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Everything posted by DrD

  1. As an older engineer, I am rather dismayed by what I see as the results of engineering education today. I have seen most young engineers come out of their college degree programs with a modest familiarity with come CAD program and a related Finite Element program, and an almost complete lack of capability to analyze machine components without the use of FEA. Thus they are unfamilliar with basic design calculations relating shear stress in a shaft to the power being transmitted, the shaft sped and the shaft diameter. Instead, they want to create an FEA model and run the numbers that way which is simply not the way you make any progress in designing a machine. It is certainly true that FEA allows for a good detailed stress analysis of complex geometries, something that was not possible in general by the older methods. But the results obtained are highly dependent on the skill of the user in applying the correct boundary conditions, something that is a bit of an art in itself. I have observed a tendency to even want to use FEA to do kinematics, rather than using simple, rigid body kinematic relations. This seems like pure foolishness in almost all cases. Are we not developing a tendency to lean too heavily on the computer, rather than to learn to think as engineers? I would be happy to hear some comments and discussion from others on these matters.
  2. The PDF file below will show you how to solve this problem. DrDPlanetaryProb.pdf
  3. As a starting point, you must know the MMOI for each individual component; without that, you cannot begin. Assuming that the individual component MMOI values are known, do a kinematic analysis of the planetary train to determine the velocity of the CM of each component. The use that information to formulate the kinetic energy of the entire system, all moving according to the kinematic relations. You will have to choose some rotation as a reference value, usually either the input or output rotation. The kinetic energy will have the form T = (1/2) theta-dot^2 (term1+term2+...+termlast) where theta is the reference rotation. The sum of terms indicated is the effective MMOI for the gear train. DrD
  4. Two stroke &four stroke

    So, what else is new? DrD

    Wow!! That was super exciting!! DrD
  6. The material you posted (from a book?) is only partially visible, and cannot be completely read. The various symbols are not defined, such as Ftc, Rc,T, Wc, and alpha_c. There is no way anyone can help you with so little information. Rather than ask for help, I suggest you draw the picture and work it out for yourself. You will learn much more that way. DrD
  7. The one thing that is very clear from your two URLs is that there is no uniform, well defined definition of "rifle barrel harmonics." There seems to be a mix of ideas about compression waves traveling down the length of the barrel and reflecting back combined with a dilatational wave propagating out from the charge ignition. The word "harmonics" seems to be more of a buzz term, to dignify the confusion. Altogether, this is simply an elastic wave propagation problem, albeit a complicated on due to the geometry involved.There are lost of people around the world who have studied wave propagation problems, and I'm sure that with the necessary experimental facilities, all of this could be studied experimentally. It may already be under investigation, either by the US Army (such as Picatinny Arsenal), or one of the rifle manufacturers (Remington, Winchester, etc). You will find the necessary mathematics quite sophisticated and well beyond the typical college graduate (lots of partial differential equations). Have fun with your inquiry. DrD
  8. The term "rifle barrel harmonics" is not one that I recognize, and could be used to describe several possible phenomena. In order to help you, I have to ask you to describe just what sort of motions you are concerned with, what phenomena it is you wish to describe. Please write out (in words) what you think happens that interest you, and then we may be able to offer some help with the mathematical description. DrD
  9. The term "Pan and Tilt Control System" does not have a universal definition, so no one can be sure what you are talking about. If you want help, give a detailed description of what you need to build, including one or more sketches of the proposed system. Only than can anyone give you a meaningful answer. DrD
  10. Without more details, it is difficult to say very much. One thought that comes to mind is that you have probably neglected friction which can be very significant in a system such as this. If you want more help, please post a sketch of your system with dimensional details. DrD
  11. Gears

    Wow!! That was absolutely fascinating!! A blank page! Who can possibly top that? DrD
  12. How To Become An Expert

    I'm glad you thought so. DrD
  13. How To Become An Expert

    Mechanics Corner A Journal of Applied Mechanics and Mathematics by DrD © Machinery Dynamics Research, 2016 How To Become An Expert Introduction This is going to be another of those personal experience/opinion pieces, so if these bore you, be warned! This may be the time to click on something else. A reader recently wrote to me asking how to become an expert. I have to tell you, I don't spend much time thinking about being an expert, but I suppose on some reflection, the shoe probably fits. (Most of the time, I see myself as simply a tired old man, still enjoying the things I have done almost all my working life.) In the discussion below, I will describe a few events and observations that seem to relate to the question at hand. Find Your Place Nobody can hope to be an expert on everything, there is simply too much to know. You have to find the area that excites you, the area that really makes you want to dig in more. If you do not really enjoy it, you will never be an expert! I was very fortunate in this regard. When I was in High School, I was rather good in Mathematics, and my school advisers all told me, "You should become an engineer." Sadly, I really had no idea what that meant, and neither did they. The town where I grew up had rather little industry, and no one in my family knew an engineer of any sort. I did a little bit of research on engineering (this was thousands of years before the Internet), and it sounded interesting in a very vague way; there seemed to be little specific information available to me. But I went off to college, intending to study mechanical engineering, whatever that was. In my first semester of college, I took a Physics course in classical mechanics, and I really enjoyed it. This was exactly what I wanted to do, I just did not know the right name for it. I thought Newton's Second Law was the greatest thing ever discovered, and when implemented with Calculus, it was really fun. I was astounded at the power of it all, the questions that could be answered. If I could just get a job doing mechanics problems, I was sure I would be happy. HowToBecomeAnExpert.pdf
  14. Yes, that is all one big URL, but it links to a video of some really impressive automation. DrD
  15. Your drawing in and of itself will not cause or prevent vibration. Its all in the way the rotor is built. You have stated a desire, but you have not asked a question. Is there anything preventing you from accomplishing that which you desire? DrD
  16. I think a person would have to be NUTS to take this position. The money, $5300/mo. sounds like a lot, but look what you would have to give up and where you would have to live. NO THANK YOU!! The money comes out to $63600 per year (tax free they say), but that is far less than most MEs are making in the USA. DrD
  17. Wind Tirbune Vibration Problems

