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A Journal of Applied Mechanics and Mathematics by DrD

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DrD

    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”).

 

DrD

    Mechanics Corner
    A Journal of Applied Mechanics and Mathematics by DrD, # 34
    © Machinery Dynamics Research, 2015


A Problem in Statics & Dynamics

    
    
Introduction

        A problem was recently posted on this Forum, requesting help, that has led me to consider a somewhat more general problem for this post. The scope of this post will include the original problem, although not by the method required there, but will also go beyond to a more general geometry. We begin here by stating the present problem; interested readers are invited to search back for the original problem posted 19 December, 2016, by iivii.
    
BodyDwg.jpg
Assembly Drawing, with Dimensions

 

StatDynProb.pdf

DrD

    Mechanics Corner
    A Journal of Applied Mechanics & Mathematics by DrD, #33
    © Machinery Dynamics Research, 2016


Advanced Polynomial Curve Fitting


    The use of polynomials to fit engineering data is a common engineering practice. In school, we learn that "A data set consisting of n data points ((x_{i},y_{i}), i=1,2,3,…n) can be exactly fitted with a polynomial of degree n-1. Thus three data points can be fitted exactly with a quadratic expression, four data points can be fitted exactly with a cubic expression, and so on. If this approach is pursued much further, something ugly appears: while a polynomial of degree n-1 will pass exactly through n data points, for large values of n, it will oscillate wildly in between the data points. Since one of the most common reason for using a polynomial fit in the first place is for interpolation -- to be able to estimate a function value at locations between the known data points -- this wild oscillation is devastating. It is at this point that least squares fitting is usually introduced to give an approximate fit using a much lower order polynomial. A different approach is employed here.

AdvancedPolynomialCurveFitting.pdf

DrD

Braced Cantilever

   

Mechanics Corner
    A Journal of Applied Mechanics and Mathematics by DrD, # 32
    © Machinery Dynamics Research, LLC, 2015


Braced Cantilever

Introduction

    Anyone who has actually gotten into machine design is familiar with this difficulty. Consider the situation where a project is well advanced, many plans have been made, and it is all based on the assumption of the adequacy of one particular part. When you finally get to the detailed analysis of that part, the calculations show that it is not adequate. What can you do?
    To make the problem much more concrete, consider the cantilever beam shown in Figure 1. It supports a weight W at the free end, and when someone finally makes the calculation, the tip deflection, δ, is unacceptably large. The whole system design has been developed on the assumed adequacy of that cantilever, and there is no room to put in a beam with a larger section to give more stiffness. What can be done?

    Any of countless machine design texts, mechanics of materials texts, etc., give the formula for the end deflection,

δ=(WL³)/(3EI)

    where
    E= Young's modulus for the beam material
    I= area moment of inertia for the beam cross section
    L= length of the beam
    W= tip load value
    While we can argue that someone should have checked this earlier, finger-pointing does not fix the problem.

 

BracedCantilever.pdf

DrD

ODE Solution --- Fail!!

   

Mechanics Corner
    A Journal of Applied Mechanics and Mathematics by DrD, # 31
      Machinery Dynamics Research, 2016


ODE Solution --- Fail!!

    

Introduction

    Digital computation has become a major tool for engineers, and it is a great benefit. It can also lead to many pitfalls for the unwary. This note is about the latter, a potential pitfall that many engineers risk on a daily basis, most of them with little awareness of the danger.
    Early in the development of digital computation, every problem required that the user write a program specific to the problem at hand. If speed was a very important issue, the programs were written in machine language, so that they would execute as fast as possible. If speed was a little less critical, programs were written in so-called "high level languages." This included FORTRAN, BASIC, ALGOL, C, C++, and a host of other such names. But even with a high level language, there was the problem of generating a program for the solution of the specific problem at hand.
    As things have continued to evolve, it was soon evident that a lot of the work in writing each program was the same from one problem to the next. The major mathematical operations, such things as numerical integration, matrix operations and the solution of systems of linear equations, plotting, and many other steps were re-usable from one problem to the next. It was natural that this would eventually lead to the development of general purpose programs, able to solve broad classes of problems. This group includes programs like Mathematica, Maple, MatLab, SciLab, Maxima, TKSolver, and numerous others. Most of those just mentioned have built-in capability to solve ordinary differential equations, in some cases by analytical means, and in practically all cases, by numerical means. This has taken the sting out of working with differential equations
from many engineering problems, and we must all be grateful for that.
    At the same time, we must also be somewhat skeptical about any general purpose solver when applied to a particular problem. How do we know that the solution generated is correct? How do we even know if it is reasonable? Most of the time, when engineers resort to numerical solutions, it is because there is no readily available analytical solution. Thus, when faced with a problem that cannot be solved in closed form, how can we know when to trust the numerical solution? This is a very serious question, one that all must consider. It you blindly trust a numerical solution, the old excuse, "The computer said it was OK" will not get you very far. The computer cannot be fined, fired, or (in extreme cases) possibly sent to prison, but all of these things can happen to an engineer!
    So, what can the engineer do when the differential equation has no known solution? Well, there are several options.
    (1) He can resort to any physical principles that apply to the situation. For example, if the system is such that energy should be conserved, then he can add code to calculate the total system energy at every instant. Just verifying that energy is conserved does not "prove" that the solution is correct, but if energy is not conserved when it should be, you can be sure there is an error in the solution.
    (2) He can try various approximations that may apply to see if they are in reasonable agreement with the computed solution.
    (3) He can verify the solution code by applying it to a similar problem for which there is a known solution. It is this last approach that I want to talk about in this post.

ODE_Soln_Fail.pdf

DrD

Where is this?

Does anyone recognize where this video is shot? Is it a group of students at a school (what school?), or is it an industrial site (what company)? I am anxious for someone to locate this for me, please.

DrD

 

Saurabh Jain, our host, has identified this location for me, and that is much appreciated.

When I watched the video, I was aghast at all those nearly bare feet in a machine shop! I can appreciate that in Indian culture, the simple sandals are socially quite acceptable, but from a safety perspective, this is an absolute horror. Think of all the opportunities for something to drop on a foot, a tool, a machine part, sparks, etc.

 

Some years ago (quite a few years ago), I worked in a steel mill. We were required to wear hard hats and steel toed shoes at all times in the mill. And these were not just any old steel toed shoes. These shoes came up ankle high, and had massive steel toes and an additional steel plate, called a metatarsal plate, that came up over the top of the foot almost to the ankle. Each shoe weighed 4 lb, and it was very tiring simply to walk around wearing them. But, .... and this is the key part .... they added much to our safety. Even today, in my advanced old age, I have a pair of steel toed boots (but not metatarsal plates) for when I go into an industrial environment.

