1. Field of the Invention
The present invention relates to a tape driving mechanism for a magnetic recording apparatus such as a video cassette recorder and a cassette tape recorder. The present invention, in particular, relates to a tape driving mechanism in which a pinch roller is set up to press a magnetic tape against a capstan to pinch the magnetic tape between the pinch roller and the capstan in order to drive the magnetic tape.
2. Description of the Related Art
FIG. 9 shows a conventional tape driving mechanism for a magnetic recording apparatus. FIG. 10 is a sectional view taken on line X--X in FIG. 9. In this mechanism, a pinch roller 2 moves toward a capstan 1 until the pinch roller 2 is pressed against the capstan 1 with a magnetic tape 3 pinched between the pinch roller 2 and the capstan 1. The outer portion of the pinch roller 2, which is rotatable, is made of an elastic material such as rubber. In FIG. 9, when the capstan 1 is rotated in the direction of an arrow A1 by a driver (not shown), the magnetic tape 3 is advanced in the direction of an arrow A3 against a load F.
Since the axial length of the capstan 1 and the axial length of the pinch roller 2 are designed to be greater than the width of the magnetic tape 3, as shown in FIG. 10, the capstan 1 and the pinch roller 2 are in direct contact with each other on both sides of the magnetic tape 3. A mechanism for transferring the driving force to the magnetic tape is made up of an indirect frictional drive mechanism and a direct frictional drive mechanism. In the indirect frictional drive mechanism, the driving force is first transferred from the capstan 1 to the pinch roller 2, which in turn drives the magnetic tape 3 with a frictional force generated between the pinch roller 2 and the magnetic tape 3. In the direct frictional drive mechanism, the capstan 1 directly drives the magnetic tape 3 with a frictional force generated between the capstan 1 and the magnetic tape 3. The capstan 1 is conventionally made of a metal, and the outer portion of the pinch roller 2 is usually made of rubber whose frictional coefficient is far greater than that of the metal. The pinch roller 2 has a diameter far larger than that of the capstan 1; thus, the contact area of the pinch roller 2 with the magnetic tape 3 is naturally larger than the contact area of the capstan 1 with the magnetic tape 3. This suggests that the indirect frictional drive mechanism plays a greater role in driving the magnetic tape 3 than the direct frictional drive mechanism. It should be noted that rubber is in general less uniform in hardness than metal. The greater role the indirect frictional drive mechanism plays, the more the rubber portion of the pinch roller 2 pronounces its effect of uneven hardness. As a result, stable tape advance may be impaired.
A factor which determines the magnitude of frictional forces in these frictional drive mechanisms is the pressure the pinch roller 2 exerts onto the capstan 1. An appropriately large pressure is required to obtain a sufficient tape driving force. A force of 1 kg or more is usually exerted in the prior art technique. In an apparatus like a video cassette recorder which uses a tape driving mechanism such as this conventional one, substantial limitations as described below arise, in particular in realizing compact and light-weight design.
To realize a compact and light-weight apparatus, any mechanical parts and other parts integrated into a chassis should be miniaturized to their extremes. If, in such the miniaturized structure, the pressure the pinch roller exerts remains as large as a pressure in the conventional mechanism, mechanical parts, such as the pinch roller arms, suffer from warpage or deformation, thereby impairing the operation of the apparatus. Since the bearings employed in the capstan is also miniaturized in their construction, their allowable load is accordingly small. If the pressure the pinch roller exerts is as large as in the conventional mechanism, the service life of the bearings may be shortened. To miniaturize the apparatus, therefore, the pressure the pinch roller 2 exerts onto the capstan. 1 should be as small as possible. The indirect frictional drive mechanism is a major driving force transmission mechanism, as already mentioned. If a large pressure is exerted, however, the rubber portion of the pinch roller quickly wears and deteriorates. As a result, the stable operation of tape advance will soon be impaired.
Conversely, if the pressure which the pinch roller 2 exerts onto the capstan 1 is set too small in the conventional tape driving mechanism, no sufficient frictional force occurs between the capstan 1, the pinch roller 2, and the magnetic tape 3. Slippages take place between the capstan 1, the pinch roller 2, and the magnetic tape 3, leading to unstable tape advance. The apparatus does not provide the performance originally intended. Its reliability is destroyed.
Japanese Laid-Open Patent Publication 2-199653 discloses a mechanism wherein a capstan has irregularities on its surface. According to the disclosure, the capstan has grains of 1-100 .mu.m diameter stuck to its surface in order to provide a larger frictional coefficient between the capstan and the magnetic tape. See FIG. 11. The disclosure states that this method assures a stable tape advance. In this method, however, a thin magnetic tape is naturally deformed in accordance with the irregularity of the capstan surface when the pinch roller exerts pressure, and a large frictional coefficient results between the magnetic tape and the pinch roller as well. The disclosure fails to mention how much of an increase in frictional force this method offers by means of the direct frictional drive mechanism and how much by means of the indirect frictional drive mechanism. Japanese Laid-Open Patent Publication 2-199653 does not analyze the tape driving mechanism from the above point of view. The disclosure also suggests impressing or sandblasting to form irregularity on the capstan surface. These methods are not satisfactory from the standpoint of durability, according to the disclosure.