting to measure MTBF is that your people must be in agreement as to what will actually constitute a failure and that consistency will always play a key to a credible and meaningful measurement, otherwise, our data will just be compromised and we are just fooling ourselves with the numbers.
Mean Time To Fail (MTTF) Explained
Case Study : The life of an incandescent bulb reached a lifespan of 3 years. Hence, the bulb was used for 3 years before It was declared fail and incapable of providing illumination. In its simplest sense, the bulb was busted. Therefore, if the life of the bulb reached a span of 3 years before it was replaced, then what is the Mean Time to Fail of that bulb? The answer is 3 years. If the MTTF of the bulb is 3 years, then what is the Mean Time Between Failure of the bulb? The answer is also 3 years. Therefore, if both MTTF (Mean Time to Fail) and MTBF (Mean Time Between Failure) is three years, is MTTF also the same as MTBF or they are different?
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Answer: The MTBF and MTTF of the incandescent bulb is 3 years but the two mean indicators are not the same. They are different. MTBF (mean time between failures) is the expected time between two successive failures of a system. Therefore, MTBF is a key reliability metric for systems that can be repaired or restored while MTTF (mean time to failure) is the expected time to failure of a system. Non-repairable systems can fail only once. Therefore, for a non-repairable system, MTTF is equivalent to the mean of its failure time distribution. For MTBF actual time to repair should be included, therefore MTBF is equal to MTTR + MTTF.
If you buy a laptop and after several months of using, it fails to function. You went to a service center and they diagnose the failure to be on the mother board. They told you that the mother board cannot be repaired but it can be replaced. Hence, if we speak about the mother board which cannot be restored or repaired then we speak about its MTTF but if we consider the laptop, it is repairable then we speak about the MTBF. Similarly, if we speak about the equipment which can be repaired or restored then we speak about its MTBF but if we speak about the actual part that had failed which cannot be restored but replaced (example a bearing that fatigue) then we speak about the MTTF of that particular part.
MTTF is a basic measure of reliability for non-repairable systems. It is the mean time expected until the first failure of a piece of equipment. MTTF is a statistical value and is meant to be the mean over a long period of time and large number of units. For constant failure rate systems, like MTBF, MTTF is also the inverse of failure rate. Technically, MTBF should be used only in reference to repairable items, while MTTF should be used for non-repairable items. However in modern language, MTBF is commonly used for both repairable and non-repairable items.
Mean Time To Repair (MTTR) Explained
When a failure occurs, it is critical to restore the equipment as soon as possible. Typically much of repair time is spend in determining the cause of the problem. Repair time should be performed at the shortest possible time and our goal will be to put back the equipment in its current operating state. For failures that keeps on repeating itself again and again, the best strategy will be to address the root cause of the problem and prevent it from recurring on its own . MTTR (Mean Time To Repair) is the average time required to repair a component. It may be defined as the time it will take to bring a failed system back to its available or operating status again. It is also the average time required to perform corrective maintenance or repair on all of the removable items in a product or system. MTTR analyzes how long repairs and maintenance tasks will take in the event of a system failure. Other terms used is Mean Time To Restore or Mean Time To Recover but the most common term used for this indicator is Mean Time To Repair. MTTR trend will be the lower the better or the shorter the time to repair the better. Improving MTTR means shortening the time to repair the machine.
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Typically, when a machine fails, the operator will find someone who can repair it. Once the maintenance arrive at the failed equipment, he will diagnose the fault and if a part or spare is affected, he will leave the equipment temporarily to check the system and acquire the part in their stockroom. Then the actual repair will take place. After the part had been replaced, the maintenance will revalidate and make some test runs then finally endorse the equipment back to the operator to continue with production.
Question: If the part is unavailable in the stockroom and it took 1 week to acquire the part in which the machine was not used and idle for that period. Once the part arrived, it took maintenance only one hour to repair and replace the part. In this case, what is the total repair time? Is it 1 week and 1 hour or only one hour? I have made a mini-survey to my small lists and as expected, there response differ from one industry to another. Here are some of the answers they provided and how they responded.
