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Everything posted by DrD

derive this wc=f/cos alpha
DrD replied to Kedar Boni's topic in Machine Dynamics & Design Considerations
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 
The physics behind rifle barrel harmonics
DrD replied to Thomas White's topic in Machine Dynamics & Design Considerations
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 
The physics behind rifle barrel harmonics
DrD replied to Thomas White's topic in Machine Dynamics & Design Considerations
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 
Pan Tilt Vibration Stability
DrD replied to Guy Balas's topic in Machine Dynamics & Design Considerations
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 
Gearbox sizing for trolley
DrD replied to Xose Portela's topic in Machine Dynamics & Design Considerations
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 
Wow!! That was absolutely fascinating!! A blank page! Who can possibly top that? DrD

I'm glad you thought so. DrD

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

https://mailattachment.googleusercontent.com/attachment/u/0/?ui=2&ik=4d910f0c03&view=att&th=15cef623deff2d23&attid=0.1&disp=safe&zw&sadnir=1&saddbat=ANGjdJYCVyI1Fbt0ODz0haTG6Mpr8ytrJvMfpB0BRymjaxekgNJSwYYJE9fyvrmGcrT2Oj29vWY2PbfZmxQ9tYSDhLX93PsKcH_O2SvVrH4ohadiRm6jRguBmswvRVB8xByQIIffbAvOXof23AEJg1thC1UlGMi79Kp97PRQwXYK58NVaCpQIZeq2Gnz8lrwTgeElFHnn0mwr3sLm0hlCOq8nnzEOmKJgJusyJ60IgwJ5wiGnDtU727bliecYGRFnrbchIXgsEpoZWSvCHSoU8Ra_fWHcnTsuvwCPeRtyveUcWHpUfC7CIZUl_NZ59xPl4F3AQyFnOHh4TiNHEaGblMsLLUGkzrW0rehiZ1JL8JB9JiHIC3aK_vvNjIKkQZlcBoDj4E49UORxp7QaGpQQZuH03HoPbLK7c4i_EHp8L0CSZRSY7B8BsZC6eMG_WzG3ZuINgz89Ec_KIfunx73I54i2L7MDhHM947L9WIRhKNthrplxKpKo8mmEKbxFOVlgVFhRsbQ0EQbfhoIOUD4NjNsIl4tMlwJsJgUmCnk8BQdxU2droCghPnckT_Ga9oG6hkmbn68xgUc_VqouED64Nvo4u1jaE9zb3IMdH5qj5yUfXAIqEA4 Yes, that is all one big URL, but it links to a video of some really impressive automation. DrD

i want to have drawing of my rotor to make it correct for low vibration
DrD replied to Mehak Preet's topic in Machine Dynamics & Design Considerations
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 
Mechanical Engineer Lecturer Needed  (Relocation to Saudi Arabia)
DrD replied to Egon Kullik's topic in R&D/Engineering Design Job feeds
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 
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

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

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. MouseTrapPendulumDynamics1.pdf

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

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

Mechanics Corner A Journal of Applied Mechanics and Mathematics by DrD, #43 (c) Machinery Dynamics Research, 2017 FourBar / 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 fourbar / toggle linkage; it is shown in Figure 1. TogglePress.pdf

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 fourbar/toggle linkage idea. The crank, the link, and the radius bar form a fourbar linage. The connecting rod drives the crank through the link, essentially a slidercrank of a strange sort. This is a type of engine called an Lhead engine, referring to the idea that the combustion chamber and the dead volume near the valves form an Lshape. 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 twostroke 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 fourstroke cycle. DrD

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 ‡attopped, 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?

4 Bar Linkage Problem
DrD replied to Robotics nerd's topic in Machine Dynamics & Design Considerations
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 NewtonRaphson to get the final numbers. DrD 
4 Bar Linkage Problem
DrD replied to Robotics nerd's topic in Machine Dynamics & Design Considerations
Crossed4Bar.pdf Dear Nerd, I have assembled the equations you want, at least as well as I understand your problem. Please let me know if this answers your question or not. DrD 
Henry, your comparison of authors between rabbits and moles is very funny! Thank you. DrD

