Mechanical Vibrations - IMP Oral Questions. Balancing

Transcription

1 Mechaical Vibratios - IMP Oral Questios Balacig ) What is balacig? As: Balacig is the process of removig either partially or completely, the effect due to resultat iertia forces ad couples (ubalace) actig o machie parts. ) Explai the eed of balacig i high speed egie? As: The static forces (force actig o statioary mass) as compare to dyamic forces (Cetrifugal forces ad Iertia forces) are very high. Fc=m r w Therefore for small icrease i agular velocity w There is large icrease i Force Fc. This force is havig tedecy to lift machie from foudatio. It is realised that o foudatio will ormally be able to withstad force of such high magitude. Hece to avoid upleasat effects, precise balacig is essetial. 3) Explai the static ad dyamic balacig. As: Static Balacig- A system of rotatig masses is said to be i static balace if combied mass cetre of system i,e C.G. lies o axes of rotatio. For the system to be statically balaced, the resultat of all dyamic forces actig o system durig rotatio must be zero. This is sufficiet coditio for balacig, if sigle or several masses are actig i same plae. Durig rotatio axis passig through C.G. of mass is parallel to axis of rotatio. Dyamic Balacig-. For the system to be dyamically balaced the resultat of all dyamic forces actig o system durig rotatio must be zero ad resultat couples due to all dyamic forces actig o system must be equal to zero This is sufficiet coditio for balacig, if several masses are actig i differet plae. Durig rotatio axis passig through C.G. of mass is ot parallel to axis of rotatio. 4) Explai balacig of sigle mass rotatig i sigle plae. As: There is formatio of oly ubalaced force, ot couple. So balacig is doe by addig balacig mass mb at radius rb ( Ubalaced Forces) = 0 m r = mb rb 5) Explai balacig of sigle mass by two masses rotatig i differet plaes. As: There is formatio of ubalaced force as well as couple. So balacig is doe by addig two balacig masses. ( Ubalaced Forces) = 0 m r = mb rb + mb rb Page of 7

2 Mechaical Vibratios - IMP Oral Questios ( Ubalaced Couples) = 0 mb rb lb = mb rb lb 6) Explai balacig of several masses rotatig i sigle plae. As: There is formatio of oly ubalaced forces, ot couple. So balacig is doe by addig balacig mass mb at radius rb i same plae of rotatio. 7) Explai balacig of several masses rotatig i differet plaes. As: There is formatio of ubalaced forces as well as couples. So balacig is doe by either addig two balacig masses i two differet plaes or balacig is doe by adjustig agular positios. ( Ubalaced Forces) = 0 m r + mb rb + mb rb = 0 ( Ubalaced Couples) = 0 m r l +mb rb lb + mb rb lb = 0 8) What do you uderstad by primary ad secodary balacig of reciprocatig masses? As: I reciprocatig masses ubalace is due to Iertia force ad is give by, FI = m r w [ cosθ + cosθ ] = m r w cosθ + m r w cosθ = FP + FS FP = m r w cosθ This compoet of FI is called as Primary ubalaced force FP ad FS = m r w cosθ This compoet of FI is called as Secodary ubalaced force Fs. 9) Explai partial balacig of reciprocatig egie. As: I reciprocatig egie ubalaced force is actig alog stroke lie, but the balacig mass is rotary ad actig o crak ad havig two compoets oe is horizotal ad other is vertical. If Horizotal compoet is balaced vertical compoet remais ubalaced ad if vertical compoet is balaced, horizotal compoet remais ubalaced. So it becomes ecessary to balace reciprocatig egie partially. 0) What is equatio for ubalace force due to reciprocatig mass? I reciprocatig egie ubalaced force is Iertia force FI = m r w [ cosθ + cosθ ] ) Give equatio for primary ad secodary ubalace forces. FP = m r w cosθ Primary ubalaced force FS = m r w cosθ Secodary ubalaced force ) What is mea by primary crak ad secodary crak method? The Primary or Direct crak ad Secodary or Reverse crak method is used to balace Radial or V-egie. Parameter Primary Primary Secodary Secodary Page of 7

