20 April 2006
shock sticker
Though I own no shortage of digital cameras*, I rarely seem to have one handy at the right times. Recently I’ve seen a number of scenes or things I’d've liked to have photographed, only to find that I couldn’t. One that I was able to capture is the label linked in this post, which if you read all the way to the bottom mentions “shocks in excess of 40 G’s” as a warranty voider. I find that funny for a few reasons:
- Not only will a 40 g** shock likely cause more problems than a voided warranty, but how could you measure it? Does Seagate have physicists on staff, poised ready with slide-rules to calculate the forces on a (shockingly) dropped drive? How could you, or they, prove it wasn’t in excess of 40 gs?
- What happens at exactly 40 gs anyway? How ‘delicate’ can this equipment be if they are willing to stick their necks out for a drive dropped at any shock under 40 g?
- I’ve been away from physics class for a long time, but last I checked 1 g was the acceleration of gravity, about ten meters per second squared. Forty times that would likely be higher than many a terminal velocity, wouldn’t it?
But that’s the only photo I’ve taken lately. Two others I didn’t were:
- A giant ‘76′ gas station sign lying at the roadside, in the overgrown grass and weeds, discarded by a company unwilling to be proud of its past.
- Two workers in a cherry-picker bucket in front of a video billboard. One has his fists raised upward in the universal pose for victory, and the ‘board is shows some nice colors.
Okay, maybe you needed to be there. Better yet, you would’ve brought me a camera, too.
* Three at last count, and I’m not counting broken or throwaway (i.e. sub-VGA resolution) ones.
** I’m italicizing the g because Wikipedia says so.
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Maybe it’s a really bad typo on their part and they meant 4. As in, don’t take it on rollercoasters. Or maybe the company is run by an alien.
You do realize that instantaneous G forces upon impact can be orders of magnitude higher than steady forces, correct?
Holding a drive in the air, it experiences one effective G. If you drop it, in freefall, it experiences close to zero Gs (discounting the effect of air, which should be negligible). When it hits the ground, it experiences a sudden deceleration, amounting to (by my back-of-the-envelope calculations) a couple hundred Gs.
So, 40G is not even remotely implausible. The rest of your points stand, though. (I think the maximum G rating is designed to keep Seagate from having to deal with head crashes.)
My HDD is rated at 600g operational shock. I wish I knew how that was tested as well…
Delighted to say the answer is simple…the forces can be calculated with very basic physics calculations (See conservation of energy and free-fall calculations) . The results are checked using accelerometers and benchmark tests. Do Seagate have physicists standing by to check? Absolutely, as a customer and chartered engineer I’ve seen them along with many other Science mathematical, chemical and engineering grads. How do they know a drive has been dropped? Disk-shift, bearing-Brunelling, gimbal distortion, designed-in witness marks, HDI media defects amongst many other forensic tests all done by every disk drive company. Why stick out one’s neck? because there are statistically sound reasons to do so as the damage threshold is not exceeded. Albeit that the 40G value is not a hard limit. Also, FYI just dropping a drive 6 inces onto a typical office desk can generate > 200 G. (sorry if you prefer it italicized). Bravo to Benjamin who seems to have a solid grasp of the world around him ! As Ben said, the shock is greater depending on the rate at which you stop !
(See jolt and jounce as derivatives of acceleration).