机械设计5第56章连接.ppt

机械设计5第56章连接
2021
Your content to play here, or through your copy, paste in this box, and select one：prevent connection from becoming loose →improve reliability, strength and tighteness
F0
When screw down (拧紧时)→pretightening force (预紧力)F0:
／bolt (螺栓)→pre-tension (预紧拉力 )
＼connecting pieces (被联接件)→
pre-compression (预紧压力)
Carbon steel bolt
Alloy steel bolt
T1: resisting moment (螺纹阻力矩)
T2: frictional resistance moment (摩擦阻力矩) at
supporting surface of the nut
Tightening torque T: T1+T2
Ｔ1＝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=
§5—4 anti-loose of thread connection
Tightened→ψ≤ v(self-lock) →anti-loose？
If impact, vibration and variable loads happen, fricti-on may disappear, so that self-lock by friction is unrel-iable, therefore, anti-loose is necessary
Friction anti-loose: Maintain longitudinal and lateral pressure between contact area of threads→ resistance moment will not disappear
Mechanical anti-loose→Constrain the screw pair by the metal elements being easy to replace
Undetachable anti-lose→weld, riveting and bonding
Anti-lose method ：Prevent screw pair from relative turning
I Strength calculation of loose bolt conn. (noＦ0)
Tight bolt conn.: have pretightening force and operating tension
Loose bolt conn.: no pretightening force, only operating
tension
§5-5 Strength calculation of bolt connection
Check
Design
Tight bolt conn.｛
II Strength calculation of tight bolt conn. (haveＦ0 )
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
1. Tight bolt connection with only pretightening force
F0 have relatio