Calculating drop test height from a g-shock value
Calculating drop test height from a g-shock value
(OP)
I have currently had some small parts fail during a vibration test and it's more than likely that the shock part of the test that made it fail.
I've come up with some new ways to protect this part, but testing these methods will be very costly. Therefore to mitigate this risk i want to devise a drop test that will mimic the shock loadings seen during the vibration test.
I know that the max shock during the test is 50g for 10ms and the weight of the object is 0.05 Kg. Now all i need is a height, but the more i look into it the more it starts to look like einsteins style of work!
Has anybody ever come accross this type of rough and ready approach before and derived a simple formula for working out a drop height in meters?
I've come up with some new ways to protect this part, but testing these methods will be very costly. Therefore to mitigate this risk i want to devise a drop test that will mimic the shock loadings seen during the vibration test.
I know that the max shock during the test is 50g for 10ms and the weight of the object is 0.05 Kg. Now all i need is a height, but the more i look into it the more it starts to look like einsteins style of work!
Has anybody ever come accross this type of rough and ready approach before and derived a simple formula for working out a drop height in meters?





RE: Calculating drop test height from a g-shock value
you want u = ?, given v = 0, a = 50g = 160ft/sec^2, t = 0.01s ... (u-v)/t = a
then h = ? given mgh = 1/2*m*u^2 ... h = u^2/(2*g)
u = 50g*0.01 = 0.5g
h = 0.25*g^2/(2*g) = 0.125*g = 4 ft
RE: Calculating drop test height from a g-shock value
I thought anything falling under gravity could only reach a maximum acceleration of 9.81m/s^2, I have missed something?
desertfox
RE: Calculating drop test height from a g-shock value
Additionally, such shocks are highly dependent on the material of the part and the surface being hit.
Without doing the full analysis, you may understress or overstress the part.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Calculating drop test height from a g-shock value
IRstuff ... the OP asked a question and i gave him an answer (to the question) ... it's hard enough gettting the right answer to the question, but getting the question right is a whole different matter !
maybe a way to calibrate the height used is to drop an original part so that it breaks (in some set up), then drop the new part.
RE: Calculating drop test height from a g-shock value
Ahhhhhhhhhhh I see now, so the acceleration would be a -Ve, sorry just saw the positive figure's anyway I agree with your 4feet answer.
desertfox
RE: Calculating drop test height from a g-shock value
Dean
RE: Calculating drop test height from a g-shock value
Just be aware that your question as posed may not accurately reflect the actual test conditions.
"50g for 10ms" implies a step function to a constant 50g acceleration for a duration of 10ms.
The equivalent MIL-STD-810 sawtooth has only a peak value of 50g that is ramped at 50g/10ms rate. The amount of equivalent crash energy is less than half that calculated using a square wave acceleration.
Note also that MIL-STD-810 has a 15% tolerance on the peak acceleration, so it could be as high as 75g in an actual test.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Calculating drop test height from a g-shock value
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Calculating drop test height from a g-shock value
RE: Calculating drop test height from a g-shock value
If the deceleration is given, the drop height is dependent upon the distance of the deceleration. Which is the deformation of the object if the impact surface is very rigid.
Maybe you can devise a device to control deceleration distance, i.e. a pad that is soft compared to the object and calculate drop height to give 50g for a reasonable deceleration distance (compression).
I'm not sure I understand this myself...
Dan
RE: Calculating drop test height from a g-shock value
You could then use something based on the formula for dynamic impact factors in Roark to estimate when you'd get 50 g by dropping the component on it. You could make sure that the impact was equal to or greater than 50 g for the required time.
The Roark formula is based on linear behaviour, so the load (and acceleration) are assumed vary linearly with deflection (hence the suggestion of a simple support).
RE: Calculating drop test height from a g-shock value
Another thing is what orientation it is in when it impacts. Just dropping in free air will not give repeatable results, it will land differently each time. Even how it is released can make a difference.
When doing this kind of test for fuzing systems we used a rig with a quick release mechanism to get repeatable results. If the rig adds mass to the falling body this can be compensated though ideally you want to make it 'light'.
For an object as light as yours, you may be able to manufacture an OK rig from fishing line or similar hung vertically, with pieces of drinking straws or similar to run down the line. I did this for a physics project at school and it worked pretty well.
KENAT,
Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Calculating drop test height from a g-shock value
I suggest starting with basic Newtonian dynamics with known stiffness's. Probably the Roark reference above is more then sufficient.
With this, you could then setup a basic test in which you attach an accelerometer to the object. (or use a vibrometer)
With this you can tell more accurately if what you have done is way off or sufficiently close.....
TTFN
Fe
RE: Calculating drop test height from a g-shock value
When the strain energy absorbed by the gear equals the kinetic energy of the falling object, the object is effectively stopped. You have to simultaneously solve for all of the deflections in the landing gear system. Depending on the landing gear, that can either be easy or extremely hard. I had a helicopter skid and cross-tube system to work with, so equations in Roark worked out well. Assuming that the fuselage is perfectly rigid gets you pretty close.
I had to use a computer program to solve all of the simultaneous equations - not an easy task for hand calculations!
If it is a helicopter, you may find hints in the Maintenance Manual or SRM under "hard landing inspections".
Steven Fahey, CET
RE: Calculating drop test height from a g-shock value
KENAT,
Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Calculating drop test height from a g-shock value
tia. Ekonkaar.
RE: Calculating drop test height from a g-shock value
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Calculating drop test height from a g-shock value
RE: Calculating drop test height from a g-shock value
While the 4 ft drop, might give you an adequate shock with a "solid wooden bench top" ala MIL-STD-810, you might get an overshock with a metal surface.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Calculating drop test height from a g-shock value
RE: Calculating drop test height from a g-shock value
TTFN
FAQ731-376: Eng-Tips.com Forum Policies