Tightening torque to preload a bolt
UNC thread :
1-64 UNC
2-56 UNC
3-48 UNC
4-40 UNC
5-40 UNC
6-32 UNC
8-32 UNC
10-24 UNC
12-24 UNC
1/4-20 UNC
5/16-18 UNC
3/8-16 UNC
7/16-14 UNC
1/2-13 UNC
9/16-12 UNC
5/8-11 UNC
3/4-10 UNC
7/8-9 UNC
1-8 UNC
1 1/8-7 UNC
1 1/4-7 UNC
1 3/8-6 UNC
1 1/2-6 UNC
1 3/4-5 UNC
2-4.5 UNC
2 1/4-4.5 UNC
2 1/2-4 UNC
2 3/4-4 UNC
3-4 UNC
3 1/4-4 UNC
3 1/2-4 UNC
3 3/4-4 UNC
4-4 UNC
SAE Grade Number :
class 1
class 2
class 4
class 5
class 5.2
class 6.8
class 8.2
Preload as percentage of proof strength :
0.2 Sp
0.3 Sp
0.4 Sp
0.5 Sp
0.6 Sp
0.7 Sp
0.8 Sp
0.9 Sp
1.0 Sp
Thread coeff. of friction :
0.0
0.08
0.1
0.12
0.14
0.15
0.16
0.18
0.20
0.22
Head face coeff. of friction :
0.0
0.08
0.1
0.12
0.14
0.15
0.16
0.18
0.20
0.22
Major diameter d
in
Threads per inch N
-
Pitch P = 1/N
10
-3
in
Pitch diameter d
p
= d - 0.649519/N
in
Minor diameter d
r
= d - 1.299038/N
in
Tensile stress area A
t
= π/4 ((d
p
+d
r
)/2)
2
10
-3
in
2
Proof strength S
p
kpsi
Initial tensile stress
σ
i
= C
1
x S
p
kpsi
Torsional stress
1)
τ = M
G
/ ( π/16 d
r
3
)
kpsi
Equivalent stress
σ
e
= (
σ
i
2
+ 3τ
2
)
½
kpsi
Thread friction M
G
lb
·in
Thread friction M
G-
lb
·in
Head face friction M
WD
=
F
i
μ
head
1.3 d/2
lb
·in
Initial preload F
i
lb
Total tightening torque M
A
= M
G
+ M
WD
lb
·in
1)
stress by torsion is relaxed after tightening.
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