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Issue No. 41 – September 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 Click here to email me. 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 September 2010 Edition of our Monthly Reliability Newsletter for this year 2010 and I hope that you enjoy reading and sharing with your people.
My Warm Regards,
Website : www.rsareliaiblity.com Inspired by Change . . . |
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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
There are a lot of Mean Time Indicators out there that is currently being used by our reliability and maintenance people in order to measure their equipment performance. The most common are MTBF, MTTF, MTTR and MTBA so this is what we will cover in this 3 part series of newsletter.
Mean Time Between Failure (MTBF) Explained
Failure simply means the inability of an equipment to perform its required function. If we speak about a failure of a spare or component, then this is viewed as terminating its life. On the other hand, reliability is the probability that no failure will occur throughout its prescribed operating period of time.
Failure of a component indicates that it had become completely or partially unusable or has deteriorated to the point that it is undependable or is unsafe for normal sustained service. It may also be defined as an event in which any part of the equipment or machine does not perform in accordance to its operational specification. In short failure can also be synonymous to the term breakdown. As Bazovskly states that the modern concept of reliability in popular language simply means as the capability of an equipment not to fail or breakdown in operation. When an equipment works well and performs to do its job for which it was designed to do, such equipment is said to be reliable.
MTBF is an average measure of reliability that indicates a device will run without failing. The most common units used for MTBF is in hours. MTBF originated from the US Military Standards (MIL-STD-217) and has been widely used in other applications in industries. The original reliability prediction handbook was MIL-HDBK-217. The Military Handbook for the Reliability Prediction of Electronic Equipment published by the Department of Defense, based on work done by the Reliability Analysis Center.
MTBF is a reliability engineering term that means the average amount of operating time between the occurrence of breakdowns that requires repair. The MTBF trend will be the higher the value the more reliable the machine or part will be. MTBF simply means the average amount of time between failures. It is based on historical data or estimated by vendors and is use as a benchmark for reliability. The formula for MTBF is :
MTBF = Operating Time / Number of Failures
Where: Loading Time = Available Time – Non-Machine Related Downtime Operating Time = Loading Time – Machine Related Downtime due to Breakdowns
• Available Time is given as the total time in a given period, 24 hours for one day, 168 hours for one week and 720 hours for 30 days or 744 hours for a 31 days period.
• Machine-Related Downtime is the time the equipment is not operating due to machine related factors. In manufacturing, failure or breakdown is not the only cause of machine related downtime. Other causes of machine related downtime includes, short stoppages, change in cutting tools, quality related problems on the product, set-up and conversion or changing an equipment to produce one product to another product, start-up losses and speed reduction losses. Only downtime due to equipment breakdowns should be included in the calculation of the operating time. Downtime due to set-up and conversion as well as short machine stoppages and assists must be excluded in the MTBF calculation. This must be made clear in an attempt to calculate a true and meaningful MTBF calculation, other related machine downtime should be excluded.
• Non-Machine Related Downtime is the time the equipment is not operating due to non-machine related factors. TPM refers to this as Planned Downtime. Factors reflecting non-machine related downtime includes operators breaktime, meetings where equipment is stopped, planned shutdowns and Preventive Maintenance overhauls and replacements, planned scheduled outages, holidays when the machine or equipment is not required to work in the absence of operators.
• Operating time is the time in which the equipment and assets are operating. It is also the time during which the equipment and asset are working without failure. Other terms to designate operating time is uptime, productive time, running time or time the equipment was utilized.
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Given : Machine Related Downtime due to breakdown = 52 hrs Non-Machine Downtime due to PM, Break time = 60 hrs Available Time in one month = 720 hrs Loading Time = Available Time - Loading Time = 660 hrs Operating Time = 608 hrs Number of times the equipment had failed = 4 times
MTBF = 608 / 4 = 152 hrs
So if the MTBF of this equipment for a specific month is 152 hrs, what does it simply means? Most will say that the machine will fail every 152 hrs. That assumption is truly wrong because the word mean simply states an average, hence the correct way of stating this is that the average failure of this particular equipment is every 152 hrs so don’t expect your equipment to fail every 152 hours because for all I care the equipment can fail more or less than 152 hrs since we are just basing this on an average.
In case where there is no breakdown or failure, the denominator will be zero and an MTBF of infinity will be obtained. This simply indicates that there is nothing wrong with the equation. So in case where there are no actual breakdowns recorded. either prolong the duration of MTBF until a breakdown happens or just assume a denominator of 1 to obtain a given value. Meaning in a months time, when the MTBF is 720 hrs then no failure occurred during that period..