 

What is shown in this video is actually a cautionary tale, a warning of just about everything not to do from a safety perspective. Take heed! Be warned, or you could easily loose all your toes on one foot of the other.

 

DrD

 

DrD

A Question for Readers

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 Pro-E?

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 work-arounds, 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

DrD

    Mechanics Corner
    A Journal of Applied Mechanics and Mathematics by DrD
    © Machinery Dynamics Research, 2016


Becoming An Expert -- Part 3

    

Introduction

    In the previous article on Becoming An Expert--Part 2, I mentioned that there were two big issues for the engineering analysis section at my Houston position, the first being the matter of seismic survivability and the second being torsional vibration. The first item was dealt with in Part 2, and in this article we will take up the second item of concern.
    When I joined the engine distributor in Houston in the mid-1970s, the company was about 65 years old, and the torsional vibration problem was not new. This was a problem that they had been dealing with, in one way or another, for many years. There were lots of old torsional vibration analysis reports available to study. I was not at all familiar with torsional vibration of machine trains; I had not studied anything quite like that in school and it had not come up in my previous industrial experience. So I eagerly began reading the old reports, and that is when the problem became acute for me: They did not seem to make any sense. I could not, with any integrity, continue to write reports like that when I thought they were complete nonsense, but I did not know how to analyze the problem correctly. I was in a jam!
    There were three major difficulties:
    1. The entire crank assembly rotates endlessly, so the stiffness matrix for the system is singular. This results in a zero eigenvalue, something that did not take too long to figure out.
    2. It is obvious that the system does more than just go round-and-around; it goes up and down as well. I was baffled for a long time about how to deal with the kinematics and their impact on the dynamics.
    3. It is apparent that there is a torque acting on the crank, but it is not directly applied to the crank by the combustion process. There is the slider-crank mechanism between the two, and I was at a loss as to how to transfer the cylinder pressure into a crank torque. This is again directly related to the kinematic problem mentioned just above.

BecomingAnExpert--Part3.pdf

DrD

A Comment Remembered

   

Mechanics Corner
    A Journal of Applied Mechanics and Mathematics by DrD
    © Machinery Dynamics Research, 2016


A Comment Remembered

    
    Recently, in connection with one of the posts on Becoming An Expert, one of the ME Forum readers made a comment to me, something about seeing everything in terms of differential equations. That comment brought to mind a comment made to me many years ago that I want to pass along to you today.
    Most of my college education was at the University of Texas at Austin. It was there that I received BS, MS, and PhD degrees in engineering, and I was there studying for most of a decade. After I finished my PhD, I was asked to stay on the faculty for a year as an Assistant Professor, so that was my first post-graduate teaching position as well.
    One of the well known faculty members at UT-Austin was Dr. E.A. Ripperger, a man with a national and international reputation for his work in plastic stress wave propagation. In addition to his teaching responsibilities, Dr. Ripperger directed a laboratory at the Balcones Research Center, a research arm of the University. He had many graduate students working under his direction, and he was riding high in terms of his reputation. He was a rather august figure, somewhat austere and above everyone else.
    While I was still a struggling and confused undergraduate, one of Ripperger's graduate students had taught my Mechanics of Materials course, and I had done well in that class. This fellow liked me, and when a job opening came up out at the lab, he let me know about it and helped me to land it. Thus I was working a few hours a week as a lab assistant for this particular graduate student who was himself working under Dr. Ripperger. Before long, I signed up to take a class in Intermediate Dynamics, and Dr. Ripperger was the assigned teacher. Truth to tell, he was only mediocre teacher, nothing to get excited about.
    The class was fairly difficult, and I was having trouble keeping up with it all. In particular, the solution of the many differential equations just overwhelmed me. Since I was working out at the lab, and Dr. Ripperger was out there from time to time, I thought it might be a good idea to go in to to see him at the lab to discuss the class. I found him at his desk one afternoon, and screwed up my courage to go into talk with him.
    I told him that I was finding the class difficult, even though I thought I had a good understanding of dynamics. I told him that my difficulty was particularly with the differential equations, not with dynamics. He listened quietly while I spoke, and then he fixed me with a withering gaze when he spoke, calling me first by name and then saying, "Did you think there was anything else besides the differential equations?"
    He said no more, and I slunk away to lick my wounds! I don't think I ever spoke to him again.
    
    DrD is a retired Professor of Mechanical Engineering in the USA. He can be reached for comments, questions, or requests via the ME Forum message system. Be sure to check back soon at www.http://mechanical-engineering.in/forum/blog/206-mechanics-corner/ for more articles.

DrD

   

Mechanics Corner
    A Journal of Applied Mechanics and Mathematics by DrD
    © Machinery Dynamics Research, 2016


Becoming An Expert -- Part 2

    

Introduction

    In the previous article on How To Become An Expert, I covered a lot of points in generalities with some short anecdotes from my own experience. In this article and the next, I will describe in considerably more detail a critical period in my own formation, an time of considerable professional embarrassment which was a real spur to learning.
    In the summer of 1974, I took a position as the head of the engineering analysis section with a large diesel engine distributor in Houston, TX. This company purchased diesel engines, mostly from General Motors (GM) and packaged them on a skid with some driven machine such as a generator, a pump, air compressor, or other driven machine, along with the required controls. For me, it was a fascinating place to be as I had always been intrigued by diesel engines. I soon found out how little I actually knew about the whole matter.
    The analysis section consisted of three other engineers (two men from India and a lady from Turkey) and myself. The men were there before I came, and I hired the lady. They were all good workers, but they were best at following directions. They did not ask "Why?" very often. If this is the way it had been done previously and nobody objected, they would repeat that same pattern over and over without wondering why we do it that way. More about that aspect later.
    This was a time of great activity in terms of nuclear power plant construction in the USA, and the company was building a lot of very large engine-generator sets to serve as standby power in nuclear power plants. In a nuke plant, pumps continuously supply cooling water to the core to take away the heat and as a means to move heat to the steam generators. If those pumps fail for any reason, the core can over heat and meltdown, a major catastrophe. The great fear was that the pumps would lose power from their regular supply, in which case the standby generators would need to start up and provide power to the pumps. The proposed cause of loss of power was an earthquake, and that meant that the standby generator set must survive the earthquake and be able to start and run.

