The Bathtub Reliability Curve

The reliability of most individual components falls on “the bathtub curve” (see Figure 1). The bathtub curve shows the expected reliability of a component throughout its life. At the beginning and end of the life cycle, the expected failure rate is high. These high failure rates are for short time periods of unknown duration. The middle of the component life cycle enjoys a low expected failure rate. This is good because the component spends most of its time in the middle part of the bathtub curve.

Figure 1 Bathtub Curve
Failures in the early part of the life cycle are infant mortality failures. These are due to imperfect manufacturing. A loose bolt, someone forgot to put in all of the parts, or there might be an invisible crack in a casting. Any number of things can go wrong and slip through the inspection process. If you bought a new American automobile in the 1970s, you were almost guaranteed in the first months of ownership to bring your car back to the dealer for multiple infant mortality failures. If you were lucky, you did not get stuck on the road, after dark, in the rain and need a tow. During 1970s and 1980s, the Japanese imported more and more cars to the U.S. The quality of the Japanese cars was so good that it caused the U.S manufactures to increase their quality. Today, you still might have an infant mortality failure, but not as likely. Whenever you install a new critical component, always remember the bathtub curve. Give your new component a chance to prove itself in fair circumstances. For example, do not install a new fuel pump and then immediately cruise across the Atlantic. Test the component close to shore on a few nice days before fully relying on it.
At the end of the life cycle, there is another period of high probability of failure. This is due to the component wearing out. When it happens, you will need to have a plan of action. Use one of the methods outline in this book to deal with any component failure.
The middle or main part of the life cycle is the longest period on the bathtub curve. During the middle or main part of the curve, the failure rate is relatively low and constant. The component can still fail at any time during this period but with the same probability, and a much lower probability than at the beginning and end of the component life cycle. The component is just as likely to fail at the beginning as at the end of the main part of the curve. I will use the bathtub curve to explain why you need to prove and test your components before completely relying on them.
Remember the bathtub curve when installing new critical components required for proper operation. For example, do not set out across the ocean with a brand-new fuel pump until you run the pump for a while or unless you carry a spare.

Leave a Reply