How to Select The Optimum Fixed Time Maintenance Intervals

Think about your maintenance program. How often are your PMs scheduled?  How were those frequencies established?   If you are in the majority, the chances are that the frequencies were either established from the OEM manual, or by someone in the department without data.

Establishing the correct frequency of maintenance activities is critical to the success of any maintenance program.   Too infrequently and the organization is subjected to failures, resulting in poor operational performance.  Too frequently, and the organization is subjected to excess planned downtime and an increased probability of maintenance induced failures.  So how do you establish the correct maintenance frequencies for your organization?   There are three different approached to use, based on the type of maintenance being performed;

• Time-Based Maintenance
• On-Condition Maintenance
• Failure Finding Maintenance

This article will focus on Time Based Maintenance Tasks.

Time-Based Maintenance Tasks

“The frequency of a scheduled task is governed by the age at which the item of or component shows a rapid increase in the conditional probability of failure” (RCM2).  When establishing frequencies for Time Based Maintenance, it is required that the life be identified for the component based on data.

With time-based failures, a safe life and useful life exists.  The safe life is when no failures occur before that date or time.  Unless the failure consequence is environmental, or safety related, the safe life would not normally be used.   The useful life (economic life limit), is when the cost of consequences of a failure starts to exceed the cost of the time-based maintenance activity.   There is a trade-off at this point between the potential lost production and the cost of planned downtime, labour, and materials.

So how is the safe life or useful life established?  It is established using failure data and history.  This history can be reviewed using a Weibull Analysis, Mean Cumulative Failure Analysis or even a Crow-AMSAA Analysis to statistically determine the life of the component.   Once that life is determined using a statistical analysis, the optimum cost effective frequency must be established.

Establishing the Optimum Economic Frequency

This formula is used to establish the economic life of the component, balancing the cost of the downtime vs. the cost of the replacement.

Where;

• C_T= The total cost per unit of time
• C_f= The cost of a failure
• C_P= The cost of the PM
• T = The time between PM activities

The formula will provide the total cost based on the maintenance frequency. Since the calculation can be time-consuming, Dodson developed a table which can be used if;

• The time to fail follows a Weibull Distribution
• PM is performed on an item at time T, at the cost of C_P
• If the item fails before time = T, a failure cost of C_f is incurred
• Each time a PM is performed, the item is returned to its initial state “as good as new”

Therefore when using the table, use formula; T=mѲ+δ.  Where;

• m is a function of the ratio of the failure cost to PM cost and the value of the shape
• Ѳ is the scale parameter of the Weibull distribution
• δ is the location parameter of the Weibull distribution

In the example below, you can see how the table can be used with the formula;

The cost for a PM activity $60.  The cost of a failure for the same item is $1800.  Given the Weibull parameter of B=3.0, O=120 days, and δ =3 how often should the PM be performed?

• C_f/ C_P = x
• 1800/60 = 30

The table value of m given a shape parameter B of 3.0 is 0.258.  Therefore;

• T=mѲ+δ
• T = (0.258)(120)+3 = 33.96
• T = 34 days for each PM

As you can see, determining the frequency of Fixed Time Maintenance tasks is not as simple as picking a number out of a manual or based on intuition.  Armed with this information, a cost effective PM frequency based on data can be developed for your Fixed Time Maintenance tasks.   This will ensure the right maintenance is done at the right time, driving your plant performance further.

Does you Fixed Time Maintenance Tasks have this level of rigor behind them?  Why, not?  After all, your plant performance (operational and financial) depends on it.   Stay tuned for next week’s post on establishing frequencies for On-Condition tasks.

Remember, to find success; you must first solve the problem, then achieve the implementation of the solution, and finally sustain winning results.

I’m James Kovacevic
Eruditio, LLC
Where Education Meets Application
Follow @EruditioLLC

References;

• RCM2 by John Moubray
• CRE Primer – Quality Council of Indiana

How To Manage Each Failure Pattern With An Effective Maintenance Strategy

Most maintenance and reliability professionals have seen the six failure patterns (or failure hazard plots), described by Nowlan and Heap.  In case you are unfamiliar with them, you can learn more about them in a previous article on them.   Here is a quick summary to jog the memory, just in case.

