Hitachi Koki WR 12DM2 Manual De Instrucciones página 21

5. 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.
6. 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.
7. 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 increase above a certain value even if the tool is driven for a long time.
(See Fig. 17 and 18)
(3) Diameter of bolt
The tightening torque differs with the diameter of the bolt as shown in Fig. 17 and 18.
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 (WR12DM2)
The tightening torque is reduced a little when an extension bar, universal joint or a
long socket is used.
(6) Clearance of the socket (WR12DM2)
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 4 and 5.
(7) Tightening torque varies, depending on the battery's charge level (WR12DM2)
Fig. 20 shows an example of the relationship between tightening torque and the number
of tightenings. 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.
English
21