Question How HDDs maintain flying height (head-platter distance)


Jun 29, 2015
I am looking into detailed HDD principles of operation and I cannot get my head around this.

Slider of the read/write head maintains about 5nm gap between itself and the platter surface using air bearing (basically flying on the cushion of air). I presume this force pushes the slider away from the platter. But in that case, what is the position of the slider when the drive is not spinning (head is parked)? And how it transitions from parked to "flying" above platter without contacting that platter?

The actuator arm is quite thin and I find it hard to believe, there is a 5nm clearance (and manufacturing precision) when heads are moving from parked outside the platter to the platter. I could imagine a situation, where there is a larger gap by default and the slider is "pressed" towards the platter once it is above it and spinning. But of cource the actuator is not spring loaded 😁. It would help though if the air bearing principle was able to keep the slider in constant distance not only by pushing away, but also by "pulling in".

So where is the bug in my presumptions? Is there really a 5nm gap that is maintained in all drive states? Is the air bearing not only pushing slider away?

Related to this, when watching people unstucking the heads, they move the head across stationary platter (sometimes rotating the platter a bit simultaneously). Are the sliders in contact with the platter in that case or not?
Drives originally parked their heads in a laser etched landing zone at the edge of the platter. Nowadays the heads are retracted onto a loading ramp, so ideally they never contact the platter.

To maintain a precise flying height, there is a heating coil which expands the slider so that it flies closer to the platter. HGST has some good whitepapers on the subject (thermal fly height control).

These days the heads are very flimsy (because they're much smaller), so it is very easy to rip off the slider after a stiction event. I think you'll find that the head is in fact under spring tension, so it naturally pushes toward the platter.
First HDDs (MFM, BF etc) used a stepper motor to move heads, needed a command to park head(s) in a safe place prior to stopping.
Newer ones have permanent magnetic levitation where a coil on the head arm positions it and when power to drive is cut off coil looses power so arm goes on a ramp without head touching anything. Same magnetic force holds it roughly in proximity of platter surface with rest of force provided by air vortex from spinning platters.
Modern HDDs can take 90g+ when parked but only couple of Gs while running.
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