"DC Fan" double ball bearings are more upscale. There are several pieces of tiny ball bearings around the axis, when the fan page or axis of rotation, that is pivoted with the ball. Because all spheres, therefore less friction, and there is no oil leakage problems. Double ball bearing fan advantage of longer life expectancy, about 40,000 to 55,000 hours; anti-aging properties, suitable for high speed fan. Dual ball bearing disadvantage is the high manufacturing cost, and at the same speed the maximum noise level (because the ball bearing friction points increased by 2 times.)