Deceleration - Slide Hammer

[chavez78]

I have a scenario that is quite similar to that of a slide hammer (thought not a slide hammer), whereas an object is dropped about a foot, then collides with a forged body. I was asked to compute the force on the forging at the moment of collision. I used conservation of energy principles to calculate a velocity and estimated several scenarios of deceleration (0.05s, 0.1s, and 1s). Is there some guideline or the deceleration of metal to metal impact?

Alternately, I provided the party asking the question with total energy absorbed, but I wanted to find out if I was reasonable in providing forces based on those types of "instantaneous" decelerations.

I realize I'm leaving out details involving the scenario, but I'm looking for general, rather than specific suggestions.

 

[IRstuff]

This question continually comes up, but there is no hard and fast answer. If you search around, you should be able to find the older threads on this subject. There is supposedly an approach using strain energy and treating the object as a stiff spring.

this is one of the hits when I ran the search:
thread404-272070


TTFN

FAQ731-376

 

[chavez78]

Just found this - think it might be what I'm looking for...any other tips are appreciated.

http://www.eng-tips.com/viewthread.cfm?qid=110558

 

[IRstuff]

Mot really, it suffers the same basic problem that all such estimates have, which is that you need to know either the deformation distance, or the impact time. Without either, you cannot determine the actual force.

Likewise, in the thread I posted, the solution is to use the strain energy to effective determine a spring constant, from which you can determine the energy stored into the spring, hence its deflection, hence the maximum force applied.

TTFN

FAQ731-376

 

[chavez78]

The impact time is the "guess", and I realize it's a matter of the strain energy in objects, so I think I'm chasing my tail on this one. I don't have enough information or data being supplied to give a great answer to those who requested this "Force", but I was trying to see if I can make some reasonable guesses, and note to them that it is simply a "guess" with too many unknown variables to be much more than that...

 

[IRstuff]

Well, a semi-rigorous "guess" might be to consider that the military test standard for impact shock is a half-sine or sawtooth waveform 11 ms long. I think that a thorough reading of MIL-STD-810 Method 516 will provide sufficient insight for you to decide on an appropriate course of action.

TTFN

FAQ731-376

 

[dhengr]

Chavez78:
A “great answer” without “guessing” or “chasing your tail on this one,” would be to apply some good educated engineering judgement; pick a few stopping times and stopping distances to start to bracket the magnitude of the forces involved. Then educated the people who asked you for this info., explaining essential what IRstuff has told you, and what you should know and brush up on a bit, by reading your physics and dynamics text books, before you respond. This is not a question which lends itself to a single, exact, answer; and you have just brushed the surface on what happens with all the energy; but you can look all the better by educating them on this matter. It is an approximation, but maybe graph how the force changes with time or distance, and let them determine if the want to spend another million dollars to halve the time or distance, or double the force. And, quit using the word “guess,” at least try using terms like ‘educated assumptions’ or ‘approximations based on experience.’

Nice response and direction IRstuff.

 

[chavez78]

dhengr:

Good suggestions, and I did what you suggested and gave a range or bracket of magnitudes based on variation in variables and my educated approximation.

Maybe using the word guess isn't appropriate in your eyes, and clarifying it as an "educated" guess is better. They aren't approximations based on experience...I DON'T have any experience in this area, so I think it's also inappropriate for us as engineers to call it otherwise. It's an approximation based on what I consider to be reasonable assumptions, but not much more. And in that regard, what I deem reasonable is subject to argument.

This is a very helpful forum. I had stumbled upon this a few years back, but had forgotten it until this issue was presented.

Thank you to everyone for your feedback.

 

[GregLocock]

Of course your guess improves markedly once you work out (or measure) the dynamic stiffness of the two systems.

Cheers

Greg Locock


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[chavez78]

Very true Greg.

 

[sreid]

If the two peices have large flat areas, there's a nice air cussion effect that limits the peak acceleration.

 

[Cockroach]

I would measure the recoil height, noting that in reverse, an equivalent mass would impart the same energy when hitting the forging. The energy absorbed would be that energy imparted by the first height drop, the difference in heights representing the energy of absorption.

Too simplistic?

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[chavez78]

Cockroach. That doesn't seem too simplistic. Obviously my constraints are unknown to this board. I may not econimically or accurately be able to measure the recoil height. But I agree with your recommendation. Thank you.

 

[amorrison]

Before energies are calculated Conservation of Momentum should evaluated.
Conservation of Momentum is always 100%.
With energy who knows what the "efficiency" is.

 

[gruntguru]

Evaluating the contact area may help with analysis of the force/displacement relationship. perhaps applying bearing blue to one surface to enable measurement of A-max?

Engineering is the art of creating things you need, from things you can get.

 

[Cockroach]

Just a dumb idea, I guess, this early in the morning. Why not put some sort of measuring background behind the impact site and video it? You could then replay frame-by-frame and get a pretty good idea of the recoil height.

Who knows, could be a full feature movie some day? LOL, give the artsies a run for their money!

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[chavez78]

Cockroach - I don't think it's a dumb idea. Logistics are a little tough (obviously it's difficult for anyone else to know that setup), but the idea itself is reasonable. Open discussion (specific to a topic) is what makes a forum great.

 

[hydtools]

A laser measuring system may work also.
Or ultrasonic measuring tape.

Ted

 

[IRstuff]

The only problem is that the typical video capture is probably too slow a frame rate to get accurate results. However, there is a relatively inexpensive Casio:

http://shop.ebay.com/i.html?_nkw=EX...o=&_trksid=m194&ssPageName=STRK:MEFSRCHX:SRCH

which can do 1000 fps at reduced resolution:

http://cgi.ebay.com/Casio-Exilim-EX...674822?pt=Digital_Cameras&hash=item43a2796a06

Alternately, you might just try gluing an accel on the hammer and see what you get:

http://www.satistronics.com/triaxia...celeration-transducersensor-module_p2445.html

TTFN

FAQ731-376

 

[IRstuff]

Oops, that accel probably won't hack it. Digikey has a some boards that have higher ranges.:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=2622557&k=accelerometer

They're probably still inadequate for a hammer strike. If you want to actually measure the shock, you'll probably need to build or buy something more appropriate, i.e., something that will read out at least 500 g

TTFN

FAQ731-376