How are hard drive bearings sealed?

[bryanl]

I know magnetic fluid was used to seal the ball bearings in hard drives, but now that they've been replaced by fluid dynamic bearings, how are those bearings sealed? If magnetic fluid is used is it kept separate from the bearing fluid, and how?

 

[PottedMeat]

hmm maybe you dont need a barrier to keep the fluid sealed because the surfaces are so close together - so surface tension perhaps?

This picture isnt all that good -
Hitachi FDB vs BB picture on Pg 3

Does it mean the bearing is ester oil in a herringbone/spiral groove?

 

[Mark R]

Not sure.

I think the way its done is 2 fold:
1: When the bearing is stationary, the lubricant is held in by surface tension
2: When the bearing is operational, the spiral groove actively pumps the lubricant back into the bearing cavity.

I've heard that the latest bearings use magnetic fluid as the actual lubricant. Apparently, the semi-solid nature of the magnetic fluid means it is more stable and stiffer when pressurised by the bearing operation, so there is less friction and the bearing runs truer. Finally, it is easily sealed by placing a magnet at the exit - the magnetic field draws the fluid into a solid at the entrance to the bearing, so the fluid itself acts as a barrier.