Читать книгу Introduction to Mechanical Vibrations - Ronald J. Anderson - Страница 10

When I first studied vibrations, as an undergraduate student, its importance was clear to our class because it was a required course for mechanical engineers. A few years later, when I started teaching vibrations and new topics were entering the field of mechanical engineering, a course on vibrations was no longer seen as being important enough to be a required so it became an elective. Now, although “mechanical engineering” is still used as an umbrella term, the students who graduate are mechanical engineers with a specialization. Students in the specialized streams do not have time to cover all of the topics that used to be expected of mechanical engineers so some graduate without thermodynamics, others without vibrations, and so on. Specialization like this is inevitable given the expanding scope of knowledge in engineering and the limited time available to undergraduate students but it means even fewer students are learning about vibrations and other important topics. While preparing this introduction to vibrations, I kept in mind the need for undergraduate students to have a better understanding of two topics that are ubiquitous in today's engineering workplace – finite element analysis (FEA) and fast Fourier transforms (FFT). FEA and FFT software tools are readily available to both students and practicing engineers and they need to be used with understanding and a degree of caution.

I was never able to find a textbook that covered just enough, and the right, material for a semester length introductory course in vibrations. I used many textbooks over the years but there was never a fit with what I thought should be in an introduction to vibrations. I was looking for something student‐friendly in that it should be readable, almost conversational, but still be mathematically rigorous. What I found on the market were mainly “reference books” as opposed to “teaching books”. Many of the textbooks I tried are very good at covering, in depth, a broad range of topics in vibrations, but students have difficulty using them as a first text in the subject, mainly because of the overwhelming amount of material presented.

This book grew from my attempt to accomplish two things in a single course in vibrations. The primary goal is, of course, to present the basics of vibrations in a manner that promotes understanding and interest while building a foundation of knowledge in the field. To do this, I have had to give only brief coverage of many important topics with the hope that some students will go on to expand their knowledge in these areas if their interest is piqued. As mentioned earlier, a secondary goal is to give students a good understanding of finite element analysis and Fourier transforms. While these two subjects fit nicely into vibrations, this book presents them in a way that emphasizes understanding of the underlying principles so that students are aware of both the power and the limitations of the methods.

Chapter 1 addresses the way in which a student has to think about previous undergraduate dynamics knowledge in order to make the transition to analysis of vibrating systems. It introduces the idea of small motions about a stable equilibrium state and addresses the details of linearization. Lagrange's Equations are introduced here and students take to them very quickly as an alternative to Newton's Laws.

Chapter 2 considers the details of analyzing single degree of freedom systems. While much of this material is obvious to those skilled in vibrations, it is vital material for developing the students' abilities. It covers topics such as preloads in springs and why gravitational forces don't need to be included because they are canceled out by the constant preloads. It looks at the constitutive relationship for a spring and shows how to draw free body diagrams consistently and accurately.

Chapter 3 is about free vibrations of single degree of freedom systems. It covers systems with and without damping and tries to make sense of what it means to solve a second‐order, linear differential equation without being too prescriptive about it.

Chapter 4 looks at time response when applying a harmonic forcing function to an undamped single degree of freedom system, thereby introducing the phenomena of beating and resonance. This is a short chapter although the subject of time response, if presented in detail, could make for a very long one. Time response is an area that I see as being of secondary importance in an introduction to vibrations.

Chapter 5 considers steady state forced vibrations, covering harmonic forcing functions, harmonic base motion, systems with a rotating unbalance, and accelerometer design. This is really the essence of vibrational analysis and is covered in detail.

Chapter 6 is devoted to the very important subject of damping. Linear viscous damping is discussed and the concept of modeling other energy removal devices as “equivalent linear viscous dampers” is introduced. Coulomb damping is covered. The concept of logarithmic decrement is introduced.

Chapter 7 recognizes that systems often have more than one degree of freedom. Deriving the equations of motion for systems with many degrees of freedom is discussed. The concept of multiple natural frequencies, each associated with a different mode shape, is covered in detail. Description of mode shapes is given a lot of time because of its importance in the field. Forced vibrations, vibration absorbers, and the method of normal modes are covered.

Chapter 8 moves on into the study of continuous systems and uses vibrations of a taut string and a cantilever beam as examples of two continuous systems where solutions can be found. The concept of infinitely many degrees of freedom is introduced.

Chapter 9 recognizes that solutions cannot always be found for continuous systems so the finite element method is introduced as an alternate way to get solutions. Shape functions, element mass and stiffness matrices, and assembly of global mass and stiffness matrices are covered, as well as application of boundary conditions and applied forces. Derivations here are handled using Lagrange's Equation because the students are familiar with that approach by the time we get to finite elements. This is certainly not the approach taken by experts in finite elements but it is a useful and appropriate way to get the students to understand the assumptions made in using FEA.

Chapter 10 is devoted to a relatively new device called the “inerter”. This device provides a force that is proportional to the relative acceleration across it. A concept for how to construct such a device and analyses showing its effects on vibrations are presented. This is presented to give the students a look at developing technology with which they can have some fun while realizing that pat answers coming from what they have learned about vibrations to this point may not apply to this device.

Chapter 11 presents a detailed description of how to analyze experimental data in studying vibrations. Topics covered include: Discrete Fourier transforms; sampling and aliasing; leakage and windowing. The approach I have taken here is very old‐fashioned in that it treats the Discrete Fourier Transform as something that is derived from a least squares curve fit. This harks back to methods used more than fifty years ago but it enhances students' understanding of what lies behind the transformation to the frequency domain. This is a long chapter that presents methods that the students can program themselves so they don't have to be tied down to packaged FFT programs. Once they have their software working, they can experiment with data sets that clearly demonstrate aliasing, leakage, and so on.

Chapter 12 discusses a variety of topics in vibrations such as how to handle the mass of a spring, flow‐induced vibrations, self‐excited vibrations in rail vehicles, rigid body modes, and things you can determine from the static deflection of a system. I find that I am usually able to get somewhere into this chapter before the semester is over. The material in this chapter is interesting but certainly doesn't need to be covered to have a complete introduction to vibrations.

It is my hope that this book strikes the right balance for professors teaching introductory vibrations and for their students. I wish them all well.

Ronald J. Anderson

September, 2019

Kingston, Canada