115 714 THS THE EFFECT GE VAMATEON OF THE- PHYSECAL COESTAhfiTS G? A THREE MASS SYS?EM LENSES THE" RESONANT FREQUENCEES Thais far {he Dagmar 36 M. S. WCMGAN STATE CGiLEGE Zigurd: jmés Levefisfséns 19547 'I‘HESIS This is to certify that the thesis entitled THE EFTECT OF‘ VARIATION OF THE PHYSICAL CONSTANTS OF A THREE MASS SYSTEM UPON TPE RESONANT FREQUENCIES presented by ZIGURDS J. LEVFNSTEINS has been accepted towards fulfillment of the requirements for MASTER OFW degree in APPLIED MECHANICS Major professor DateDeQembgr 6, 125’; 0.169 THE EFFECT OF VARIATION OF THE PHYSICAL CONSTANTS OF A THREE MASS SYSTEM UPON THE RESONANT FREQUENCIES by Zigurds Juris Egyensteins A THESIS Submitted to the School of Graduate Studies of Michigan State College of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Applied Mechanics 1951+ ACKNOWLEDGEMENT The author wishes to express his sincere thanks to Dr. Charles 0. Harris, under whose lofty inspiration, constant supervision, and unfailing interest this investigation was undertaken and to whom the results are herewith dedicated. THE EFFECT OF VARIATION OF THE PHYSICAL CONSTANTS OF A THREE MASS SYSTEM UPON THE RESONANT FREQUENCIES by Zigurds Juris Levensteins AN ABSTRACT Submitted to the School of Graduate Studies of Michigan State College of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Applied Mechanics Year 195% Approved ._.—r._...- ‘RA Zigurds Juris Levensteins The object of this study was to investigate the rela- tionship between the resonant frequencies and the physical constants in a three mass system. In particular, it was desired to see the effect that variation of the physical constants of the system has upon the distribution of the resonant frequencies. Two methods of analysis were used. First the sixth degree algebraic resonance equation was solved by a direct method, obtaining expressions for the three resonant frequencies in a trigonometric form as functions of the physical constants of the system. Second, by an inverse method of approach, relationships between the resonant ' frequencies and the physical constants of the system were derived in an algebraic form. It was concluded from the direct method of solution that the expressions were well suited for the computation of the resonant frequencies in any given three mass system. Also, the results obtained by the direct method of analysis shdhd that it is possible to have only two distinct reso- nant frequencies in a three mass system, provided that the physical constants of the system have a particular rela- tionship. However, this relationship is so complicated that it appears to be difficult to devise a physical system in which the masses and springs fulfill the required rela- tionship. Zigurds Juris Levensteins For the purpose of determining the effect that varia- tion of the physical constants of the system has upon the distribution of the resonant frequencies, it was found that the inverse method of solution was simpler than the trigonometric form as found by the direct method. Some numerical calculations were made with the inverse method solution. By varying one physical constant of the system at a time and keeping all others unchanged, the effect that each constant has on the distribution of the resonant frequencies was determined. TABLE OF CONTENTS PAGE Object .............................. ..... ......... 1 Introduction ................................ ...... The Steady State Metion of a Three Mass System .... Resonant Frequen01es .OIOOOOOOOOOOOOOO0.0.0.0000... 00mm»: Trigonometric Form of Solution .................... Conditions for Two Resonant Frequencies to be Equal .................................... 11 Distribution of Resonant Frequencies .............. 13 Numerical Examples ................................ 16 Conclusions ....................................... 