Select double ball Micro Cooling Fan pros and cons

Double ball structure "Micro Cooling Fan" is the use of two types namely bearing ball bearing, what advantages and disadvantages does this structure, we have to analyze
  1. Double ball dustproof structure .2 ball bearings (Bearing) + S420 + iron axis dust-proof structure design First, he is a dust-proof design
◆ "Micro Cooling Fan" advantages:
a) a more stable speed fan operation, fan installation bit different aspects of disposal will not change the fan speed
b) a relatively long life
C) production and processing and assembly easy
◆ "Micro Cooling Fan" shortcomings
a) high cost
b) four-pole miniature fan motor is more difficult in the low-speed design, start