Sprocket mechanism for cameras

A sprocket mechanism for cameras adapted for advancing a film provided with two juxtaposed sprocket holes at regular space intervals includes a sprocket with a body having a couple of pawls spaced apart at a pre-determined distance at a circumferential rim thereof and a first gear disposed on a top side of the body and having a notch at a suitable position thereof, a transmission gear having a second gear formed on its top side, the second gear having a shaft extending from a bottom side thereof and being inserted into a central hole of the first gear, the shaft capable of free movement within the central hole and having a stop element fitted thereon, the stop element being provided with a vertical segment at an extreme end thereof to be retained in the notch of the first gear. When the pawls of the sprocket engage the sprocket holes of the film, the sprocket, the transmission gear and the stop element rotate synchronously. After the sprocket has rotated through a certain angle, the pawls will come into contact with the film and the stop element will absorb the transmitted rotational speed to ensure the positioning of a single frame of the film.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a sprocket mechanism for 
cameras, and more particularly to a sprocket mechanism adapted for use 
with a newly developed film construction to ensure positioning of a single 
frame of the film at every advancing action. 
2. Description of the Prior Art 
FIG. 1 shows an existing film roll structure 10 which comprises a 
substantially hollow cylindrical magazine 11 and a spool 13 axially 
disposed in the center of the magazine 11. A film 14 is wound around the 
spool 13 and may be pulled via a notch 12 at a side of the magazine 11. 
The top and the bottom sides of the film 14 are provided with a 
multiplicity of sprocket holes or perforations 15. This kind of film rolls 
have been used for many years and are quite practical indeed. 
With the development of modern technology, some manufacturers have recently 
developed a new kind of film rolls which are more performing and compact 
in size than conventional ones. Referring to FIG. 2, such a new film roll 
20 is mainly characterized in that a top side of a magazine 21 is 
configured to have four different signs 22 and that a location plate 23 
located at an inner wall of the top side of the magazine 21 will, 
following the action of the film roll 24, be situated at one of the signs 
22 to represent the use status of the film roll 20. And besides, a film 25 
of the film roll is provided with two juxtaposed sprocket holes 26 at 
regular space intervals (such as those illustrated in FIG. 2) for 
stressing the uniqueness of each individual frame. In other words, the 
exposure position of each frame is predetermined, unlike the conventional 
film rolls in which the exposure position of each frame is determined by 
the length of the leader pulled out when mounting the roll into the camera 
and by the length of the film wound upon film advancing, hence the lack of 
efficient film planning. The new film roll also provides an improvement 
for matching some structural parts of the camera to permit tight winding 
of the film 25, further reducing the size of a film compartment defined 
between the magazine 21 and the spool 24 for accommodating the film 25. In 
order to match the new kind of film rolls recently developed, novel 
improvement on the construction of the camera is therefore necessary. 
With reference to FIG. 3 which shows a sprocket mechanism 30 and a roller 
mechanism 31 for assisting the advancing of films in conventional cameras, 
both the sprocket mechanism 30 and the roller mechanism 31 are driven by a 
motor via a plurality of gears 33 and planetary gears 34 to perform an 
advancing action on the film. However, it may be found that the sprocket 
mechanism 30 is proportionally provided at its circumferential rim with 
eight pawls 301 which may randomly engage with the series of sprocket 
holes of the film for transmission purposes. This arrangement, however, 
cannot and do not suit the new kind of film roll 20 with pre-determined 
film planning. 
SUMMARY OF THE INVENTION 
A primary object of the present invention is to provide an improved 
sprocket mechanism for cameras, which not only provides the function of 
film advancing as in the prior art but also ensures positioning of each 
frame of the film.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The sprocket mechanism according to the present invention is a transmission 
mechanism for film advancing used in cooperation with the new film system 
lately available on the market. FIGS. 4 and 5 show a preferred embodiment 
of the present invention, which comprises a sprocket 40, a transmission 
gear 50 and a stop element 60. 
The sprocket 40 has a body 41 circumferentially provided with a couple of 
pawls spaced apart from each other at a pre-determined distance. The 
sprocket 40 further has a first gear 43 with a certain thickness provided 
on a top side thereof. The first gear 43 is provided with a notch 44 at a 
suitable position of its circumference. 
The transmission gear 50 is provided with a second gear 51 at a top side 
thereof. The second gear 51 is engageably connected to a transmission gear 
of a motor (not shown). The second gear 51 further has a shaft 52 
extending from a bottom side thereof, the shaft 52 having a diameter 
slightly smaller than that of a central circular hole 45 of the first gear 
43, so that there is a clearance permitting movement of the shaft 52 when 
the shaft 52 is fitted into the hole 45. 
The stop element 60 is fitted onto the shaft 52. The stop element 60 may be 
a friction resilient piece or a spring. In this embodiment, a spring 60 is 
adopted. The spring 60 further has a vertical segment 61 provided at an 
extreme end thereof. The vertical segment 61 is retained in the notch 44 
of the first gear 43. 
The structural elements of the sprocket mechanism according to the present 
invention and their relationship are described as above. Reference is now 
made to FIG. 6 which illustrate the actions of the sprocket mechanism of 
the present invention. As shown in FIG. 6, a new kind of film 70 is 
provided with two juxtaposed sprocket holes 71 at regular space intervals. 
When the film 70 is being advanced, the second gear 51 engageably 
connected to the transmission gear of the motor (not shown) starts 
turning. At this moment, the sprocket 40, the transmission gear 50 and the 
stop element 60 are synchronously turned therewith. By means of its pawls 
52 engaging the sprocket holes 71 of the film 70, the sprocket 40 assists 
in the transmission action. After the sprocket 40 has turned through a 
certain angle, its pawls 42 will come into contact with the film 70. Since 
the sprocket 40, the transmission gear 50 and the stop element 60 are 
interconnected, under the condition that any one of them is not subjected 
to an external checking force, they will turn as an integral whole. 
However, when any one of them is subjected to an external force, they will 
be compelled to move individually on their own. Thus, when the sprocket 40 
is checked by the film 70, the rotational speed intended to transmit to 
the sprocket 40 by the second gear 51 is absorbed by the stop element 60, 
that is, the second gear 51 will be in idle rotation and the sprocket 40 
will not rotate therewith temporarily. On a second advancing action, the 
couple of sprocket holes 71 of the film 70 will approach the sprocket 40 
and engage the pawls 42. Then the sprocket 40 will, again, synchronously 
rotate with the transmission gear 50 to advance another frame of the film 
70. By this arrangement, it may be ensured that a single frame of the film 
is positioned upon every advancing action. 
In view of the aforesaid, the sprocket mechanism for cameras according to 
the present invention is directed to match a new kind of film rolls and to 
ensure positioning of a single frame of the film upon every advancing 
action, providing substantial improvement over the prior art as well as a 
novel construction. 
Although the present invention has been illustrated and described with 
reference to the preferred embodiment thereof, it should be understood 
that it is in no way limited to the details of such embodiment but is 
capable of numerous modifications within the scope of the appended claims.