Hitachi DH 24PF3 Technical Data And Service Manual page 19

Hammering function
The piston reciprocates within the cylinder to move the striker in the same manner as in conventional rotary
hammers. As the piston reciprocates, the changing air pressure inside the air chamber between the piston
and the striker causes the striker to move and repeatedly 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
weakens the air-cushion effect and reduces 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 hammering prevention mechanism
The idle hammering prevention mechanism in the Model DH 24PF3 is different from that of conventional rotary
hammers. When the drill bit is lifted from the work surface on completion of drilling, the second hammer
moves to the position indicated by the continuous lines in Fig. 4 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 vent hole is blocked by the inner wall of the
cylinder, the air in the air chamber will pass through the large piston vent hole and be released through the air-
escape slot and large cylinder vent 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
mount system that practically no pressing force at all is required to restart the hammering operation.
Gear cover
First gear
Reciprocating bearing
Striker
Air-escape slot
Small piston vent hole
Piston
Clutch claw
Large piston vent hole
Second gear
Claw
Coil spring
Fig. 4
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Moving
distance
Front cap
Cylinder
Second hammer
O-ring (C)