18 Bibliography OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO 19 92129.2 The object of this study was to investigate the reso- nance conditions of a vibrating three mass system and deter- mine the relationship between the resonant frequencies and the physical constants of the system. In particular, it was desired to see the effect that variation of the physical constants of the system have upon the distribution of the resonant frequencies. Introduction Vibration is a repetitive motion. It can develop on every occasion where there is an elastic body and a repeti- tive force acting on it. Therefore, it is a very important problem in engineering. To describe completely the vibrating system at any instant, a certain number of independent coordinates is necessary. This number of independent coordinates is called the number of degrees of freedom of the system. There are systems with a single degree of freedom as well as systems with infinitely many degrees of freedom. The simplest vi- brating system is with one degree of freedom and is compar- atively easy to analyze, but as the number of degrees of freedom increases the problem becomes exceedingly difficult. A system with three degrees of freedom was chosen for this analysis, so let us turn to the investigation of that par- ticular type. ‘Ihg Steady Stggg Motion 9; a Th;gg-Mgg§ System Figure one shows a system with three degrees of freedom. It consists of three masses, M1, M2, and M3 and three springs with spring constants k” k; , and In“, respectively. It is assumed that the springs have negligible mass. Furthermore, all friction in the system (causing damping of the vibrations) is neglected. A harmonic driving force mmtslne caused by a rotating unbalance is applied to the mass M1. Thus vi- bration of the system will occur. It seems reasonable to assume the frequency of the vibration to be the same as the frequency of the driving force. This assumption is supported by experimental results and theory. Figure la shows the configuration of the system in static equilibrium, and it is assumed that the vibration will be about this configuration. Only the vertical vibrations of the system will be considered in this analysis; thus in this case the independent coordinates necessary to describe the system are the three vertical displacements.'9¢., 96;, and 9¢,of the masses M1, M2, and M3, respectively. Only steady state vibrations of the system will be considered, neglecting the transient vibrations. A Figure 1b shows the configuration of the system when it is vibrating in the steady state. Also Figure lb shows all forces acting on the masses M1, M2, and M3 at any time'* . By applying Newton's Second Law of Motion to the system three \(WK maxim: .\ .Uxhx MET.» m. 33 EMERSON an‘ML u 2‘ ZOE «$303200 3c URva k0 ZDRQQDUEZOU «U» \\\\\\\\\\\\\\\\\\\\\\x \\\\\\\\\\\\\\\\\\e \\\\.. t dadvx dunuv‘ k W m as, m a w » w 1 a a I — _ M2 _ a - MKX . - « AQRIJGMsx « 1 » ANRI~RV\V\ » .aea . 1 ”S ”H m. .I.‘ m. A R SC «.111. k mauéillfw A e t d .E as, QWOUNOQKE .41 X mestdndKE F simultaneous second order differential equations are obtained. They are: M153. + 1999-761) 4- k,('x..- x,) = mrm‘sinw, (1) M1521" k, (9". " x1.) "' kz'xn.‘ O , (2) M36t5—k,(x,-x,)-o . (3) Where it is assumed that the frequency of the driving force ""135an is constant, so that 9303+ and 9 can be replaced by art . For steady state vibrations, the solutions of equations (1), (2), and (3) can be assumed in the forms: x..= X,sinwf , (tr) xf-‘i nginmt, ( 5) 9": X’sinwt . (6) Where X1, X2, and X3 are the amplitudes of motion of M1, M2, and M3, respectively. Substition of equations (H), (5), and (6) in equations (1), (2), and (3) yields, (kf’ k’-utM.)X. - k'xz " k3x3= mrw" 3 ~k,X. + (k.+ k,-<3'M,_)X._= o , <7) "" k; X. "' (ks " th1)X3= O 3 which is a system of three simultaneous algebraic equations that can be solved for X1, X2, and X3. Solving the system by determinants and introducing the notations, I t t k a «ta-=12, Cow—k"; w.