Pendulum Hammer Impact Force Calculation

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Discussion Overview

The discussion revolves around calculating the impact force exerted on an anvil table by a pendulum hammer. Participants explore the complexities involved in estimating this force, considering various factors such as the angle of release, the dynamics of the collision, and the materials involved. The conversation includes both theoretical and practical aspects of the problem.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • Some participants suggest that the problem lacks sufficient information to arrive at a clear answer, questioning the purpose of the inquiry.
  • Others propose that if the pivot is frictionless, the impact force can be analyzed as if the anvil were directly below the hammer, implying a simplification of the dynamics involved.
  • Several participants highlight the importance of knowing the hammer's release angle and the construction of the hammer and anvil, as these factors significantly influence the impact force.
  • There is a discussion about the continuous change in acceleration during the collision, indicating that the answer is not a single value but a function dependent on various parameters.
  • Some participants mention that practical testing, such as drop tests, might provide more reliable data than analytical calculations due to the numerous unknowns involved.
  • One participant notes that the discussion relates to a specific military shock test standard (MIL-DTL-901e), which lacks detailed modeling information, complicating the analytical approach.
  • There is a suggestion that using an accelerometer could provide the maximum Gs experienced by the anvil during testing, although some participants emphasize the need for an analytical estimate before testing.

Areas of Agreement / Disagreement

Participants generally agree that the problem is complex and lacks sufficient information for a definitive analytical solution. Multiple competing views remain regarding the best approach to estimate the impact force, and the discussion remains unresolved.

Contextual Notes

Limitations include the vague setup of the problem, missing parameters such as the hammer's weight and the specifics of the collision dynamics, and the dependence on material properties and construction of the hammer and anvil.

  • #31
Indeed the machine's value is to provide a reproducible scenario for percussive damage. What is in fact involved is not a step function but a large (not infinite) acceleration whose details are the reason for this elaborate machine. The maximum acceleration is in fact of primary interest.
 
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  • #32
I have used every online search variation I can think of and cannot find any actual manufacturer/supplier of this type of machine to see if I could find some type of a performance spec. for it without any luck.
It is easy to calculate the impact velocity for this machine as configured on the above spec drawing; but, while there are multiple papers on testing with this type of machine, none, including the above reference paper, seem to fall within the performance range of this machine. The majority are at higher hammer impact velocities.
 
  • #33
hutchphd said:
The maximum acceleration is in fact of primary interest.
That is where we differ.

I believe the mechanism is being used to provide a velocity step function, to stimulate an equipment mounting system. That step is being used to test the high frequency components of the transfer function. Only the bandwidth of the test is determined by the acceleration of the anvil.
 
  • #34
Baluncore said:
That is where we differ.

I believe the mechanism is being used to provide a velocity step function, to stimulate an equipment mounting system. That step is being used to test the high frequency components of the transfer function. Only the bandwidth of the test is determined by the acceleration of the anvil.
Are saying that the frequencies of interest for testing the mounting system are low enough that any sharp step is effectively instantaneous ?
 
  • #35
hutchphd said:
Are saying that the frequencies of interest for testing the mounting system are low enough that any sharp step is effectively instantaneous ?
How low is “low enough” ?
How sharp is “any sharp step” ?
How do you define “effectively instantaneous” ?
 
  • #36
If your review the above referenced document http://www.sandv.com/downloads/1611alex.pdf starting on page 10 you will see the the response wave(s) form are sinusoidal. This same result is also repeated in other papers I reviewed on testing of this machine. It is used for shock testing for shipboard mounted equipment in accordance with MIL-S-901D.

For more specifics go to:
https://en.wikipedia.org/wiki/MIL-S-901
 
  • #37
  • #38
Baluncore said:
How low is “low enough” ?
How sharp is “any sharp step” ?
How do you define “effectively instantaneous” ?
I was trying to understand your point.
If you are interested in the high frequency components of the transfer function then the exact shape of the velocity step is important.
 
  • #39
I thought we put this one to bed over at the ME Forum. Is there something new required?
 
  • #40
This is the ME (Mechanical Engineering) Forum and hopefully about all that can be covered given the available information now has been.
 