3 Mechaical Vibratios - IMP Oral Questios direct reverse crak direct crak reverse crak crak Crak Radius r R r r 4 4 Agular Positio θ -θ θ -θ Agular Speed w W w w 3) Explai V egie, W egie ad radial egies? V-egie- It is a two cylider radial egie i which coectig rods are fixed to the commo crak. W-egie - It is a three cylider radial egie i which coectig rods are fixed to the commo crak. Radial egie - It is a multi-cylider egie i which coectig rods are fixed to the commo crak. I above egies crak ad all coectig rods are rotatig i oe plae. So there is formatio of oly ubalaced forces ot couples. 4) What is firig order? It is order i which firig takes place i a multi-cylider egie is called firig order. Best suitable firig order is that order which gives miimum ubalaced force/couple. ***** Page 3 of 7

4 Sigle degree of Freedom system Mechaical Vibratios - IMP Oral Questios ) What is vibratio? Ay motio which repeats itself after a iterval of time is called vibratio or Oscillatio. Geerally it is to ad fro motio. ) Metio the uses of vibratio. I the brach of egieerig, vibratio is useful i the aalysis, desig, costructio, operatio ad maiteace of complex structures. 3) What are desirable vibratios? Vibratio is occasioally desirable. For example the motio of a tuig fork, the reed i a woodwid istrumet or harmoica, or the coe of a loudspeaker is desirable vibratio, ecessary for the correct fuctioig of the various devices. 4) Defie ad give uit. a) Frequecy Number of cycles per uit time is called frequecy. Uit- Hertz b) Period It is time required to complete oe cycle. Uit - Secods c) Amplitude It is maximum displacemet from mea positio. Uit - mm d) Simple harmoic motio (SHM) A periodic motio of particles whose acceleratio is always directed towards mea positio ad proportioal to its distace from mea positio. e) Stiffess of sprig. It is force per uit deflectio. Uit- N/m f) Degree of freedom (DOF) It is miimum umber of idepedet parameters require to defie a system. It is uit less. g) Natural frequecy- Natural vibratio refers to mechaical oscillatios about a equilibrium poit. The oscillatios may be periodic such as the motio of a pedulum or radom such as the movemet of a tire o a gravel road. It is frequecy of free vibratios of a system. h) Damper ad its types. Damper is device used to create resistat to the motio of vibratig body. Types- Viscous dampig, Dry frictio or colombs dampig, Material or Hysteresis dampig. i) Equivalet sprig stiffess I may practical applicatios more tha oe sprig may be used. To covert such system ito equivalet mathematical model, it is ecessary to covert sprigs ito equivalet sprig. Equivalet sprig stiffess for Sprigs i series K e = K + K Equivalet sprig stiffess for Sprigs i Parallel K e = K + K 5) Classificatio of vibratio with defiitio. Accordig to differet criteria vibratios are classified as, A] Accordig to actuatig force Free ad Forced vibratios. B] Accordig to exteral resistat to vibratig body Udamped ad Damped vibratios C] Accordig to motio of system w.r.t. axis Logitudial, Trasverse ad Torsioal vibratios Page 4 of 7

5 Mechaical Vibratios - IMP Oral Questios D] Accordig to behaviour of vibratig system Liear ad No-liear vibratios 6) Why is it importat to fid the atural frequecy of a vibratig system? Whe the frequecy of exterally excited system equal to atural frequecy of vibratio system it get failure due to resoace. So to avoid the resoace at vibratig system atural frequecy must be kow. 7) What happes to the respose of a udamped system at resoace? I udamped vibratig system; the system get vibrate till it s frequecy reaches to the atural frequecy. So it likely cause to failure of body. So if system is havig udamped vibratio it leads s to failure of body or system. 8) Calculate equivalet stiffess of the sprig for the system show below, which has sprig stiffess of 3000 N/m As 50 N/mm 9) If oe sprig of stiffess K, cut i two equal parts, what will be the sprig stiffess of each part? As K 0) What is D Alemberts priciple? Accordig to D Alemberts priciple,a body or a system which is ot i static equilibrium due to acceleratio it possesses ca be brought to static equilibrium by itroducig iertia force o it. This iertia force is equal to mass times acceleratio Hece, [iertia forces ad exteral forces]=0 ) Explai Eergy method ad Releigh s method. Page 5 of 7

