Tightening torque to preload a bolt
UNF thread :
0-80 UNF
1-72 UNF
2-64 UNF
3-56 UNF
4-48 UNF
5-44 UNF
6-40 UNF
8-36 UNF
10-32 UNF
12-28 UNF
1/4-28 UNF
5/16-24 UNF
3/8-24 UNF
7/16-20 UNF
1/2-20 UNF
9/16-18 UNF
5/8-18 UNF
3/4-16 UNF
7/8-14 UNF
1-12 UNF
1 1/8-12 UNF
1 1/4-12 UNF
1 3/8-12 UNF
1 1/2-12 UNF
SAE Grade Number :
grade 1
grade 2
grade 4
grade 5
grade 5.2
grade 7
grade 8
grade 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)
the equivalent stress is a result of the stress by tension and torsion.
2)
the torsional stress may vanish by relaxation.
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