What is the Bathtub Curve and how it impacts your maintenance plan

An asset’s performance will vary throughout its life, until it finally reaches its retirement age, and needs to be replaced. Several hazard and reliability functions allow us to monitor this evolution, such as the failure density and cumulative hazard functions. In this article, we will talk about one of those hazard functions: the failure rate curve, also known as the “bathtub curve” due to its shape. 

What is the Bathtub Curve?

The bathtub curve represents the probability of a certain asset failing over time. It allows us to clearly map three distinct phases in the asset’s life cycle, as you can see below. Understanding these three stages enables you to adapt your maintenance plan throughout the lifecycle.

Since the bathtub curve helps us determine the asset’s expected useful life and reliability, it’s easier to keep it under control. But the main advantage is undoubtedly being able to more accurately plan how and when to perform maintenance.

The ultimate goal, as always for facility managers, is to extend the asset’s useful life and increase availability without unnecessary maintenance.

What are the 3 stages of the Bathtub Curve?

📌 Stage 1: Infant mortality and early failures

At the beginning of the asset’s life, in “childhood”, the number of failures is high, but the failure rate is decreasing, and the function has a negative slope. This period is known as infant mortality.

At this stage, failures occur due to root problems in the design, lack of quality control, errors in the installation, manufacturing defects, inadequate components, or insufficient burn-in. 

How to prevent infant mortality equipment failures

There are several strategies that can decrease infant mortality and avoid early failures. Here are some suggestions:

• Purging – perform preliminary tests with automatic technologies and sensors. Defective equipment is discarded or repaired before leaving the factory.
• Acceptance and reliability tests – equipment revaluations whenever parts are replaced or there are changes to the design, tools or processes. 
• Quality control – use early fault detection techniques, such as vibration analysis, to detect problems.
• Accelerated tests and burn-in tests – tests that consist of subjecting the equipment to equal or even more demanding conditions than usual, to observe its behaviour under stress or to measure the results obtained in a certain time.

What is the best maintenance strategy for new assets?

For young assets, the best option is reactive maintenance, as it is the only solution to prevent failure from reoccurring. Besides, you can also activate the warranty, and demand a repair from the manufacturer or the installation company.

A brand new lift can fail two or three times a month. These failures are due to installation errors or problems in starting up the lift. These problems are usually fixed with adjustments between the lift, the running box, and the lift shaft of the building. This period lasts between two months (at best) to twelve months (at worst), depending on the quality of the materials and workmanship.

📌 Stage 2: Useful life and random failures

At this point, the asset is already an “adult”. The failure rate is constant because everyone already knows how to handle it, and the manufacturing errors were fixed. This stage is known as the asset’s “useful life”. 

Throughout the useful life of the asset, the failure rate is low and constant. Failures are random and due to human errors, overuse or overload, and accidental breakdowns. Also during this stage, the MTBF is the inverse of the failure rate.

What’s the best maintenance strategy during the asset’s useful life?

During the asset’s useful life, we recommend that you follow the manufacturer’s advice for periodic maintenance. The most appropriate maintenance strategy for an adult asset is preventive maintenance or, if your operations are prepared for it, predictive maintenance. The latter will allow you to foresee an inflection of the curve and prevent stage 3 altogether.

Let’s get back to our lift. Now it only fails once every three months, almost always due to random causes. There are malfunctions due to misuse (for example, broken buttons in the cabin), energy overloads, excessive vibration, mechanical impacts from usage, temperature variations, human errors while operating it or during maintenance.

📌 Stage 3: Late life and wear-out failures

At this stage, the asset is already a “senior”. The failure rate increases gradually and the bathtub curve function has a positive slope. Not all assets come to this end, as some are phased out before their wear-out stage. For example, some electronics are replaced when their technology becomes obsolete, even though they are still working.

At this stage, failures are a result of wear and tear, poor maintenance or lack thereof. Repairs get more expensive and security risks increase.

What’s the best maintenance strategy for late life asset management?

At this stage of the asset’s life cycle, the best maintenance is preventive maintenance, which is more effective in preserving reliability and extending the useful life. Ideally, you should establish new preventive maintenance measures as soon as you notice an increase in the failure rate. This will allow you to avoid a sharp rise, and keep a failure rate close to that of stage 2. 

Which preventive maintenance activities prevent wear-out failures?

• Maintenance and inspection plan – make a detailed plan with periodic maintenance checks and activities that must be carried out through the year. Prioritise the assets that begin to show signs of wear.
• Preventive replacements – don’t wait for equipment to fail and break down with no chance of repair. At this stage, you should consider preventive replacements of key parts (for example, batteries and engines), especially if the new parts have extended warranties.
• Use high-quality spare parts – sometimes, in older assets, there’s the temptation to use parts that are different from the original ones, but “interchangeable” with the equipment. However, if you’re investing in repairs, it’s better to acquire durable spare parts approved by the manufacturer.
• Suppression of harmful agents – try to keep the equipment in the most appropriate conditions, with the recommended temperature and humidity level in the air.

Our beloved lift is back to two or three failures per month, but this time around it is not a matter of manufacturing or installation errors. Corrosion and fatigue, which translate into the visible deterioration of mechanical and electrical components, are the main culprits. Occasionally, insufficient maintenance, or poor maintenance, cause breakdowns. The lift is no longer even with the floors, and sometimes stops between floors. Soon, we’ll have to choose between replacing the lift, which is quite expensive, or replacing essential parts, such as the frequency inverter.

Is it better to repair or replace?

It’s up to the maintenance department to decide whether to repair or replace the asset. If you act right at the beginning of the wear-out stage, you can modernise it so that it goes back to phase one. After this “reset”, you will notice an increase in the failure rate, as in the infant mortality phase.

However, these failures will decrease and, over time, regain a predictable pace. If the cost of the improvement is lower than the cost of a new asset and comes with warranties… we’re ready to start the cycle again!