1. Field of the Invention
The present invention relates to a screwdriver. More particularly, the present invention relates to a screwdriver of the type with a ratchet mechanism allowing a change in the driving rotational direction for driving a screw and in a free rotational direction not driving the screw.
2. Description of the Related Art
U.S. Pat. No. 6,047,802 discloses a ratchet driving mechanism for a screwdriver. The ratchet driving mechanism includes a main body having two slots. A detent is received in each slot. The detents are selectively biased outward by springs to engage with teeth of a cylindrical rotary seat, thereby allowing a change in the driving rotational direction for driving a screw and in a free rotational direction not driving the screw. In manufacture, formation of the slots requires two elongate mold cores that are placed in a cavity of a mold. Since each slot does not extend through the main body, each mold core has an end unsupported in the cavity. When molten metal is filled into the cavity, the mold cores sway and wobble due to high temperature and high pressure in the mold such that errors are incurred in the specification of the slots. Namely, if the slots are not processed subsequently, the movement of the detents in the slots would not be smooth, leading to risks of getting stuck and malfunction. The cost is thus increased. Further, only two detents are used, and only one of them is engaged with the teeth of the cylindrical rotary seat such that the detent might slide relative to the teeth or the detent and/or the teeth might be damaged when a large torque is applied to the screwdriver. Hence, such a screwdriver can only be driven by a small torque, leading to limited application of the screwdriver, and if in any event a torque exceeds what the screwdriver can withstand, the detent which moved to a position in engagement with the teeth would be pushed out of the slot by the teeth.
U.S. Patent Application No. 2006/0096422 discloses a ratcheting mechanism for a driving tool. The ratcheting mechanism is utilized for permitting the driving tool to make a change in the driving rotational direction. The ratcheting mechanism includes two pairs of detents and each pair has a spring connected therebetween. The two pairs of detents are received in two spaces respectively defined in a driving member and each detent has one side abutted and supported by a wall of the space and an opposite side selectively engagable with a plurality of teeth defined in a control ring. Additionally, the control ring is rotatable and operably moves one of one pair of detents to a position in engagement with the teeth and has a stir bar that moves the other of one pair of detents to a position away from the teeth. Thus, when the driving tool is rotated in one driving direction two detents (i.e. upper left and bottom right or upper right and bottom left) would engage with the teeth at the same time, and this results that the driving tool is more capable of withstanding higher driving torque in comparison to the prior admitted invention. Further, while the two detents engage with the teeth, the other two detents are kept away from the teeth.
But, it is noticed that this invention has a practical difficulty because in operation when the control ring is rotated counterclockwise, the upper right and bottom left stir bars cause the upper right and bottom left detents to disengage or separate from the wall of the space and coact with the teeth, and even worse, after subsequent direction changes the detents will not mesh with the teeth correctly. Likewise, when the control ring is rotated clockwise, the upper left and bottom right stir bars would cause the upper left and bottom right detents to disengage from the wall of the space and coact with the teeth, leading to incorrect meshing after subsequent direction changes.