impact force estimate/calculation
impact force estimate/calculation
(OP)
I have an application of a trolley hoist traveling on a monorail track. Though it's not supposed to be run into the trolley stop at the end of the rail, sometimes operators do it anyway. I've been asked to determine the force produced if this occurs.
A moving object hitting an immoveable object generates a theoretically infinite force. Is there a way to estimate a force assuming virtually zero deflection? Or is it best to assume a very small deflection, say 0.01"?
Open to suggestions.
A moving object hitting an immoveable object generates a theoretically infinite force. Is there a way to estimate a force assuming virtually zero deflection? Or is it best to assume a very small deflection, say 0.01"?
Open to suggestions.





RE: impact force estimate/calculation
RE: impact force estimate/calculation
Accelleration, not velocity!
You need to decide the maximum deflection that will be acceptable, and work back from that to get the force for a given volcity.
You might well end up having to accept a bigger deflection than you first thought!
Remember that there will be some deflection in the trolly as well as the stop.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: impact force estimate/calculation
RE: impact force estimate/calculation
Tobalcane
"If you avoid failure, you also avoid success."
"Luck is where preparation meets opportunity"
RE: impact force estimate/calculation
You can certainly feel it through the building when that happens. At the top of a column, it's going to a matter of how many quarter inches of movement, not tenths.
RE: impact force estimate/calculation
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: impact force estimate/calculation
RE: impact force estimate/calculation
How to figure the stiffness? Well, I don't know what the parts look like, but perhaps they could be idealized. If you really want to get fancy, an explicit FEA model will give you the most accurate results.
RE: impact force estimate/calculation
1. cart = trolley hoist
2. When considering the PE, be sure to consider combined stiffness of the two parts (think two springs in series).
3. an implicit FEA solver can probably handle this sort of impact these days with enough horsepower.
RE: impact force estimate/calculation
RE: impact force estimate/calculation
Fe
RE: impact force estimate/calculation
RE: impact force estimate/calculation
1) test it if an accelerometer on the cart or a load cell on the stop
2) add a soft (comfy?) cushion to the stop (to increase t)
3) train the operators
RE: impact force estimate/calculation
is not the way you do this, since you only know V, the velocity and mass, m. You don't know a and you don't have clue for the time of impact. Guessing at a is a non-starter.
Also since I assume you haven't destroyed the trolley or stop, we have stresses that are less than the yield.
As a few others have said, the problem is mainly linear motion and the energy equation yields.
1/2*m*v^2=1/2* k*x^2=F^2/2k
and
F=v*square root(k*m)
which assumes constant k implying (since you haven't previously destroyed the trolley or stop) we have stresses that are within the elastic limit
In this all you have to do is get k, the "spring constant".
I would do it empirically by a controlled experiment allowing the trolley to move at a very low known velocity and get the deflection, x by simple visual means.
Then for the experiment
use the energy equation to get k
k=mv^2/x^2
and then the force equation above to get F,namely
F=v*square root(k*m)
RE: impact force estimate/calculation
What was I thinking?!?!? A failed attempt at a quick answer I guess. When I was thinking of a quick estimation I short-cutted it too much.
mass * velocity squared / your assumed small deflection
F = m * V^2 / (2 * 0.005), or maybe 0.002/0.003
(rough estimation but good enough for your purpose I suppose)
{that's where my mind went, when I was thinking of getting rid of the inch squared, I threw away the whole V squared, and then when I was thinking of an assumed deflection of 0.005 x 2 being equal to your proposed 0.01, I threw away the whole bottom term. Haste makes waste, I learn again.}
RE: impact force estimate/calculation
RE: impact force estimate/calculation
Thanks for your help.
RE: impact force estimate/calculation
Here is a link to a vendor that put out a nice little white paper on the subject.
http://w
-Dustin
Professional Engineer
Pretty good with SolidWorks
RE: impact force estimate/calculation
TTFN
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RE: impact force estimate/calculation
RE: impact force estimate/calculation
I don't think you need to know the duration (or pulse like 0.001 sec half sine wave) unless there is something that is frequencies sensitive that you want to avoid shock amplification.
Tobalcane
"If you avoid failure, you also avoid success."
"Luck is where preparation meets opportunity"
RE: impact force estimate/calculation
In fact, if you want a tolerable approximation, I would just use an 11-ms half-sine or sawtooth as a starting point.
TTFN
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RE: impact force estimate/calculation
F*t = m*(v2 - v1), assuming no mass change
F = force
t = time during which force acts
m = mass of the moving object
v1 = velocity, initial
v2 = velocity, final
Yes, time is a factor.
Ted
RE: impact force estimate/calculation
Tobalcane
"If you avoid failure, you also avoid success."
"Luck is where preparation meets opportunity"
RE: impact force estimate/calculation
RE: impact force estimate/calculation
TTFN
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Chinese prisoner wins Nobel Peace Prize
RE: impact force estimate/calculation
Determine k statically:
- put the trolley in contact with the barrier
- apply a measured force (preferrably at the CG) to a rigid part of the trolley, pushing towards the barrier
- measure the deflection at the same point
- use k=F/x
There are a number of assumptions in this method, mostly concerning the location and magnitude of compliances in the system but the results should be close enough for your purpose.
Engineering is the art of creating things you need, from things you can get.
RE: impact force estimate/calculation
RE: impact force estimate/calculation
TTFN
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Chinese prisoner wins Nobel Peace Prize
RE: impact force estimate/calculation
RE: impact force estimate/calculation