Fayat Group BOMAG BW 27 RH-4i, Service Manual, Page: 25 / 41

General repair instructions
BOMAG 25 008 922 89
1.3
Strength classes, metric screws
The strength classes (from 3.6 to 12.9) are specified
for all strength classes from a nominal diameter of
5mm. The corresponding identification can be found
where allowed for by the shape of the screw.
Fig. 8 Identification of screws
Example: A screw is identified with 12.9.
The first number corresponds with 1/100 of the nomi-
nal tensile strength (minimum tensile strength) in N/
mm
2
.
l
The nominal tensile strength is 12 X 100 N/mm
2
=
1200 N/mm
2
.
The second number specifies 10-times the ration be-
tween lower yield point and nominal tensile strength
(yield point ratio).
i Note
When exceeding the lower yield point, the material will
return to its original shape when being relieved (plas-
tic deformation).
When exceeding the upper yield point the material will
not restore its original shape after being relieved.
l
The lower tensile strength is 9/10 X 1200 N/mm
2
=
1080 N/mm
2
.
i
Note
However, these values are by no means identical with
the tightening torques, which are to be set on a torque
wrench. The corresponding calculation requires a
higher effort and, in the end, depends on the materials
to be bolted together.
Strength classes of metric nuts
Nuts are differentiated by three load groups. Each
load group has a special designation system for the
strength class assigned, so that the load group can be
clearly identified.
Nuts for screw joints with full load capability (4, 5,
6, 8, 10, 12)
Fig. 9 Identification of nuts
In a connection with a screw, these nuts 1 (Fig. 9)
must be able to bear the full pre-load at the yield point.
Nut height above 0.8 d (d = nominal dimension).
Nuts for screw joints with limited load factor (04,
05)
The preceding "0" indicates that, due to their low
height, nuts 2 (Fig. 9) in this group are only able to
withstand the force of a screw to a limited extent.
Nut height below 0,8 d (d = nominal dimension).
Nuts for screw joints without specified load factor
(11H, 14H, 17H, 22H)
This standard contains strength classes (hardness
classes) for nuts 3 (Fig. 9), for which no load values
can be specified, e.g. because of their shape and di-
mensions, but which can only be classified by their
hardness.
Nut height below 0,5 d (d = nominal dimension).
Strength class of
nut Strength class of associated
screw
4 3.6, 4.6, 4.8
5 3.6, 4.6, 4.8
5.6, 5.8
6 6.8
8 8.8
9 9.8
10 10.8
12 12.8