DrD

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Everything posted by DrD

  1. The sad part, Rick, is that today (2017), critical thinking is essentially prohibited at most schools; they have become simply indoctrination institutions. That said, however, there is more to the Doonesbury strip. The point, as I see it, is the complete lack of student participation in the learning process. Students then, and students today, want the faculty to simply open their skulls, pour in a spoonful of knowledge, and close the lid tightly lest any leak out and be visible. They are unwilling to do even so much as to ask a question. My reason for posting this on ME Forum is to point to the same problem. Most readers on ME Forums are unwilling to participate. They are usually unwilling to ask any questions. They are unwilling to think through a problem presentation. They seem to simply want to have the knowledge wash over them, in the hope that they will absorb enough by osmosis. No one ever really learned anything that way. DrD
  2. The following is a verbal description of a Doonesbury cartoon of unknown date by Garry Trudeau. Doonesbury has long been one of America’s major cartoon strips, with a very dry wit and a decidedly left-of-center outlook. I found this today in going through some old files. SCENE: A college classroom, the teacher lecturing in a rather absent minded fashion, the students silently bent over, taking notes and keeping their heads down. TEACHER: Of course, in his deliberations on American capitalism, Hamilton could not have foreseen the awesome private fortunes that would be amassed at the expense of the common good. TEACHER: Take the modern example of the inventor of the radar detector. In less than ten years, he made $175 million selling a device whose sole purpose is to help millions of people break the law. TEACHER: In other words ... STUDENT (suddenly sitting up and interjecting): Maybe the fuzz buster is a form of Libertarian civil disobedience, man. You know, like a blow for individual freedom. TEACHER: I ... I don’t believe it! STUDENT: Believe what, man? TEACHER (smiling in happy elation!): A Response! I finally got a thinking response from one of you. And I thought you were all stenographers! I have a student! A student LIVES! TEACHER (kneeling down, hand extended like one might approach a shy animal): Who are you lad? Where did you come from? Don’t be frightened ... STUDENT: (looking around himself): What’s the deal here? Am I in trouble? The above all appeared in print many years ago, but it is an apt description of Mechanics Corner.
  3. Recently I was looking at the well known textbook, Mechanical Vibrations by S.S. Rao. I found an interesting problem there that seemed to be worth investigating further. The attached note is the result of that investigation. I challenge all of you to work through the details! DrD TwoSprings.pdf
  4. To design something implies an objective, a goal to be accomplished. We design a machine, a system, to accomplish that goal. What does this accomplish? That was the thrust of my previous comment. DrD
  5. Hey, JAG ... You're telling me that you think you have a book older than I am? Why, when I was learning to write, it was still done with a stylus pressed into damp clay (later to be fired); papyrus and ink had not yet been invented! DrD
  6. And the point is ......? DrD
  7. Questions like this make me crazy!! The difference is that they are made differently, rather like the difference between a pig and a bird. DrD
  8. Ah, once again, we have a communication gap!! When I look at the quadrant labels in the original question, I see that the labelling convention at at least some of you are familiar with is not the one I know. The labelling sequence shown in the question moves clockwise from the first quadrant; the convention that I (and I suspect others in the USA at least) am familiar with is the opposite. Thus, I would exchange the labels for the 2nd and 4th quadrant. Neither is more right or wrong than the other, but it points to the need to communicate carefully. If you say "Second quadrant" to me, I have to ask, "just what do you mean?" As to why one system is used and not the other, I think it is purely a matter of convention and long accepted practice. We can use any view that is useful to show the necessary information about a part. DrD
  9. You will get more useful feedback if you do a couple of things different: 1. Put more information in your question title. To simply say, "Engineering" does not tell the reader anything at all about what you are interested in learning. Most will simply ignore your questions. 2. Elaborate your question more completely. I do not know exactly what you mean when you say "iron grades." What kind of iron are you talking about? Cast iron? Wrought iron? Pig iron? Or something else? Tell us more abut what you mean and why you want to know. In many cases, as in this example, the terminology is not completely universal, so some of us will not know what you mean by a particular term. DrD
  10. That question has no general answer. The contact ratio for a gear pair depends upon the particular gears and the way they are mounted (it cannot be determined for an unmounted pair). Tell your "question paper" that it is nuts, that the question is ill posed, and shame on it! DrD
  11. The contact ratio is the average number of teeth engaged in a gear mesh over a full mesh cycle. Higher contact ratios generally mean more evenly spread loading and quieter operation. DrD
  12. The fundamental law of gearing states that for a constant velocity ratio, the pitch point must be stationary. DrD
  13. I regret to point out that ianbates1 has led us on a wild goose chase with his mathematical presentation. 1. The universal gravitational law, describing the force of attraction between two masses, is not relevant to a discussion of the bending of an I-section. 2. The resulting expression for a, the acceleration, is irrelevant as the beam bending problem is usually applied to static structures. 3. The bending deflection expression, FL^3/(3EI) is relevant for a uniform cantilever tip deflection only, but has no general applicability. 4. The expression for the area MOI, bh^3/12, is for a rectangular section as shown, which has no significance for the I-sect ion. 5. The stress-strain relation through Young's modulus explains nothing about the I-section. Other than that, yeah, sure .... why not? DrD
