ML20100P354
| ML20100P354 | |
| Person / Time | |
|---|---|
| Site: | Shoreham File:Long Island Lighting Company icon.png |
| Issue date: | 09/17/1984 |
| From: | AFFILIATION NOT ASSIGNED |
| To: | |
| References | |
| OL-I-014, OL-I-14, NUDOCS 8412140147 | |
| Download: ML20100P354 (5) | |
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Teesional Vibentlen
( 7 pnf \\,Y A. Notweal (Teesional) Feequency et ais}as.E l
Hystem-Torsiossal tibratioes occurs in any rotating mass elastic system where perind.c forces are present.
A mass. elastic system consists basically of two or more anasus connecteel by an elastic shaft. If one end of this
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systens is twid risul, amt the other end twisted al.out its amis asial then released, the free mass or masws will oscillate aisi.t tlw amis at a certain frequency called the natural frev)uency of torsional vibration. The r.atural frequency et toessonal vibration is determined by the mass distribution and the tofsional elasticity between masus.
These vibrations may cause dangerous stresses in the
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syssem if the frequency of the periodic forces which eacite these vibrations occurs in, or near, resonance with the natural frequency of the system.
Lt' B. Teesiemal Critical Speed-The mass elastic system described in alw previous paragraph involves a nontosating v
situation but, if the system were free and rotating, the natural freituency would be the sanw. In actual shaft f
systense, the situation is considerably more complica'ed.
Periodse impulus are applied to the e 5gine crankshaft at each of the cranks. When the frequericy of these im.
pulses, or some harmonic thereof, equals the natural frequency of the system, resonance occurs. At resonance, the amplitude increases with corresponding increase of shalt streseen. The rotative speeds of the engine at which resonance oreurs are known as torsional critical speeds.
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C. Teesional Vibesteer Systems Characteristico-A critical speed may, oc snay not, be dangerous, dependieg upose the forces intelsed and the arrangement of the vibeasery syseems. Samme ei the excitation forces whwh eemse vibrations seiny coner from the engine but others may conw from tlw driven equipment. Although it is not possible, within the secpe of this boolr. to undertake
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a treatise on metho<ls of torvonal analysis. it is important that every installation be analysed during the design stages to efetermine critical speeds and torsional stresses on com.
ponents at these speeds. This can be done readdy by established calculation procedures. Such esiculations must be based on complete design data covering the mass.
elastic system.
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Thew ilata iew hale :
- 1. All shafting in'ormatines, sigh as physical character.
istses of materials. tent;ths. shameters, etc.
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- 2. leension of rotating nunes along the shaft fength.
- 3. WK' values of rotating masws (and I', safues f.
l generatuts).
- 4. hiethod of securing masses to shalting, lightness i 60s lengtle of 64. Leyways, bolt anel dumet sises, eti S. Arrangement. type, beation, and torsional sti#ne.
(actees of clutches and couplings.
- 6. Nasure e( the load.
- 7. Speed and lead ranges tequired.
Serb analysis makes possible alie prediction of oper ation that is free of harmful torsional criticals or indi cases a need to mobe desat[n changes that will alleviate
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a terseenal stresa problem. The mass. elastic system mus be considered as a whole and, therefore, the e#ect of a:
elements such as gests, couplan,;s pumps, generaw retors, and connecting shalting, meat be taken into ac count.
Centa introduce a fatter in the torsional vibrat'or problem becaesse of the, vibratory tooth leading. Cear
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chatter er teeth separation will occur if the supefim.
pond eibenting torqtse eneweds the transmitted torou.
A,,bessery torque of a system, which may not be g-eneegh to cause shaft damage or tceth separation, rr still cause consaderable torque variation on the gear tootn.
and such cases should be speci6cally si'pfosed by the gear snanufacturer.
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Coupleags may have considerable eMect on the torsionst natural frequencies of a mass. elastic system. For instance.
coupimes of the same torque rating may haie a wide range ei mass and seesional stiMness safues. For this renamn, ech are further seleeted to provide the most estesiersery torsional sness elastic thate:teristics. In fact, some electrie oc hydraubt couplings, because of theit inhecent slip features. isolate the machinety elements on
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either side of the slip coupling from each other, and separate the entire system into two independent torsional systems.
