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Mechanical Engineering


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Everything posted by DrD

  1. The motor you describe is probably very large and heavy. Consider instead going with a small, lighter, less expensive motor and a speed reduction (gears, belts, etc). DrD
  2. It would help if you would post a figure to make your geometry clear. DrD
  3. Gabe, that's just standard US Government quality service (I'm really quite familiar with it all; I used to work for the US Navy as a civilian). DrD
  4. Looks like you now have enough reading material to keep you busy for quite a while! I notice that, as usual, the quality of the document from DTIC is dismal; they must have the worst Xerox machines in captivity. DrD
  5. Gabe, I found the attached article on the 'net. I have not read it through, but I'll bet it has some information for you. The internet also has tons of other information on this machine. DrD MediamWeightShockMachine.pdf
  6. Ah, Gabe! I knew I had seen that figure before, but I certainly did not remember the mil-spec designation! Sadly, really nothing is conserved all the way through the motion. Energy is conserved before the impact and after the impact, but energy is not conserved during the impact. This is where the coefficient of restitution stuff comes in. I would strongly suggest that you keep looking for more definition from the military. There has to be a more complete description, because without such, you cannot possibly have a standard test. If everybody has a different test machine, there is nothing standardized or comparable about the results. DrD
  7. This looks like a standard military shock test. If that is the case, there should be standards that provide the necessary information in tabular form. DrD
  8. The short answer, Gabe, is that there is no answer; you have asked an impossible question. By conservation of energy, we can give a pretty good estimate for the speed at which the hammer approaches the underside of the anvil. This assumes good bearings, negligible air friction, etc. But after that, you have an impact problem and we need to know what appens to the hammer and anvil after impact. Do they stick together? Do they separate, with the anvil moving up while the hammer moves down? If they separate, what is the coefficient of restitution? Let me also note that force is never measured in g's. A g-value is an acceleration, not a force. I suggest that you look in a good elementary mechanics text under "Impact." Beer & Johnston is the text I would suggest, but there are many that will do. DrD
  9. Sorry, Rakesh, but this is not "nice;" it is foolishness. Let's look at a few quotes from what NonGrata has written. 1) "Very few scientists dare to doubt conservation of energy ..." Wrong!!! Very few folks with any education at all would believe that energy is conserved in all situations. In fact, what we know is that energy is conserved under some conditions, and not conserved when it is not. 2) The quote from Walker, Halliday, & Resnick, "Energy cannot magically appear or disappear," shows the confusion in the mind of NonGrata. The book authors are speaking of "total energy," while the engineering concept of energy used when engineers speak of "conservation of energy" is mechanical energy, specifically excluding thermal energy. When mechanical energy is converted to thermal energy, there is no direct means of recovery. It can only be recovered by complex processes such as producing steam to pass through a turbine or steam engine. 3) Speaking of gyroscopes, NonGrata says, "RPM is proportional to kinetic energy." Wrong!!! He clearly does not know anything about mechanics. Kinetic energy of rotation is proportion to the square of the RPM. 4) Still speaking of gyroscopes, NonGrata describes a proposed experiment involving two different gyros, or alternatively, two gyros in different situations at the same time. But notice this: He says that in each case, the gyro comes to rest. If he believes in the absolute conservation of energy (with out distinction between mechanical and thermal), where did the energy go when the gyro comes to rest? 5) As a final point, look at the writer's name: NonGrataEngineering. The first part, Non Grata, is Latin meaning "an unwelcome person." Well, he certainly got that part correct! We should not welcome those who spread ignorance, foolishness, and confusion, whether it be deliberate (with the intent to mislead others), or simply through their own ignorance. DrD
  10. What aspect did you not understand? It looks pretty straight forward. DrD
  11. DrD

    Linear actuator

    There are many ways that such a device can be made, and what Jek says may be the way this one works. It can also be done with a ball screw powered by a small electric motor. Without more details, no one will be able to say with certainty how t this one works. DrD
  12. Pete Wills has the right idea. By "off the shelf" if the right equipment exists, but don't buy the wrong equipment just because it is "off the shelf." You will get what yo pay for. DrD
  13. DrD


