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2. Tightening torque
(1) Diameter of thread 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. The tables 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/cm
2
) x 0.8 x Effective sectional area of the thread (mm
2
)
k: Torque coefficient (0.17)
• Strength grade and rated axial stress of thread
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
Precessed: Hard material
• Diameter and effective sectional area of thread
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”)
Effective section area
of thread (mm
2
)
14.2
20.1
36.6
58.0
84.3
• Diameter of thread and appropriate tightening torque