  14. Several of the previous comments speak of "stress concentration" avoidance as the reason for the use of the cylindrical form. This is a misunderstanding of the term "stress concentration." A stress concentration occurs when there is a sharp change in section area such as a hole, a notch, or a reenternant corner. This would not be the case with a pressure vessel with flat sides such as a rectangular box. If a flat sided vessel is used, the problem that dominates is the bending stresses at the edges of the flat plate. These are required to sustain the required angle at the corner, and they can often be severe. DrD
  15. There is a problem with the original question. The term "superior material" is undefined. What is a "superior material"? Is it superior with respect to strength? With respect to wear? With respect to heat conduction? With respect to workability? With respect to ductility? With respect to toughness? Etc, etc. Until this is defined, the question is not very meaningful. DrD
  16. Mechanics Corner A Journal of Applied Mechanics and Mathematics by DrD © Machinery Dynamics Research, 2017 Last Post Time to Hang It Up This will be the final post of Mechanics Corner here on Mechanical Engineering Forums. It has run almost exactly two years, and there have been ups and downs along the way. In this final post, I want to reflect a bit on my original goals for the blog, and also on what has actually happened. When our host first proposed to me that I might write a blog for ME Forums, I was excited about it. About half of my career had been spent in engineering education, and I always loved working with students. It seemed like a way to get back to something that I had long enjoyed, and so I accepted his suggestion. A long time ago, back when I was about 14 or 15 years old, in Junior High School, my shop teacher mentioned, in an off-hand way in class, that various curves could be described mathematically. I’d never heard that before, but I thought immediately, “This has interesting possibilities.” Moving ahead a few years, I discovered that I wanted to study and build my career around was the area known as Applied Mechanics, although it was a time before I first heard that term. In my freshman physics class, I discovered the laws of motion, and thought to myself, “This is great stuff! I can use math to describe how things move!” All of that happened back in the 1950s, and I’m still doing the same thing today (some might say I am in a rut!). As a teacher, I taught mostly undergraduate engineering courses, although I taught my share of graduate courses as well. It was the undergraduate courses that I liked most, because I firmly believe that the economy of a nation is strongly dependent on the quality of the baccalaureate level engineers produced in that nation. Engineers with graduate degrees are valuable as well, but the vast majority of the national engineering workload falls to BS level engineers. Thus, I envisioned Mechanics Corner as a sort of continuation of the several undergraduate courses I most enjoyed teaching — kinematics, dynamics of machines, vibrations, and mechanics of materials. For the most part, I have stuck to the plan, so that most of the technical posts I have made have dealt with problems that I considered suitable for undergraduate engineering students, say perhaps, junior level. I have posted a few topics from my industrial experience, but those have been situations that baccalaureate level engineers would be expected to handle. Now I knew it would not be exactly like continuing to teach my classes. In particular, you would not have any homework or tests, and I would not have any grading to do – a win-win, or so I thought. I did hope, that even with no assigned homework, readers would take an interest in the problems discussed, even to the point of working through the details for themselves (I was terribly naive, apparently!). I knew from my own experience that the only way I ever really learned a new idea was to get in and work with it, work some problems, make some numbers, plot some curves, until I really understood what it was all about. I’ll venture to say that nobody ever learned any technical material simply by reading only. In actual fact, in the early days, I had one or two folks say that they would in fact work through the problems, so I was encouraged. What I was not prepared for, however, was the fact that the vast majority never seemed to even read very carefully, much less work through the problems! The questions that have come, and there have been a few, have largely been about matters totally unrelated to the posts. The most common question has been, “Suggest a topic for my final project,” which relates to not a single post. Needless to say, that aspect of my vision was totally unfulfilled. But there is another side. I ventured to write a few “philosophical” articles, items dealing with academic integrity and cheating, with how to ask for help, with how to write a report or a paper, and various other matters. I really thought all of this would be considered obvious and trivial, so I was completely unprepared for the excitement that some of these articles generated. There were, in some cases, many, many comments, and people seemed to really be interested. I’m left to wonder: why? Are these ideas foreign to the culture of India and SE Asia? Are these things not all taught at home and in the public schools? I don’t know, but there was a lot of interest in these matters. But Mechanics Corner was intended to be primarily a technical blog, and there, it just did not excite the interest of the readership. As time passed, there was less and less interest. First, the comments dropped off to just about zero, and later, there were fewer and fewer who even bothered to “like” the articles. Finally, the number of reads has dropped to almost nothing (there may be no one left to read this final note). Well, there could hardly be any more clear indication that it is time to stop. I asked for opinions about this from some of the administrators, and was told that the blog was just over the heads of the readership. That makes me sad; that was never the intent. If it is true, I do not see how engineering has a very bright future among this readership. Even so, I wish all of you the best for your careers. I hope that you are able to find rewarding and beneficial work in which you will be happy and make a real contribution to your societies. To use an old cowboy metaphor perhaps familiar to many of you from Bollywood, “It is time to hang up the bridle and saddle, and say, ‘Adios’ (Adios is literally, “to God”).
