All Life Time values in catalogues or brochures have to be considered as rough estimates and are not that as it should be. Under no circumstances these values are valid for comparison. This time value does not say anything about the real lifetime of a product. This is a statistic number nothing more and can be bended to any desired value, depending on the calculation and test method used. The term lifetime 80.000h L10 at 25°C says nothing about the test and how it was done. It just says that maximum 10% is the failure rate when operating at 25°C under testing conditions.
One of our customers did a test with 10 fans type 1606KL. The time for the test was limited to 18 month. That means that the customer had to consider accelerated aging and tested at 105 °C. After the test, the results from the statistics should be calculated back to the standard operating temperature of 40°C. The customer had used the assumption that after reducing the temperature by 10°C the lifetime is doubling (1). The first fan failed after only 1300 hours, due to a non typical failure. This first failure was ignored (2), because it was a statistic outlier the customer mentioned. After 18 month the test ended. During this time two other fans failed by electronic components failure.
Taken this into account and using standard statistic calculations you get the following results. Statistic according to Gauss with the outlier: 91.200 hours L10 at 40°C Statistic according to Gauss ignoring the outlier: 131.200 hours L10 at 40°C Customer’s own statistic with safety factor: 77.120 hours L10 at 40°C Statistic according to Weibull (NMB Standard) 112.000 hours L10 at 40°C
The above example does make clear, that it is not so easy to compare lifetime values of different companies, when the testing method is unknown. Even different divisions in one company may use a different method that leads to a total different result. There is room for a lot of pitfalls you can step into. 1. The assumption that the lifetime is doubling when reducing the temperature 10°C. Some companies are using not factor 2 but factor 1,4 or other algorithm. The difference between 2 and 1,4 seams to be little, but in the end it is a factor 24 respective factor 5,7 when calculating back from testing temperature 85°C to 40°C L10. 2. What happens to statistic outlier? 3. Is the FAIL a total fail or increase in current by 15%, decrease in speed, increase in noise? 4. How to take failures into account based on too high testing temperature? 5. What is the lifetime of the grease of the bearing? 6. What are the safety factors the company is using? 7. How optimistic is the designer (high or low safety factor) ? 8. Which statistic calculation (3) was used ? 9. How high is the risk the management is going to take to publish higher values?
So as you can see, this is a wide range for mistakes, interpretation and manipulation. Because of these uncertainties it is nearly impossible to prove that the stated lifetime is true or not.
The actual Life Time Expectation
The actual lifetime is very much depending on the environmental situation. Temperature is of course one factor, but what about the humidity, dust, dirt and temperature cycles. Also the operating conditions are important. Does the product work continuously, with interrupts and what is the supply voltage like. What kind of components has been designed in? Is it low cost ball bearing, low cost electronics or high reliable components?
After been over 30 years successful in the market of fans and motors and over 50 years successful in the market of precision ball-bearings, we think that we have enough background to manufacture a high reliable product. The stated life time values from NMB Minebea have been proven in practice and exceed our too conservative specification values
The high quality standards of NMB Minebea, the continuous quality control after each single step in the production line and the final inspection will guaranty that the customer gets a reliable product over years.
Construction and Life Time Expectation
NMB Minebea has different types of fan designs. Some designs have been done to accommodate constructions for the low cost market and some for the high reliable industrial markets or telecom markets. A few remarkable differences are plastic liner or brass liner, welded shaft to the yoke, moulded shaft into the yoke, bearings with shields or without shields, standard PCB board or sealed electronics. All these will have an effect in the life time of a product.
NMB Minebea Life Time Statements
The binding lifetime values are only part of the latest issue of the NMB Minebea product specification. In catalogues, brochures and old specifications there might be different values because of the long evolution of products and designs. Updated specification and lifetime values are available on request.