D. Design Objeettees and Criterie-In variable spe f installations the objectise is generally to prmide an erwr.
sting speed range as fru as possible from liarn.le I totssonal vibratory stresws in the tante from 10 pr eene
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below minimuni operating speed to 10 pt cent at ne the mamienum rated operating sped. In the case of cen.
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stant speil units. siwls as genef ator was, the objectne is to insure that no harmful torsional uteratory stresws accur within hie lve cent shore anil below the rated sieed.
For erankshafts, connecting shafis, flange or couphng conipenents, etc., enade ed conventional materials. toes.
ional vehraseaty consistions shall generally tw considered sale when they induce a supeeimposed stress of less than M00 psi, eteated by a single order el uheateen. or a sug s, mi stress of less than 7000 psi, created by the senwnation of the nisiet orders of ubration which might come inee phase periodicady.
For the case of shalt elements estiously known as
" quilt shells," " tuning shafts. or "teesionally resshent torque shafts " and other elements whwh are speci6cally
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desagged let the application, and manulutured from material of adetuale physetal propecties, with careful attenteen te design and machining e( beyways, 61Lets.
etc., superimposed sibratory stresses at mwh hightf leiels may be acceptable. The design of such elevvwfus is always estrelated in the teeseenal analysis. In the case ei vibessery teeque acrose gears the gear mano.
fattestee shalt review the setsional analysis and has full attepeance thereof is the purchasee's best guide to the adequacy ei the gear design for the service intended.
E..Medleestion of the Maas.Elastle System-It is usually imptutstal to onstaen all the reqmted data to ninke a cenop.'ete torsional analysts befute a proposal heuenues a centreet. However, w hen the complete analysis is nnally made, the enistence of any harmful N
J toes enal vibrasson will become apparent. The solution leasieng to the neeestacy alterations in the system can l
then be woebed ses. At this tiene. negotiations may be necessaey to ansend the conteact.
F. Caleeletiene and fleepensibility-The responubility lot the preisstation ei a torsional analysis usually de.
j voltes uten the engine manuluturer, even el he does not lurmsat senw. or any, el the deisen enmpment. The accurney of the results of the engirie manufxturer's calculaisons depends upon the ucutwy of the infwma.
twas suppleed to him asul obviously his respons.
limited accordingly, if inweurate or inerwnpicte intu<...
tives is furnished to the engine manufxturef, changes may be sweestaty to make a satisfutory installation and
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ashle' entiente anal delay enay result. When the engine manufnetutee furnislws the denen equipment, he 39 l
aumes restemsileihty fut us gafformanes wuh reyect l
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- to torsional tibratinn. If he locs not furnish the dri en
- i y, ":*. t equipnient,Iw shoeild be responsible only for t'.e accuracy of slw calculations and fnr the engine. but not for the
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perfornance of the drisen equipawns. The engine manu.
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facturer should not ue responsible for vibrations thar are emeited by sources esternal to the engine. Alterations.
additions to, or renoval of equipnwnt from an approved N
torsional system should not ne made without nest con.
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sulting etw engine manufacturer as to the effect the change would have on critical speeds. It is, likewise.
evident that the operating speed range of a unit should not be changed without eensulting the engine manu.
facturer, as the moulined speed range niay include critical speeds or come close enough to one or more of them to be dangerous.
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Aslal Shaft Vibrellon-Crankshafts base a natural asial frequency. When torques are appliert to a crankshaft, it is alternately shortened and lengthened. Emperience has shown that these motions become t.ra':
.w only when an asial natural frequency is cloes to a torsional frequency. When this happens, large asial shalt vibrations, sometimes called longtitudinal vi.
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beations, are poseeble. Failures frons this conditen are rare.
though the force from the asially viberting shaft may cause vibra.
tion and noise at other points in alw installation. Asial shaft vibration of a troublesome nature is suf5ciently rare to ma's calculation unwarranted in most esws and is mentioned here to complete the subject of vibration possibilities.
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