BecomingAnExpert--Part2.pdf

DrD

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

DrD

   

Mechanics Corner
    A Journal of Applied Mechanics and Mathematics by DrD, # 28
    © Machinery Dynamics Research, 2016


Analytical Design -- Part III

    

Introduction

    In Parts I and II of this series, we began looking into the design analysis for an emergency steam cut-off valve for a nuclear power plant. Part I simply posed the problem, and Part II got into the detailed kinematic analysis of the four bar linkage that is to control the motion of the plug as it shuts off the steam flow. We would like to move on to the dynamic simulation of the motion to get an idea about the time required to close and ultimately to a force analysis so that the links can be appropriately sized. Before that can be done, it is necessary to spend some effort on the design of the plug itself so that the dynamic properties of the plug (mass, mass moment of inertia) can be estimated.
    In the stress analysis of the plug, I am at somewhat of a disadvantage with respect to most of you. I know that many of you have access to Finite Element programs which enable you to do a fairly complete and correct stress analysis. Because I am no longer connected with a university or an industry that would give me that access, I have do not have access to FEA. Thus the analysis that I will present here is somewhat crude, rather approximate, and not recommended where better methods are available. You may choose to look at this as somewhat of a "historical approach" to the stress analysis, the way it would have been done before the arrival of FEA. The model that I will present is overly simplified, but it is the best that I can provide for this series.
    The maximum temperature and pressure for the steam system are given as P = 1000 psi, T = 1000 deg F or equivalently, P = 68.95 bar, T = 538 deg C. These are the targets specified for the plug design.

AnalyticalDesign-Pt3.pdf

DrD

More than once, I have remarked in this blog about the lack of participation. I have been mystified by the lack of questions and comments on my articles. Even those that get several thousand views, often have no comments at all. I just don't understand it!

Yesterday, I had a visit. My visitor is a former student of mine, a fellow that I taught back in the early 1980s. His name is Bob, and Bob was an Ag Engineering student, if I recall correctly. He took my theory of machines class as an elective (something not many people would do), and he really got into it. One of my earliest recollections of Bob in class was having him yell out, all of a sudden while I was lecturing on some mechanism, "That's just like on the combine." Bob was from a farming background, and had grown up with ag machinery. A combine (for those who are not familiar with the term) is a very large, very expensive, harvesting machine. With different attachments, they can harvest a variety of crops; in my area, we see them primarily in the fall, harvesting corn about 12 rows at a time as they lumber along at a fast walk. Bob had made a connection between what we were studying in class and what he was familiar with on the farm. After finishing that class, he graded that class for me for a couple of semesters until he graduated and left.

Even while he was an undergraduate student, Bob knew what he wanted to do. He wanted to work for John Deere & Co (JD). John Deere is the premier ag machinery manufacturer in the USA. They have a number of competitors, but JD is always ranked right at the top. When Bob graduated, JD was not hiring, and he was crushed. He went for a Masters degree, and when that was finished, he got on with JD.

I live in the US state called Iowa (after an American Indian tribe), a very agrarian state in the upper midwest, pretty much in the middle of the USA. JD has several really large manufacturing plants in Iowa, and a few test facilities, etc scattered across the country. The JD plant in my city builds construction machinery and forestry products. Another of my former students runs the plant where I live. They also have plants around the world. I am aware of them having one or more plants in Russia, in India, and no doubt elsewhere. Their machines are characteristically painted a light green with bright yellow details. Bob now works at the JD Research Center in Waterloo, IA, about 100 miles west from where I live. They are very secretive about what they do there; JD is very sensitive about their company secrets! He has finished a PhD about 10 years ago, and is now one of their senior people in the area of gearing. He tells me that he is now being asked to teach some of the younger engineers, so the wheel continues to turn!

As we were visiting yesterday, I mentioned writing this blog and the lack of participation from readers. That prompted Bob to tell a story. He said that an engineer from one of their plants in India has been assigned to come over here for a 6 month period, a short while back. When the man first came, he hung back, reluctant to speak up, to get his hands dirty, to get involved with the equipment. Evidently the Research Center in Waterloo was quite a shock for him, because JD encourages their engineers to get involved with the equipment, to get dirty, to drive tractors around, etc. As the 6 month period went by, the man began to eventually get into the "American way," that is, to get more personally involved with his work. He took those attitudes with him when he returned to India. The report came back to Waterloo from his manager in India that the man was a much, much better engineer after his 6 months in the US. Rather than standing aloft, he was getting into the machinery, getting dirty and working with the machinery.

The point of this story is that, through the agency of international business, attitudes about work and many other matters are being changed. New ideas that work better are spread to areas in need of them, and there is progress around the world, one person at a time. We all benefit from this.

DrD

 

DrD

   

Mechanics Corner
    A Journal of Applied Mechanics and Mathematics by DrD, # 27
    © Machinery Dynamics Research, 2016


Analytical Design -- Part II

    

Introduction

    In Part I of this series, the problem posed was to apply, as far as possible, the available knowledge of mathematics and science to the design of an Emergency Steam Shut-Off Valve, to be gravity driven as in the sketch below, Fig. 1. The inlet and outlet for the steam flow are both specified to be 0.85 m in diameter, so the valve plug will be a little bit greater than this amount in diameter. That suggest that, as a first approximation, L2 =0.85 m. Now, where do we go from here?
    
MechCorn.JPG
    Fig. 1  Proposed Gravity Actuated Emergency Steam Cut-Off Valve.

    The draftsman's sketch has been redrawn to scale, and it appears that this is a workable geometry (we will have to investigate that further, but it is a place to start). Scaling his sketch in such a manner that L2 =0.85 m, the first estimate for the dimensions is
    L1 = 3.460 m
    L2 = 0.85 m
    L3 = 0.902 m
    L4 = 1.912 m
    D = 0.638 m
    These are only preliminary, subject to change as needed, but they give us a place to start the kinematic considerations.
    There are two issues of major importance to be address early on:

1.Will the plug enter straight into the valve seat without binding or impacting on an edge?
2.How long will it take for the plug to fall into place?

    These two issues will be our initial concern.