=—'Ma I t 3 M, , 9: Ms the amplitudes of motion are, x3315 (5;) )0" (wlgfig'fihg' H65] (8) x1: WSW-Silas; (9, D a (a? [A' (at (at a $1 X33; . D (10) D is the coefficient determinant of the system of equations (7), and is, I. D. 04%,) J [mi—:3;1(ai-(sa‘][(aa‘-)+v-(ai1 amp“ -v[-;= a} at -(-$-.>‘J (ll) Resonant EIeguencies Whenever the denomination D is equal to zero the ampli- tudes of motion X1, X2, and X3 will become infinite. The condition under which an amplitude tends to increase without limit is called a resonant condition. It is very important to know for what values of the driving frequency on the amplitudes become infinite in any given vibrating system, because at those frequencies an extremely violent shaking of the system will result. In some problems this condition is desired, in others it is to be avoided. To determine the values of as for which resonance occurs, the coefficient determinant D must be equated to zero. Thus an algebraic equation of sixth degree in'gga is obtained, 6 It a, a» w w __ ._ (ag1'A(m,1+B(zs,1 C-O- <12) The coefficients A, B, and C are functions of the physical constants of the vibrating system, namely, 031, 002, (.03, ft, and I, . They are: A=('+#1%;1*('3‘:1* v+I . <13) ‘ w .‘ §1+F(%, + VS“ 1. (11+) IS ashes-:1) 334a)? 8 I. I. 3.316%: . (15) An equation like (12) must have six roots, but only three of them are different numerically; that is, in absolute value there are only three roots. This is true because equation (12) can be written, {—Aaf-o-sa-c-o , (16) by letting, '- 1(E§:)IISK . Thus it can be seen that a system with three degrees of free- dom has three distinct values of the driving frequency for which resonance occurs. Since the coefficients of the reso- nance equation (12) are functions of the physical constants of the system, the location of the resonant points can be varied by changing the masses and spring constants in the system. I Now assume that the roots of equation (16) are 5"1’ A2, and 9s 3. Then the following relationship between the coefficients A, B, and C and the roots A1, $2, and 9&3 must be true, A: A. +AL+75$ , (l7) B=7M31+Atzt+atxt ) (l8) c=7\u7\z7‘s ' (19) ,4. 4 From equation (19) it can be seen that 2-3-3'30 can never be a resonant point, since that would require constant C to be zero, but for the system to be of three degrees of freedom C must be different from zero as can be seen from equation (15). Igigonometric Eorm.g£ Solution A trigonometric form of solution of equation (16) will now be attempted. First, to eliminate the quadratic term let, A=z+£§ - Then, 23+3Hz+G=0. (20) where, H: 35 ‘Al ’ ' TE) (21) and, G__AB-ZA::_Z'IC ’ - 21 and A, B, and C are as previously given by equations (13), (22) (1%), and (15). From physical considerations it can be assumed that the roots 7s 1, A2, and 9x3 of equation (16) and therefore also' fill, .252, and 153 of equation (20) are distinct positive real quantities. Hence the dis- criminant 42; of equation (20) must be positive. The discriminant of a reduced cubic equation is, I. A=-Z'1(G +4H’) . 1- 5 Therefore, €3‘*‘LF44{(> and since G2 cannot be negative, H must be, from which it follows that A">55 . This is one condition that must be fulfilled in order for a system with three degrees of freedom to have three resonant points. It can be proven that the solution of equation (20) can be given in a trigonometric form* as, MI(‘2?W' +2.1“! z= 2.\/-H C05[ *5 vfi] 3 (231 where kao,l,2. . Substitution of equations (21) and (22) into equation (23) yields, 1 1 (aAe-zA-z'IcNA-ss ] ONCOSE' ' ch -35 +ka k.o,u,z . *Conkwright, N. B. Introduction to the filheorz 26. Equations. New York: Ginn and Company, 19 1. 10 A Noting that A=E+fl5 , this expression gives the three desired roots of equation (16), 3 $2» ~2A-21c2th- as 9. 5%- A"-56 cos 1 3 + g 3 (21+) o,[_(fl-ZK-21€2‘\/At-bb 3+2 m=§ 555 c05{ 2. g“ r +55- <25) a"m[_caAs-2A‘-z1c)\IN-SD 4" ' aft—3'; ZOE-351‘ ?