Fan Life and Reliability
Life expectancy of a cooling fan is a critical element in thermal design. The benefit of fan life and reliability testing is a fan that sets the quality and reliability standard for the cooling industry.
NMB evaluates fan life and reliability during the design phase using accelerated life testing in conjunction with ORT (Ongoing Reliability Testing). Accelerated life testing is used to compress the amount of time required to conduct life testing.
Development testing occurs early in the product design, prior to product release. It is vital to characterized the reliability of the product in the initial stages of design to allow for improvements and to meet the reliability specifications prior to release to manufacturing.
Once the design has been through design verification testing and is turned over to manufacturing, ORT is conducted. For some models, ORT evaluation has continued beyond 10 years. The value of ORT is a continued refinement of the accuracy of the accelerated life testing and constant review of the design of the fan. This continued process improvement allows for ongoing evaluation and increase in fan life and reliability.
Under accelerated life testing, NMB fans are tested at extreme environmental conditions, with temperature stress factors above standard operating levels. In order to gather meaningful data within a reasonable time frame, the stress factors must be accelerated to simulate different operating environments. High temperature stress is the most common stress factor used for these purposes.
NMB uses parametric failure modes during life testing to calculate for life expectancy. Speed (RPN) and Current (mA) failures include both “hard failures” (where the fan is non-functional) and “parametric failures” (defined as 15% decrease in RPM and increase in mA of 15%).
Proper understanding of accelerated stresses and design limits are necessary to implement a meaningful accelerated reliability test. NMB uses the Arrhenius model for determining acceleration factors (AF) during life testing. This is the most commonly used model in accelerated life testing where thermal stress is the primary factor affecting life.
Life test data gathered from different types of fans and blowers lends to highly accurate statistical analysis. This data can produce very detailed information about the behavior of the product for reliability and prediction of fan performance in the field. The Weibull Distribution is a typical method employed by NMB for statistical analysis.
An explanation of this calculation model is shown below.
Arrhenius Weibull Model:
The Arrhenius Life-Stress Relation is given by:
L represents a quantifiable life measure, which is the scale parameter or characteristic life of the Weibull Distribution.
V represents the stress level (formulated for temperature and temperature values in absolute units, i.e. degrees Kelvin or degrees Rankine).
C is one of the model parameters to be determined ( C > 0 ).
B is another model parameter to be determined.
Mean Life or MTTF
The mean, T , also called MTTF or Mean Time To Failure, or the Arrhenius Weibull relationship is given by:
The Arrhenius Weibull Distribution model predicts the length of time at which a defined percentage of a product population will still be operating without failing to meet pre-set criteria. For cooling fans, this is normally characterized as L10 life expectancy, or the time at which 10% of a population will have failed and 90% of a population will continue to operate within specifications.
For the Arrhenius-Weibull relationship, the reliable life, TR, of a unit for a specified reliability and starting mission at age zero is given by:
This is the life for which the unit will function successfully with a reliability of R(TR).If R (TR) = 0.90 then RF=90% reliability or 10% unreliability (L10) or the life by which 90% of the units will survive.
NMB uses parametric failure modes, or the condition at which a performance parameter fails to meet pre-set criteria, to record failures during accelerated life testing. This produces a more accurate prediction of field reliability than methods which use only non-operating failure modes to record failures.
Example: Life Experiment Data Using Arrhenius Weibull
Product L10 life expectancy for NMB fans ranges from 30,000 hours to 200,000 hours of continuous operation at room temperature depending on fan speed, frame size, design structure, size of ball bearings and the type of ball bearings used.
NMB, a world leader in miniature precision ball bearing design and manufacturing, uses precision, long life bearings, produced in house to ensure extended fan life.