Aerospace and Electronic Systems Magazine April 2017 - 22
Electromechanical Actuator Fan Failure Analysis and Safety-Critical Design
corrosion, and yielding are various failure modes and mechanisms
depending on the application load. Lubrication deterioration due to
thermal overloads can be in the form of either physical or chemical
degradation. Physical degradation is due to lubricant evaporation.
Chemical degradation is caused either by antioxidant consumption
or lubricant oxidation. Lubricant deterioration leads to poor lubrication between bearing elements, resulting in seizure, the state of
stopping relative motion between bearing components as a result
of interfacial friction. Deterioration of grease/oil in permanently
lubricated ball bearings is caused by thermal overloads.
In order to assess the reliability of a fan, environmental and
operational loads must be understood. The application loads during
a fan's life cycle can occur in manufacturing, assembly, storage,
handling, transportation, and operation. The IPC-9591 standard
 specifies two distinct loading conditions to be satisfied during
the fan life cycle: nonoperating and operating. The loads during
the nonoperating portion of the life cycle include temperature, humidity, shock, and vibration. Here, all the discussion is within the
range of IPC-9591 standard.
To estimate bearings' life span, for example, the bearing life expectancy due to lubricant deterioration caused by thermal overloads
is calculated. The grease life equation has different constants depending on the type of lubricant. A grease life equation for general
purpose grease used in fan's one/single bearing is as follows :
log10 L50 = 6.54 − 2.6
− 0.025 − 0.012
where L50 is time to 50% accumulated probability failure in hours;
n and Nmax are fan speed (rpm) and limiting speed (rpm) with grease
lubrication, respectively; and T is operating temperature (°C). In a
typical EMA fan cooling application, a radial ball bearing considered to be used has an outer diameter of 8 mm, an inner diameter
of 3 mm and a width of 4 mm in a BLDC fan. If the rated fan
speed was 8,000 rpm for the bearings, the maximum limiting speed
100,000 rpm and the operating temperature is 20° C, by substituting the above values into the (3), the calculated grease life, L50, is
found to be 105.872 = 745,000 hours if the bearing is used continu-
Fault tree analysis on dual-bearing system.
ously. Considering a nonoperation factor of 2, the fault probability
is about 1/1490,000 = 6.7 × 10−7. It is noted in Bennett's paper 
their motor bearings failure probability is ∼O(6.6 × 10−7) which is
very close to the above estimate.
In addition, it is noted that the life expectancy of fan ball bearings can also include calculations related to fatigue mechanisms
-. The life expectancy associated with fatigue is extremely
long ∼O(5 × 1011) hours so fatigue failure is ignored here -.
From Figure 5, it also can be seen that even though dual-bearing system could have a lower failure 1.3 × 10−8 , the overall loss
of output still remains at 2 × 10−5. So, there is no overall improvement in the reliability. This is also consistent with the fan manufacturers estimates too. Therefore, implementing dual-motor bearing
alone is not sufficient to improve the system's failure probability
down to ∼O(10−8) due to the fact that the overall fault probability
is determined by the failure probability of electronics components.
The bearing failure probability of ∼O(6 × 10−7) is already very low
Fan motor dual-winding system design.
IEEE A&E SYSTEMS MAGAZINE