Hitachi DH 24PM Technical And Service Manual page 19

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 a
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 break down of the
insulation.
Speed control
The Model DH 24PM is equipped with a variable speed control switch which permits free change of the
rotation speed and hammering force. When drilling in fragile materials, pull the switch trigger gently for low
rotation speed (hammering force) to achieve optimum results.
Note that the switch trigger cannot be pulled to the full but up to the half in the reverse drilling, and the speed
is about half of the forward drilling. In addition, the switch stopper cannot be used in the reverse drilling.
Internal pressure adjustment mechanism
An air passage is provided as shown in Fig. 5 to let out air inside the gear cover and let outside air in the gear
cover. This passage is sandwiched between felt packings to pass only air (pressure). Thus variations in the
internal pressure are minimized for stable hammering operation and prevention of idling and grease leakage.
(Inside the gear cover)
A-A (2:1)
Air passage
(Outside the
gear cover)
Felt packing (B)
Valve
Felt packing (A)
Fig. 5
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Inner cover