Why Mean Time to Defect Matters with Condition-Based Maintenance

In a quick Google search, you will find that MTTD has several meanings depending on the industry. It’s associated with mean time to detect and mean time to diagnose. But, we’re going to cover mean time to defect as a maintenance metric and how it can improve your preventive and condition-based maintenance.

Mean time to defect is a measurement of the time between identifiable defects or issues that lead to equipment failure.

In many cases, a defect or issue doesn’t cause an immediate breakdown. Although it won’t operate as safely or efficiently, the equipment will still be able to function. For example, your equipment motor temperature may go above the recommended threshold, that’s “the defect”. Typically, it will continue running for hours before functional failure. So, MTTD is the first failure metric, followed by MTBF, MTTF, and MTTR.

Why Should You Measure MTTD?

Mean time to defect is an important KPI for scheduling preventive maintenance and condition-based maintenance. Identifying early signs of asset failure and knowing the operating time left before functional failure can help you reduce the overall downtime. You have advance notice to purchase parts and schedule your technicians to do the work. This results in a lesser impact on production. Knowing the MTTD for your assets can also help you plan proactive inspections. The key is to inspect at an interval that can catch a failure after it’s detectable but before functional breakdown.

How Does it Help with Condition-Based Maintenance

Tracking defects through MTTD enables you to optimize the maintenance plan for your organization. Identifying failures before a breakdown occurs moves you more in the direction of preventive and condition-based maintenance. This helps you counter the cycle of constant reactive maintenance that is hard to get out of. So, what exactly is condition-based maintenance? How does it compare to preventive maintenance?

The perfect example is manufacturer-recommended maintenance for automobiles, specifically for oil changes. The manufacturer recommends based on average performance data and experience, but the actual condition of the oil in your vehicle may differ from these averages. Preventive maintenance would be following the manufacturer’s recommendation to change the oil every 5,000 miles. However, condition-based maintenance takes into account the actual condition of your oil and planning an oil change at the optimal time for you, which may be shorter or longer than the manufacturer’s recommendation.

This example can illustrate MTTD as well. Checking your oil level at planned intervals allows you to identify when your oil is low. You may even catch the early signs of an oil leak if it has dropped quickly. The issue or defect of low oil won’t cause an immediate breakdown of your car, but you do need to schedule an oil change. If you simply waited until a functional breakdown, it would be a far more expensive oil change. Additionally, there would likely be collateral damage caused by running the engine to that point of failure.

The benefit of condition-based maintenance is that it is optimized for your actual use, which may require maintenance tasks to be done less frequently. In the vehicle example, that may mean getting an additional 2,000 miles in between oil changes. In your organization, that could result in cost savings that really add up.

How Do You Measure MTTD?

The simple calculation is to divide an asset’s total number of operational hours by its total number of defects over a period of time. Continuing with the oil change example, let’s say you drive 60 miles per day for a total of one hour per day. That is 21,900 miles and 365 hours per year, and you got four oil changes.

365 hours / 4 defects (the oil changes) = 91.25 hours

The mean time to defect is 91.25 hours. This would allow you to plan inspections starting around hour 80 to check oil levels. You could then proactively plan the oil change that will be needed soon after.

How Can You Improve Mean Time to Defect?

The key component to improving your MTTD is using it to adjust your maintenance strategy. This is done by pairing a strategic plan of inspections and maintenance with root cause analysis. It enables you to identify and potentially prevent the defect, or at least increase the time between defects.

There is technology available for every step of the process, but they are often siloed from each other and end up creating more work for you and your maintenance team. With Redlist, you have an all-in-one integrated system. You can automate your feedback loop to remove the manual entry of sensor or lab data, configure the inspection module to fit your needs, and even digitize your work order system to sync with all of this. To implement condition-based maintenance, you have to have an efficient and effective system to collect and analyze data to base decisions on. Improving your MTTD, or any maintenance metric, has to be viewed in the big picture of how everything is working together in your organization.

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