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Impact force from tipped cabinet

Impact force from tipped cabinet

Impact force from tipped cabinet

(OP)
I'm trying to calculate an impact force from a tipped electronic cabinet.

I realize that are a lot of unknowns, but I'll replace those with some reasonable assumptions and see what my range of results will be.

My goal is to see the range of forces that PWB's inside the cabinet may have experienced.

Thanks,
JunkforJG

RE: Impact force from tipped cabinet

Impulse = change in momentum.
F*t = m*(v2 - v1).

Ted

RE: Impact force from tipped cabinet

I've seen a similar problem come up several times through the years. Basically, I'm going to drop Item X on a hard surface, and what's the force generated. Well, you can do the F=ma and all of that, but you can't solve the problem without detailed analysis of deformations in the object and in the surface it's hitting, which is usually impractical, and in many cases, impossible. You can assume the distance that it decelerates in, or assume the time it takes to decelerate, but then again, you could assume the force and save some effort.

One approach- see the diagram below- I'm showing widgets attached to a shelf in the cabinet. Assuming the cabinet tips over, hopefully, you can calculate the velocity when it hits. If you have any way of calculating the spring coefficient for that shelf, and know the mass of the widget, you can work it out from there. If the shelf is "heavy" relative to the widget, you're left assuming some portion of the shelf weight in that mass as well. Still very approximate, all in all, though.

RE: Impact force from tipped cabinet

The velocity at impact will be sqrt(2*g*h), where h is the vertical distance through which mass m falls.

Ted

RE: Impact force from tipped cabinet

It won't do any good unless one has a good idea what forces the contents can survive.

If you do, you can back out the acceleration and from that the distance over which the acceleration will take place. Then decide if the contents have that much travel.

I've had occasion to make such a calculation and unless the boards can accept 100 Gs or more it may be a problem as the travel will be rather large.

RE: Impact force from tipped cabinet

"If you do, you can back out the acceleration and from that the distance over which the acceleration will take place. "

This requires you to guess at a distance or a time. Alternately, you could use MIL-STD-810's Method 516.6 Table 516.6-II default terminal peak sawtooth pulse of 11-ms duration.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Impact force from tipped cabinet

You know what velocity it will hit with, so you can directly calculate the time based on the acceptable acceleration.

RE: Impact force from tipped cabinet

JunkforJG, double check on the definition of impact before you get too far on F=ma. Normally, an impact is defined as a jerk which is a change in acceleration I= dF/dt=d(ma)/dt.

RE: Impact force from tipped cabinet

junkforJG,

Your cabinet will fall to the floor, and it will decelerate to a stop in some distance. You need to make assumptions about that distance. Everything else can be calculated. If you lift something and drop it, your kinetic energy is the height times the weight (not the mass).

--
JHG

RE: Impact force from tipped cabinet

Is not F = m(v2-v1)/t sufficient to calculate impact force F? As IRSTUFF referenced, use t=.011secs.

Ted

RE: Impact force from tipped cabinet

hydtools,

I don't know what the velocity is. I can very easily work out what the energy is, as I noted above. If I (think I) know the deceleration distance, then I can use work energy to figure out the force. This may be an iterative process, but that is what spreadsheets are for.

--
JHG

RE: Impact force from tipped cabinet

Drawoh, the velocity, v2, is sqrt(2*g*h), h being the vertical fall distance. V1 is zero. It is a free fall from rest.

Ted

RE: Impact force from tipped cabinet

yeah, but the velocity at time of impact doesn't tell you anything about the force of the impact. That depends on the time interval of the impact, and how the box (and it's contents) responds to the impact (is it rigid ?).

Do you want an accurate number or is a ballpark (0.1sec, 0.01sec) sufficient ??

another day in paradise, or is paradise one day closer ?

RE: Impact force from tipped cabinet

Finding the deceleration distance or the time are essentially equivalent in complexity, since that requires some knowledge of how the structure reacts to the impact. For an electronic cabinet, there may be some flex that would stretch the impact time.

The only sure way is to do the actual test. You could possibly use mass simulators to minimize the potential damage to the subcomponents. Instrument as much as possible and tip the cabinet and see what you get.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Impact force from tipped cabinet

Predicting acceleration due to shocks is possible if you know an amazing amount of detail about the system, or if there is one dominant spring and one dominant mass. Since an accelerometer costs about $5 there is no excuse for not doing the test.

http://www.analog.com/en/products/mems/acceleromet...

In fact your phone may well have accelerometers built in and there may be an app you can use to interrogate the accelerometers. Mine does. If yours does, stick your phone to the cabinet shelves with hot glue or modelling clay and push the sucker over.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Impact force from tipped cabinet

The op was looking for a range of forces. That could be 1 to 100, depending on the impulse time 100 to 1 msecs. The calculation is good enough for that.

Ted

RE: Impact force from tipped cabinet

(OP)
Thanks guys for the suggestions!

I do realize that the actual impact force will depend on a lot of factors that I can't determine easily and at low cost, such as the response of the cabinet, how far it bounces back up (I don't want to repeat this on a test set that is very expensive). Some of the attachment sheets of metal have bent as a result of the fall, so that would have absorbed some of the energy too. What I'm trying to do is bound the problem to determine maximum impact force and maybe also make some assumptions about the elasticity of the system that would reduce the impact force (by how much? order of magnitude?).

One of the product engineers has provided me the Cg of the cabinet. Would I simply use the diagonal height of Cg from the tipping point as the height from the ground and calculate the final velocity as v2 = sqrt(2*g*h)?

RE: Impact force from tipped cabinet

Sure. The diagonal would become the max vertical cg height as the rack rotates about the tipping edge.

Ted

RE: Impact force from tipped cabinet

but, again, what does final velocity tell you ? other than saying "ok, now how fast does it decelerate?" 0.01sec, 0.0001sec ??

and is it falling on a flat floor ? (it's been our assumption, but ...) if it falls can contacts in a specific area, away from the CG, the results are different.

and are you trying to answer "why did these things break/bend ?" ... 'cause this wouldn't tell you that.

again, do you want a precise answer or near enough ??

another day in paradise, or is paradise one day closer ?

RE: Impact force from tipped cabinet

(OP)
near enough

RE: Impact force from tipped cabinet

One thing to note is that, in tipping, the corner of the base may have no impact while the top will have far greater impact.

RE: Impact force from tipped cabinet

Maybe the corner will have higher velocity. Summing moments around the corner at the instant of impact would indicate lower force at the top.

Ted

RE: Impact force from tipped cabinet

Does that include the rotational inertia? I mean, if I fell stiffly face down I would expect my face to hurt a lot more than my knees.

RE: Impact force from tipped cabinet

Quote (JunkforJG)

I realize that are a lot of unknowns

Well you're not wrong there - rather too many for any real figuring out I feel.

This cabinet of yours - did it tip over on one corner, fall from a height, fall on one corner, flat face
Did it fall onto a concrete surface? something else?
Did anything get deformed in the shell
Did anything get deformed inside the shell (seems like it)
What is the mounting of the bits inside, their mass, any attachment points?
Is the PWB fragile?

Far too many unknowns to get even close to a force and what then? How much force is OK and how much is in the grey zone?

Nope, just replace it all or test it all and visually inspect every last bit.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Impact force from tipped cabinet

Quote (OP)

Some of the attachment sheets of metal have bent as a result of the fall

but others didn't?

If you're trying to work out what this particular set of cards might have suffered with a view to working out whether they're fit for continued service, you might learn something useful by trying to work out what force it would have taken to mangle the weakest-looking bracket that actually survived.

A.

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