 

 

AnalyticalDesign-Pt2.pdf

DrD

   

Mechanics Corner
    A Journal of Applied Mechanics and Mathematics by DrD, # 26
    © Machinery Dynamics Research, 2016


Analytical Design -- Part I

    

Introduction

    On ME Forums, there have been a number of questions raised about design. Many students are writing in asking for ideas for a senior design project, others are asking for help with a design problem, etc. There has been some discussion about "Just what is design?" That was definitely a question in the minds of many faculty at one school where I taught a number of years ago. The engineering accreditation board in the US is called ABET (Accrediting Board for Engineering and Technology), and just before I joined this particular faculty, ABET had denied the school accreditation "because they were not teaching nearly enough design." When the faculty heard that, they were shocked, and we spent several years getting everyone on board with what design is.
    I am not going to attempt to launch into a full scale discussion of what constitutes "design," but rather, with this post I am beginning a short series on one narrow aspect of design. Please understand that this is not the full scope of design, but simply a part of it.
    In most jurisdictions where engineering is defined legally, the definition will include something like, "being qualified to design," and will also speak of having a sufficient knowledge of mathematics and science to apply those fields to the engineer's work. With this series, I hope to show the application of mathematics and science to a typical design problem. The problem I have chosen is fairly typical of a first cut at a design, but it must not be taken to represent all such problems.

The Problem

    Suppose that you are a young engineer, working in the nuclear power industry. Because of our natural fear of uncontrolled nuclear energy, one of the constant concerns in the nuclear power industry is how the plant will handle various possible accidents, particular things like an earth quake, bomb strike, or airplane crash onto the nuclear reactor. Any of these events could cause a need to shut down the nuclear steam generation, and bring the whole plant to an orderly halt. We are not about to tackle the entire problem, but just one small part of it, stopping the flow of steam from the system.
    In the event of a catastrophic accident, it is not wise to rely on the ordinary control systems that function using electrical actuators, pneumatic systems, or in some cases, hydraulics. All of these energy systems are likely to be disrupted by the emergency, so what is left to actuate a steam valve?

AnalyticalDesign-Pt1.pdf

DrD

    Mechanics Corner
    A Journal of Applied Mechanics and Mathematics by DrD
      Machinery Dynamics Research, (c)  2016


Professional Societies -- Or Not?


Introduction
    This post is written in response to questions raised by one of our regular participants, a young engineer in Australia. What I have to say here is based entirely on my own, very American, experience, and others may have different ideas. I would encourage a general discussion in the comments, so that we may see how various folks look at this question.
    The questioner asked, "... have you maintained a membership to such an organization for the purposes of networking and staying active in the engineering world?" I'd like to re-word the question just slightly to read, "What is the purpose for being in a professional society, and is it worth the costs?" My experience in this matter is limited strictly to American professional societies, but I suspect there will be some carry over to other nations as well.


What are we talking about?
    Before getting into the good and bad points, it is worthwhile to cite some examples of various professional societies, so that everyone will understand the kind of organizations are under consideration. Let me name a few that come to mind, just off the top of my head:
    ASME --- The American Society of Mechanical Engineers
    SAE --- Originally called the Society of Automotive Engineers, now legally simply SAE
    ASHRAE --- American Society of Heating, Refrigeration, and Air-conditioning Engineers
    NSPE --- National Society of Professional Engineers
    ASNE --- American Society of Naval Engineers
    SNAME --- Society of Naval Architects and Marine Engineers
    AIAA --- American Institute of Aeronautics and Astronautics
    IEEE --- Institution of Electrical and Electronic Engineers
    SME --- Society of Manufacturing Engineers
    ASCE --- American Society of Civil Engineers
    AIChE --- American Institute of Chemical Engineers
    IIE --- Institute of Industrial Engineers
    SAME --- Society of American Military Engineers
    SPE --- Society of Petroleum Engineers
    AWS --- American Welding Society
    This list is certainly not exhaustive; there are many more such organizations. As this brief list suggests, there is an organization for every interest! Let me talk a little bit about a few of these, the ones of which I am a member and one other that I am familiar with.


  ASME
    In many respects, ASME has long been the premier mechanical engineering society in the US. It first rose to prominence in the early 20th century when there was a nation-wide problem with boiler explosions. ASME took the lead in developing boiler standards, and today the ASME Boiler and Pressure Vessel Code is recognized world wide as a guide to safe design of such systems. Sales of the Code documents are a major source of income for the organization, and ASME is very active in many areas of Codes & Standards. It also published a number of technical journals, such as the Transaction of ASME, Journal of Applied Mechanics. They also organize and host many conferences around the country, and indeed around the world, on various topics of specialist interest.
    When I first joined ASME as a student in the early 1960s, it was a very membership oriented organization. Each year, when you paid your dues, you received five coupons that could be redeemed for technical papers that were available from ASME. At that time, the membership really ran the organization. Today, it has all changed. It is simply a business, run by a bunch of folks in New York, for their own benefit, and they simply do not care about the needs of the membership. If I want an ASME paper, I can buy it for about $25, exactly the same as anyone else can get it. There are ASME Student Sections at most American engineering schools, but ASME does little or nothing to support them. Over the years, I have attended many ASME functions, from local section meetings to national conferences. They are often well attended, and they are an opportunity to meet others in the field.
    I am a Life Member of ASME which simply means that I was foolish enough to pay dues for 30 years, and now I am exempt from further dues. I probably would not join ASME if I were starting today with what I know now.


 SAE
    SAE was originally known as the Society of Automotive Engineers, a fairly self-explanatory name. It has dropped the name to become simply SAE, and refers to itself as "the mobility society." It incorporates folks interested in mechanics, materials, fuels, lubricants, combustion, controls, electronics, etc., and deals with automobiles, trucks, ag machinery, boats, and aircraft. It is very, very broad, and almost any technical person can find a role in SAE. SAE, like ASME, also publishes many Codes and Standards, and organizes a number of technical meetings each year.
    One thing that I think is important about SAE is the way it supports engineering education. SAE sponsors, with significant amounts of money, many student design competitions in which undergraduates design, build, and test various real projects. One of the most popular of these is what is called the Mini-Baja Competition, a reference to the famous off-road racing done in Baja California (Lower Califormia, Mexico, a very primitive area located directly south of the US state of Califormia). Let me tell you a bit more about my own involvement with SAE.
    In the mid-1980s, I was teaching in a small engineering school in Wisconsin. I was in my office one afternoon when one of my former students burst in, eager to talk to me. Brad said, "Are you a member SAE?" to which I replied, "No, I am a member of ASME." He came back with "Well, would you be?" which really puzzled me. Why did he care whether I would join SAE or not? As the whole story came out, he was nearing graduation, and he wanted to do something of lasting value for the school. His idea was to organize a SAE Student Chapter and get them involved with the Mini-Baja race car competition. Such a group would need a faculty adviser, and he wanted me to be that adviser. To make a long story short, I joined SAE and we got approval to organize a Student Chapter and got started on the construction of a race car. I was amazed at the student enthusiasm, and also at the financial support we got from the Senior Section (the adult SAE Section in our area). Money poured in from the Senior Section, and we received a donated engine (everyone uses the same engine) and many items of donated hardware. We did not win that first year, but we did the next year, and I'm proud to say, the group continues to this day, doing very well year after year! A couple of years ago, I attended an SAE student competition where they were racing Formula I race cars. We had about 25 schools represented, coming from as far as 1000 miles away for the competition! This is a serious boost for engineering education, and a real service!