6 Mechaical Vibratios - IMP Oral Questios Eergy method It is based o eergy method, I free udamped vibratio o eergy is added or removed from system, but coversio of Kietic Eergy to Potetial Eergy ad vice versa. Therefore at ay momet total eergy remais costat. [ KE + PE ] = Costat Releigh s Method- Accordig to Releigh s method maximum KE at mea positio is equal to maximum PE at extreme positio. KEmea positio = PEextreme positio m x = K x ) Give two examples each of the bad ad good effects of vibratio Bad effects. Proper readigs of the istrumet caot be take. May buildig, structures ad bridges may fall Good effects:. Useful for the propagatio of soud. Vibratory coveyors 3. Musical istrumets 3) Defie degree of freedom of a vibratig system The miimum umber of idepedet coordiates required to specify the motio of a system at ay istat is kow as degrees of freedom of the system 4) I vibratio aalysis, ca we always disregard dampig? No 5) Ca we idetify a oliear vibratio problem by lookig at its goverig differetial equatio? Yes 6) What is the differece betwee determiistic ad radom vibratio? I determiistic the magitude of excitatio force is kow but i radom magitude of excitatio is ot kow. 7) What methods are available for solvig the goverig equatios of a vibratio problem? Rayleigh method, eergy method, ad equilibrium method 8) How do you coect several sprigs to icrease the overall stiffess? By coect sprigs i parallel 9) Defie with equatio. i) Over damped system. - Whe dampig factor ζ >, dampig co-efficiet c > cc the system is said to be over damped. ii) Uder damped system. - Whe dampig factor ζ <, dampig co-efficiet c < cc the system is said to be over damped. iii) Critically damped system - Whe dampig factor ζ =, dampig co-efficiet c = cc the system is said to be over damped. iv) Logarithmic decremet - It is atural logarithm of ratio of two successive amplitudes o the same side of mea positio. 0) Defie Page 6 of 7

7 Mechaical Vibratios - IMP Oral Questios a) Dampig factor - It is ratio of critical dampig co-efficiet c to the critical dampig co-efficiet cc. b) Dampig co-efficiet - It is ratio resistig force o vibratory system to the velocity. c) Critical dampig co-efficiet Critical dampig coefficiet cc is that value of dampig coefficiet c at which frequecy of damped vibratio is zero ad motio is aperiodic. ***** Page 7 of 7

8 Mechaical Vibratios - IMP Oral Questios Page 8 of 7 Sigle DOF Forced Vibratio ) Defie with the equatio followig terms a) Forced vibratios If the system vibrates uder the actio of exteral periodic force or impressed force, the vibratios are kow as FORCED VIBRATIONS. Eg Vibratios i compressors, IC Egie vibratios Differetial equatio of motio of forced vibratio t e m kx cx x m si 0 b) Magificatio Factor It is ratio of amplitude of steady state vibratio to the steady state deflectio M.F. = X X st c) Amplitudes of steady-state vibratios 0 ) ( m m e X d) Phase agle ta e) Agle of lag ta ta

9 Mechaical Vibratios - IMP Oral Questios Page 9 of 7 f) Force Trasmissibility - is defied as ratio of force trasmitted to the supportig structure or foudatio, to that force impressed upo system. o T r F F T g) Motio Trasmissibility - - is defied as ratio of motio trasmitted to the supportig structure or foudatio, to that motio impressed upo system. Y X T M.. = ) ( h) Relative steady state amplitude. Y z i) Steady-state absolute amplitude. ) ( / k c k Y X ) Explai vector diagram of graphical represetatio of forces 3) What is complemetary fuctio (Xc) ad particular itegral (Xp). Complemetary fuctio(xc): It is obtaied by cosiderig o force actig o body. i.e. 0 kx cx x m This equatio is same as obtaied for damped free vibratio system. The solutio of equatio is,

10 Mechaical Vibratios - IMP Oral Questios Page 0 of 7 ] si[ t e X X t C Particular itegral (Xp): Particular itegral is obtaied by ay of two methods. ) Aalytical method (differetial equatio method) ad ) Graphical method. 0 ta ) si( ) si( p p k t F X t X X 4) Explai followig curves. a) Frequecy respose curve, Magificatio factor Vs Frequecy ratio. The magificatio factor is maximum whe w/w= This coditio is kow as resoace. As dampig factor decreases, the maximum value of magificatio factor icreases. Whe there is o dampig =0 M.F. reaches ifiity at frequecy ratio equal to oe. At zero frequecy of excitatio w=0, the M.F. is oe(uity). At very high frequecy of excitatio, M.F. teds to zero. For dampig factor more tha 0.707, M.F. is below uity.

