Fan life can be measured using different techniques.  Engineers in the industry use either L₁₀ or MTTF.  L₁₀ measures life by assuming certain conditions of fan life distribution, at which ten percent of the sample units are tested and failed.  L₁₀ is calculated based on temperature acceleration method defined in MIL-STD-781.  Testing is usually conducted under 70-80^oC

MTTF or the mean time to failure is the average time before fans fail.  Failure is defined as one of the following: 1) fan does not work, 2) RPM is 30% of origin, and 3) rated current is 30% of origin. Usually, a test batch of samples is taken to ensure that the probability test is reliable.

Calculations of Life

L₁₀ Life (the minimum service life)

T (forecast) = T (actual) x FCC

Where:

T (forecast) = the forecasted L10 life under working temperature T_f usually 25^oC

T (actual) = the actual tested life under temperature T_a.

FCC  = the temperature accelerator = 2EXP[(T_f - T_a) / 10]

MTTF (Mean Time To Failure)

A.F. = e ^(DH/K)   x  ((1/273 + T_L) - 1/(273 + T_H))

Where,

A.F.   = Acceleration Factor is number of fan failed at T_L test divided by number of fan failed at T_H.

273 = is absolute temperature

e = natural log

DH = Activation Energy

K = Bottzmann's Constant = 8.623x10^-5

T_L = tested temperature low = 50^oC

T_H = tested temperature high = 80^oC

Life characteristics are dependent on the combination of voltage, frequency, ambient temperature, mounting altitude, environment, and restriction to airflow conditions encountered in an individual application.  The most dependent factor to failure, however, is bearing and lubrication of the bearing.

Bearing Type

The relationship between the bearing and the fan is a positive relationship.  If the bearing runs breaks, so does the fan.  There are different bearing types to consider.  There are sleeve, ball/sleeve, and ball bearing.  The life hours increase as you move from left to right.  Sleeve bearing, compared to ball bearing, tend to be quieter at the initial start and relatively less expensive.  As for the ball bearing, it can operate better under extreme temperature.

Sleeve bearings have a life around 30,000 hours or the equivalent of 6 years under normal, 8-hour day operating conditions.  At the initial point, a sleeve bearing fan is much quieter than a ball bearing.  As time goes by, however, the sleeve bearing will start losing lubrication and will become noisier than its counterpart.  The life on ball bearing fans are somewhat different.  The life of a ball bearing will, typically, last twice as long than that of a sleeve.  At initial use, the sleeve is much quieter.  But if there is a rise in temperature, the sleeve bearing will lose lubrication much faster, leading to faster thermal breakdown.  Still, sleeve fans remain relatively quiet up until the lubrication is gone; whereas ball bearing fans get noisier shortly after it starts.