機(jī)械設(shè)計(jì)5第56章連接-文庫吧
2024-12-24 15:12 本頁面
【正文】 Cclass. Aclass has the highest degree of accuracy, it is applied for important thread connections requiring exact fit and vibrationproof； Bclass is used to thread connections subjected to heavy loads or fluctuating loads and those needing frequent adjustment or dismounting. ； Cclass is often used to ordinary thread connections. 167。 5— 3 Pretension of thread connection Purpose： prevent connection from being loose →improve reliability, strength and tighteness F0 When screw down (擰緊時(shí) )→pretightening force (預(yù)緊力 )F0: ／ bolt (螺栓 )→pretension (預(yù)緊拉力 ) ＼ connecting pieces (被聯(lián)接件 )→ prepression (預(yù)緊壓力 ) Carbon steel bolt Alloy steel bolt T1: resisting moment (螺紋阻力矩 ) T2: frictional resistance moment (摩擦阻力矩 ) at supporting surface of the nut Tightening torque T: T1+T2 Ｔ 1＝ F d2/2＝ F0 tg(ψ＋ ?v) d2 /2 T＝ T1＋ T2≈ Because F0 =5T/d, if d is small， F0 will be big. It is suggested that the major diameter should be larger than M12 T2＝ fc F0 r m ?: Helix angle； Equivalent friction angle ?v= 167。 5— 4 antiloose of thread connection Tightened→ψ≤ ?v(selflock) →antiloose？ If impact, vibration and variable loads happen, friction may disappear, so that selflock by friction is unreliable, therefore, antiloose is necessary Friction antiloose: Maintain longitudinal and lateral pressure between contact area of threads→ resistance moment will not disappear Mechanical antiloose→Constrain the screw pair by the metal elements being easy to replace Undetachable antilose→weld, riveting and bonding Antilose method ： Prevent screw pair from relative turning I Strength calculation of loose bolt conn. (noＦ 0) Tight bolt conn.: have pretightening force and operating tension Loose bolt conn.: no pretightening force, only operating tension ? ?? ??????FddF44121???167。 55 Strength calculation of bolt connection Check Design Tight bolt conn.｛ II Strength calculation of tight bolt conn. (haveＦ 0 ) Bolts with M10~M64： τ≈ Calculation stress： σca= Pretightening force and operat ing tension Only pretightening force Tight bolt conn. ｛ Screw down— tightening torque—shear stress τ Screw down— tightening torque—tension stress σ ??? 22 ??? ???? ??2104dFca1. Tight bolt connection with only pretightening force F0 have relation with lateral loads 2. Pretightening force Ｆ 0+operating tensionＦ Tight bolt conn.｛ Screw down— pretightening tension Load— operating tension Total force F2 ≠Ｆ 0 +Ｆ = ？ Tightened: elongate λb F0 shorten λm F0 Bolt Connecting pieces Loaded: elongate again △ λ relax △ λ Pressure of connecting pieces： Residual pretightening force: F1= F0 － △ F Total force F2： F2= F0 +△ F =F +F1 F2 Force 變形 F0 △ F F1 F △ λ Deformation patibility condition：△ λ is the same Cb = F0 ／ λb bolt stiffness Cm = F0／ λm Connecting pieces stiffness ????? bCF?????? mCFFmbCCFFF ????FCC CFmbb???FCC CFFFFmbb?????? 002mbbCCC?—— Relative stiffness of the bolt Relative s
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