I was wondering if someone could explain shock tolerance in simple terms...
what I am asking is: what exactly might a real-world example be of a 250g shock, which is the non-operating tolerance of many modern hard drives.
That's an excellent question that I've often wondered myself... like a rollercoaster pulls anywhere from 3-6 Gs (yes, they go that high... there's one in Texas I believe) and for anything more than a second that sort of force could put someone out... so 250G sounds insanely high. But the rollercoaster accelerates/decelerates with/against gravity... a hard drive that has taken a fall will come to a near-instantaneous stop... I think that explains the difference in the numbers... although I have no clue what 250Gs is comparable to.
Ok, I'm done rambling and will just wait for an educated poster.
250Gs is equivalent to a stopping force of 2500m/s/s (that's 2.5kilometres per second per second). If you could accelerate something at that acceleration for 1 second, it would be travelling at 2.5km/s - which is 3600*2.5=9000 km/h.
Put like that, it sounds *huge* - after all, how hard could you throw a hard drive? maybe 160km/h (100mph - the speed of the fastest of fastballs/cricket deliveries).
However, the acceleration (which is what "Gs" mean) is dependant on *time*, as well as on speed.
Let's look at some more maths;
A hard drive travelling at 1m/s (roughly 3.6kmh or 2.5mph - a slow walking pace) hitting a hard, concrete floor will stop almost instantly. Let's say it stops in a hundredth of a second (which is quite reasonable).
Your acceleration (in this case a negative acceleration, often called a "deceleration") is 1 metre per second times 0.01 seconds.
1*0.01 is 100G.
Under gravity, an object which is dropped will accelerate to 1m/s in approximately a tenth of a second. Think about dropping something off your desk, how long does it take to hit the floor? Longer than a tenth of a second?
I'm *really* not going to go into the maths, because it's a pain
Suffice it to say that shock measurements such as 250G are more confusing than they need to be. A more sensible measure would be "will survive a drop of 'x'cm onto concrete".
We're stuck with G's because they're nice and emotive and everyone* knows* what they mean*.
Hope this helps!
Wikipedia has a good article on Acceleration - worth reading.
Very informative, mkaibear! Let me try another example.
A few years ago I asked the same question: Just what do these shock tolerance levels mean? Well, my local PC guru had a nice easy answer for me. He took a spare hard drive that was on the counter and banged it (not gently, but not roughly) against the counter and told me that the HD would probably be damaged if it were in use at the time. Seems about right to me, considering you don't want to be banging a hard drive in use against a counter (or dropping your PC or something). What about when it's not operating, though? Well to demonstrate that, he took said hard drive and hurled it at the non-carpeted floor so hard that I was a little taken aback. The drive was still in once piece, but he told me that was about how much force it could take before breaking.
"So you just broke that hard drive?" I asked.
"Yeah, probably." was his answer. But it's okay, as it was an obsolete 5.25" HD.
Well, it is worth mentioning that some laptops now have built-in sensors that can detect when they've been dropped. I know that Lenovo ThinkPads have this feature and when a drop is in progress the computer will park the hard drive heads and essentially brace itself for the impact. Does my Dell do that? Probably not... did my Dell cost as much as a ThinkPad? Nope.
I know it has been a while since I have done g-force calcs but something does not look right with the calculations mkaibear presented. I thought acceleration was (final velocity - initial velocity) divided by time - not mulitplied by time.
A = ( V(final) - V(initial) ) / T(interval)
and this needs to be converted to g-force using 1grav = 9.8m/s/s
I found the message below and had some interesting comparisons.
Re: How do you calculate 'g-forces'?
Date: Tue Sep 8 12:21:33 1998
Posted By: Jason Goodman, Graduate Student, Massachusetts Institute of Technology
Area of science: Physics
"g"-forces are really a measure of acceleration, which is the rate of change of velocity of an object. If a car accelerates from zero to sixty miles/hour in 6 seconds, it has an acceleration of 4.3 meters per second per second. That is, every second, its speed is 4.3 meters per second greater.
To convert this into "g-force", we compare this acceleration to the acceleration produced by gravity, which is 9.8 meters per second per second. The car is accelerating at about 0.4 times gravity, or 0.4 G's.
The "g" can be thought of as a unit of acceleration whose value is 9.8 m/s^2. In many cases (such as the one above), gravity has nothing to do with the situation. In others, it does. For example, the strength of gravity at the cloudtops of Jupiter is 2.5 G's, because objects would fall at 24.5 m/s^2 there.
So the "g" is simply a unit of measurement, but it's a convenient one for describing accelerations in terms people can identify with. Here's a handy table, reproduced from Physics, by O'hanian, 1989:
Protons in Fermilab accelerator 9e12 G
Ultracentrifuge 3e5 G (300,000 G)
Baseball struck by bat 3000 G
Soccer ball struck by foot 300 G
Automobile crash (100 km/h into wall) 100 G
Parachutist during opening of parachute 33 G
Gravity on surface of Sun 27 G
Explosive seat ejection from aircraft 15 G
F16 aircraft pulling out of dive 8 G
Loss of consciousness in man ("blackout") 7 G
Gravity on surface of Earth 1 G
Braking of automobile 0.8 G
Gravity on surface of Moon .17 G
Extra bonus factoid: if you lock your knees when you hit the ground after jumping off a 1-meter platform, the acceleration is about 50 G. So don't.
Yeah, and not to mention that I'm "old" now. When you're a kid you can get slammed in the face with a soccer ball and them white blood cells go ape-sh*t crazy repairing any damage so you can get back to playing in no time. If I tried a stunt like that now I'd have a headache for two days!
I can;t give an example of a 250G shock but to put how durable they are (when switched off):
At my school, the kids use the pcs as stools, they get kicked frequently when people get pissed off at admin, they get knocked from kids fighting, and just abused from assholes that like watching the admins face go purple. All these pc's are still running fine now.