PRODUCT INFORMATION
INDEX
DATE
Dep.2
IM-C-C-00-002e
B
February 2012
TIGHTENING TORQUES FOR COMMERCIAL BOLTS & NUTS
2/3
Coating
Coefficient of friction
Normal
Lubricated
Steel with cadmium
0.07
0.07
Cadmium with zinc
0.1
0.08
Cadmium with cadmium
0.1
0.07
Steel with phosphate
0.11
0.1
Phosphate with cadmium
0.11
0.08
Phosphate with zinc
0.11
0.11
Phosphate with phosphate
0.12
0.1
Steel with zinc
0.14
0.07
Table 2 – Coefficient of friction for different coatings
In any case, the normal coefficient of friction used is 0.14 for clean and non-lubricated steel-to-steel assembly. If
the screw and the mating threads are of steel, clean and lubricated, this coefficient is 0.1.
From a known coefficient of friction, it is possible to calculate the pre-stressing force using this formula.
To determine whether a screw will resist a pre-stressing force (as calculated from a known tightening torque and
coefficient of friction), it is necessary to know the mechanical properties of that screw. The following table shows
those properties.
GRADE
8.8
10.9
12.9
Tensile strength (N/mm
2
)
Min.
784
980
1176
Max.
980
1176
1372
Yield strength (N/mm
2
)
627
882
1058
Extension to the breakage (%) min
12
9
8
Resilience (N.m/cm
2
)
59
39
29,4
Hardness HRC
Min.
18
27
34
Max.
31
38
44
Table 3 – Mechanical properties of screws used
(acc. to DIN 267)
M
P
p
dn
S
df
v
torque
with the
obtained
force
stressing
-
Pre
pitch
Screw
diameter
nominal
Screw
diameter
Washer´s
thread
part´s
and
screw
in
friction
of
Coeficient
part
and
screw
between
friction
of
Coeficient
:
of
friction
of
coeficient
average
The
diameter
side
2
1
]
16
.
0
)
58
.
0
4
(
[
p
df
dn
S
P
M
v
2.1.38
O&M_2.002211.810_A_10_2016
