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Mechanical Engineering

DrD

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Everything posted by DrD

  1. 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
  2. Cracks do not grow by themselves. Most cracking is due to fatigue, and that requires motion. A motionless car will not crack. DrD
  3. 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
  4. 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
  5. 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
  6. 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
  7. DrD

    What Makes A Ship Move?

    The nature of the prime mover really does not matter from the system point of view. Steam turbine, gas turbine, diesel engine, whatever, all simply provide shaft torque. It is the critical interaction between the thrust bearing and the hull structure that drives the ship. DrD
  8. DrD

    What Makes A Ship Move?

    What a delight and a marvel, to have two people comment!! Thank you both. Yuli makes the point that we still cannot solve this system because we lack many of the necessary parameters. That is certainly true, but the point is simply this: Now we know what we are lacking and go to seek further information on these matters. As I mentioned in my previous comment, this was originally written for some EEs who had no concept of the whole system. In particular, they did not comprehend the critical role of the thrust bearing in pushing the boat forward. In their minds, motors only produce torque, not thrust. HKS mentions developing some models himself, which is a good idea. Rather than me reviewing them, why not post them here so that the entire community can review them? If I see them (which I probably will), I will comment also. Go to it, and see what you can create. DrD
  9. Mechanics Corner A Journal of Applied Mechanics and Mathematics by DrD, #48 Mass On Multiple Spring Supports Introduction One of the classic problems of elementary mechanics is to determine the load on each leg of a four-legged table. The table top is assumed to be a uniform rectangle, the legs are of equal length, the center of mass is the centroid of the table top, and the floor is level. What is the load in each of the legs? MassOnMultipleSpringMounts.pdf
  10. DrD

    What Makes A Ship Move?

    What has happened? Is the sky falling? This is unprecedented!! A well thought out response to one of my posts! I am simply stunned!!! Thank you, HKS! This is the first time I have ever seen such a good response to a post. You asked about learning whole system level modeling. I suggest that you might simply want to pick some everyday systems and try to model them. A car or motorcycle is an obvious choice, but there are others such as a railroad train (starting/stopping/running down the track), a rolling mill (if you are familiar with how such systems are built), a diesel engine driven generator starting a pump motor load, or perhaps a soda bottle filling machine. At the time I first wrote this article, some 15 years ago, I was working for the US Navy in a propulsion engineering group. Most of the engineers around me were EEs, because we were looking at electric drive for ships. I found that most of them did not know much about motors, much less the rest of the system. Their knowledge was largely limited to power electronics for speed control of electric motors. They has no idea how that related to ship motion. DrD
  11. Mechanics Corner A Journal of Applied Mechanics and Mathematics by DrD, #47 What Makes a Ship Move? One of the problems that often confronts engineers is the description of large, interconnected systems. Engineers tend to specialize, so that one is very knowledgeable on gears, another knows bearings, a third knows pumps, but none of them are comfortable with the whole system. In the automotive context, this is often expressed as, "How does the engine cause the car to move forward?" On the one hand, most engineers can describe the process in words, but they are far less quick to talk about equations for the whole system. Here, I will address a similar modeling question in the marine context, "What makes a ship move?" WhatMakesAShipMove.pdf
  12. It would help to have a sketch of what you have built/propose to build, and also you must specify the frequency. The frequency is a critical parameter. DrD
  13. Perilun, you evidently do not like to read. Everything you need for this problem, and a whole lot more, is contained in the book that I gave you above. In Ch 2 you will find the kinematics, in Ch 6 you will find the statics, and in Ch 7 you will find the statics of this system. The PDF from Alban Kronenberger is also fine, although there is no reason to use the approximate kinematics when the exact relations are just as easy. DrD
  14. You can find a lot about this and many other mechanisms in the newly released textbook Mechanics of Machines, 2nd ed. Please download a copy and encourage your friends to do the same; it is offered without charge to everyone. DrD MoM2DraftMar2019r1-compressed.pdf
  15. I did not see any mention of negative torques, and without them, there will be nothing to slow the system down. What are your load torques? DrD
  16. Hetal, Please publish your results here so that we may all benefit from them. DrD
  17. There are two types of flexibility readily evident with a belt drive, and to a lesser degree with a chain drive. They are 1) Lateral flexibility that allows the pulleys to be slightly out of plane with each others. 2) longitudinal flexibility that allows the belt to stretch and shrink under load variation. For a polymer or rubber like material, the existence of both forms of flexibility is readily evident; the material is stretchy. For a steel chain, the material is also "stretchy" although to a much reduced degree. We tend to think of chain links as rigid, but in fact, they are merely stiff. They deform under load, just like any other material. In addition for the chain, there are small clearances around the pins that opens and close, allowing for both types of flexibility. All in all, it is simply a matter of degree. The belt material is far more compliant, while the chain material is more stiff. But neither is truly rigid. DrD
  18. And Carrie Underwood has what to do with "economical joints?" DrD
  19. Is it really necessary to ask such a question? Consider what would happen if the heat were not dissipated, and you'll quickly come to the answer.+
  20. Can this question really be answered in general, that is, without specifying the machine element? For some, strength is a major consideration, while for others corrosion resistance it the main determinant. The list goes on and on. This question has no answer.
  21. Whatever in the world is an "economical joint"? In American slang, a "joint" is sometimes a bar or restaurant, so an "economical joint" might be one where the prices are modest. In other American slang usage, a "joint" refers to a marijuana cigarette, so presumably an "economical joint" might be inexpensive marijuana. Just what in the world did the Administrator have in mind for an "economical joint"?+
  22. Mechanics Corner A Journal of Applied Mechanics and Mathematics by DrD, #46 Comments on A Calculus Challenge I would like to thank all those who took part in the Challenge. My solutions and comments are attached. 46 CalcChallengeComments.pdf
  23. DrD

    A Calculus Challenge

    I'll get to you William. Please be patient. I have a few other things going on right now. DrD
  24. DrD

    A Calculus Challenge

    Dear Mucugia, The challenge is now closed, and I will not be reviewing additional work any longer. Thanks for the effort. DrD
  25. DrD

    A Calculus Challenge

    Dear Mucugia, Two problems: (1) It looks like you may have left out a page. You sent Page 1 and Page 3, but what about Page 2? (2) Neither of the pages that you sent are complete. The PDF does not show the final result on either page. You might do better to copy these pages over , writing a bit smaller so t hat the scan will capture the whole page. DrD
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