SNAME
    SNAME is the place where most naval engineering is focused. They publish a high quality series of technical journals, and deal with real marine engineering, including ships, off-shore platforms, ice-related problems, etc. I joined primarily because they are the only ones who seem to be concerned with a particular vibration problem that interests me. I have not solved that problem yet, but when I do, I'm sure I will publish the results in a SNAME journal.

ASNE
    For contrast with SNAME, there is the American Society of Naval Engineers. I became aware of this organization when I worked for the US Navy, and found that it is mostly Navy officers, government big-shots, government contractors, and others, all pretending to be engineers. Their meetings (I went to several) are about as technical as a comic book. The only reason to be a member here is for the contacts one might make; the technical content is just about nil.


Summary
    So, back to the main question: Is it worth it?
    That depend entirely on your own goals and values. Are you interested in it for a social outlet? Are you interested in terms of community service? Are you hoping it may provide you a contact that could lead to a better job? Are you hoping to learn serious new engineering content? Before I would commit to one of these organization, I would try to investigate just how it fits into your own hopes.
    Most such organizations welcome visitors to your meetings, so you can probably visit a time or two and get some feel for the organization at your local level. Ask what does it do? What projects has it undertaken? Ask yourself if the meeting is well run and organized (I'd stay clear of disorganized groups; they are simply too boring for words!) Ask how often they meet, and what they do in their meetings. I have been on some really excellent field trips as part of various society meetings, and I have often taken students with me to these meeting where there was a field trip involved. I have also been to some really terrible meetings, with a dinner of rubberized chicken and a meandering, dull-as-dust speaker. They vary all over the map.
    I would suggest that every engineer should probably be a part of some such organization, but that should be chosen with real care. Check out prospects very carefully, find out what you might expect to get out of it, what you might expect to contribute, and what the financial cost is. Their can be real benefits, but not every possible choice leads to them. Make a wise choice!
    
    DrD is a retired Professor of Mechanical Engineering in the USA. He can be reached for comments, questions, or requests through the ME Forum message system.. Be sure to check back soon at www.http://mechanical-engineering.in/forum/blog/206-mechanics-corner/ for more articles.

DrD

War Stories

   

Mechanics Corner
    A Journal of Applied Mechanics and Mathematics by DrD
    © Machinery Dynamics Research, 2016


War Stories -- Dr. Jack Levedahl

Today I want to tell a few war stories, war stories in the literal sense of the word, in that they all relate in one way or another to World War II. They all center on my friend Dr. Jack Levedahl, a most excellent mechanical engineer and adventurer. Jack was an elderly man when I first met him about 15 years ago, and he has since passed on, but I remember him with great fondness and respect.

Jack grew up in a Swedish American family in Aurora, Illinois, southwest of Chicago. He grew up hunting small game in the fields around the town and tinkering in the machine shop his family ran. At the time that World War II began for the US, Jack was a mechanical engineering student at MIT. When the US entered the war, Jack dropped out of college and joined the US Army Air Force, as it was then called. Jack became a pilot and flew the North American P-51, the finest propeller driven fighter the US has ever built. The P-51 was originally designed for an American-made Continental Lycoming engine, but that didn't have enough umph! It wasn't long before the design was revised to include a Rolls-Royce Merlin engine with about 1600 hp. This gave the P-51 a top speed of around 450 mph, the fastest mass produced aircraft of WW II. It was quite a excellent machine!

The Merlin engine was a marvelous design, but it was not easy to build. In England, the Brits built these engines one-by-one, essentially by hand, hand fitting all the bearings and other precision parts. There was simply no possibility that Rolls-Royce could supply engines at the rate needed by the US Army Air Force. For that reason, the design was licensed to the Packard automobile company in the US. Packard engineers reworked the design, loosening tolerances wherever possible, and came up with an engine that could be mass-produced. This produced an engine that was in many ways superior to the handmade engines from the UK. The Rolls-Royce engines were like a fine watch, but they had practically no truly interchangeable parts. This made field repairs extremely difficult and time-consuming. The Packard built engines ran a little more noisily, but they all had fully interchangeable parts and could be repaired with ease. This last proved to be a great benefit in the theater of war.

When Jack had completed his training, he was sent to the Mediterranean where he was based on Sicily, flying missions north up the Italian peninsula. Gradually the Allied effort was driving the Germans back, and they were retreating back toward Germany. Jack told me in some detail about one such mission.

Jack said that he and his wing man were attacking a German train headed north through the Brenner Pass into Austria. This was not an ordinary train but rather was an armored train. It carried on board a number of antiaircraft cannons, capable of firing at attacking aircraft. As they bored in on the train, Jack said his wing man was hit and went down, but he got through and shot-up the locomotive, causing the boiler to explode.

I visited Jack in his apartment in Annapolis, Maryland one day where I saw a strip of aluminum about 15 inches wide and about 7 feet long hanging over a wide doorway. It was literally torn to pieces, with large gaping holes gouged through it, rather clearly the work of large bullets (probably 30 mm cannon fire). I asked Jack what the piece of metal was. He replied, "Oh, that was the canopy fairing from my plane. That was right behind me in the cockpit." When he had returned from one of his missions with his plane shot full of holes, the aircraft mechanics had salvage this one piece for him for a souvenir. When I asked if there had been anything else between him and the bullets he explained that there was an aluminum armor plate, about 1/2 inch thick behind his seat and that was all.

When he had completed 50 missions, Jack was mustered out of the Army Air Force, even though the war was continuing. He did not want to leave, but there was a rule that if you survived to complete 50 missions you had to be retired. Not too many pilots survived 50 missions, so it wasn't a problem for very many people. Reluctantly, Jack returned to the US and enrolled again in MIT where he finished his degree. He went on and got a master's degree as well.