What is the total repair time? a) 1 week + 1 hour (Total repair time is equal to the total downtime) b) Only 1 hour (Total repair time is equal to the actual time the machine is repaired regardless if the part is available or not)
Response 1: It is a case to case basis. when an equipment fails, downtime for us is when the machine does not give a good product or there is no output at all. When we install an alternative part vs OEM, it is a risk we have to decide equating the cost of additional repair to the profit we can make to produce the good product. From Ryan Ben R. Sabilala, Philippine Overseas Working in the Middle East
Response 2: Answer (B) only 1 hour. For record purpose, we usually put actual time to repair which is 1 hour. The waiting time for spares to arrive can be devoted to other activities. If materials/spares are not available, this will automatically be considered pending work orders and will be active upon arrival of required materials. Hope this would help. From Rusty Peralta working in a Power Plant, Philippines
Response 3: Good afternoon Rolly, we would consider the following conditions. if the machine was restored to the following conditions: the speed prior to breakdown is same and the quality prior to breakdown is the same then, we consider only 1 hour. If we were not able to operate the equipment, then repair time will be equal to the total downtime. I hope this will help. From Albert Floresca, working in an Automotive Plant, Philippines
Response 4: Greetings! My answer to your question is letter (B), only 1 hour, the reason for that is the maintenance, will not consider the downtime due to the procurement of the parts considering that procurement is not under the maintenance. We only consider the time upon the arrival of the said parts. From, Kenn R. Rio, Head, Maintenance Group, Philippines
Response 5: Rolly good morning. My opinion to your posed question is the following, the repair time is only 1 hour. Since the rest of the machine downtime is just waiting time for the part to arrive. However, the machine downtime is 1 week + 1 hour. Thank you for considering me as one of your respondent. From Fortunato G. Dequit, working in a power plant, Philippines
Response 6: Hi Rolly, my answer to your question is letter (B). The total repair time is only one hour, which is equivalent to the actual repair time executed. From Joemar Apolinario, working in a semi-conductor in the Philippines
Response 7: Rolly, my answer is (A). Once you receive the permit to work, that is time that the clock of downtime started. Once you receive the PTW (Permit to Work) they cannot use the equipment that is already been isolated. From Joseleo Vicaldo, working in a power plant, Philippines
Response 8: Hi Rolly, one hour downtime plus one week waiting time are two different scenarios. When we are talking about maintenance particularly skills, the computation should refer to letter B. The 1 week waiting time will become assignable cause when trending the downtime. On the other hand, when we are talking about OEE, I believe letter A is more appropriate. From Anthony Bathan working in a semiconductor industry, Philippines
Response 9: Rolly, I would choose (A). For me, repair time means exactly what it is because I consider the equipment under repair starting when it broke down and until it is restored, regardless of delays in shipment, parts, etc. From Mac Chan Lee, working in a power plant, Philippines
Response 10: Hi Rolly, Good day, for us, we measure repair time as the actual time in doing the repair. If the reason is non-availability of parts then the machine done is of different reason. From Charles D. Mendoza working in a semiconductor plant, Philippines
We will continue with the responses on the third part series of this newsletter and this time the responses are mostly from consultant. Keep on reading. |
RELIABILITY NEWSLETTER |
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Inspired by change |
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• Gallery: New pictures for 2011 added from the gallery portion. • Newsletter: Aug. 2011 Edition finally released to our subscribers. Email me if you want to subscribe to our Newsletter. • 2012 Training Schedule: If you are interested to attend in any of our public workshop and master class for 2011. Send us an email or register online. • WCM Book: If you’re interested in buying the book on World Class Maintenance, The 12 Disciplines. Send me an email to reserve. WCM Book is now available. Get your copy now. • In-house Training :Should your industry be interested for in-house trainings we are now accepting regular schedules for in-house training for both local and international countries on courses that we currently offered. Contact us |
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Our Reliability Newsletter will be provided once a month to our valued subscribers. It provides highlights as well as issues and lessons regarding our most common link which is all about improving the reliability of equipment. |
