Maximum 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
Maximum equivalent stress : σ
e
=
xR
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
equivalent stress σ
e
= (σ
t
2
+ 3 τ
2
)
½
MPa
torsion stress τ = M
G
/ ( π/16 d
3
3
)
MPa
tension stress σ
t
= F
i
/A
t
MPa
thread friction M
G
Nm
thread friction M
G-
Nm
head face friction M
WD
Nm
Total tightening torque M
A
= M
G
+ M
WD
Nm
Stress relaxation (σ
e
- σ
t
)/ σ
e
%
Initial preload F
i
=
σ
t
·A
t
kN
Load reserve P
b
= F
0.2
- F
i
kN
1)
the equivalent stress is a result of the stress by tension and torsion.
2)
stress relaxation is the reduction of the (equivalent) stress by setting, where torsional stresses fade away.
3)
load at yield F
0.2
=R
p0.2
.A
t
4)
torque to yield stress TTY is obtained with σ
e
=R
p0.2
.
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