SCREWS, ASTENERS AND NON PERMANENT JOINTS 2 MOW 227 2014
Yeldng take place n the frst thread wth cold work strengthenng Never re-use nuts n structural connectons because ther thread gets daaged
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?
countersunk
BOLTED JOINTS STINESS Table 8.7 l` h t 2 l
BOLT STINESS Tensle Stress Area
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 1 1 1 L k k k 1 2 3 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
k d k t
COMPRESSION O BOLTED MEMBERS 2 2 2 2 D d A π ( r0 r ) π xtanα 2 2 D d D d π xtanα xtanα 2 2
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 )
COMPRESSION O BOLTED MEMBERS Assung: d w 1.5 d, and α 30, and l2t k 0.5774 π Ed 0.5774 l 0.5d 2ln 5 0.5774 l 2.5d
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.
Read secton 8-6 *Use sae grade nuts as the bolt
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
BOLTED JOINTS P <0 b >0
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
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
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
BOLTED JOINTS Increase n deforaton of the bolt δ P k Copresson of ebers b b P δ k
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
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
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
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
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
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
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
BOLT MATERIALS
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
Endurance Lt ully corrected (ncludng K f ) axal endurance strength for rolled threads
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
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
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
160 kn 40 Table A-29 nut thckness 14.8 4 40 14.8 4 58.8.. M16 x 60 Grade 8.8 20 20
2 x 16 6 38 60 38 22 18 157 2 0.016 2 201 2 60 201 157 207 6 10 201 18 157 22 923.68 MN/ 100 GPa 0.5774π100 16 0.5774 40 0.5 16 2 ln 5 0.5774 40 2.5 16 1609.08 MN/ 100 16 0.77871 1594.19 MN / e 0.61616 16 / 40
0.9 923.68 923.68 1609.08 0.365 11, S p 600MPa (157)600 70.65kN 5 0.365 2 160 600 157 70650 600 157 70650 2.02 0.365 (160000 5) 4.96 5
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
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