Seagate Begins Volume Shipments of Helium-Filled HDDs, Reveals Their Final Specs

65 Comments

View All Comments

• FunBunny2 - Friday, April 29, 2016 - link

  -- Air cushioning of the heads have been a known fact for as long as winchester-based DASD has existed.
  
  longer actually. IIRC, the first DASD from IBM mainframes exploited "floating heads", at least the 1301 drive. that was 1961:
  "The 1301 is the first disk drive to use heads that are aerodynamically designed to fly over the surface of the disk on a thin layer of air."
  
  here: https://en.wikipedia.org/wiki/History_of_IBM_magne...

• Kalessian - Friday, April 29, 2016 - link

  I personally don't think the things he said warranted this kind of attitude

• dueckadam - Saturday, April 30, 2016 - link

  I really don't see what he's doing so wrong. He has some knowledge that makes him think one thing, and he's presented with information that conflicts with that, and he's trying to reconcile the two. Really, the conclusions he's drawing do seem reasonable, even if they're incorrect.
  
  As far as googling it, it's not as easy as it sounds. Sure, it's easy to find stuff saying that the head rides on a cushion of air, but I wasn't able to find much explaining the phenomenon, i.e. the mechanism by which the air bearing forms or how it acts as a cushion. So if you're as knowledgeable about engineering and physics as the condescending tone of your post implies, would you care to try and explain, or provide a source that does?

• Solandri - Monday, May 2, 2016 - link

  He's not spouting pseudo-science. What he's describing is exactly what happens with an incompressible fluid like water. It's why ships need tugboats to push them away from docks - they can't do it under their own power (unless they have a bow thruster), since any forward or backwards movement just pushes them closer to the dock. The QE2 ran aground partly due to this effect (high speed over shallow water caused it to be "sucked" down by Bernoulli forces).
  https://en.wikipedia.org/wiki/Squat_effect
  
  Air (and helium) however are compressible, which leads to a different phenomenon (ground effect) which counters Bernoulli.

• sor - Thursday, April 28, 2016 - link

  You forget what would be causing the gas to move at all:the platter. The gas is actually caught by the platter as it spins, sucking it under the heads.
  
  If you lay a piece of paper on a table, lift one side, and try to blow under it, it won't magically stick harder to the table. Yes, there is air moving, but you can't overcome displacement. The pressure is actually higher as it is being compressed/forced, and the movement is alleviating the pressure.

• sor - Thursday, April 28, 2016 - link

  Sucking is probably a bad way to describe it... The platter actually pushes the air it contacts, which increases pressure under the heads and creates the cushion.

• BurntMyBacon - Monday, May 2, 2016 - link

  @sor: "The platter actually pushes the air it contacts, which increases pressure under the heads and creates the cushion."
  
  Which is possible because air/helium is compressible. In fact, it is this compressibility that stabilizes the heads. It makes it act as a shock absorber of sorts.
  
  @Solandri: " What he's describing is exactly what happens with an incompressible fluid like water. ... Air (and helium) however are compressible, which leads to a different phenomenon (ground effect) which counters Bernoulli."
  
  This. The Bernoulli effect doesn't disappear, it simply ceases to be the dominant force. Interestingly, IIRC, it actually helps dampen vibrations by resisting the repulsing part of the vibration.

• MrSpadge - Friday, April 29, 2016 - link

  As far as I know you're correct - the only piece missing is that the heads are indeed aerodynamically designed to provide the proper force repelling them from the platter. That's what Azethoth described in fancy scientific language ("riding the cushion of air").

• freeskier93 - Saturday, April 30, 2016 - link

  Your critical flaw is the misguided use of Bernoulli's principal. Bernoulli's principal is actually derived from Navier-Stokes through certain assumptions, which are not applicable here. What we're actually looking at in the case of spinning platters is a Couette flow, where a velocity is created through shear forces in the fluid and NOT a pressure gradient. There is no pressure gradient in the tangential direction.

• Solandri - Monday, May 2, 2016 - link

  That's a pretty good recollection of physics - just enough to get you into trouble. What you're describing happens if the fluid is a liquid. We ran into it all the time with our robot submersibles - if you get too close while running parallel to a wall, the submersible gets "sucked" against it by Bernoulli forces and you're stuck (need to turn so a thruster can push you away... if you have enough room to turn).
  
  It's different when the fluid is a gas. At moderate distances, Bernoulli rules. But as the two surfaces get very close, the compression of the gas as it squeezes between the head and platter overpowers Bernoulli, and you get a high pressure zone. In aircraft, it's called ground effect, and can be used to greatly extend cruising range because the higher pressure between the wing and ground (or water) creates more lift, reducing drag.