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.

 

[Bobber1]

Kenetic energy of the moving crane equals the elastic energy stored in the deflection of the crane stop/crane body.

 

[Twoballcane]

"what was the duration of the event... that is why you can't simply do F=ma."

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”

 

[IRstuff]

F = d/dt(m*v), which requires a time element. An impact that takes 100ms is substantially less forceful than an impact that takes 11ms. That's why the legacy military shock waveform was a specific shape with an 11-ms duration.

In fact, if you want a tolerable approximation, I would just use an 11-ms half-sine or sawtooth as a starting point.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[hydtools]

Impulse = change in momentum
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

 

[Twoballcane]

I would agree with he both of you, but how are we defining duration are we talking about shock frequency coupling with the Fn of anything on the trolley or are you saying duration is time from Vo to final V just before impact? If Fn of anything on the trolley is close to the shock frequency then we would have a problem, but this is not what the OP was asking, hence my post. The latter would give you the force, but I’ve never heard of this quantity as “duration”, but I could be wrong.

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

 

[IFRs]

You should look at building codes, AISC Design Guide 7, AISE Technical report 13, ASCE 7, etc. For instance, ASCE 7 says "The lateral force on crane runway beams with electrically powered trolleys shall be calculated as 20 percent of the sum of the rated capacity of the crane and the weight of the hoist and trolley." etc.

 

[IRstuff]

If you want to do the complex analysis, then there is no simple duration, although one could probably still come up with some equivalent "rms" force or somesuch. But for some analyses, such as that suggested in the OP, then a simple force profile and a time duration will provide some sort of answer.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[gruntguru]

You don't need to know deflection or duration. Use Zekeman's equation F(max)=v*square root(k*m)

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.

 

[bsmet95]

Regarding the comment from IFRs: the building codes as well as crane and hoist specs limit the travel speed for estimating lateral forces, and we design our equipment accordingly. The hoist in question is in use by one of our customers and we'd like to convince them to put energy-absorbing bumpers on the trolley.

 

[IRstuff]

So, why not use the 11-ms sawtooth, and see what you get?

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[bsmet95]

Thanks for your help. I managed to come up with a value (rather large) which I believe has convinced the end user to install shock-absorbing bumpers. Who knows what operators will do despite what the manual says?

 

[rb1957]

"convinced the end user to install shock-absorbing bumpers" ... brillant ! ... it is truly a good analysis that convinces the end user to do the right solution all along !