In the NMB specifications you will find statements like this: · 50.000 h @ 20°C +/- 15°C, 65% +/- 20% · 30.000 h @ 20°C +/- 15°C, 65% +/- 20% humidity, increase of noise 5 dBA · 30.000 h @ 20°C +/- 15°C, 65% +/- 20% humidity, increase of current 15% and decrease of speed by 15% · 100.000 h @ 25°C L10 increase of current 15% and decrease of speed by 15%
Of course all these methods and results can be matter of discussion. But this is not the important issue. The most important question is, have these test all done under the same conditions and calculations over years. Only if this is true, the information about life time is reliable. For NMB Minebea this question can be answered with YES. We are using the Weibull statistic over years now and test the samples every time under the same conditions, 85°C till the fans fail. A fail is defined when one of the following conditions occur 15% increase in current, 15% decrease in speed of 3 dB increase in noise.
These test can last several years. One test started in September 1995 had been finished in 2001. (55.000 h)
Life Time Calculation according to DIN 622
Manufacturers of fans sometimes give life time values indicating for instance L10 50.000 hours @ 40°C in accordance to DIN 622. But this is misleading, because DIN 622 is a specification dealing with forces and transmission on ball bearings. Lifetime calculations are also included but for very high speed and heavy loads. Both do not apply to the small and lightweight box fans. Using these formulas results in a much too high lifetime, 300.000 hours and more.
Comparing life time values of different vendors
Comparing values in specifications of different vendors is difficult. To compare the lifetime values of different vendors is impossible. You know this already from the example I have described earlier. As long as there is no common specification how to setup the test and how to interpret the results, it is impossible to compare any of these values. These are just numbers and they depend on statistic and how it was done.
This is now also the right time to clear the wrong understanding that the lifetime of the fan is limited by the lifetime of the bearing. That is not true in any case. There are much more parts involved. Even a simple retainer ring can fail, due to a quality issue. More important for my understanding for the lifetime in reality is the quality of components used, the production process and production process control.
What does the customer get from NMB Minebea?
Customer wants high life time to make sure that the components give reliability and a zero failure rate within the first three years to overcome the warranty issue. Five years continuous operation is the desired time. Five years is 43800 hours, so a request of 80.000 hours is another safety factor. With some suppliers it is necessary to choose such high additional safety factors, but not with the NMB Minebea fan with two precision ball bearings and its high quality standards.
NMB Minebea is using quality standards, as there is ISO9002, ISO14001, and QS9000. NMB Minebea is the worldwide leading company with fans and blowers NMB Minebea monthly output on fans, 9 millions, shows it strength and reliability NMB Minebea has experience over 50 years with precision ball bearings. NMB Minebea has experience over 30 years with fans and brushless DC motors.
NMB (Nippon Miniature Bearings) NMB-MAT MINEBEA MOTOR CORPORATION is the world largest manufactures of fans and blowers.
NMB-MAT is a joint venture between NMB and Matsushita created in order to manufacture NMB fans as well as Matsushita's PANASONICS brand of fans.
The company is manufacturing all parts incorporated in the fans.
The products include DC and AC fans DC blowers.
To put an end to this scourge of the workplace, the Minebea-Matsushita joint venture has developed a new range of silent fans whose noise emission is limited to a tolerable 20 dbA.
NMB is one of the world largest manufactures of rod ends, spherical bearing, ball bearings, dc motors and stepping motors.
NMB-Minebea offers its customers competent system support with:
· Electrical Tests
· Thermal Simulation
· Fan Support
· Ball Bearing Support
· Motor Support
· EMC Tests
The ever increasing performance density is associated with mounting problems related to equipment cooling. During the development phase, thermal simulations with state-of-the-art computer software provide NMB with crucial information about airflow distribution as well as air and component temperature, which not only eliminates the need for thermal redesigns, but also facilitates shorter development times and optimized equipment cooling.
NMB provides its customers with complete system solutions: for any combination of fans / heat sinks / power supplies in conjunction with optimal layout and dimensioning, tailored precisely to the customer’s equipment.