But before anyone can calculate and use MTBF, it is very important that they have a crystal clear understanding on what will eventually constitute a failure in the first place. Let me give you an example. I hope you take the time to answer these cases if this will be part of the MTBF calculation or not. The answer will be provided in the 2nd part of our newsletter on Most Common Mean Time Indicators Part 2.
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. 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?
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?
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 the machine and the equipment runs again. Usually it takes seconds or minutes to address this kind of problem. Will Minor Stops or short stoppages be included in the MTBF calculation or not?
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 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?
Case 5 : A Safety Officer audited one of your equipment to have severe leak and was provided a non-conformance ticket. 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?
Case 6 : In some manufacturing plants, some equipment are used to process more than 1 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?
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.
Limitations of MTBF
Collecting failure data to calculate MTBF in order to determine the maintenance interval is wrong and should not be done since MTBF is just a measure of the average. Failures fail predominantly into 3 categories, age related which make up to 20% of all failures and the bigger portion will be both infant and random failures which constitute around 80% of all failures. And for age-related failures it is not MTBF but rather the remaining useful life that is significant when attempting to determine the best replacement period. In short, PM replacements and overhauls should be based on the useful life and not on the average life of the part being replaced or overhauled..
Another limitation of MTBF is that both failure and assists are not distinguished properly. Some industries make their own set of rules and assume that an unattended assists for six minutes will eventually turn out to be a failure because the waiting time is far becoming long. An assists or error will still be an error and must be separated from failure itself to get an accurate MTBF value.
MTBF may vary up to ten times depending on the installation site, environment and most especially how it is being operated. Example, an environment with a high sulfur compound can provide a shorter life for electronic components. Field conditions vary enormously, from power supply voltage to corrosive atmospheres, ambient temperatures, relative humidity, moisture, air cleanliness all of which will affect the MTBF of the same component depending upon the conditions in which the part is being operated. Hence, vendors MTBF may not be the same , if the operating conditions will be different.
MTBF for Group of Machines
If you plan to use MTBF as one of your indices, first we need to understand how does your plant intent on measuring MTBF in the first place. The formula can be quite confusing and will depend a lot on the type of industry you are currently working. MTBF can be used to compute for the following.
MTBF by critical part (usually called MTTF to be explained in the Part 2 of this newsletter)
MTBF by sub-assembly - so we can pinpoint the worst sub-assembly for a particular machine
MTBF by machine - to determine the MTBF of a particular machine irregardless on what part fails
MTBF by process/system - so we can determine the component in the process that fails frequently
MTBF by group of machines - so we can determine the total MTBF for a group of machines. In calculating MTBF for a number of machines, you have the following three options to get the total MTBF. In this case, a software will highly be recommended to compute the total MTBF for a group of machines. The important point in this case is to identify the machine in that group with the lowest MTBF and determine what particular part frequently fails or breaks down in operation.
First, if you want to get the total MTBF for these 4 machines, you can add the total MTBF, say for a given week or period of time. MTBF = (36 + 45 + 74 + 90) hrs
Second, you may get the average MTBF and divide it by the total number of machines. MTBF = (36 + 45 + 74 + 90) / 4
Third, you may get the percentage MTBF in which a perfect MTBF in a weeks time is 168 hrs. MTBF = (36 + 45 + 74 + 90) / 4 x 168 hrs
When to use MTBF
MTBF can be used to compare two or more identical parts from different vendors. When we sub-contract some repairs of our equipment to some of our local vendors, one of the justifications in awarding the part will be its MTBF. A part or spare that has a higher MTBF may require a higher initial cost but a lower life cycle costs can be achieved in the long run if we consider the cost of doing repairs, spare parts, overtime costs, overhead costs, downtime costs just to name a few. The part with the higher MTBF will definitely have a lower cost in the end. A part or component with a higher MTBF will yield better return in terms of cost savings in the future. Although, it is not recommended to use MTBF in determining the frequency of overhauls and replacement for our Preventive Maintenance schedule, MTBF can be used to determine the frequency of functionality inspection or Failure Finding tasks for protective devices in order to eliminate the chance of a multiple failure. A multiple failure is when the protective device is in a failed state simply because the protected function has already failed. Failure Finding tasks are done on hidden failures.
MTBF is used to determine the main contributor on why equipment keeps on failing. A group of maintenance people can conduct an MTBF study in order to determine what specific parts in their equipment keeps on failing and analyze the root cause of failure on that part so that they can modify or redesign the part with inherent design weakness.
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? The answer will be revealed in our next issue of this 2 part series of reliability newsletter.
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