Jack told me that one day about 1947 or 1948, he encountered a German-speaking man on the streets of Milwaukee, Wisconsin. The man was asking, in German, for directions to Mader’s, a famous German restaurant in Milwaukee. Jack spoke some German and was able to communicate with him, so he walked the visitor a few blocks over to the restaurant where they had a meal together. It turned out that the visitor was a famous mechanical engineering professor from the University of Aachen in Germany. He invited Jack to come to Aachen to study with him, and that's where Jack did his doctoral work.

During the time that he was studying in Germany in the postwar period, Jack said that many of his fellow students were former German army personnel. One day, in a graduate student bull session, one of the other students described what he had done during the war. He explained that he had been a gunner on an armored train. He said that all came to an end one particular day in the Brenner Pass when his train was attacked by two American P-51s. He went on to say that he had shot down one of the attacking aircraft, but the other one got through and destroyed the locomotive of his train. It was a bit strange for Jack to have to tell him that he was the one who got through and blew up the locomotive!

For many years after the war Jack was a research engineer for the U.S. Navy, working on a variety of projects. It was in that capacity that I encountered him when I worked for the U.S. Navy in a similar situation at the end of my working career. The Navy had a program for its research personnel that was called "Scientist-to-Sea" in which research engineers were allowed to sail on US naval ships under certain circumstances. Jack and I signed up for one of these cruise opportunities, and we sailed together from New York harbor down to Pascagoula, Mississippi on the destroyer Roosevelt, DDG-80. It was a marvelous eight days at sea, and we were free to explore the ship from stem to stern, talking to anyone and everyone and asking any question. So imagine two elderly men, acting like two kids, roaming the ship and asking to be let into out-of-the-way places such as the chain locker for the anchor chain and the steering gear compartment at the stern of the ship. We would go together and look in a section of the ship, and then we would return to the state room that we shared. There we would discuss what we saw, and frequently disagree about exact details of what we had seen. When we disagreed, we would get up and return to that part of the ship to take a second look, and thrash out just how each ship system worked. It was a grand experience for me, to be able to explore this fascinating mechanical system called a destroyer and continually discuss parts of it with this highly experienced and vastly knowledgeable mechanical engineer. I miss him greatly!

DrD

Mechanics Corner
    A Journal of Applied Mechanics and Mathematics by DrD, # 25
    © Machinery Dynamics Research, 2016


A Textbook Statics Problem

    

Introduction

    In the course of each day, I visit many web sites, always keeping an eye out for interesting engineering problems. The problem that is discussed here is one such that came from an American site (more about that web site later). The problem was presented as a simple statics problem, and the poster was asking for help in the solution. There were several replies with suggestions, but no one seemed to be able to really put their finger on the difficulty. As it turns out, this problem presents multiple difficulties, and offers an opportunity to look at several matters of interest to this readership.
    The question was posed with a hand drawing comparable to that shown in Fig. 1 here.
    
  Fig1-25.JPG
    Fig. 1  Schematic Drawing as Given

    The person posting this problem implied, without ever clearly stating it, that the problem is to find Fex when the applied load T=100 N as indicated. He says, "So it's been a while since I've done FBDs, and what seems like a simple problem is causing me grief. ... Can someone tell me what I've done wrong here? I seriously was looking at this all day and was just scratching my head." Can any readers relate to the dilemma of this poster?
    Just looking at the sketch, it seems like a reasonable problem. When the load T is applied, it will tend to rotate the crank ABC in a clockwise direction, straightening the joint at C. This will push the roller at E against the wall, developing the reaction force Fex. There is no friction at either D or E, so what is the difficulty?

TextbokStaticsProb.pdf

DrD

   

Mechanics Corner
    A Journal of Applied Mechanics and Mathematics by DrD, # 24
    © Machinery Dynamics Research, 2016


Torsional Stiffness of a Shaft -- Part III

   
    Introduction
    The discussion in previous parts of this series has focused on stiffness (or compliance) estimates for various shaft geometries. There has been nothing said yet about joining parts together, although most readers will readily agree that integral (single piece) shaft assemblies are very rare in practice. It is time to discuss joining multiple components together to form a shaft system.
    The list of possible coupling types is almost endless, so this article will simply focus on a few of the more common types to illustrate the thought processes.

Keyway or Spline

    Before considering actually joining to a second member, if the connection is to be made by means of a key or a spline, it is appropriate to look at the way in which the keyway (or spline) itself increases compliance in the section where there is no torque transfer.

Fig1KeywaySpline.JPG
    

    [Fig. 1  Shaft Sections with Keyway (left) and Spine (right)]

    Where a key is used, the keyway is usually cut significantly longer than the key itself. This results in a section of the shaft that effectively has reduced diameter. Experience has shown that this can be treated adequately by considering it to be a uniform solid shaft of diameter Deff as shown in Fig. 1.
    Similarly, where a spline is used, it is not difficult to see that the ribs that form the spline teeth carry no significant shear. Thus the part of the shaft that is splined is also properly modeled as a uniform solid shaft with diameter Deff as shown in Fig. 1.

ShaftStiff-Pt3.pdf

DrD

   

Mechanics Corner
    A Journal of Applied Mechanics and Mathematics by DrD, # 23
    © Machinery Dynamics Research, 2016


Torsional Stiffness of a Shaft -- Part II

    
    Introduction
    
    In the previous post of this series, a variety of shaft forms involving both solid and hollow sections were considered. A general approach was developed, applicable to various sorts of non-uniform shafts, but always subject to the provision that the variation in section was gradual; no sudden changes in section were permitted among the forms considered. This leads directly to the question that is the focus of this post: "How are sudden changes in diameter taken into account?" Steps, also called "shoulders" are a common feature of many shaft designs, used to locate rotating elements (fans, flywheels, pulleys, etc) on the shaft. They are also used in connection with bearings and seals. It is important that the means be established to account for shoulders in the shaft stiffness calculations.
    

