What Is the Mean Time to Fail?
Being able to quantify product and facility reliability through a variety of calculations makes it easier to form comparisons between operation times and facilities. However, mean times do not directly correlate with reliability but give aspects of it. Understanding what average times show and how they differ from reliability will make interpreting the data more exact. Knowing how to make these calculations is also vital in getting a bigger picture of probabilities of success.
Why Is It Important to Calculate Failure Rates?
Failures at your facility are guaranteed, but their frequency and the downtime they create can impact your reliability and productivity. Quantifying failures through many calculations lies at the heart of reliability engineering, which focuses on mitigating risk.
Identifying where failure rates exceed tolerable levels is a critical step for any facility seeking to improve its operations. Determining the best time to replace old equipment comes down to knowing when the cost in downtime surpasses the price of a new device. Maintenance can also tie tightly into calculating reliability and failure rates.
Many production facilities increasingly use Reliability Centered Maintenance, or RCM, which requires information about equipment operations. Unfortunately, the industry is only slowly integrating failure rate calculations. Determining failure rates and reliability of equipment makes it easier to quantify how well a facility functions.
While making multiple reliability calculations has not become the industry norm, that doesn’t mean your facility shouldn’t adopt it. Using this information can make it possible to schedule maintenance and part replacements at the best times. When consultants evaluate a facility for optimizing productivity, they look at reliability metrics and other factors to least impact operations with scheduled maintenance. Your facility should balance production, downtime and maintenance costs in the best way to optimize productivity.
Mean Time to Failure (MTTF)
Only calculate MTTF for irreparable equipment. Use mean time between failures (MTBF) for reparable items. The MTTF calculation determines the lifespan of single-use goods by giving an average time until a part breakdown. Because MTTF is a statistical measurement, it could easily exceed the actual lifespan of a part. For example, if you only have one failure from a combined 100 years of operation from 10 motors, the MTTF would be 100 years. Rarely will parts and equipment actually last 100 years. The measurement instead indicates the equipment that has a low likelihood of breaking.
Regular inspections and maintenance of equipment can extend its lifespan. In this way, scheduling maintenance can increase the MTTF because well-kept components will last longer. As with any measurement of equipment reliability, it’s dependent on how carefully parts get serviced. Being proactive with maintenance rather than reacting to breakdowns can help increase the reliability calculations and the numbers used to determine reliability, such as MTTF.
Related to MTTF is the failure rate. Unlike the former, which indicates lifespan, failure rate gives how many breakdowns occurred over a given time when looking at disposable parts. Other reliability measurements use failure rate as a part of their calculations, but, like MTTF, failure rate gives only a small part of the reliability picture for a part or facility.
While the MTTF gives a small part of the picture of equipment reliability, it should not be the only metric used to determine reliability. Actual reliability of equipment in a facility can vary wildly from the calculated MTTF value. For example, even if the MTTF shows a part has an average lifespan of 40 years, many parts will fail before that, reducing the actual reliability.
MTTF is only an average and should not be considered an exact measurement used to predict failure. Never replace regular maintenance and inspections with estimated calculations. The MTTF will, however, give an excellent big-picture view of the performance of parts on the factory floor.
How to Calculate MTTF
MTTF is a type of failure rate measured in terms of days, hours, weeks, months or years to the point of breakdown. Take the total number of operating hours or years for all similar parts and divide by the number of failures.
For instance, when calculating MTTF for motors, combine the operating period of all engines. If three have run for 10 years and four operated for five, the total span would be 50 years. Should five motors have broken down over that time, the MTTF would be the result of dividing 50 by 5 to get 10 years of average life:
- MTTF = operating time/number of breakdowns
- MTTF = 50 years/5 breakdowns = 10 years until failure
A related metric to MTTF is the failure rate. To calculate it, take the number of failures and divide by the total running time for all equipment, including that which did not fail. For example, if you operate three motors with a total time of 20 years among them, and one failure occurs in the bunch, this information will help calculate the failure rate.
- Failure Rate = 1 failure/20 years = 0.5 failures per year.
