A bathtub curve is a graphic representation of the failure rate of an asset over its life cycle. It is called a bathtub curve because the average graph matches the shape of a bathtub. Many consider it the most valuable graph for the reliability of assets, as it can help you better predict asset failure and identify the root cause. When you plot failures over time, the bathtub curve will outline three main periods of failure: Infant mortality period, normal life period, and wear-out period. However, the charting of your asset failures may not precisely match the expected bathtub curve shape. In that case, you should focus on the trend of failure rates instead of having concerns over the shape.
The 3 Stages of the Bathtub Curve
Infant Mortality Period
While it is commonly called the infant mortality period, this stage is referring to early failures. Assets in this stage exhibit a decreasing failure rate. As these assets are typically brand new, failures in this stage are due to defects or issues with installation. Additionally, early failures may be caused by operator error, lack of training, or incorrect start-up procedures.
Normal Life Period
The normal life period of a bathtub curve is also called the useful life period. It is mostly random failures that occur after the initial operation and early failures level out. You can expect assets in this stage to have a constant failure rate. This does not mean the asset is constantly failing. Instead, this means that the failure rate is neither increasing nor decreasing. Your maintenance team’s goal during this stage would be regular maintenance and inspections to keep assets in good working order. The majority of failures in this stage are due to defects, collision, operator error, system overload, or improper maintenance. If the failure rate begins to increase, your asset may be entering the next period, and you should perform a root cause analysis to identify any issues.
As assets age and experience normal wear from usage, they enter the wear-out period. This final stage of the bathtub curve is an increasing failure rate. Asset failure is more predictable in this period, as it is due to deterioration, corrosion, or gradual wear and not the random events of the normal life period. The length of time before an asset reaches the wear-out period will be different for every asset. It depends entirely on the asset’s maintenance, your usage, and its expected life cycle. It is important to track the cost of repairs and downtime for assets in this period. In some cases, it may be more cost-effective to replace an asset than to continue repairing it.
How Do You Use the Bathtub Curve?
The bathtub curve can provide you with valuable insight to drive your maintenance strategy. If your maintenance team wants to prolong your asset’s useful life, then the failure predictions will assist with preventive maintenance planning. Effective preventive maintenance catches problems before they cause catastrophic breakdowns and reduces downtime. Having a system that tracks asset failure and provides easy data reporting allows you to put the bathtub curve to work for you. Noting when an asset’s failure rate begins increasing will alert your team to pay more attention to the inspections and maintenance of that asset.
The bathtub curve is an excellent general guide. However, you still have to analyze your data with your assets, your usage, and your production goals in mind. Not every asset will follow the bathtub curve graph. For example, infrequent or decreased usage of an asset may result in a much more gradual bathtub curve toward the wear-out period. Using sensors combined with Enterprise Asset Management (EAM) software will assist you in collecting, tracking, and analyzing your bathtub curve data.