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6-2-2. Screw diameter and appropriate tightening torque
Generally speaking, the appropriate tightening torque for a screw can be determined by the strength grade of the
screw and the material tightened. Tables 1 and 2, and Fig. 5 below list data relative to the strength grade of
various screws and the appropriate tightening torque. For further reference, appropriate tightening torque is
calculated with the following formula. Study and use this formula for accurate selection of tightening torque.
T = 9.8 k•d•p/100
T: Appropriate tightening torque (N•m)
k: Torque coefficient (0.17)
d: Nominal diameter for the screw (mm)
p: Recommended axial tightening force to be applied to the screw (kgf)
p = rated axial stress (kgf/mm
2
) x 0.8 x effective sectional area of the thread (mm
2
)
Strength grade and rated axial stress of threads
Table 1
Strength grade
Material
Rated axial stress (kgf/mm
2
)
Heat treatment
4.8
6.8
8.8
12.9
29.1
43.7
58.2
95
Mild steel
Alloy steel including Ni, Mn, Cr, etc.
None
Processed-hard material
Diameter and effective sectional areas of threads
Table 2
Thread diameter and appropriate tightening torque
Fig. 5
Effective sectinal area of
thread (mm
2
)
Kind of thread (x pitch)
M5 x 0.8 mm
(3/16")
M6 x 1 mm
(1/4")
M8 x 1.25 mm
(5/16")
M10 x 1.5 mm
(3/8")
14.2
20.1
36.6
58.0
M12 x 1.75 mm
(15/32")
M14 x 2 mm
(9/16")
84.3
115
120
100
80
60
40
20
0
0
5
6
8
10
12
14
(9/16")
(15/32")
(3/8")
(5/16")
(1/4")
(3/16")
Thread diameter
1200
1000
800
600
400
200
(1042 in-lbs.)
(868 in-lbs.)
(694 in-lbs.)
(521 in-lbs.)
(347 in-lbs.)
(174 in-lbs.)
0
(N
•
m)
(kgf
•
cm)
Appropr
iate tightening torque
0
200
400
600
800
1000
(in-lbs.)
Strength grade 12.9
(high-strength bolt)
Strength grade 8.8
(high-strength bolt)
Strength grade 6.8
(ordinary bolt)
Strength grade 4.8
(ordinary bolt)