Compliance of a Stepped Shaft

    At typical shaft section involving a step or shoulder is shown in Fig. 1. As usual, it is assumed that all of the dimensional data is known. The difficulty is the rather abrupt step from diameter D1  up to D2 . For the development below, it is always understood that D1 < D2. By the methods previously established in Part I, the compliance of each shaft segment can easily be computed,
    

Fig1-SteppedShaft.jpg.ac41748e565fefe247

ShaftStiff-Pt2.pdf

DrD

   
Mechanics Corner
 A Journal of Applied Mechanics and Mathematics by DrD, # 22
(c)  Machinery Dynamics Research, 2016


Torsional Stiffness of a Shaft -- Part I
    
    Introduction
    
    A shaft is a common machine element, used to transmit rotational motion and torque from one component to the next. It is clear that the length of the shaft must be sufficient to span the distance from the first component to the second, but what should the diameter be? The answer to this question requires first answering two related questions:
  -->  How much torque is to be transmitted? This will define the strength requirements for the shaft so that it does not fail under load.
  -->  What stiffness is required for the shaft? This will determine the angular relation between the two ends of the shaft, important in cases where angular displacement accuracy is a concern and also a major factor in the torsional vibrations of the system.
    As is typical with a design problem, the designer is not able to simply specify what is required and compute the required dimensions. Rather, it is necessary to propose a design, that is, to propose both shape and dimensions, and then see if the strength and stiffness requirements are met by the proposed design. There are many possible designs for a shaft, so it is important to deal with the necessary stiffness calculations for a variety of geometries (the matter of strength is left to another time).
    The basic theory required for this work is commonly found in all Mechanics of Materials textbooks (see for example, Timoshenko). There are also two well known references that deal specifically with this material. The first is the BICERA Handbook (which is the source for many of the ideas in this note) and also the work of Wilson.

TorsionalStiffnessOfAShaft.pdf

DrD

Professional Responsibility — Do The Right Thing

In Western culture, three professions are recognized. They are medicine, the law, and theology. The practitioners of these professions have long been accorded a special status in society in recognition of their special knowledge. The physician is expected to use his knowledge of healing only for good, and never to murder or maim a person. The lawyer is expected to give correct legal advice to his clients, to keep them free from legal entanglements. The theologian is expected to always correctly guide folk in the right way according to their faith, so that they avoid eternal condemnation. In every one of these professional roles, the professional person is seen as a protector of society at large.

Engineering has much more humble beginnings. In the earliest days, engineers were simply builders of whatever was needed, whether it be a canal, a windmill, a bridge, or a fortification. As such, they were carpenters, stone masons, blacksmiths, etc. Over time, with the application of mathematics, engineering has gradually become recognized as a profession, similar to the classical professions. It is based on a core of specialized knowledge, and it has a responsibility to society at large to protect that society. Thus the man who designs and constructs a bridge assumes responsibility for assuring that the bridge will not fall. The man who designs a complex assembly machine assumes a responsibility that (1) the machine will function and not be a waste of the investor’s money, and (2) that the machine will function safely and not endanger the workers nearby.

Everyone recognizes that professional people are still just people, and thus prone to failure. Doctors sometimes perform surgeries that kill the patient. Lawyers sometimes give unsound advice and their clients suffer as a result. Engineers occasionally design systems that do not function as expected, but in every case, the intent of the professional person must be the protection of the individual client first and also of society at large.

Many readers of ME Forum are either engineering students or new graduates. As you enter into the role of an engineer, you are acquiring a respected status, and you are also taking on professional responsibilities. You have a duty to assure that society is well served, that people are not endangered, either in their physical safety or their finances. This does not mean that you cannot participate in high risk ventures. But it does mean that it is your duty to be sure everyone is fully informed of the risks involved. An example of this is the US moon landing program. This was certainly a high risk program, and there were a few failures along the way. The good part, from a professional perspective, is that everyone was kept fully informed of the risks, so that no one was mislead.

One of the areas where there is much misinformation today is energy sources. This is a “hot area,” in the sense that there is much excitement about “new energy sources.” Solar power, wind power, wave power, in-situ coal combustion, and so forth, are all ideas being explored as people look for cheaper and non-polluting energy sources. All of these are really not new; they have been exploited to a very limited degree for many years. They are “new” only in the sense that there is an expanded interest in them, particularly driven by current economics.

At the beginning of this article, I discussed the characteristics of a profession, both in terms of status of the practitioners and their responsibilities to society. There is another class of people who also claim to possess special knowledge, the Con Man. These are magicians, astrologers, soothsayers, witch doctors, etc. They gain the confidence of their clients by trickery, and deliberately mislead them to the advantage of the con man. (The term Con Man refers to someone who operates by first gaining the confidence of his victim.)

In the 19th century American west, the "snake oil salesman" was a commonly encountered con man. He typically traveled from town to town, peddling his patent medicines which he claimed could cure everything from gout, cancer, tuberculosis, appendicitis, liver failure, blindness, deafness, and failed love affairs. While no such medication has ever existed, this did not stop those suffering these various aliments from seeking his products. They wanted to believe him because of their suffering. The term “snake oil,” implying an oil extracted from some breed of snake, was commonly used to describe these phony medications that were usually nothing more than alcoholic drinks in medicine bottles.

The critical thing about the snake oil salesman is that his customers wanted to believe what he was telling them. They wanted relief from their ailments, and thus they believed the most absurd claims. We have a similar situation today in terms of the energy area. There are all sorts of con men operating, promoting schemes that cannot possibly work but that will make them rich if they can be sold to the public. This is where the engineer’s responsibility comes into the picture.

The energy con men often speak in terms of perpetual motion machines, “over unity” magnetic systems (over unity means with efficiencies greater than 100%), and other similar imaginary concepts. I say “imaginary” in that, while we can imagine them and desire them, the laws of physic preclude their existence. In particular, the Second Law of Thermodynamics and the concept of Entropy make such devices impossible. But impossibility has never stopped the con man!

When such confidence schemes are promoted to the public, engineers have a professional responsibility to speak out. We have studied, we have acquired the specialized knowledge required to evaluate such schemes, and we have a duty to protect the public from such con men who only seek to profit from the ignorance of the general public. We have two such confidence schemes in operation here at ME Forums, even though there has been, to the best of my knowledge, no attempt to profit monetarily from these tricks.

The first is presented in an article titled “Gravitation - Energy of the Future,” begun by a poster called Sentally, on 8 October, 2015 (http://mechanical-engg.com/forum/topic/12485-gravitation-energy-of-the-future/#comment-17044     not clickable, but copy and paste will work). If you read his article and the comments that follow, you will see a demonstration of the classic confidence operation. He presents his false information, and when challenged, he simply claims superior knowledge that enables him to dismiss his critics without any real explanation at all.

The second is VEProject1's Blog, ( http://mechanical-engg.com/forum/blogs/blog/179-veproject1s-blog/ )  On this blog, the author has presented a number of interesting devices that appear to contradict known scientific principles. Some of these are described as perpetual motion machines, other simply as curiosities. The title stands for “Visual Education Project,” but if he was interested in true education, he would explain how his devices work.

