Hitachi DH 24PF3 Technical Data And Service Manual page 21

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)
Air passage
(Outside the
gear cover)
Felt packing (B)
Valve
Felt packing (A)
Fig. 5
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Inner cover