If instead of years, the calculation used the total number of hours, the result would indicate the number of failures per hour. The method used for calculating the MTTF and failure rate would remain the same, however.
Mean Time Between Failures (MTBF)
For repairable machines and parts, the correct calculation to use is the MTBF. Another way to think of the MTBF is how long the pieces last between repairs or rebuilds. The extent of the damage when the machine breaks down determines whether simple repair or more complex rebuild is the best option. When a piece of machinery stops working and requires downtime for repairs, count that as a failure for the MTBF calculation.
In most instances, facilities calculate MTBF for highly reliable parts or equipment with low expected failure rates. The period expected before a part needs repairs should have a high value, indicating a more extended time between services. Ideally, the units used for total operating measurement should be in days, weeks, months or years rather than hours, especially for reliable machines that break down after years of operation.
Use the MTBF calculation to schedule preventative maintenance and planned repairs for equipment. For example, if a motor has an MTBF of two years, align the repair schedule to that. Knowing when machinery will be out for repairs will make it easier for your employees to work around the inconvenience to decrease the loss of productivity that occurs from the repairs.
Just as regular maintenance can help prevent failure of disposable parts, it can also increase the span until repairable parts need fixing. For example, regular servicing of a machine will help it work more efficiently, reducing wear and raising the time until it fails.
Look at the trend of the MTBF for equipment over the years. If the value shows a downward trend, the machine may be nearing its end of usefulness. Should the MTTR increase for the same equipment as its time between failures drops, your facility may save money by replacing the part.
How to Calculate MTBF
The method to calculate the MTBF resembles that used for mean time to failure. Both look at the time divided by failures, which is the reciprocal of the failure rate.
To find the MTBF, calculate the total operating period. Divide this value by the number of equipment failures. This value gives the time before a part requires operation-restoring repairs.
- MTBF = total time/number of failures
For example, if a set of 10 hydraulic pumps runs for 100 months and experiences four breakdowns, the MTBF would be equal to 25 months between repairs.
- MTBF = 100 months/4 breakdowns = 25 months
Just as the failure rate is related to the MTTF for irreparable items, it also has a relationship to the MTBF for repairable parts. The failure rate gives how many parts fail over a given period. As with MTTF, the failure rate for repairable items is the reciprocal of the MTBF.
- Failure rate = number of failures/total time
The calculated values of MTBF do not give reliability, but these numbers provide an estimate of the period between needed repairs. If your facility uses a preventative maintenance schedule to determine when parts need service, repairs, inspections or maintenance chores done, the MTBF makes it easier to know when to fit in repairs for equipment.
While some parts may fail sooner than the MTBF calculates, others will last longer. Always remember that ideal calculated values offer only estimates rather than real values. The estimations are invaluable in creating service plans and schedules. With them, your facility’s maintenance staff can have a set of guidelines for what equipment they need to work on to maximize productivity and uptime.
If your team knows how to use them, the numbers found from these calculations will be valuable planning tools for your facility.
How Average Values and Reliability Can Affect Your Business
When equipment breaks down, keep track of the frequency and the period required for repairs. This information will prove vital to calculating MTTF, MTBF and MTTR. These three numbers will help determine the probability of failure, which relates to reliability. Ideally, your business should strive for the highest reliability possible. However, breakdowns do occur. To increase reliability metrics, reduce the time for repairs. At Global Electronic Services, we can help speed repairs with our rush service, which reduces our standard repair time of one to five days down to one to two days.
In addition to knowing where to send your equipment in case of breakdowns, having staff on hand with the ability to troubleshoot and conduct minor repairs when needed can also help reduce downtime and improve reliability. As equipment ages, its reliability decreases. Keeping track of the frequency of failures and repair times can make it easier to come to a fiscally responsible decision on the best time to replace worn parts and equipment and create service schedules.
Innumerable factors will affect your facility’s productivity, including equipment reliability and efficiency. To keep up with other ways to improve your company’s reliability, subscribe to our blog. We post news, information and tips on helping your manufacturing business thrive.