You may ask how the systems show on the VEProject blog can be denied when they appear to work in video demonstrations. The answer is simply that the video does not show everything that is involved. There is more to these systems than meets the eye in the video. In a previous post at Mechanics Corner, I examined one of these systems in detail, based upon what is visible in the video. I showed there that the system cannot possibly move under gravity because the center of mass is stationary (The VEProject --- Shifted Levers --- A Critical Assessment).

So, how do these devices work? I cannot say for certain, but there are several possibilities:
1. Perhaps the most obvious is computer graphic trickery, where the video has been manipulated to show something that never really happened.
2. Perhaps there is a hidden motor, driving the system through concealed belts and/or gears. This would have to be done with considerable skill, but it is certainly possible.
3. One of the most interesting possibilities is that of manipulation of powerful magnets below the table.

This last is interesting from a technological standpoint. Permanent magnets have been known to man for a very long time, but really powerful permanent magnets are a relatively recent improvement. This has been brought about the application of various rare earth elements such as Samarium-Cobalt. Today, using rare earths, we have permanent magnets far more powerful than the permanent magnets of previous generations, and many people have looked in this direction for a “new” energy source. I think such a search is misguided, but I cannot say that it is impossible. But if it is necessary to move the permanent magnets, that movement constitutes a work input to the system, and must be taken properly into account.

So, where does professional responsibility come into the ME Forum discussion? As engineers, we have the duty, the obligation, to call out false demonstrations wherever they are shown. If we fail to do this, we are tacitly endorsing the false representations. We do not want to be put in the position of having someone invest in ignorance in such schemes, thinking that we approve of them. We have a responsibility to speak out against falsehood wherever it is found.

For this reason, I urge every reader of ME Forums to review the material presented by these two frauds and then to protest to the site owner by an internal e-mail (saurabhjain Administrator). These sites should be urged first to make a correct, honest presentation of their ideas. If that is not done, they should be removed from ME Forums. It is time for all Forum readers to speak up! We have to do the right thing!

DrD

   

Mechanics Corner
    A Journal of Applied Mechanics and Mathematics by DrD, #21
    © Machinery Dynamics Research, 2015


Rigid Body Rotordynamic
Instability, Part I

    

Introduction

    The rotating elements of machinery are usually balanced to avoid vibration which results in noise and fatigue damage. That said, perfect balance is not possible, and there are practical and economic limits to the effort that can be expended to balance machine components. Consequently, every rotating element has some degree of unbalance. This is usually slight, but it is always present.
    As rotating elements increase in speed, the forces arising from unbalance increase as the square of the speed. At very low speeds, this usually causes no problem at all, but at higher speed, the increased deviation of the rotating element will eventually cause a collision or a rub. If this is allowed to persist for any length of time, it will often mean a wreck on the machine, with major damage to the rotor and the support structure. This is considered a loss of stability because the operating state deviates ever more and more with increasing speed. Interestingly, in many cases, if it is possible to get through the critical speed (the speed at which maximum deviation occurs), it is often possible to reach lower levels of deviation at higher speeds.
    Because rotating elements are so very common in continuously operating machinery, the whole area of rotor dynamics has become extremely important and major efforts are made to understand the phenomena. The applications range from simple motor driven machinery to jet engines. Steam and gas turbines are included as well as all turbo pumps and compressors, so it is evident that this is a very broad topic. One of the most famous rotor dynamic stability problems involved the turbo pump on the space shuttle main engine. This problem was solved by my friend, Prof. Dara Childs. Here we consider only an elementary example as an introduction to a very broad and complex field.
    For this short introductory article, consider the system shown in the upper part of Fig. 1, a rigid rotor enclosed in a rigid housing. The entire assembly is mounted to the wall at left with a spring and damper assembly. The center of mass of the rotor is off the axis by an amount ε. As drawn, ε appears quite large, but in actual practice it will be some tiny amount, typically less than one millimeter. The rotor remains centered in the housing at all times, and rotates at a constant angular speed Ω rad/sec. The horizontal displacement of the rotor axis is x(t), where x=0 is the stress free state of the spring.

 

RigidBodyRotorDyn-Part1.pdf

Fig1.JPG

DrD

    Mechanics Corner
    A Journal of Applied Mechanics and Mathematics by DrD, #20
    © Machinery Dynamics Research, 2015


A Question of Stability

Introduction

    The word stability in its several forms is widely used in nontechnical communication. A person whose life it highly consistent from day to day is said to have a stable life. When the political situation in a particular area appears to be unlikely to change, it is said to be stable. A person who is well balanced and unlikely to be easily provoked to anger is said to be a stable person. When the medical condition of a sick or injured person ceases to get worse, the person is said to be stabilized. A company on the verge of bankruptcy is said to be an unstable company. But what does the word stability mean in a technical context? Each of the foregoing examples hints at the technical meaning without really being explicit about it.

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A factor g = accel of gravity was missing in the potential energy expression. That is now corrected.
    
 
 

 

Stability.pdf

DrD

Value Engineering

A toothpaste factory had a problem.  They sometimes shipped empty boxes, boxes without the tube inside. This challenged their perceived quality with the buyers and distributors. Understanding how important the relationship with them was, the CEO of the company assembled his top people. They decided to hire an external engineering company to solve their empty boxes problem. The project followed the usual process: budget and project sponsor allocated, RFP, and third-parties selected.  Six months (and $8 million) later they had a fantastic solution - on time, on budget, and high quality.  Everyone in the project was pleased.

 They solved the problem by using a high-tech precision scale that would sound a bell and flash lights whenever a toothpaste box weighed less than it should. The line would stop, someone would walk over, remove the defective box, and then press another button to re-start the line. As a result of the new package monitoring process, no empty boxes were being shipped out of the factory.

With no more customer complaints, the CEO felt the $8 million was well spent. He then reviewed the line statistics report and discovered the number of empty boxes picked up by the scale in the first week was consistent with projections, however, the next three weeks were zero! The estimated rate should have been at least a dozen boxes a day. He had the engineers check the equipment, they verified the report as accurate.

 Puzzled, the CEO traveled down to the factory, viewed the part of the line where the precision scale was installed, and observed just ahead of the new $8 million dollar solution sat a $20 desk fan blowing the empty boxes off the belt and into a bin.  He asked the line supervisor what that was about.

 "Oh, that," the supervisor replied, "Bert, the kid from maintenance, put it there because he was tired of walking over, removing the box and re-starting the line every time the bell rang.”