Hitachi DH 24PM Technical And Service Manual page 17

Piston reciprocating mechanism
In conventional rotary hammers, a piston is caused to reciprocate by a connecting rod and crank shaft, and
the crank shaft and the cylinder axes are at right angles to each other. Accordingly, the armature shaft and the
cylinder axes are also at right angles to each other. In the Model DH 24PM, through adoption of a spiral drive
system (a mechanism using a reciprocating bearing), a more compact design has been achieved by arranging
the armature shaft in parallel with the cylinder axis. Referring to Fig. 3, the armature's rotation is transmitted to
the second shaft via the first gear. The second shaft's rotation is then transmitted through a spline to the
clutch, which engages with a reciprocating bearing and causes it to rotate. However, as illustrated, circular
grooves on the inner race of the reciprocating bearing are positioned on an angle of inclination with relation to
the second shaft. The rotation of the inner race and the shaft causes the angle of inclination to change
regularly forward and back with relation to the second shaft, and produces a rocking motion in the outer race
of the reciprocating bearing. Finally, a rod extending from the outer race of the reciprocating bearing is
connected to the piston by a piston pin, and causes the reciprocating motion of the piston.
Second gear
Piston pin Piston
Cylinder
Armature
Steel ball
Motor shaft
Key rail (2 pcs.)
Bit holder
Second shaft
First gear
Reciprocating bearing
Clutch
Annular groove
Fig. 3
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 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.
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