ML20100P363
| ML20100P363 | |
| Person / Time | |
|---|---|
| Site: | Shoreham File:Long Island Lighting Company icon.png |
| Issue date: | 09/17/1984 |
| From: | Jones F, Oberg E, Shigley J AFFILIATION NOT ASSIGNED |
| To: | |
| References | |
| OL-I-026, OL-I-26, NUDOCS 8412140156 | |
| Download: ML20100P363 (9) | |
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I MACHINERY'S H ANDBOOK i
v A Reference flook for she AfechanicalEngineer, Draftsman, Toohnaker and Afachinist By Emis Outsc ut:J F. D. Jones 1
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282 Fundamenesule..) niecisanical Unsists 2 The nature and magnitmle of the applied loud. Ilow was the load in general, it wouhl seem that if it.c seinimass properties, tu,L the average, obtained in the first place? Ily test? Or ley estimation of some kindt is of the material acre determined by many tests, amt if there is no deterions-the load a static one, a fatigue load, or a combination of the two? lias the Lion in strength during the lifetime of it.e part, a snargin of to to 15 per cent lessihihty of impact 1,cen considered? llave all the factors influencing the shoubt he sullicient allonance for the us. certainties in strength. Asul if the loading 1.cen consideretl? Many designers, for example, do su excellent job stressrs are accurately determined amt if the nature of the part is such that of engineering a new design and then fail to consider the lonels whicle are it canimt 1.c overlanded, or if the overled is known and accounted for, an allowance of 15 to 20 twr cent should ine sufficient to account for tiie oppheel to the equipment during shipment or sasembly.
- 3. lliscontiimitics and stress concentrations. Sometimes discontinuilles uncertaintics inn stress. These antil up to a margin of safety of about :10 whicli cause stress concentration are introduced after the drawings have per cent, or a....J uum factor of safety ol 1.30.
Iwen released by Lt.e design engineering department. These discontinuities When (langer tr., man life is involved, these recommendeal values r.houbt he increased. Ilut the ticsigner also has a nsweial responsitaility to Imild into may occur in priah.ction, assembly, or inspection, for a variety of reasons.
Instwetors, for example, may not appreciate the reasons for maintaining a his design extra safeguards to prevent failure. The principles of " fail-safe" can frequently he used; alternatively, one can employ reduntlant enemtwrs targe fillet rmhus.
in the structures to take the luail if another fails.
Each time a factor of safety is chosen it is necessary for the engineer to carrfully evaluate all the unknowns relating to strength and all those The range of factors of safety recommended in this imok (1.25 to 4 H) relating to starss. Even in the design of a single part the designer may differs sharply from many to lm foumi in handbooks on machinery, falui.a employ several ihfierent factors of safety because of the changes in these tion, aml mechanical engisu crisig, aiul tlic erader sliould be aware of diis dillerence. It is imt uncomnum, for instance, to see vnhics of n as high as uncertaintics for cach set of calculations. For example, it may he neccesary 20 recommemled for use. When the line print is atmlied, it is oftcu foumi to check a part for die possibility of a fatigue failure as well as for a statto that these recouunendations are haa.ed am using an average value of the failure. f)ne should not expect that the same factor of safety will be used for Imth these calculations; the reason for this is that the degree of uncer.
ultimate strength (not the yicht strength or cmlurance limit), with no corrections for size, surface finish, stress concentration, amt the like, anul tainly for the two types of failure may lie quite different.
ficIcetion of n.
Now that the reader knows the meaning of factor of that the stresses are only nominal stresses obtained without taking into consideration such factors as combined stresses, fatigue loads, and overknads.
safety, he is ready to ask the question, Ilow do I know when I have selected o suitable value? This is indeed a dillicult spicstion to answer.
Design Calculations. All stress espiations may he represented by the formula Ex wrience, more than anything else, teaches one what factors to choose.
One learns what an ample factor el safety is by having ilcaigned an itani of e - Cf(rs,ze,
,r.)F(Fi,F,..,F.)
