Issue No. 43 – November 2010

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 There is no question on having highly skilled people in our maintenance team.  Technically speaking the goal of MTTR is to reduce the repair time, but the real goal of maintenance is to analyze failures by addressing the root cause of the problem.

By Rolly Angeles

 

Mean Time To Repair (MTTR) Explained

 

Going back to our survey question, when a machine fails, typically, 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 begin to 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. The 11th to 15th responses came from consultants.

 

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)

 

 

 

 

 

 

 

 

Here are the responses of consultants who have participated in this survey question and there choice is almost unanimous except for response 14.

 

Response 11: Rolly, answer (A), Repair time = Active repair time + Preparation time.  The reason the machine is there is to produce and not sit idle.  An airplane only makes money when it is in the air.  A machine can only be in one of two states, working or not working. In the second case if we don't need it to produce, it is still available to work, so we can treat it as working.  All the rest of the time it cannot work, so it is down for maintenance, so no $$$.  An efficient maintenance system will minimize the preparation time, and scheduled work so we minimize loss of $$$.  Finally, maintenance is a Busines Process, not a Department.  From Vee Narayan, Lead Author, 100 Years of Maintenance: Practical Lessons from Three Lifetimes, Industrial Press, U.K.

 

Response 12: Hello Rolly, answer (A), a failure event needs to be seen in terms of the total lost value and costs incurred to the business.  Total repair time is a measure that means exactly what it says: The time from the first second the machine stopped being available for production to the second the machine came back up to its required production rate.  That includes commissioning time and hand-back time removing tags and completing permits.  What is most important to the business is the full impact of a machine outage on production uptime.  From the point of view of the business the repair took 1 week and 1 hour.  That is the correct business measure to be used if you want to truly understand the impact of the outage on the business.  From Mike Sondalini, http://www.lifetime-reliability.com, Australia

 

Response 13:  Rolly,  my selection is (A) if maintenance function objective is "equipment ready to run" and the maintenance function has authority over resources required, diagnostics, preventive, spare parts, etc. If the maintenance function is only "repair failure" then "b" could be an the answer. I like the maintenance goal defined as "equipment ready to run". However, the resource consumption required needs to be logical to the cost of down time. IE. No need to own an expensive spare part and pay interest $ when the part can be obtained in a short relative to lost production time $. So, the maintenance function needs to be closely aligned to the value added activity of production. From Greg Peitz, Affiliate, Failsafe Network, USA

 

Response 14: Interesting question Rolly, I am not an expert on maintenance metrics, but I always believe that you measure what you want to hold to a certain performance standard or to improve and the measures need to reflect the actual problem. In this case the downtime needs to be recorded but what bucket do you put it in. Most places have reduced spare inventory but if it causes a week delay then it needs to be visible that spares or inventory was the issue, not maintenance working on the repair. So my answer is total repair time is 1 hour, total down time is 1week + 1 hour and cause is inventory. From Virginia Edley of SBK Consulting, LLC, USA

 

Response 15: Rolly, anyway, I really don’t have anything to add to either Greg or Virginia’s response. I do like Virginia’s inference that we measure what we’re interested in improving.  So, if we’re interested in improving total availability, then I’d opt for a. And as Greg said, if I’m only interested in maintenance response time, then I’d opt for b. Most people and organizations might be a bit too myopic, and don’t measure from a broad enough perspective. So, from the two options you’ve giving, I personally would opt for “A.”  Thanks for asking. From C. Robert (Bob) Nelms, President, Failsafe Network

 

My response:  In my own opinion the total repair time will be 1 week and 1 hour which includes the time the spare had to be on hand up to the time the equipment had actually been repaired hence, I choose (A) because spare parts management is one of the responsibilities of the maintenance function.  If the part is not available when needed then maintenance should be held accountable since we are the ones who control our spares.  When the equipment is idle because a part or spare is not available, then the industry is not making any profit, consideration of the repair time should include the time the part or spare is acquired in the first place. The reason for this simple survey question is that if one industry opt to choose a or b, MTTR computation will vary a great deal.  If consideration of the total repair time is only 1 hour then we can have the following computation.

