Hitachi DH 24PB2 Technical Data And Service Manual page 18

Hammering function
The piston reciprocated 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 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 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 hammering prevention mechanism
The idle hammering prevention mechanism in the Model DH 24PB2 is different from that of conventional rotary
hammers. 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. 3, 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.
Moving
distance
Front cap
Cylinder
Second hammer
Claw
Second gear
Large cylinder
respiratory hole
Clutch spring
Second shaft spline
Change lever
Striker
Coil spring
Air-escape slot
Fig. 3
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Piston
Reciprocating bearing
Gear cover
First gear
Clutch claw
Small piston respiratory hole