Hitachi WR 12DH Mode D'emploi page 11

kg–cm
1200
800
400
2. Tightening torque
Refer to Fig. 12, 13 and 14 for the tightening torque
of bolts (according to size), under the conditions
shown in Fig. 15. Please use this example as a
general reference, as tightening torque will vary
according to tightening conditions.
Tightening torque varies, depending on the battery's
charge level. Fig. 16 shows an example of the
relationship between tightening torque and the
number of tightenings, for a M14 × 50 high tension
bolt. As shown, tightening torque gradually weakens
with the increase in the number of tightenings. In
particular, as the torque decreases very close to the
complete discharge ("a" margin in graph), the unit's
impact weakens, the number of time impacts
declines and tightening torque drops off abruptly.
If this occurs, check torque level, then recharge the
battery if necessary.
3. Work at a tightening torque suitable for the bolt
under impact
The optimum tightening torque for nuts or bolts
differs with material and size of the nuts or bolts.
An excessively large tightening torque for a small
bolt may stretch or break the bolt. The tightening
torque increases in proportionate to the operaton
time. Use the correct operating time for the bolt.
4. Holding the tool
Hold the impact wrench firmly with both hands. In
this case hold the wrench in line with the bolt.
It is not necessary to push the wrench very hard.
Hold the wrench with a force just sufficient to
counteract the impact force.
5. Confirm the tightening torque
The following factors contribute to a reduction of
the tightening torque. So confirm the actual
tightening torque needed by screwing up some
bolts before the job with a hand torque wrench.
Factors affecting the tightening torque are as follows.
(1) Voltage
When the discharge margin is reached, voltage
decreases and tightening torque is lowered.
(2) Operating time
The tightening torque increases when the operating
time increases. But the tightening torque does not
M14 × 50 High tensile bolt (tightening time 3 sec)
When full recharged
When completely discharged
0
0 20 40
Number of tightenings (PCS)/charging
60 80 100 120
Fig. 16
increase above a certain value even if the tool is
driven for a long time. (See Fig. 12, 13 and 14)
(3) Diameter of bolt
The tightening torque differs with the diameter of
the bolt as shown in Fig. 12, 13 and 14. Generally
a larger diameter bolt requires larger tightening
torque.
(4) Tightening conditions
The tightening torque differs according to the torque
ratio; class, and length of bolts even when bolts
with the same size threads are used. The tightening
torque also differs according to the condition of the
surface of workpiece through which the bolts are
to be tightened. When the bolt and nut turn together,
torque is greatly reduced.
(5) Using optional parts
The tightening torque is reduced a little when an
extension bar, universal joint or a long socketis
used.
(6) Clearance of the socket
A worn or deformed hex or a square-holed socket
will not give an adequate tightness to the fitting
between the nut or anvil, consequently resulting in
loss of tightening torque.
Using an improper socket which does not match
to the bolt will result in an insufficient tightening
torque. Matching socket and bolt sizes are shown
in Table 1 and 2.
MAINTENANCE AND INSPECTION
1. Inspecting the socket
A worn or deformed hex or a square-holed socket
will not give an adequate tightness to the fitting
between the nut or anvil, consequently resulting in
loss of tightening torque. Pay attention to wear of
a socket holes periodically, and replace with a new
one if needed.
2. Inspecting the mounting screws
Regularly inspect all mounting screws and ensure
that they are properly tightened. Should any of the
screws be loose, retighten them immediately. Fail-
ure to do so may result in serious hazard.
a
English
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