Hitachi DH 24PE (S) Technical Data And Service Manual page 17

• Striking 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 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 striking 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 a 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 24PE (S) is different from that of conventional
hammer drills. 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 the O-ring mounted on the inner wall of the cylinder. 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 striking operation) is prevented. The gripping force of
the O-ring 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 striking operation.
Front cap
Cylinder
Second hammer
Large piston vent hole
Second gear
Travel distance
Fig. 4
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Claw
Coil spring
Striker
Air-escape slot
Piston
Reciprocating bearing
Small piston vent hole