Hitachi DH 24DVA Technical Data And Service Manual page 24

First gear
Reciprocating bearing
Slip mechanism
The slip mechanism in the Model DH 24DVA consists of a coil spring which applies a pre-set amount of
pressure to ensure the interlocking of three claws provided on the flange of the cylinder (the final rotating
shaft) and three matching claws provided on the face of the second gear, by which rotation is transmitted to
the cylinder. The second gear is fitted to the cylinder with a certain amount of play.
If an excessively large torque is applied to the tool shaft (cylinder), the force of the torque will exceed the
pressure of the coil spring and cause the claws on the second gear to disengage from and ride over the claws
on the cylinder so that the second gear idles and does not transmit rotation. Even should the drill bit come in
contact with a reinforcing bar within the concrete, causing sudden excessive torque, the slip mechanism
functions to prevent damage to the gears, and possible loss of control of the tool by the operator.
Sealed and dustproof construction
The gear cover is totally enclosed by oil seals, O-rings and other devices to prevent leakage of lubricating
grease, and to keep dust and dirt out of the internal mechanisms. The drill bit chuck portion is protected by the
rubber front cap to keep out dust and chips which could cause improper fitting of the drill bit and/or other faulty
operation of the chuck portion. The speed control switch is also a fully dust-proofed type to prevent dust and
chips from entering the handle section and causing possible operational trouble or a breakdown of the
insulation.
Speed control
The Model DH 24DVA is equipped with a variable speed control switch which permits free change of the
rotation speed and striking force. When drilling in fragile materials, pull the switch trigger gently for low
rotation speed (striking force) to achieve optimum results.
Air-escape slot
Piston
Gear cover
Clutch claw
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Second gear
Striker
Claw
Coil spring
Small piston respiratory hole
Fig. 6
Travel distance
Front cap
Second
Cylinder
hammer
O-ring (C)
Large piston respiratory hole