(64) cepiipment that never fails. Himilarly, mye learns what an inadequate factor where C is a constant,1. are the dimensions of the part to be designed and of safety is by having des,gned immething which thics fail or which falls F. are th forca or loads awhd to W part. Fquation (6-4) is intendbl s.
i smm times. Industries and companies buihl up a background of experlanos l>e vfry Erncrd and can represent any kind of strras or stress component. n in this manner, which they can extrapolate for new designs.
normal stress, a sheer stress, a von Miscs stress, or a stress amplitude for Vidosic' states that commonly used factors of safety in bas.ic design vary g g g
gg from 1.25 to 4, depending on the uncertainties mvolved. Ile apphes these Eq. (6-4) in this lumk. Note that the right-hand side of the equation con-to the yield strength for ductile materials, to the ultimate strength for brittle tains the dimensions and the forces. In design, the forces are usually snuterials, and to the fatigue strength for parta subjected to fatigue loads.
ki% d b dimb m W k hmid hp d g h y Lipson' states that the stresses and strengths should lie carefully ami tions in the form of Eq. (6-4), and substitute in it the kamwn quantilirs leaving the appropriate symlnds ahsignating the dimensions to be found b
thoroughly determined, and then the factor of safety can he chosen froin the Now, in place of a substitute 3/n on the left-hand side of the equation
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'ange id ving 6N l.35n 620
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3 MECHANICAL DESIGN d
AND SYSTEMS HANDBOOK
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L HAROLD A. ROTHBART, Editor-in-Chief l
Dean, College of Science and Engineenng l'
Fairleigh Dickinson Univeredy
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Teaneck, New Jersey
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..s MECHANICAL DESIGN AND SYSTEMS HANDBOOK 18-4 f
iI3 if the uncertainti> J are great enough to cause severe weight, volume, or economic e{
penalttee. testing and/or more thorough analyses should be performed rather than p
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relytag upon very large factors of safety.
8 Typical values of design safety factors are:
f.s. = 1.25 to 1.5 for excepuonally reliable matettals used under controllable condi.
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tions and subjected to loads and streseos that esa be determined with certamty.
Used O
almost invariably where low weight is a particularly impcrtant considerauon.
f.s. = 1.5 to 2 for wellAnown msterials under reasonably constant environmental k
conditions, subjected to loads and stresses that can be determined readily.
f.s. - 2 to 2.5 for average matermis operated m ordanary envtroaments and sub-jected to loads and stresses that can be determined.
f.s. = 2.5 to 3 for less tried as well se for brittle materials under average conditions of environment, load, and stress.
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f.s. = 3 to 4 for untried materials used under average conditions of environment.
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3 load, and stress.
f.s. = 3 to 4 should also be employed with bet er-known matertala that are to be 1
used in uncertain environments or subjected to uncertam stresses.
f.s. = 2 for impact of very ductde materials where the omsil tader of sensitivity results in low streso concentration factors.
f.s. = 1.5 for less ductile materials where a higher seesttivity will provide a larger i
factor of strese concentration.
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f.s. = 1.5 for danism as hasher temperaturen. based on the creep strength of the materal thas wdl ruumia inn a permanible pummame dudmmunassen over a preestablished life
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permd.
1 Nors: 1. For repeated loads, the factors of safety established are acceptable but j
must be applied to the endarance limst rsdmar than the yield strength of the mater:al.
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- 2. For causames, faeiomen. -m sad wehend examponents, factors of safety S
bare ammd de ans emmmay very..
' i h iemmme ymumented above.
- 3. Factors of safeey cm he used wish sammderd demian e6mnents, commeretally avad.
f
' imr the by sediminin manufacturers and/or by
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established codes for design of machines.
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- 4. Where higher factors of safety mtsht seem destrable, a more thorough analysis 3
should be undertaken before deciding upon their use.
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18.7. TRUE FACTOR OF SAFETY
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The true factor of safety, which may be deEned in terms of load, stresa, defection.
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creep, wear, etc.. is the ratto of the magnitude of any of the above parameters resulttrig in damage. to its actual value in service. For example:
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- True factor of safety =
maximum load part sustams in service j
9 The tme f actor of safety is determined after a part is budt and tested under serviev
_a conditions.
18.L STRESS CONCENTRATIONt' m6 Abrupt increases in local stress due to stress rassers, such as notches, holes. fillett
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< breads, shoulders, and scratches, are termed utess concentracons.
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-3 The theorettcal (or elaauc) stress-conceutrauon factor is dedned as
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maximum stren at soeuon j
g,, average stress at secuan based upon net area
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The theoretical stress.concentrauon factor is a funcuan of geometry only and is g
ietermined from photoeimatte studies, theory of elastsetty. or actual straan measure-q
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Table 13.1 hats I
K. does not consider the miugaung edects of local yteidtag.
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ralues of K
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-3 Strese concentration should be considered with respect to its edees upon the strength in staticady loaded ductile matertais. yielding at taa reducuan af the spectmen,
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