Hitachi WR 12DAF Handling Instructions Manual page 12

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. 15).
(3) Diameter of bolt
The tightening torque differs with the diameter of
the bolt as shown in Fig. 15. Generally a larger
diameter bolt requires larger tightening torque.
M8 × 30
kgf cm N m
• •
1000 100
800 80
High tension bolt
600 60
400 40
200 20
Ordinary bolt
0 0
0 1
Tightening time: sec
(Steel plate thickness
t = 10 mm)
Bolt
Nut
11
kgf cm N
•
1000
800
600
400
200
0
2 3
Washer
Steel plate thickness t
Washer
(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 socket is
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.
M10 × 30
m
•
100
High tension bolt
80
60
40
Ordinary bolt
20
0
0 1 2 3
Tightening time: sec
(Steel plate thickness
t = 10 mm)
Fig. 15
*The following bolt is used.
Ordinary bolt: Strength grade 4.8
High tensile bolt: Strength grade 12.9
(
Explanation of strength grade:
4 — Yield point of bolt: 32 kgf/mm
8 — Pulling strength of bolt: 40 kgf/mm
Fig. 16
M12 × 45
kgf cm N m
• •
1200 120
High tension
bolt
1000 100
800 80
600 60
Ordinary bolt
400 40
200 20
0 0
0 1
Tightening time: sec
(Steel plate thickness
t = 10 mm)
2 3
)
2
2