Learn (or review) the difference between MTBF and lifetime

There has been confusion in understanding the difference between mean time between failures (MTBF) and lifetime. A product might have an MTBF of 500,000 hours, but a lifetime expectancy of 20,000 hours. So why is there such a large discrepancy? Puls LP says the answer is easy if you understand...

09/24/2008


St. Charles, IL – There has been confusion in understanding the difference between mean time between failures (MTBF) and lifetime. A product might have an MTBF of 500,000 hours, but a lifetime expectancy of 20,000 hours. So why is there such a large discrepancy? Puls LP says the answer is easy if you understand the difference between the terms, because one does not have anything to do with the other.


Puls explains the difference between MTBF and lifetime.

MTBF represents the statistical approximation of how long a number of units should operate before a failure can be expected. It is expressed in hours and does not represent how long the unit will last. There are many ways of calculating MTBF. Use calculations based on models such as SN 29500, MIL HDBK-217 or Belcore; use field failures, or Field MTBF; or use laboratory testing, or demonstrated MTBF. For instance to test 10,000 units for 1000 hours with 10 failures, the MTBF would be 1 million hours. This does not suggest the unit will operate for 114 years. A better representation would be if 500 units operate at the same time, a failure could be expected every 2,000 hours, or 83 days.
Unlike the hours from the MTBF calculations, lifetime indicates operating hours expected under normal operating conditions. It is the period of time between starting to use the device and the beginning of the wear-out phase. This is determined by the life expectancy of components used in assembly of the unit. The weakest component with the shortest life expectancy determines the life of the whole product. For power supplies, electrolytic capacitors have the shortest lifetime expectancy.
To understand MTBF versus lifetime, think of a product going through three phases over its lifetime. In the first, the failure rate is high and is referred to as the “infant mortality” phase. In the second, the failure rate is low and fairly constant. In the third, the failure rate begins to increase and is called the “wear out” phase. The complete graph is the “bathtub curve” because it looks like one. MTBF is a way of determining how many spare parts you might need to support 500 units, but a poor guide on when those parts should be changed. A unit that operates eight hours a day will last three times longer than a device operating around the clock. However, MTBF is the same because both units receive the same number of hours in service.
Many factors determine reliability. Low failure rate and long life are two. A good quality process control and a high degree of automation during production can lower the defect rate and improve reliability. A rugged design using high-quality components can improve reliability system-wide. Environmental conditions such as vibration and temperature can play a major role in defect rate and reliability. For power supplies, heat is the enemy and can shorten the life of electrolytic capacitors dramatically. The industry rule states that every 10 °C increase in temperature reduces the life of the capacitor by half.
As that relates to products, for instance, Puls uses large-diameter, high-quality capacitors, allowing the Dimension series to have a rated life of at least 50,000 hours. The older Puls SilverLine Series use capacitors rated with a longer life than competitors with current product, the company says.
For more information on mean time between failures and mean time to failure (MTTF), read

How do you determine MTBF

?
–  Control Engineering System Integration eNewsletter
Register here and scroll down to select your choice of eNewsletters free .





No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
The true cost of lubrication: Three keys to consider when evaluating oils; Plant Engineering Lubrication Guide; 11 ways to protect bearing assets; Is lubrication part of your KPIs?
Contract maintenance: 5 ways to keep things humming while keeping an eye on costs; Pneumatic systems; Energy monitoring; The sixth 'S' is safety
Transport your data: Supply chain information critical to operational excellence; High-voltage faults; Portable cooling; Safety automation isn't automatic
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Plant Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.

Maintaining low data center PUE; Using eco mode in UPS systems; Commissioning electrical and power systems; Exploring dc power distribution alternatives
Synchronizing industrial Ethernet networks; Selecting protocol conversion gateways; Integrating HMIs with PLCs and PACs
Why manufacturers need to see energy in a different light: Current approaches to energy management yield quick savings, but leave plant managers searching for ways of improving on those early gains.

Annual Salary Survey

Participate in the 2013 Salary Survey

In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.

Average salary across all job titles for plant floor management rose 3.5% to $95,446, and bonus compensation jumped to $15,162, a 4.2% increase from the 2010 level and double the 2011 total, which showed a sharp drop in bonus.

2012 Salary Survey Analysis

2012 Salary Survey Results

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.