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Raul Martins

Using time based maintenance to prevent failures

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Hi all,

Time based maintenance (TBM) is part of a preventive maintenance approach, which aims at replacing a piece of equipment or component based on how long it has been running.

Such activities are scheduled, have a determined interval, and should contribute effectively to prevent the occurrence of a failure mode, and are based on the economic life cycle of the asset, or on the appropriate interval for a component to be replaced.

In order to determine this interval, do not forget of going through your asset maintenance history, check with experienced colleagues their thoughts and use as much quantitative analysis as you can, such as Life Data Analysis (LDA) and Reliability Growth Analysis (RGA).

Yet, one of the most important steps when creating your time based maintenance plan is understanding the characteristics of the failure.

A TBM will help you with age-related issues, such as some wear out or corrosion problems. This is because age-related failures, allow you to estimate an optimal time to replace an item.

On the other hand, when a TBM strategy is used for a random failure, like operational problems, or using the wrong lubricant, this will make your costs go through the roof, as you will be replacing parts unnecessarily on your PM, as well as the failures will keep happening.

In short, time based maintenance can be one of your biggest allies when building a reliability program and improving your plant’s results. However, determining the failures modes and their characteristics are vital to make it happen in an efficient way.

 

Regards,

Raul Martins

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Well described Raul. The failure mode must have a time or usage based distribution. In Weibull analysis it has a beta value greater than 1. The larger the value, the more strongly related to aging the failure is. As in all proactive work we are aiming to reduce the risks of failure. We do that by reducing what happens when the failure occurs (consequence mitigation) or by reducing the probability it will happen. With age related failures, where preventive measures can be effective, they actually prevent most of the failures if they are performed at an early enough time. 

I've seen quite a few instances where preventive replacements were used, expecting to improve performance of the assets but failing. When asked about task frequency, a number have studied their work order histories and determined MTBF, then used it as the task frequency. Big mistake! MTBF is the average age at which failure can be expected, i.e.: 50% will have failed by then, the other 50% will still be working. The task frequency needs to be quite a bit less than MTBF.

If MTBF is known then you probably have enough data to perform a Weibull analysis and can determine Beta. You then have a good idea what the probability curves look like. The area under the probability density function represents is the % of failures that have occurred up to that time. If you can tolerate say, 10% failing before reaching the PM interval, then choose that time as your interval. 

If you do this, you will experience roughly 10% failing by the time you get to the interval you chose and 90% still running. You'll replace the 90% and restore or discard the items. It is expensive to do that and most of the items will appear to be just fine. Many trades will argue that you are throwing away good parts (and you are). The whole purpose is to reduce the risk. To make sure you aren't doing that needlessly, you need to be certain that you are dealing with an aging failure, and that it is worth the costs of discarding all those items when they are replaced. That's a simple economic calculation and it must consider the costs of downtime and any other secondary damage that might be associated with the failures if they are allowed to occur.

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Hi @UptimeJim,

That MTBF mistake is something very commonly made. Not only when it comes to task frequency, but also for spare part strategy.

Great explation about using risk and economic as an approach to achieve positive outcomes from your PM. 

By the way, I went through a situation this weekend that a component wore through, causing significant damage to a piece of equipment. This looks like a good opportunity to use some LDA concepts to improve it.

 

Regards,
Raul Martins

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On 5/1/2020 at 6:50 AM, Raul Martins said:

Hi all,

Time based maintenance (TBM) is part of a preventive maintenance approach, which aims at replacing a piece of equipment or component based on how long it has been running.

 

Such activities are scheduled, have a determined interval, and should contribute effectively to prevent the occurrence of a failure mode, and are based on the economic life cycle of the asset, or on the appropriate interval for a component to be replaced.

 

In order to determine this interval, do not forget of going through your asset maintenance history, check with experienced colleagues their thoughts and use as much quantitative analysis as you can, such as Life Data Analysis (LDA) and Reliability Growth Analysis (RGA).

 

Yet, one of the most important steps when creating your time based maintenance plan is understanding the characteristics of the failure.

A TBM will help you with age-related issues, such as some wear out or corrosion problems. This is because age-related failures, allow you to estimate an optimal time to replace an item.

On the other hand, when a TBM strategy is used for a random failure, like operational problems, or using the wrong lubricant, this will make your costs go through the roof, as you will be replacing parts unnecessarily on your PM, as well as the failures will keep happening.

 

In short, time based maintenance can be one of your biggest allies when building a reliability program and improving your plant’s results. However, determining the failures modes and their characteristics are vital to make it happen in an efficient way.

 

 

 

Regards,

Raul Martins

 

Hi Raul my best greetings.  In your written does not mention the Predictive Maintenance, I would like to know your thoughts. Thank you . Seidel Muriel , Cali Colombia , e-mail.  seidelmuriel@hotmail.com

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Hi Raul my best greetings.  In your written does not mention the Predictive Maintenance, I would like to know your thoughts. Thank you . Seidel Muriel , Cali Colombia , e-mail.  seidelmuriel@hotmail.com

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Hi @SEIDEL MURIEL,

Every week we post a different topic. Other maintenance strategies, such as detective and predcitive maintenance are on my list and will be posted soon.

Stay tuned to our community and feel free to create independent topics so we and other users can share their thoughts.

Regards,

Raul Martins

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Raul , my best greeting and  congratulations for this page where we can dialoge about our specialty and learn a litter more

Please continue going on this and include others themes like  RFA Root Failure Analysis, Non Destructive Tests to apply in the industry and so on.

Thank you

Seidel Muriel

seidelmuriel@hotmail.com

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