Tightening torque to preload a bolt
Metric thread :
M1
M1.2
M1.6
M2
M3
M4
M5
M6
M8
M10
M12
M14
M16
M20
M24
M30
M36
M42
-
3.6
4.6
4.8
5.6
5.8
6.8
8.8
9.8
10.9
12.9
Initial tensile stress : σ
t
=
·R
p0.2
Thread coeff. of friction : µ
thread
=
Head - face coeff. of friction : µ
head
=
pitch P
mm
pitch diameter d
2
mm
root diameter d
3
mm
tensile stress area A
t
=π/4 d
0
2
; d
0
=(d
2
+d
3
)/2
mm
2
ultimate tensile strength R
m
MPa
yield strength R
p0.2
MPa
tensile stress σ
t
= σ'
t
x R
p0.2
MPa
torsional stress τ = M
G
/ ( π/16 d
3
3
)
MPa
equivalent stress σ
e
= (σ
t
2
+ 3τ
2
)
½
MPa
thread friction M
G
Nm
thread friction M
G-
Nm
head face friction M
WD
=
F
i
μ
head
1.3 d/2
Nm
Total tightening torque M
A
= M
G
+ M
WD
Nm
Initial preload F
i
=
σ
t
·A
t
kN
Load at Yield F
02
=
R
p02
·A
t
kN
Load reserve P
b
=F
0.2
- F
i
kN
1)
stress by torsion is relaxed after tightening.
2)
equivalent stress should remain below the yield strength (linear elastic).
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