Why does an HDD vibrate?

bartNL · Jun 24, 2014

Hi,

Just curious, maybe it's a stupid question but i'd like to know why the following happens:
Even though a hdd platter is extremely well balanced/aligned, it still buzzes and vibrates when spinning.
A hdd platter must be produced with an almost zero tolerance, it's almost perfectly round and flat so the center of gravity is almost exactly at the center, where the axis of the platter is. And the spindle motor is brushless. So you shouldn't be able to feel any vibrations when you hold or touch an hdd. And i'm not talking about when it's busy reading or writing, you can still feel it when it's not handling read/write requests.

USAFRet · Jun 24, 2014

almost zero tolerance, it's almost perfectly round

Almost.

bartNL · Jun 24, 2014

USAFRet :

But this almost is negligible, you should by far not be able to feel it vibrating as those magnetic sectors are only several nanometers big aren't they? If it vibrates more than a certain amount of nanometers, the disk would be unreadable i assume. Hdd manufacuresrs say the head hovers a space equal to 100 atoms,(don't know which atoms but still) well i assume you wouldn't feel a vibration of a few hundred atoms.

USAFRet · Jun 24, 2014

Right. Almost negligible. Almost.

7200RPM is a not insignificant speed. For a standard drive, the platter edge is going 80MPH.

But also, the platter and the head/actuator arm are not operating independently. They are all bolted to the same rigid case.

bartNL · Jun 24, 2014

USAFRet :

I don't know how you came on that answer but a calculation gives me this: (i'm metric)
(((2,5*2,54*pi*7200)/60)/100)*3,6=86km an hour or if you divide it by 1,6 it's 53mph
I've based this on a standard 2,5" disk.
But 1,8" 5400rpm drives vibrate too and are going with a speed of less than 30mhp at the edge.
Anyway, since it's perfectly in balance, the vector sum of all forces is zero. So it shouldn't vibrate.
And when the disk is not handling r/w requests, the arms don't move. and then they don't contribute to the vibrations. I'm not trying to be a wiseacre, i'm just curious.

USAFRet · Jun 24, 2014

bartNL :

USAFRet :

I don't know how you came on that answer but a calculation gives me this: (i'm metric)
(((2,5*2,54*pi*7200)/60)/100)*3,6=86km an hour or if you divide it by 1,6 it's 53mph
I've based this on a standard 2,5" disk.
But 1,8" 5400rpm drives vibrate too and are going with a speed of less than 30mhp at the edge.
Anyway, since it's perfectly in balance, the vector sum of all forces is zero. So it shouldn't vibrate.
And when the disk is not handling r/w requests, the arms don't move. and then they don't contribute to the vibrations. I'm not trying to be a wiseacre, i'm just curious.

I was calculating on a 3.5", 7200 RPM drive
3.75 diameter = 11.8" circumference = 80mph edge speed.

Could they be made perfectly vibration free? Yes. But you or I couldn't afford them.

bartNL · Jun 24, 2014

USAFRet :

Okay. I still can't imagine why the slightest movement(e.g. those caused by vibration) wouldn't disturb the head so it moves only a few nanometers. enough to read/write the wrong sector.
Those arms must be extremely accurate, and this is affordable though?
And i'd assume if they expand if the temp rises, this would cause problems too.

USAFRet · Jun 24, 2014

bartNL :

Because the people who design hard drives are much smarter than you or I.

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Why does an HDD vibrate?

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