Mean time to failure

Free guide to maintenance metrics

What is mean time to failure?

Mean time to failure (MTTF) is a maintenance metric that measures the average amount of time a non-repairable asset operates before it fails. Because MTTF is relevant only for assets and equipment that cannot or should not be repaired, MTTF can also be thought of as the average lifespan of an asset.

How to calculate MTTF

To calculate MTTF, divide the total number of hours of operation by the total number of assets in use.

MTTF = Total hours of operation ÷ Total assets in use

MTTF = 10,000 hours ÷ 40 assets

MTTF = 250 hours

Calculating MTTF with a larger number of assets will lead to a more result as MTTF represents the average time to failure.

Example of MTTF calculation

Let’s say you want to calculate the MTTF of the conveyor belt rollers at your facility. There are 125 identical rollers, which operated a total of 60,000 hours in the last year. Your MTTF calculation would look like this:

MTTF = Total hours of operation ÷ Total assets in use

MTTF = 60,000 hours ÷ 125 assets

MTTF = 480 hours

You can conclude that the average lifespan of a roller at your facility is 480 hours.

How to use MTTF

Non-repairable assets

MTTF applies to assets that are non-repairable—when they fail, they are replaced.

There are a number of scenarios in which an asset might not be repaired, but the most common reason is that it costs less and takes less time to replace the asset. For example, replacing a tire that costs a few hundred dollars in parts and labour is probably more cost-effective than removing the wheel and trying to repair it. It just isn’t worth the effort or the cost. Some examples of this type of equipment include:

  • Transistors
  • Fan belts in motors and engines
  • Idler balls/rollers on conveyor belts
  • Wheels on a forklift
  • Lightbulbs

These assets can be part of run to fail maintenance, preventive maintenance, or condition-based maintenance.

When to use MTTF

There are a few situations where calculating MTTF can help you improve your maintenance and asset management strategy.

  1. MTTF can be used to schedule maintenance on non-repairable assets in cases where regular preventive maintenance can prolong the life of the part and a larger, mission-critical asset. For example, lubricating bearings on a larger machine.
  2. MTTF can also be used to make decisions about purchasing parts and equipment. Higher-quality and more durable materials will lead to a higher MTTF, which means fewer resources are spent buying new parts and replacing old parts.
  3. MTTF can play an important role in establishing a just-in-time inventory strategy. If a facility knows a certain part has an MTTF of 10,000 hours and it takes 100 hours to get a replacement, they can order a part every 9,900 hours.

MTTF and MTBF

Mean time to failure sounds a lot like mean time between failure (MTBF), but they’re not the same.

The key difference is the type of asset used in the calculation. Where MTTF uses non-repairable assets while MTBF deals with assets that are repairable—when they break down, they can be easily repaired without spending too much.

Summary

The MTTF formula is a key part of the overall reliability equation. When failure of an individual component (such as a fan belt) will lead to failure of a motor that will shut down an entire system or production line, being able to predict when that component is likely to fail, and replacing it before the failure occurs, is essential to reduce costly repairs, minimize downtime and maximize equipment lifespan.

Knowing and understanding MTTF can help organizations reduce downtime and develop better maintenance strategies, driven by reduced dependence on reactive maintenance and increased predictive or planned maintenance.

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