\gprir l‘fl' C:H5 31 Thus it can be seen that the location of the resonant points represented above by equations (2h), (25), and (26) are de- pendent on all the physical constants of the system ‘0 1, m2, and (03, ft , and 9’ , but the functional relationship between them is extremely complicated. Since the cosine funtion cannot be larger than plus one or smaller than minus one, for any given system the maximum and minimum values of resonant frequencies are, 11 A+ m“('é's1n:+5 AI,” ’ (27) min(€,—Q ;%-% A"- 35‘ . (28) Conditions £2; T g Resonant Frequencieg £9 be Equal Now let us look at the possibility of making the roots of the resonance equation equal. Assume that the roots of equation (20) are 21 and 32- 2.3. The latter can be true if and only if the discriminant 1:5 is equal to zero, that is 6241-3. 0 . (29) In equation (23) let ¢IOVCCOJ- am 3 2 but H3 '“- ~%-; if there are repeated roots, therefore, (baarccos- G sarccostl, 20‘s 12 depending on whether G is positive or negative. If G>O , ¢=arccos -| ; and «ts-11' . If 640 ; ¢=GPCCOS+l ; and (P80 . Therefore, if there are repeated roots of the resonance equation (16), they must be, 9\,=—% At-SB +% a .._L 1-... A (30) we +1;- : semi-(#535 +% . <31) If 6O’ or E5=>() ,*which is always true, since all quantities in equation (1%) are positive. Distribution g;_B§sonant frequencies To see the effect that variation of the physical constants of the system have upon the distribution of the resonant fre- quencies, as given by equations (2%), (2S), and (26), differ- ent values of 001, (02, Q3, ,4. , and I, must be substi- tuted in the equations (13), (1%), and (15) to obtain the quantities A, B, and G. Then in turn the known A, B, and C must be substituted in equations (2%), (25), and (26); and the variation of 7N1, $2, and $3 noted. However, due to the complexity of the expressions for the resonant fre- quencies, and also quantities A, B, and C, the substitution and computation would be a very tedious andlengthy process. Therefore, let us try another method of approach. Equations (13), (1%), and (15) give the quantities A, B, and C (they are coefficients of the resonance equation (16)) in terms of the physical constants of the system. Equations (17), (18), and (19) give the relationship that must exist between the roots of the resonance equation 9‘1, $2, and 1h 9K3 and A, B, and C. Therefore, the right hand sides of equations (13), (1%), and (15) may be equated to the right hand sides of equations (17), (18), and (19), respectively, because the left hand sides of the above equations are also equal, respectively. This gives: 7s,+7\,_+ 7x3=(l+-,!;)(%j-+ 3:14-04- | (32) “schemes -(""r~)(" mg)+(‘a’;)+(a,3¢$—5 0' “3'1. aJa'L +7r(a's,)+"(‘5.) , (33> .....,=(%;a§ . a) In this system of three simultaneous algebraic equations, eight quantities are involved, namely, 75 1, $2, 7N3, cal, :02, 003, It , and P . By assuming five of them, the other three can be uniquely determined. Suppose we assume values for $1, 002, O0 3, ’4. , and 9 and solve for 00 l, 9K2, and $3. From equation (31+), %)=u $.7V19\ (”1) ' (35) 15 Substituting this expression in equations (32) and (33) and solving for 2 3 yields, mm:- "9'” [@‘j—AJ - <36) (I’N)i(|+fi)%3-I 41%? Substitution of equation (36) into equation (35) gives the .923. unknown (‘95 , (a 1'_ N-P- ”[95:31:- 3"] p , (37) w’ —(":;l('+fi)J L531} 71" It can be seen from equation (37) that an interesting rela- tionship is obtained if 5N.1 is chosen to be unity; then, 1 't 6' =’£[|— 93-")1 . (38) no t ’ Recalling that 12%) , man means that con-w,- Hence, if the frequency of the driving force is equal to the natural frequency of the mass M3 and spring F(3 considered alone, then the equation (38) holds true. Substituting in this equation the previously defined quan- tities of C01, C02, and It we get, 16 I. k.