SCREWS, FASTENERS AND NON PERMANENT JOINTS 2 MOW Department of Mechanical and Aeronautical Engineering

Transcription

1 SCREWS, ASTENERS AND NON PERMANENT JOINTS 2 MOW

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4 Yeldng take place n the frst thread wth cold work strengthenng Never re-use nuts n structural connectons because ther thread gets daaged

5 Washers underneath bolt head Purpose of a bolt s to clap two or ore parts together Turn the nut not the bolt f possble. Why?

6 countersunk

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8 BOLTED JOINTS STINESS Table 8.7 l` h t 2 l

9 BOLT STINESS Tensle Stress Area

10 COMPRESSION O BOLTED MEMBERS There ay be ore than two ebers n the grp of the bolt The total eber stffness of the jont 1 k L k k k k n If one of the ebers s a gasket the stffness s so soft relatve to the other ebers that for all practcal purpose, the others can be neglected and only the gasket used 1

11 k d k t

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13 COMPRESSION O BOLTED MEMBERS D d A π ( r0 r ) π xtanα 2 2 D d D d π xtanα xtanα 2 2

14 The contracton of an eleent n the cone of thckness dx, subjected to a copressve force P s: Substtutng A nto the prevous equaton and ntegratng the resultng equaton fro 0 to t gves. δ k P (2ttan α D d)( D d) ln πedtan α (2ttan α D d)( D d) P δ Pdx dδ EA πedtanα (2ttan α D d)( D d) ln (2 t tan α D d )( D d )

15 COMPRESSION O BOLTED MEMBERS Assung: d w 1.5 d, and α 30, and l2t k π Ed l 0.5d 2ln l 2.5d

16 Bolt Strength Bolt strength s specfed by: 1. nu proof strength S p 2. or nu proof load p, 3. and nu tensle strength, S ut Table 8-9 to 8-11 The proof load s the axu load (force) that a bolt can wthstand wthout acqurng a peranent set. The proof strength s the quotent of the proof load and the tensle-stress area.

17 Read secton 8-6 *Use sae grade nuts as the bolt

18 BOLTED JOINTS IN TENSION Clearance between the bolt and hole Preload - External tensle load P External shear load P s Collar frcton: f c Thread frcton: f t P

19 BOLTED JOINTS P <0 b >0

20 BOLTED JOINTS Purpose of preload s to place the bolted eber coponents n copresson for better resstance to the external tensle load and to create a frcton force between the parts to resst shear load Jont has faled when resultng loadng between ebers equals 0 The load n the bolt exceeds yeld

21 BOLTED JOINTS Sprng constant or stffness constant δ : Deflecton : orce A : Area δ L A E E : Modulus of elastcty (Steel: E200GPa) Stffness constant, k k δ A E L

22 BOLTED JOINTS P total external load on bolted assebly Preload due to tghtenng P b Porton of P taken by bolt P Porton of P taken by ebers b Resultant bolt load P b Resultant load on ebers P - C racton of P taken by bolt P P b P

23 BOLTED JOINTS Increase n deforaton of the bolt δ P k Copresson of ebers b b P δ k

24 BOLTED JOINTS On the assupton that the ebers have not separated, the ncrease n deforaton of the bolt ust equal the decrease n deforaton of the ebers δ P b k b P k But P P b P rearrangng P b k b kb k P CP

25 BOLTED JOINTS The resultant load on the bolt s The resultant load on the eber s THESE EQUATIONS ARE ONLY VALID AS LONG AS SOME O THE INITIAL COMPRESSION REMAINS IN THE MEMBERS b b P C P P C P ) (1 b b k k k C

26 RELATING BOLT TORQUE TO BOLT TENSION Torque to produce preload: Splfyng T threads T collar T 2 sec sec 2 c c d f l f d d f l d T α π α π d K T *K Torque factor

27 Tenson, not torque, ndcates proper jont tghtness. Often, preload can be set ore accurately by controllng the nuber of turns rather than nput torque. Turn-of-nut ethod After the snug-tghtenng procedure s copleted, each bolt n the connecton s pretensoned addtonally by the applcable aount of relatve rotaton

28 STATICALLY LOADED TENSION JOINT Tensle stress falure Stress n bolt Introducng a load factor n: Where S p s the proof stress b P C t t b A A P C p t t b S A A P n C

29 STATICALLY LOADED TENSION JOINT Jont separaton The value of the load that wll cause separaton P 0 At Jont Separaton The Bolt Is Carryng All The Load ( 1 C) P0 0 Porton of external load carred by eber P0 n0 P Safety factor aganst separaton, n 0 : n o P ( 1 C ) b P 0 Rato of openng load copared to external load

30 PRELOAD It s recoended that for statc and fatgue loadng the followng be used for preload 0.75 p (reused connectons) 0.90 p (peranent connectons) Where p s the proof load p A t S p

31 BOLT MATERIALS

32 ATIGUE LOADING Already corrected for notch senstvty and surface fnsh Peterson typcal bolt falures 15% under head 20% at end of shank 65% n the thread usually safe to assue that the fasteners have rolled threads unless specfc nforaton s avalable Use achned fnsh for the body of the bolt f nothng s stated

33 Endurance Lt ully corrected (ncludng K f ) axal endurance strength for rolled threads

34 ATIGUE LOADING External load fluctuates between 0 and soe axu force P ax b n Alternatng coponent: a ( ax - n )/2 ( b - )/2 Mean coponent: ( ax n )/2 ( b )/2 Alternatng stress: Mean stress: t t t b a A P C A P C A 2 2 ) ( ) 2 ( t t t t b A A P C A P C A 2 2 ) ( ) 2 ( a Load pre-tenson Pre-tenson

35 Mean Stress Goodan Soderberg Gerber ASME ellptc 1 u e a S S 1 ) ( 2 u a e a S S 1 y e a S S 1 ) ( ) ( 2 2 y e a S S

36 ATIGUE LOADING Goodan But (Applyng the load factor to the alternatng coponent only) 1 u e a S S a 1 u a e a S S 1 u a f e a f S n S n ) ( ) ( 2 e u t u e f S S P C A S S n

37 160 kn 40 Table A-29 nut thckness M16 x 60 Grade

38 2 x MN/ 100 GPa π ln MN/ MN / e / 40

39 , S p 600MPa (157) kN ( )

40 Gasketed Jonts Pressure p on gasket p A g N Load actor orces on ebers (1 C ) np Gasket pressure p 3 [ np (1 C )] To antan adequate unforty of pressure the bolts ust be spaced, not ore than sx daeters apart (rough rule) πd Nd b 6 N A g

41 Suary: BOLT SIZING actors used durng bolt szng Bolt sze Nuber of bolts Bolt ateral Type of loadng Bolt preload Safety factor Load/pressure at falure Meber ateral Note: Any of the above can be used as an entry pont for the szng of a bolted connectons