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DrD

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Everything posted by DrD

  1. 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
  2. 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
  3. 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
  4. 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
  5. 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
  6. 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
  7. 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
  8. 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
  9. 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
  10. 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
  11. Hetal, Please publish your results here so that we may all benefit from them. DrD
  12. 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
  13. And Carrie Underwood has what to do with "economical joints?" DrD
  14. 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.+
  15. 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.
  16. 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"?+
  17. 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
  18. DrD

    A Calculus Challenge

    I'll get to you William. Please be patient. I have a few other things going on right now. DrD
  19. 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
  20. 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
  21. DrD

    A Calculus Challenge

    Dear Amin, Well, I have both the 2nd and 5th editions of Roark, so I am unable to tell whether the book is correct (or if you read the book correctly). That's the problem with using a reference source (although Roark is widely recognized). I'm amused that you said, "When they ask "how do you gget formula?" the answer will be "here is the formula in the book." Does that really convince you? Are you more convinced by something in a book, or be a carefully worked out derivation that you can follow for yourself? I'll take the derivation every time!! It is certainly true that handbooks have their uses. The problem is, they also have their limitations. If there are assumptions made in the derivation, looking the formula up in a book may not tell you about those assumptions. I'm sorry that you evidently find the area moment of inertia expressions just too tedious to be worth your time. For those who want to find out if they really can use calculus, this remains a very useful exercise. DrD Dear HKS, You promised results after the New Year. Just which New Year did you have in mind? DrD
  22. DrD

    A Calculus Challenge

    Thanks for the information, Jarek. I do not like MS Word, and would never use it given any choice in the matter. But, I know many others like it, so this is useful. DrD
  23. DrD

    A Calculus Challenge

    Dear Jarek, Thanks for the comment on Mathtype. I have heard of it, but know little about it. I use LaTeX for all of my work, but I like the way your work looks very much. DrD
  24. DrD

    A Calculus Challenge

    Dear Jarek, Just as a matter of curiosity, how do you write the equations in your work? They are clearly type written, but I'm curious what software you use for this. Is it MS Word, some other word processor, or what? It looks good, but I'm interested to know more. DrD
  25. DrD

    A Calculus Challenge

    Dear Jarek, I cannot quite agree with your result. I think you may have an algebraic error in the calculation of y2-bar. I'd look there first. Interesting approach, I must say. DrD
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