--k.,+ M103, . <39) as 1‘ Knowing 23;) from equation (37) the quantities A, B, and C may be computed. Then by knowing A, B, and C the other two resonant frequencies may be found from the equation, M-(m-Auflm—Af-‘i-i 2- (40) where using the plus sign before the radical gives one root and the minus sign the other. Thus this inverse method of approach gives eXpressions that are simpler for variation and computation of the para- meters. Numegical Examples Now let us apply the inverse method developed in the latter part of the previous section to obtain results in graphical form. ‘1 First, parameters 9 and (a) were chosen as variables and the resonant frequency ratio 7\, was held constant at 1.1. The parameter t, was chosen to vary from 0.01 to 0.1 and (%:)t was given the values 0.01, 0.02, 0.03, and 0.0%, successively. Figure 2 shows the results obtained for the variation of the reso- nant frequency ratios; and Figure 3 the variation of l7 frequency, and spring characteristics of the mass Ml. Figure 2 shows that the middle resonant frequency ratio is quite sensitive to variations in t, , but its variation is negli- '1 act. VODL gible with respect to (“’3 , at least up to‘@ 20.04- . 0n the other hand, the lowest resonant frequency ratio seems to have no variation with respect to I, , but it increases 1. with increasing values of (agi' . Figure 3 shows that the I. parameter 94) , along with the spring constant k1, is 9 greatly- influenced by variations in I, , but the variation of @553 has negligible effect on both of them, at least in the range of values used. Second, for two different values of the parameter I) (9:0.l and 1’20.06) , the parameter [It was made to vary from 1.5 to 2.5. The effect of this variation on the resonant frequency ratios is shown in Figure h. For the lowest resonant frequency ratio, the magnitudes for it, obtained by the two different values of the parameter I’ were so nearly equal that it was impossible to draw two separate curves. 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I H . 1. ¢ .- . -..- .+. .. I. . ..I. ..... -. - . ... .«. .......9I ..o.. - . . . .. -....- .. .. . . . +.- ... ... v .....4 . . ‘. . . .a .. I ..w.. r. .. ... A y... . . .. .. .. I a .. . ... 4 ...-9. ... «. .vA.v..mA_U .2013: IUIDQmFFIu >Z(O.IOU PhomJJm (K" H.1(Iu 1.42. . 0» ON x ON 18 Conclusions From this study of a vibrating three mass system it may be concluded: I. A direct method of solution of the sixth degree algebraic resonance equation, where the solution is expressed in a trigonometric form, offers good possibilities for com- puting the resonant frequencies of any given vibrating system with three degrees of freedom. 2. It appears to be possible to have a vibrating system with three degrees of freedom that has only two distinct reso- nant frequencies; that is, the resonance equation may have repeated roots. However, it seems difficult to devise such a system, because of the extremely complicated relationships that the physical constants of the system have to satisfy. 3. An inverse method of solution gives simpler relation- ships between the resonant frequencies and the physical con- stants of the system than the trigonometric form. The ex- pressions obtained by the inverse method are well suited for determining the effect that variations of the physical con- stants have upon the distribution of the resonant frequencies. H. A notable simplification in the relationships between the resonant frequencies and the physical constants of the system is obtained by making the frequency of the driving force equal to the natural frequency of mass M3 and spring k3 alone . l9 BIBLIOGRAPHY Conkwright, N. B. Introduction to the TheOry g§_§guation . New York: Ginn and Company, 195T. Thomson, W. T. Mechanical Vibrations. New York: Prentice- Hall, Incorporated, 1958. Timoshenko, S. Vibration Eroblems in Engineering. New York: D. Van Nostrand Company, Incorporated, 1937. Timoshenko, S., and D. H. Young. Advanced Dynamic . New York: Mc Graw-Hill Book Company, Incorporated, l9h8. ( Haw h “M. ., ‘arm