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# Becoming An Expert -- Part 3

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Mechanics Corner
A Journal of Applied Mechanics and Mathematics by DrD

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.

Thanks, dear teacher

An amazing thing that I found it in your essay:

you have an exact man that all of the things in your project is clearly in your mind. It seems you are living with projects, still.

The best

Dear Arash,

That is an interesting observation. It is true; in a very real sense, I have lived all my life with the major problems that I have studied. They don't come to an end for me, but rather they continue and grow as time passes. I am still working on problems I began at least 50 years ago.

DrD

Thanks Dr. D

I am quite a young ME, graduated in Mozambique ( Southern Africa)  and I would like to know the reasons that you had to leave the industry and choose being a teacher. Because in my country we choose the industry because there is higher incomes that in educational section.

Regards

Chissano

Dear Antonio,

That is an interesting comment; thank you. I never "had to leave industry to become a teacher." It was always by my own choice because I enjoy teaching. I enjoy working with young people, and seeing "the light come on" when they finally understand a difficult concept.

It is much the same in the USA with regard to salaries. Engineers in industry usually are paid more than those in academia. But I was always paid enough as a faculty member to support my family comfortably, and as I said above, I enjoyed working with students.

This should not be construed to mean that I did not enjoy my time in industry because I most certainly did. However, if you could see my resume, you would see that I never spent too long in any one industry. To do so, would be for me, very boring. Most of my industrial jobs had some major challenge when I first went to them, but after a while, that was resolved and there was no more real challenge. In one of my early industrial positions, I was hired primarily to help prepare the company's case in a very large law suit involving a product design matter. After I worked on that for about a year and a half, I gave my testimony over three days in court, and then I left the company to go to a teaching position. That was fortunate for me because the company lost the law suit and went broke while I got out whole.

In academia, I had certain classes to teach, but I was free to pursue whatever research interests I wanted. That was important to me also. The university did not care what my research was, only that it was something useful and would result in publications.

DrD

Posted (edited)

Sir (@Dr.D) how can i contact you?

You can contact me the way you just did, through this message system. It works quite well.

DrD

Pardon me sir, but it is a long story (say personal). Would you like to hear?

I need some ISO standards for ABS material testing. sample fabricated by FDM machine.

ISO 179:1982 (impact testing)

ISO 604:1973 (compression testing)

ISO standards are not subscribed by my institute and the sample is fabricated as per these standards. so it is not possible for me to use any other ISO, ASTM standard.

plz send these ISO standards at my mail address: ajaysolanki1993@gmail.com

What a strange request! What make you think I would have the ISO standards? If you institute does not have them, why would you think that I, a private, retired, individual would have them?

DrD

On 5/3/2016 at 10:34 AM, Ajay Solanki said:

I need some ISO standards for ABS material testing. sample fabricated by FDM machine.

ISO 179:1982 (impact testing)

ISO 604:1973 (compression testing)

ISO standards are not subscribed by my institute and the sample is fabricated as per these standards. so it is not possible for me to use any other ISO, ASTM standard.

plz send these ISO standards at my mail address: ajaysolanki1993@gmail.com

this is weird approach..........and is at wrong place to write...  subject of post is not related to the request

Saurabh,

If you read them all, many of the comments that come have little or nothing to do with the subject post. I think it is a logical thinking defect, but it seems quite common.

DrD

Hy DrD,

I really have very little knowledge about what kind of technical problems and challenges one faces as an engineer in real life. Questions like this always pop in my mind, like- will these differential equations still be needed to solve any industrial problem?

Will the mechanics problems i face in my engineering life be like the ones i solve in my class or book? Will they be harder? How will i model them?

In school we're only taught about techniques to solve problems that are already modeled. We don't get any idea about the challenges and problems industries face.

Your post was really great and I think this is the only way of knowing the technical problems faced, i.e talking to an engineer. I googled many a times but none of the results were satisfactory.

Thanx

Sangeet,

You raise some interesting questions, and I will try to address some of them.

1. Will these differential equations still be needed to solve any industrial problem?

I'd hazard a guess that most engineers in industry solve very few differential equations on a day-to-day basis. But that does not make learning to solve them unprofitable. (a) By being able to solve them, you are more able to understand the meaning of the solution, much more so than if someone simply drew you a picture and said, "this is the solution for that system." (b) There are lots of situations where it would be profitable to be able to solve the DE, but the average engineer has simply forgotten how to do it. So, he has to fumble along, fudge an answer one way or another, and never be very sure what he is doing. (c) There are certain DEs for which you need to know the solution right off the top of your head; I'm thinking particularly of the undamped and damped SDOF spring-mass oscillator. With that, you can solve multitudes of problems. (d) If you can correctly formulate the DE, even if you cannot solve it with pencil and paper, there is always the possibility of a numerical solution. But the numerical solution is only good if you had the correct DE to begin with.

2. Will the mechanics problems i face in my engineering life be like the ones i solve in my class or book?

Most of them will be more difficult in at least one respect. They will contain more features, more things to consider, because textbook examples are chosen specifically to focus your attention on the important parts. Real problems will contain much that really does not make much difference, but it is up to you to sort out what  matters and what does not.

3. How will i model them?

This is an interesting question. While you are in college and working textbook problems, you should be thinking about where have you seen similar situations outside the classroom. Have you see a machine at work repairing the road that used some principle that is discussed in class? Have you seen a workman use a tool that looks like the problem you are considering right now? The textbook problems are intended to be similar to engineering practice while still being simple enough for you to deal with them.

4. In school we're only taught about techniques to solve problems that are already modeled. We don't get any idea about the challenges and problems industries face.

Well, that is true, but only partly so. If you are searching for applications outside the classroom, you will soon learn to make the connection between the real hardware and the model. The model almost always represents significant simplification. Consider this situation: In an actual machine or structure, the parts are often bolted together. How is the load transferred from one member to the next? In the book, we usually consider that the members are integral (continuously connected at all points in contact) or we assume a point connection, but neither is quite correct. This is where actual practice becomes much harder than the classroom.

I hope that this helps.

DrD

DrD,

Thanks a lot for your elaborate answer. I will try to revise DEs from time to time. We were taught this chapter in our first semester.

Yeah you are right, the book i'm following for dynamics has tonnes of practical situations, like cams, slider crank mechanisms, amusement park ride systems and author's favorite- radars and planes.

Well, for improving modelling skills and applying science in everyday life I found out an online course on EDx .

Thank you again!

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