-11-
7-2-2. 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. 4 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=k
•
d
•
p
T: Appropriate tightening torque (kgf
•
cm)
d: Nominal diameter of thread (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 thread (mm
2
)
k: Torque coefficient (0.17)
• Strength grade and rated axial stress of thread
Table 1
Strength grade
4.8
6.8
8.8
12.9
Rated axial stress (kgf/mm
2
)
29.1
43.7
58.2
95
Material
Mild steel
Alloy steel including Ni, Mn, Cr, etc.
Heat treatment
Not processed
Processed: Hard material
• Diameter and effective sectional area of thread
Table 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
(3/18")
M12 x 1.75 mm
(15/32")
M14 x 2 mm
(9/16")
Effective sectional area
of thread (mm
2
)
14.2
20.1
36.6
58.0
84.3
115
• Thread diameter and appropriate tightening torque
Fig. 4
