Hitachi DH 24DVA Technical Data And Service Manual page 23

Armature
Hammering function
The piston reciprocates within the cylinder to move the striker in the same manner as in conventional hammer
drills. As the piston reciprocates, the changing air pressure inside the air chamber between the piston and the
striker causes the striker to move and continuously strike against the end of the second hammer. At the same
time, the changing air pressure within the air chamber which moves the striker also provides an "air cushion"
which absorbs the impact of the hammering action. As any air leakage from the air chamber would weaken
the air-cushion effect and reduce impact absorption, the O-ring (mounted on the striker) is extremely important
to seal the air. Although special rubber material is utilized in construction of the O-ring to make its effective
service life as long as possible, wear cannot be fully avoided. Accordingly, it is recommended that the O-ring
be replaced approximately once a year, depending on the frequency of usage of the tool.
Idle striking prevention mechanism
The idle striking prevention mechanism in the Model DH 24DVA is different from that of conventional hammer
drills. When the drill bit is lifted from the concrete surface on completion of drilling, the second hammer moves
to the position indicated by the continuous lines in Fig. 6, and the protruding (lip) portion at the tip of the striker
is gripped by O-ring (C) mounted between the hammer holder and the damper holder. In this state, should the
piston continue to move so that the small piston respiratory hole is blocked by the inner wall of the cylinder, the
air in the air chamber will pass through the large piston respiratory hole and be released through the air-
escape slot and large cylinder respiratory hole provided on the inner wall of the cylinder. Accordingly, there is
no change in the air pressure within the air chamber, and movement of the striker (idle hammering operation)
is prevented. The gripping force of O-ring (C) on the striker is so small in comparison with the conventional
mouth system that practically no pressing force at all is required to restart the hammering operation.
Piston pin
Reciprocating
First gear
bearing
Second gear
Piston
Second pinion
Clutch
Circular groove
Fig. 5
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Cylinder
Steel ball
Tool shaft
Key rail (2 pcs.)
Second shaft