1. Field of the Invention
The present invention relates to a power driven screwdriver having a clutch mechanism for transmitting rotation of a drive motor to a spindle with a driver bit.
2. Description of the Prior Art
In a power driven screwdriver, a clutch mechanism is provided for transmitting and disconnecting the rotation of a drive motor to a spindle with a driver bit. The clutch mechanism is normally constructed as a claw clutch and includes a pair of clutch members, one of which is mounted on the spindle and the other of which is mounted on a main gear driven by the drive motor. The spindle is movable in an axial direction for engaging and disengaging the clutch members. With such a clutch mechanism constructed by a simple claw clutch, since the rotation of the spindle is restrained, for example, at the completion of a screw driving operation, the clutch mechanism temporarily repeats its engaging and disengaging operation. This will generate clanging sounds, giving unpleasant feeling to the operator, and cause early wear of the clutch mechanism.
U.S. Pat. No. 4,655,103 discloses a power driven screwdriver including stopper for adjusting the driving
a amount of a screw by a driver bit. A claw clutch mechanism is provided between a driver shaft and a spindle movable in an axial direction. The claw clutch mechanism includes a first and a second clutch member formed on the driver shaft and the spindle, respectively. A clutch disc is interposed between the driver shaft and the spindle and includes a third and a fourth clutch member for engagement with the first and second clutch members respectively. A spring is interposed between the first and third clutch members for normally keeping them at a disengaging position. The second and fourth clutch members includes relief portions which serves not to transmit rotation. When the stopper abuts on a work to be screwed, the driver shaft continues rotation while the rotation of the spindle is prevented. This may cause the operation of the relief portions of the second and fourth clutch members to positively disengage the first and the third clutch members with the aid of the spring.
U.S. Pat. No. 4,809,572 discloses a power driven screwdriver including a stopper sleeve for adjusting the driving amount of a screw and a claw clutch mechanism having a pair of clutch members, one of which is mounted on a main gear driven by a drive motor, while the other of which is mounted on a spindle. A spring is provided for normally keeping the clutch member of the spindle out of engagement with the clutch member of the main gear. A control mechanism is provided between the spindle and the clutch member mounted on the spindle. The control mechanism includes oblique recesses and a ball for engagement with the recesses. With such construction, when the stopper sleeve abuts on a work to be screwed, the main gear continues its rotation while the rotation of the spindle is prevented. In this stage, the control mechanism operates to positively move the clutch member of the spindle out of engagement with the clutch member of the main gear with the aid of the spring.
However, with the above prior U.S. Patents, the operation of the clutch mechanism must accompany a reciprocal movement of the spindle at a long distance. In general, a power driven screwdriver is provided with a seal member for sealing between a spindle and a housing to prevent entry of dust within the housing. In case the spindle reciprocally moves at a long distance, the dust may be absorbed into the housing through the ga between the seal member and the spindle or the housing by the pumping effect. Thus, when the spindle moves into the housing, negative pressure will be created in the housing. Such dust entered into the housing may cause early wear or damage of the clutch mechanism or bearings disposed within the housing.
Further, with the clutch mechanism of the above U.S. Patents, after the stopper or the stopper sleeve has abutted on the work, no further driving operation cannot be made even if the driving of a screw wa insufficient.