Fayat Group BOMAG BW 900-50, Руководство по обслуживанию

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1.3
General repair instructions
BOMAG 18 008 921 86
Screws and nuts
Tightening torque
!
Caution
Tighten nuts or screws with the tightening tor-
ques specified in the following tables of tighten-
ing torques. Tightening torques deviating from
the ones in the table are specially mentioned in
the repair instructions.
Damaged screws must under no circumstances
be used any longer. Recutting threads with thread
cutters or taps adversely affects the strength and
leak tightness of the screw joint. Damaged or cor-
roded thread pitches can cause incorrect torque
value readings.
Self-locking nuts must generally be replaced after
disassembly.
The use of screws with too high strength can
cause damage!
l
Nut of a higher strength can generally be used in-
stead of nuts of a lower strength classification.
l
When checking or retightening screw joints to the
specified tightening torque you should first relieve
by a quarter turn and then tighten to the correct
torque.
l
Before tightening you should lightly oil the thread, in
order to ensure low friction movement.
The same
applies for self-locking nuts.
l
Make sure that no oil or grease will enter into blind
tapped bores. The hydraulic power generated when
turning in the screw could cause breakage of the ef-
fected part.
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. 7 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.
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