11 Mechaical Vibratios - IMP Oral Questios b) Phase agle Vs Frequecy ratio. The phase agle varies from 0 0 at low frequecy ratio to 80 0 at very high frequecy ratio. At resoace frequecy i.e. w = w the phase agle is 90 0 ad dampig does ot have ay effect o phase agle. 5) What is Vibratio isolatio ad explai types of vibratio isolators. Vibratio Isolatio: It is process of reducig vibratio of machie ad hece reducig the trasmitted force to the foudatio by usig isolatig material like sprig rubber pads etc. Mai aim is either protect machie from groud excitatio or to protect structures 6) Explai a) Quality factor ad b) Bad width We kow, Magificatio Factor is ratio of amplitude of steady state vibratio to the steady state deflectio M.F. = X X st = ( ) The amplitude ratio at resoace is called quality factor or Q-factor i,e at w=w Q = X X st = Let r ad r are the frequecy ratio where amplitude or magificatio ratio falls to Q/ are called half power poits Page of 7

12 Mechaical Vibratios - IMP Oral Questios The differece betwee frequecies associated with the half power poits r ad r is called Badwidth. 7) What is critical speed of shaft? The speed at which the shaft starts to vibrate violetly i the directio perpedicular to the axis of shaft is kow as critical speed or whirlig speed 8) What are rages of shaft speed? Which is safe rage i actual practice? Rages of shaft speed ) shaft speed(ω) < critical speed (ωc), ) shaft speed(ω) = critical speed (ωc), 3) shaft speed(ω) > critical speed (ωc). As shaft speed icreases, the cetre of gravity of rotor G approaches the axis of rotatio O ad rotor rotates about its C.G. This priciple is used i ruig high speed turbies by speedig up rotor rapidly beyod the critical speed. So whe ω>>>>ωc the rotor rus steadily. ***** Page of 7

13 Mechaical Vibratios - IMP Oral Questios Two Degree of Freedom System - Damped Vibratio 9) Explai two degree of freedom system with example. Examples of DOF systems Elastic pedulum (radial ad agular) Motios of a ship (roll ad pitch) 0) What is priciple mode shapes? The motio where every poit i system executes harmoic motio with oe atural frequecy is called as pricipal mode of vibratio or atural mode of vibratio. A system with two degree of freedom ca vibrate with two pricipal modes of vibratios correspodig to its two atural frequecies. ) What is free torsioal vibratio of two rotor system? Free Torsioal Vibratios of Two Rotor System a)whe two rotors rotates i same directio - If two rotors rotates i same directio, the shaft is said to vibrate with zero frequecy. Such behaviour is called as zero ode behavior. The amplitude of vibratio at both eds will be i same directio. (b)whe two rotors rotates i opposite directios. There is a poit or sectio of shaft which remais utwisted. This poit or sectio where amplitude of vibratio is zero is kow as ode poit or odal sectio. ) What is free torsioal vibratio of three rotor system? Free Torsioal vibratios of Three Rotor System - Two ode vibratios :- Whe middle rotor rotates i opposite directio of outer two rotors, the torsioal vibratios occurs with two odes. Sigle ode vibratios :- Whe oe of the outer rotor rotates i opposite directio of other two rotors, the torsioal vibratios occurs with sigle ode. Page 3 of 7

14 3) What is torsioally equivalet shaft? Mechaical Vibratios - IMP Oral Questios I two rotor ad three rotor system, it is assumed that diameter of shaft is uiform. But i actual practice, shaft may have differet diameters for differet legths. Such shaft ca be replaced by theoretically equivalet shaft of uiform diameter. All the shafts are replaced by a sigle shaft (equivalet shaft) of uiform diameter (de ) ad legth (le ). l e d l l ( ) d l ( ) d d3 ***** Page 4 of 7

