Hitachi WR 8DH Mode D'emploi Et Instructions De Securite page 18

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3. Use a tightening time suitable for the screw
The appropriate torque for a screw differs according to the material and size of the screw,
and the material being screwed etc., so please use a tightening time suitable for the
screw. In particular, if a long tightening time is used in the case of screws smaller than
5/16" (8 mm), there is a danger of the screw breaking, so please confirm the tightening
time and the tightening torque beforehand.
4. 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.
5. 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.
6. 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. 8, 9 and 10)
(3) Diameter of bolt
The tightening torque differs with the diameter of the bolt as shown in Fig. 8, 9 and 10.
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 4 and 5.
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