  • #41
hutchphd said:
I was trying to understand your point.
If you are interested in the high frequency components of the transfer function then the exact shape of the velocity step is important.
That is a false assumption.

If the hammer and anvil is being used for destructive failure testing, then the step profile is a standard defined by the construction of the machine, as specified in the standard. The exact shape is irrelevant as it is standard.

The acceleration can be read from accelerometers mounted on the anvil. Indeed, I would instrument both the anvil and the device being tested to get acceleration and compute displacement. I would then compute the FFT of both those synchronous records, and divide the device spectrum by the anvil spectrum to get the transfer function of the system being tested.
That eliminates any assumptions about the exact shape and spectrum of the stimulus from the analysis.
 
  • #42
Baluncore said:
If the hammer and anvil is being used for destructive failure testing, then the step profile is a standard defined by the construction of the machine, as specified in the standard. The exact shape is irrelevant as it is standard.

The acceleration can be read from accelerometers mounted on the anvil. Indeed, I would instrument both the anvil and the device being tested to get acceleration and compute displacement. I would then compute the FFT of both those synchronous records, and divide the device spectrum by the anvil spectrum to get the transfer function of the system being tested.
That eliminates any assumptions about the exact shape and spectrum of the stimulus from the analysis.
All correct and on point. But I disagree with
Baluncore said:
That is a false assumption.

...
That eliminates any assumptions about the exact shape and spectrum of the stimulus from the analysis.
Practically, if the step function is not sharp, the high frequency components of the anvil spectrum will be effectively zero (smaller than the noise) and it does not test the high frequency response. Further, when you deconvolute by dividing the FFT you will get junk for the high frequency parts of the transfer function because you are dividing by ~(zero +noise). This is a fact, not an assumption.
 
  • #43
hutchphd said:
Practically, if the step function is not sharp, the high frequency components of the anvil spectrum will be effectively zero (smaller than the noise) and it does not test the high frequency response.
Further, when you deconvolute by dividing the FFT you will get junk for the high frequency parts of the transfer function because you are dividing by ~(zero +noise). This is a fact, not an assumption.
There is a fine line between presenting a ridiculous argument, and appearing ridiculous. Your reductio ad absurdum argument is clearly ridiculous. If the required high frequency components were not present, then the mechanism could not be used for testing.

Since the mechanism is used for testing, the required HF components must be present, and so they cannot be completely obliterated by noise when computing the transfer function.
 
  • #44
Baluncore said:
That step is being used to test the high frequency components of the transfer function
What makes you think this? I believe they may be more interested in the low frequencies where indeed the step shape is less relevant. But your circular argument relies upon this "fact":
Baluncore said:
Since the mechanism is used for testing, the required HF components must be present, and so they cannot be completely obliterated by noise when computing the transfer function

I do not find this in the test protocol.
 
  • #45
In the referenced MIL spec there no criteria as to the magnitude or frequency etc. of the applied shock or any other elements related to the applied shock profiles or levels. All references to the test results criteria are based strictly upon the condition of the item being tested after testing.
The only statements related to the actual performance of the test is that "the machine approved the Class of Testing required shall be used", "the hammer height used shall be determined by the machine indicator marking", "the minimum travel height for the anvil table for that class shall be as per table no. ...", the maximum allowed equipment weight for each of the three classes of testing i.e. Light, Medium or Heavy and the mounting of the test equipment, etc.

For more details on all of these issues, see the referenced MIL spec.
 
  • #46
JBA said:
In the referenced MIL spec there no criteria as to the magnitude or frequency etc. of the applied shock or any other elements related to the applied shock profiles or levels. All references to the test results criteria are based strictly upon the condition of the item being tested after testing.
The only statements related to the actual performance of the test is that "the machine approved the Class of Testing required shall be used", "the hammer height used shall be determined by the machine indicator marking", "the minimum travel height for the anvil table for that class shall be as per table no. ...", the maximum allowed equipment weight for each of the three classes of testing i.e. Light, Medium or Heavy and the mounting of the test equipment, etc.

For more details on all of these issues, see the referenced MIL spec.
Very good. I'm sure the test does what they want. I just wanted to be sure our analysis was coherent.
 

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