15 Mechaical Vibratios - IMP Oral Questios Vibratio Measuremet ad Cotrol ) What is importace of vibratio measuremet? To measure of atural frequecy of machie/structure, it is useful to select operatioal speed of machie to avoid resoace coditio. To desig ad develop vibratio isolatio system, it is ecessary to kow atural frequecy of vibratio ad forces actig o machie. To kow groud vibratios due to earthquake wid velocity fluctuatios o structures ad buildigs etc. ) Classificatio of vibratio measuremets a) Based o cotact betwee vibratig system ad istrumet Cotact type istrumet o cotact type istrumet b) Based o requiremet of power source Active istrumet Passive istrumet c) Based o method of measuremet Idicatig type istrumet Recordig type istrumet 3) What is use of followig istrumets a) Vibrometers used for amplitude measuremets b) Velometer used for velocity pick up c) Accelerometers - used for acceleratio pick up d) Frequecy measurig istrumets. to measure frequecy Eg Frahm tachometer ad Stroboscope 4) What is vibratio exciter ad its types? Exciters are used to give required cyclic excitatio force at required frequecy. It is used to study its dyamic characteristics Types a. Mechaical Exciters b. Electrodyamic Exciters c. Hydraulic Exciters Page 5 of 7

16 Mechaical Vibratios - IMP Oral Questios 5) What is FFT? Give its types? Sigal represeted by a equatio or graph or set of data poits with time as idepedet variable is trasformed to frequecy as idepedet variable The mathematical set of data poits coverted ito spectrum usig Fourier trasformatio program i digital computer Thus method of obtaiig spectrum is called Fast Fourier trasform (FFT) ad istrumet is called FFT Aalyzer. 6) What are various coditio moitorig techiques? a) Visual ad aural moitorig techique. b) Operatioal variables moitorig c) Temperature moitorig d) Wear debris moitorig e) Vibratio moitorig 7) What are methods of vibratio cotrol? a) Excitatio reductio at source b) Source isolatio c) System modificatio d) Active feedback system 8) What is vibratio absorbers? Explai its types. Whe exterally excited structure or machies vibratio is removed by attachig vibratory system, this vibratory system is called as Vibratio absorber or Neutraliser. Types a) Udamped Dyamic Vibratio Absorber b) Torsioal Absorber System c) Rig type Torsioal Vibratio Absorber d) Cetrifugal Pedulum Vibratio Absorber e) Bifilar Type Cetrifugal Pedulum Absorber f) Dry Frictio Torsioal Vibratio Absorber g) Utued Viscous Damper (Houdaille Damper) 9) What is vibratio isolatio? ad explai its methods. It is process of reducig vibratio of machie ad hece reducig the trasmitted force to the foudatio by usig isolatig material like sprig rubber pads etc. Mai aim is either protect machie from groud excitatio or to protect structures Types a)passive Vibratio Isolatio No exteral power is used. e.g. Sprigs, Rubbers or Elastomers, Peumatic Isolators, Ribbed Elastomers. b) Active Vibratio Isolatio Exteral power is used for isolatio. These isolators are cosistig of sprig, mass, damper, actuator ad sesors Natural respose due to mass sprig ad dampers, while artificial respose due to cotrolled force give by loop of sesor cotroller ad actuators. Page 6 of 7

17 Mechaical Vibratios - IMP Oral Questios 0) What are referece stadards for vibratio moitorig techiques? Stadards are the criteria established by authority, custom or geeral cocet The various stadards used for vibratio moitorig ad aalysis are, a) ISO Stadards for Vibratio Moitorig ad Aalysis b) ISO Stadards for Evolutio of Vibratio Severity c) ISO Stadards for Vibratio Measuremets d) ISO Stadards for Traiig ad Certificatio e) Other Stadards for Vibratio Moitorig ad Aalysis ) Which of the followig methods ca be used to reduce excitatio level at the source? a. Lubricatio of joits b. Balacig iertia forces c. Both a. ad b. d. Noe of the above As Both a ad b ***** Best of Luck >>>>> Page 7 of 7