Mechanics Corner A Journal of Applied Mechanics and Mathematics by DrD, #39 (c) Machinery Dynamics Research, 2017 Comments on a Textbook Theory of Machines by R.S. Khurmi & J.K. Gupta 1 Introduction Recently, through the wonders of the Internet, I have come across a copy of the textbook Theory of Machines by R.S. Khurmi and J.K. Gupta (S.Chand & Co., Ltd., 2005). Since theory of machines has been my primary technical interest since the early 1980s, I was interested to see what would be in this book, particularly in view of the many favorable comments posted in regard to it. Many people seem to think that this is a most excellent book, and I’m always interested to see what brings forth comments of that sort. As I looked through the Table of Contents, I saw that one of the last chapters was given to the topic of Torsional Vibrations (Ch. 24). Since the area of torsional vibrations has been a topic of intense personal interest for 40+ years, I was naturally drawn to this chapter. The comments that follow are based on what I found in that chapter; I have not reviewed the remainder of the book at all. In my comments below, I will refer to the authors, Khurmi and Gupta, simply as K&G to avoid writing their names out repeatedly. One of the things I think is necessary in a textbook is that it should be directed toward teaching students to solve real problems, not simply textbook examples. Certainly, textbook examples should be simple so that they can be easily understood, but they should also be as general as possible. Where they involve special, limiting assumptions that may likely not be true in actual practice, this should be made clear. Failure to do that marks an author as one who has never actually done engineering in the real world. If the assumptions are not made clear, there is a tendency for students to later want to simply apply directly the results from the textbook problem, not realizing that they may not apply at all. So, what did I find? Comments on Textbook  Khurmi.pdf

Many of you have asked me various questions, so now it is my turn. Let me lay a bit of background first, and then the questions. I have had some conversations recently with JAG (one of the other writers here at ME Forums) regarding the choice of software for 3D modeling and analysis. JAG has made some excellent suggestions, specifically a cloud based program called Onshape. Unfortunately, for reasons that are unclear, my computer cannot run Onshape; I have worked with their help people for several hours, all to no avail. JAG recommends this in part because there is a "free version for the hobbyist" and a relatively inexpensive "full version for the professional." That is pretty attractive, but since I can't run it, I'm stuck. I gather that virtually all engineering colleges these days are teaching some sort of 3D modeling and analysis software, but that raises a few questions in my mind. 1. If your college teaches brandX 3D software, what will you do when you go to work for a small company that cannot afford anything more than 2D drafting (simple CAD), with no analysis capability at all? How will you do your job then? You probably have your own pocket calculator, but will you have your own copy of ANSYS or ProE? 2. What software does your school teach (every students should have an answer to this question, so I expect lots of replies on this one!)? 3. If you have used software extensively for analysis of engineering problems (beam deflections, stress analysis, fluid flow, heat transfer, etc), are you confident that you will be able to work all of those problems if there is no such software available to you on the job? I might add, as sort of a postscript, most of you know that I am older than dirt (I just had another birthday, so the situation is even worse!), so I tend to look at things from an elderly perspective. One of my great fears as a working engineer was "What will happen when I'm ask to do something that I don't know how to do?" It happened more than once, and it usually resulted in a flurry of intense research to come up to speed on whatever topic was involved. I could usually do that because I have a pretty good library, and I knew how to use a university library as well. But in terms of software, I was always concerned that I had no FEA program, so how could I do problems that others were doing by FEA? I have come up with some interesting workarounds, including writing my own FEA for some problems, but I never wanted to be dependent on software that I could not afford to own. So, back to my questions about: How are you going to buy your own copy of ANSYS? DrD

Mechanics Corner A Journal of Applied Mechanics and Mathematics by DrD July 31, 2017 Triple Rocker Over at the Kinematics of Machines club, I recently ask if anyone could show me an example of a fourbar linkage that would be classed as a triple rocker. In the terminology of fourbar linkages, a link is classed as either a crank or a rocker: Crank  can rotate in a complete circle Rocker  cannot rotate in a complete circle] Thus my question was for an example of a fourbar linkage where no link is able to rotate around a full circle. My request has not generated any answers, but fortunately, I stumbled onto one. Since the definition of a rocker is a link that cannot rotate completely, it is evident that the linkage shown is in fact a Triple Rocker. None of the links is able to move through a complete revolution. If we try to rotate the input (left) link further down, it cannot happen without stretching the combination of the coupler and the output (right) links. When the input link (left side) gets to the top, again its motion is stopped by the need to stretch the coupler and output link. Thus, a figure I drew as an illustration for something else turns out to be a Triple Rocker, the item I was looking to find. In connection with fourbar linkages, some readers will have heard of Grashof's theorem. Let s = length of shortest link L = length of the longest link p, q = lengths of the two intermediate links Grashof's theorem says that a necessary and sufficient condition for at least one link to be a crank (able to rotate entirely around), it is necessary that s + L < p + q This inequality is not satisfied for the fourbar that I drew by chance, so Grashof's theorem says that none of the links can be a crank. That is precisely the condition required for a Triple Rocker (a ground link plus three moving but not fully rotating links). So, there you have it. That is an example of a Triple Rocker, and we now have the criteria for identifying such as a fourbar linkage that does not satisfy Grashof's Theorem.