Comment by defanor | Hacker News Reader

Comment by defanor | Hacker News Reader

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Mechanical shocks are indeed the concern.

> you could drive it right in to a computer non-stop until the battery runs out and it's not going to do any meaningful damage

What is this based on (i.e., are HDDs and all the smaller components commonly designed to withstand such shocks)?

Now I've looked up a random WD Blue HDD data sheet [1], which mentions 30G shock (read/write, 2 ms). If a vacuum cleaner was to hit it directly while moving at about 1 m/s (less than 4 km/h, and it seems to walk at around that speed, though I haven't found exact values), and assuming that it would accelerate the HDD to its own speed in those 2 ms (maybe it's not realistic, though not obvious to me), that'd be about 50G (1 m/s / 0.002 s / 9.8). Even if it is so, a vacuum cleaner has a plastic bumper, a computer case is heavy, and a vacuum won't hit any of those components directly, but it seems dangerously close and I'd rather not hit the computer.

[1] https://documents.westerndigital.com/content/dam/doc-library...

> accelerate the HDD to its own speed in those 2 ms (maybe it's not realistic, though not obvious to me)

You need to conserve the momentum and kinetic energy, so assuming an elastic collision you're going to accelerate the PC to about half the speed of roomba[1] so you're looking at closer to 25G. With the plastic bumper, you're not getting anywhere near an elastic collision and your contact time is probably a lot longer than .002 due to the deflection. I would not worry too much about this.

http://www.convertalot.com/elastic_collision_calculator.html

Three things you should keep in mind regarding those numbers:

1. They're worst case numbers, as in the hard drive being mounted perpendicular to the impact force. Some OEM crapbox PCs may have spinning rust mounted to the front of the case directly in a vertical orientation, but most PCs have them mounted horizontally and many use shock absorbing rubber bushings.

2. As noted, your numbers also assume an inelastic collision. Almost all PC cases have a plastic front, and all robo-vacs I'm aware of have the same.

3. The absolute peak potential impact force is significantly less than a single accidental kick or kid/pet running in to it.

If you're really concerned about impact, you should get your PC off the floor. That said, I'd happily frisbee a Roomba at my PC and not be concerned for anything other than the front panel of the case itself.

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