Hitachi DH 50MB Technical Data And Service Manual page 19

Mechanism to prevent idle hammering
The arrangement against idle hammering of this hammer drill is about the same as for the DH 40MA/MB in
which, when the drill bit or bull point is no longer pressed against the concrete or similar material, the second
hammer moves to a position shown in Fig. 10 so that the striker is displaced from its hammering position. This
opens the air vent so that the piston movement causes no change in air pressure in the pressure chamber,
thus stopping the hammering action.
Travel distance of the second hammer
Slip mechanism
The slip mechanism is described below with reference to Fig. 11. The bevel pinion and the gear holder are
coupled together by the key and press-fitting. The springs (C) and the needle pins are housed in elongated
grooves of the gear holder. The needle pin is pressed against the inner face of the second gear by the springs
(C) to allow idle rotation of the second gear relative to the gear holder. When an excess torque is exerted on
the bevel pinion shaft, the needle pin is raised upon the projection of the second gear against the load of the
springs (C) to allow idle rotation of the second gear. With this arrangement, the clutch slips when an
excessive torque is applied to the working tool as when the drill bit contacts steel bar/wire in the concrete,
protecting the operator from unexpected motion of the side handle.
Bevel pinion
Gear holder
A
Second hammer
Fig. 10
Gear holder
Bevel pinion
Second gear
A
Elongated grooves
of the gear holder
Fig. 11
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Striker Pressure chamber
Air vent
Section A-A
Key
Needle pin
Spring (C)