  17. And the point of this is ......????? Does this have something to do with Mechanical Engineering? If so, what???????? DrD
    Many very good pictures, but not enough good text closely connected to the pictures. All in all, much of the same stuff that has been presented on ME Forum hundreds of times previously. DrD
  18. Dear Harish, You asked how to plot an acceleration diagram easily in a few steps. It cannot always be done in a few steps, but it is not hard to do, provided you use a computer to do the calculations. You can describe the position relations in your device using vector loop equations. Solving those equations will give you numerical values for all position variables for any assigned position. You can also differentiate the position equations to obtain velocity equations. These can be solved to give all of the velocities associated with a particular position and input velocity. You can differentiate the velocity equations to obtain the acceleration equations. These can be solved to obtain the accelerations associated with a particular position, input velocity, and input acceleration. Now, if you want a dynamic acceleration curve, you must solve the equation of motion for your device; this usually means a numerical integration of the equation of motion. These numerical solution will give you the positions, velocities, and accelerations to apply in expressing the various component accelerations. For more on this, read some of my early posts at the Mechanics Corner (my now discontinued blog on ME Forum) where I showed exactly how to do most of this. DrD
    Is there anything here that is not covered exhaustively in hundreds of thermo and IC engine textbooks? If there is, I missed it. In fairness, I must admit I did not spend time to look at every file; too many downloads required. My conclusion is, this is the same old stuff, almost entirely qualitative and very little quantitative. Don't you folks ever see the need to be able to calculate anything? How can you design if you cannot make numbers? I'm puzzled. DrD
  19. Yes, that is expected. In most cases, a finer mesh will more accurately capture the physics at the expense of more computer cost (resources, run time). There may be cases where this is not true, but they are not the norm. DrD
    This presentation is like so many others on the subject. It is very long on classifications (much like a biology text), but short on the means to describe mechanisms mathematically. What good has the term "four bar linkage" or "kinematic inversion" ever done anyone? None, that I can see. It is only when we learn how to describe these things mathematically that we have real capability. I see classifications are largely useless. There are numerous technical flaws in this presentation, starting with a misspelled word on the title page. There are many places where figures have been placed over text, obscuring the text. This is really poor formatting of the material. I strongly suggest others not waste their time on this presentation.
  20. The DIAGRAM itself is of very little use, in my opinion. The information it contains, however is another matter. When you speak of a diagram, I presume you refer to something actually drawn on paper, in which case, to get numerical data from it, you must scale the diagram. This is never very accurate. A knowledge of velocities and accelerations is essential if we are to describe the dynamics of a machine system. All of this can be done mathematically, and it that form, this information is of great value. We often need to calculate the forces in various components to determine their required strength. This means that we must be able to write a full mathematical description of the system dynamics, and that always involves velocities and accelerations. To sum up then, personally speaking, I think the diagram is next to useless, but the knowledge of velocities and accelerations is essential for design. DrD
  21. The force that I called Fext, the external force acting on the nut, would presumably include friction opposing the motion of the tool assembly being moved. It would probably be appropriate to include friction as either dry friction, viscous friction, or a combination of both. It the mounted assembly is being significantly accelerated, it must include the force required to drive that acceleration. If the cutting tool is requiring force in the direction of the screw axis, there must be a term to represent this. Note, however, that if the cutting force is perpendicular to the screw axis, this force is absent. Thus it looks like you have several situations to consider, as listed above. DrD
  22. As you said, there are many ways to classify engines, so what is your question? What is it you want to know? DrD