    Friction eventually imposes a limit. If there were no friction, you could just add pulleys ad infinitum, but in reality, each new pulley adds friction to the system. Eventually you would get to a point of diminishing returns. DrD
  14. Sure, all you need is the appropriate set of gears from each of A,B, and C to D. DrD
  15. I would hardly call looking for a job suffering. I presume no one is beating you or otherwise torturing you, so I don't see any real suffering involved. DrD
  16. Looks like you have the dimensions of the channels (10 x 15.3). Do you know if those are inside dimension, outside dimension, or what? I would suggest you go to a steel design book which will probably give you I for the section. The loading diagram indicates that w = w(x), a variable loading, so you will need to express that mathematically and then integrate twice to get the moment, M(x). From there on, it should be easy. DrD
  17. JAG - If the units do not work, there has to be a dimensional constant (a constant that has dimensions, not a pure number) involved. This is not uncommon with empirical relationships, but it means that the whole equation is only valid in the original system of units. DrD
  18. I'm not familiar with that process (I've seen it, but never really analyzed it). You can bet that if the units do not work, the expression is not correct. Where did you get this? DrD
  19. There is no simple formula for the force you are asking about. It depends upon the nature of the motions involved. If you would like to post a drawing of your machine (or send it to me by PM), then perhaps I can help you. DrD
  20. Cracks do not grow by themselves. Most cracking is due to fatigue, and that requires motion. A motionless car will not crack. DrD
  21. Many years ago, I was involved as an expert in a legal dispute. My company was suing the US Army in a contract dispute. I got a very bad taste for lawyers as a result. An Army facility had designed a mechanical system and put out drawings for the system as the basis for suppliers to bid on building the system. There were five major suppliers bidding to build these devices for the Army, and the bidding was extremely competitive. Being higher by half a cent ($0.005) per unit would mean losing the bid. My employer had the misfortune to win the bidding, and we were building this rather complicated mechanical system (around 200 parts) for about $15 per unit with many 1000s to be built. Each week's production would be declared to be a "lot," typically around 20,000 units. The inspectors would draw a random sample for testing, and if all tests were passed, the lot was accepted and the company was paid for the material. If any tests were failed (and there were about a dozen different tests), the lot was rejected and the company received no payment. Dimensional tolerances were extremely critical, and the company had taken then into account in the bidding. In suing the Army, my employer was alleging that the tolerances had been deliberately set loose for the bidding while the Army knew that closer tolerances were required in order to consistently meet all the specifications and pass the tests. The Army denied this. My job for 18 months was to demonstrate mathematically that there were dimensional combinations within the allowed tolerances that would fail some of the tests, and therefore that the Army had misled the bidders in the original contract negotiations. I did this quite fully, showing that some combinations would work while others would not, even though all were within the specifications on the drawings. I was completely disgusted by the attitudes of the lawyers on both sides. The had ZERO interest in the truth; they only care about winning the argument. They were only interested in information that supported their side of the case. If a part of my study did not support my employer's case, our attorneys ordered me to destroy that paper work and to deny that I had ever done it if asked. I refused to do that; the truth is the truth. Most of my encounters with lawyers since that time has supported my early evaluation; they are evil men (and women) intent only on winning. Truth means nothing to a lawyer! DrD
  22. Backlash depends upon clearances at the joints. You will have to specify all of these, and then look at the various possible combinations to find the worst case. It is a long, tedious process. DrD
  23. Mechanics Corner A Journal of Applied Mechanics and Mathematics by DrD, #48-B Correction To Mass On Multiple Spring Supports Correction The previous post, #48 titled Mass on Multiple Spring Supports had several typographical errors. Strangely, they are all in the brief introductory example regarding the loads on the four legs of a table, at least these are the only ones that have been called to my attention. As many of you know, I am older than dirt, and my typing skill is degrading; this leads to errors at times. I apologize for misleading anyone. Fortunately, one reader notified me that there were errors. MassOnMultipleSpringMountsB.pdf
  24. With 381 views, no one has seen anything worth commenting on. Wow!! So all of you already knew this? Or perhaps you thought this was not a significant problem? DrD
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