 

MTTR = Repair Time / Breakdown Occurrences.  If we select (B) as our answer as the total repair time, then the actual repair time in one week is just 1 hour. since in one week, MTTR value will be 1/1 or 1 hour, while the rest of the week it is idle and not working since spare is unavailable. The MTTR in this case is one hour in a weeks time but the rest of the days the machine sits idle.  Although MTTR value is giving us a good figure since repair time is low but in this case, we are just fooling ourselves with the numbers, since the rest of the time the machine is not functioning and sits idle.  On the other hand, if we consider both the waiting time for the spare which is 1 week and 1 hour to be the total repair time then MTTR value will be 168 hrs in one week which is giving us a more truthful and realistic value. A true and correct MTTR starts at the time of failure and continues until the stem is operational once again, regardless if a system part or component will be available or not.

 

Mean Time To Assists (MTBA) Explained

 

Assists or errors is any unplanned interruption or variance from specification of equipment that requires human intervention on the equipment.  In some industries, if the time fix an assists goes more than 6 minutes or more, they considered this to be a breakdown.  Causes of prolonged assists may include waiting time for the maintenance to arrived at the equipment or it takes too much time to troubleshoot the assists.  In my opinion an assists is always an assists and is actually different from a breakdown.  When a part breaks down caused by the assists, then this is the time to consider it as a breakdown. 

 

Assists is often encountered when the equipment is fully automated and full of electronic parts.  Frequency of assists encountered will be much more than the breakdowns actually occurring on that particular equipment.  Meaning in a weeks time, if you provide me with a data of 5 assists and 40 breakdown in a weeks time, this is quite unlikely as assists occur more frequently than breakdown.  Second point I would like to point out is that if breakdowns are not clearly distinguished from assists then there is a temptation of including this downtime in the MTBF calculation.

 

For industries implementing TPM or Total Productive Maintenance, they are more familiar with the term minor stoppages or chokotei.  Minor stoppage is an equipment stoppage due to a failure or an error in automatic handling, processing or assembly of parts and work pieces. It sometimes occurs due to quality related abnormalities. Errors in automated processes where work piece flow stops, operator reset and the machine runs again.

 

In most manufacturing plants that encounter this type of problem on their equipment, it is quite a tedious job for the operator to record every single assists and error that can occur on a particular day and with the absence of software and system it is unlikely that the operator can capture every single assists that can actually occur on a particular equipment, in this case this is where an MTBA Snapshot can be taken down on that particular equipment. 

 

Mean time between assists is the average time the equipment performs its intended function between assists.  It is also the productive or the operating time divided by the number of assists, hence MTBA is the average time between assist on any stoppage of a machine caused by unwanted operation.  Formula for MTBA  is as follows :

 

MTBA = Operating Time / Frequency of Assists or

MTBA = Productive Time / Frequency of Assists where

 

Operating Time = Loading Time - Machine Downtime due to assists

 

PROCEDURE FOR PREPARING MTBA SNAPSHOTS

 

Step 1 : Prepare MTBA Snapshot Form

Step 2 : Select an equipment that have a high frequency of assists and errors

Step 3 : Sectionalize the equipment per station or sub-assembly and record all possible assists that can                occur on  each sub-assembly or station

Step 4 : Perform a Snapshot (min of 2 hours) and write the duration, frequency and type of assists that    

               occur on the machine

Step 5 : Generate corrective measures and perform modifications

Step 6 : Horizontal Replication of the MTBA improvement to same equipment similar problems

 

Below is an actual MTBA Snapshot for a reference sample

 

 

 

  

Machine Type : EDF + EPL 2400S Automatic load and unload stripto strip deflash + plating machine with the capability to process around 2400 strips per hour

MTBA CASE STUDY FOR MECO MACHINE

An actual MTBA Snapshot was taken from this machine which was calculated to be around 4.24 minutes meaning that on an average this equipment encountered an assists or error every 4.24 minutes.  Most of the assists encountered on this machine occurred on the loading process and modifications were done on the loading station to increase its MTBA from 4.24 to 115.7 minutes..

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