impact force estimate/calculation 4

[bsmet95]

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.

 

[tr1ntx]

Force = mass x velocity, per Mr. Newton

 

[IDS]

Force = mass x velocity, per Mr. Newton

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/

 

[MiketheEngineer]

You might want to search this site for similar threads - I believe that subject has been beaten to death a few times.

 

[Twoballcane]

I think you want to work this problem the other way around. I'm assuming that there is a bumper on the trolley. So calculate the force needed to deflect the material to either to yield or ultimate. Once you know this, you can calculate the max acceleration or velocity the trolley can travel before damage (F=ma). The way that you are attempting is more of kinematic / kinetic study that will lead to conservation of energy equations, but you will still need to know the deflection.

Tobalcane
"If you avoid failure, you also avoid success."
“Luck is where preparation meets opportunity”

 

[DLite30]

The building or support columns are going to be the most flexible item in that system.

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.

 

[KENAT]

erm, isn't there a T missing there?

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(probably not aimed specifically at you)
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[btrueblood]

Don't forget that the mass hanging on the hoist chain below the trolley reacts as a pendulum, i.e. on a different time constant than the trolley+hoist...

 

[flash3780]

You'll want to figure in the stiffness of the two parts. All things being equal, KE_cart = PE_deformed shape. Or, in other words: 0.5*m*v^2 = 0.5*k*x^2.

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.

 

[flash3780]

^^ per my last post.
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.

 

[chicopee]

I think that the operators need more safety training on hoist operation.

 

[FeX32]

It's not simply F=m*a. Search. I remember commenting on at least 2 threads on the exact topic.


Fe

 

[bsmet95]

Thanks. I have been searching. There are so many to be found searching keywords such as "Impact", "Shock", and "Collision" that I haven't yet been able to sift thru all of them.

 

[rb1957]

the cart's momentum (m*v) applies a force to the stop depending on the time of the impact (m*v/t) ...

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

 

[zekeman]

F=Ma
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)

 

[tr1ntx]

Doh!

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.}

 

[ione]

Although the approach proposed by Zekeman is conservative as it does not take into account friction effects that are involved in the process and considers the impact as fully elastic, I think is quite appropriate for your case.

 

[bsmet95]

I know we're trying to hit an unknown target. This is even worse than the problems about dropping a mass onto a spring.

Thanks for your help.

 

[ShaggyPE]

zekeman points out the key that a lot of people miss when discussing impact... what was the duration of the event... that is why you can't simply do F=ma.

Here is a link to a vendor that put out a nice little white paper on the subject.

http://www.brushwellman.com/alloy/tech_lit/julycom7_99.pdf

-Dustin
Professional Engineer
Pretty good with SolidWorks

 

[IRstuff]

This subject has been discussed on several older threads. The closest solution appears to be modeling the materials as springs, and using the calculated spring constant to determine the deflection. Given the deflection, you can then use the kinetic energy to solve for the deflection in the stored spring energy.

TTFN

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