• A. Bathtub Curve – accounts for approximately 4% of failures
• B. Wear Out – accounts for approximately 2% of failures
• C. Fatigue – accounts for approximately 5% of failures
• D. Initial Break-In – accounts for approximately 7% of failures
• E. Random – accounts for appoximately 14% of failures
• F. Infant Mortality – accounts for approximately 68% of failures

From the above, you can see that the majority of failures experenced are not directly related to age, but are the result of random or induced failures.   So how does this help when establishing a maintenance program?   First, we must understand what the patterns tell us.

What Types of Failure Modes Do The Failure Patterns Relate to?

Looking at the different failure patterns, we can group the types of failures into three unique groups;

• Age-Related failures – The term “life” is used to describe the point at which there is a rapid increase in the likelihood of failure.     This is the point on the failure pattern before it curves up.  Typically these types of failures can be contributed to wear, erosion, or corrosion and involve simple components that are in contact with the product.
• Random failures  – The term “life” cannot be used to describe the point of rapid increase in the likelihood of failure, as there is no specific point.  These are the flat parts of the failure curve.  These types of failures occur due to some introduced defect
• Infant Mortality – The term “life” cannot be used here either.  Instead, there is a distinct point at which the likelihood of failure drops dramatically and transitions to a random level.

Understanding these unique differences, an effective maintenance strategy can be developed.

What Maintenance Needs to Be Done for Each Failure Pattern?

The maintenance activity selected has to be right for the specific failure pattern.   When looking at the failure patterns, there are three unique types of activities that can be put in place to address all points in the failure curve.

• Age-Related – These types of failures can be addressed through fixed time maintenance.  Fixed time maintenance includes replacements, overhauls, and basic cleaning and lubrication.  While cleaning and lubrication will not prevent the wear out or corrosion, it can extend the “life” of the equipment.
• Random – These types of failures need to be detected, as they are not predictable, or based on a defined “life.”   The equipment must be monitored for specific indicators.  These indicators may be changes in vibration, temperature, flow rates, etc.   These types of failures must be monitored using Predictive or Condition monitoring equipment.    Cleaning and basic lubrication can prevent the defects from occurring in the first place if done properly.
• Infant Mortality – These types of failures cannot necessarily be addressed through fixed time, predictive or condition-based maintenance programs.  Instead, the failures must be prevented through proper design & installation, repeatable work procedures, proper specifications and quality assurance of parts.

Only when a maintenance program encompasses all of the above activities, can plant performance improve.

Determining the Right Frequency of Maintenance Activities for Each Failure Pattern

So with all of the activities taking place, how is it possible to know when each fixed time activity or condition monitoring inspection take place?   The approach to determining the frequency of activities for fixed time and condition monitoring inspections are different.  However, before the approaches are discussed, it should be noted that MTBF should NOT be used to determine the approach… EVER (sorry, the rant is over).

• Fixed Time Maintenance – The frequency for fixed time maintenance activities should be determined using a Weibull analysis.   Also, there may be regulatory requirements which specify the frequency of these activities. This will provide an ideal frequency to perform these types of activities
• Condition Monitoring – The frequency for condition monitoring activities should be determined by using the P-F Curve and P-F Interval.   This approach requires an understanding of the ability of monitoring technology, the defect being monitored, degradation rates, and the ability of the organization to react to the information gathered during the monitoring program.   This will be furthered discussed in next weeks post.

I hope this has provided some clarity around how you should be using the six failure patterns in your maintenance strategy.   Do you have specific activities in your program to address age-related, random and infant mortality failures?  If you only have fixed time maintenance activities in your program, what are leaving on the table?

Remember, to find success; you must first solve the problem, then achieve the implementation of the solution, and finally sustain winning results.

I’m James Kovacevic
Eruditio, LLC
Where Education Meets Application
Follow @EruditioLLC

References;

• RCM2
• Understanding the 6 Failure Patterns
• Weibull Analysis
• www.NoMTBF.com