    The question is so broad as to be unanswerable. Yes, there can be vibration problems in wind turbines. But there are so many possibilities that it would require a whole library to address them all. Please be much, much more specific. DrD
  18. Really, Henry!! Brain cells boiling ... over a simply little mouse trap? You must have a very low boiling point! No, this is not the same mechanism as a trebuchet, although that too is an interesting problem. Cool down a bit and give this a try. It really is not so impossible. DrD
  19. Mechanics Corner A Journal of Applied Mechanics and Mathematics by DrD, # 44 Machinery Dynamics Research, 2017 Mouse Trap / Pendulum Dynamics Challenge - Part I Introduction Mice are a problem all over the world, and as a result, I'm sure that there are mouse traps of various sorts found everywhere. It would be utterly amazing if this were not true! In the USA, there is a very common type of mouse trap that I have seen used all my life, the sort of system shown below in Figure 1. I want to spend a few minutes discussing this mouse trap, to be certain that all readers understand how it works, before moving on to the main part of the post. MouseTrapPendulumDynamics-1.pdf
  20. Be sure to check out the new post at Mechanics Corner. It poses a challenge problem for each of you to work on. Do you really know kinematics of machines? Find out!! Try the simple problem posted over at Mechanics Corner now. DrD
  21. #43 Four-Bar / Toggle Linkage Mechanism

    Wow, Henry!! I freely admit to being older than dirt, but these books are old even for me! You are correct; they are very interesting. Sadly, many of the figures don't really tell enough to make clear how the things work. But some do, and that makes for fun browsing. Thanks, DrD
  22. Mechanics Corner A Journal of Applied Mechanics and Mathematics by DrD, #43 (c) Machinery Dynamics Research, 2017 Four-Bar / Toggle Linkage Mechanism Introduction I believe that it would be correct to say that all of the single degree of freedom mechanisms that I have discussed on ME Forums have involved only a single loop. This might lead a reader to conclude that a single degree of freedom implies only a single loop, and vice versa, that a single loop implies only a single degree of freedom. Neither of these statements is true. In this note, I want to discuss a counter example, a mechanism called the four-bar / toggle linkage; it is shown in Figure 1. TogglePress.pdf
  23. #43 Four-Bar / Toggle Linkage Mechanism

    Been "speed reading" again, Henry? That is indeed an interesting image you posted. If I recall correctly, this is a variable compression ratio engine mechanism. It is particularly interesting that you post it here. It is another variant on the four-bar/toggle linkage idea. The crank, the link, and the radius bar form a four-bar linage. The connecting rod drives the crank through the link, essentially a slider-crank of a strange sort. This is a type of engine called an L-head engine, referring to the idea that the combustion chamber and the dead volume near the valves form an L-shape. It is interesting that one valve is in the block while the other is directly opposite in the head. I wonder how well that works? In a two-stroke cycle, you would risk pulling most of the mixture right through from inlet to outlet without burning at all! Most likely it is for a four-stroke cycle. DrD
  24. #43 Four-Bar / Toggle Linkage Mechanism

    Henry, did you read the blog post? Let me quote: If a tool of some sort is attached to the orange block, it is repeatedly brought down to bear against the work piece located below. This may be a punch to make a hole, a die to form a shape, a welding contact to make a spot weld, our countless other operations that require only momentary contact between the tool and the work. And again: Notice the shape of the solid curve. It is rather ‡at-topped, with something like a dwell in the down position. This would be useful for a situation where extended contact time between the tool and the work is required, such as in a spot welding operation. It would not be preferred in a punching operation where the best operation is to complete the punch and withdraw the tool quickly. The shape of the curve can be modified to some extent by adjusting the link lengths. Doesn't that suggest some applications?
  25. 4 Bar Linkage Problem

    Dear Nerd, Please post your work, both solutions and the final results from each. I think a lot of folks would find it very interesting. My intentions, from the equations I gave you was simply to use Newton-Raphson to get the final numbers. DrD