1. Field of the Invention
The present invention relates to a driving force transmission apparatus having a toothed clutch that transmits a driving force received from a driving source, and a sheet conveyance apparatus.
2. Description of the Related Art
An image forming apparatus, such as a copier, a printer, a facsimile machine or a multifunctional apparatus having all these functions, mounts a thermal fixing device and a sheet conveyance mechanism, such as a recording material conveying belt mechanism. This sheet conveyance mechanism includes a sheet conveyance roller (or belt), which forms a sheet conveyance nip portion; a drive motor, for driving a sheet conveyance roller; and a driving force transmission mechanism, which transfers the driving force produced by a driving motor to the sheet conveyance roller.
When the image forming apparatus is abnormally halted because of a sheet jam in the sheet conveying mechanism and the sheet is caught at the conveyance nip portion, the sheet must be manually removed by a user, i.e., the jam must be cleared by the user. While the jam is being cleared, the sheet conveyance roller is rotated by friction between it and the sheet. At this time, because of the operating arrangement of the image forming apparatus, a driving force generated by the rotation of the sheet conveyance roller is transmitted to the driving motor via the driving force transmission mechanism, that is, a load produced by the rotation is imposed on the gear of the driving force transmission mechanism and thence on the driving motor. Therefore, removing the sheet is difficult and the sheet may sometimes be torn.
In order to reduce such a load, a driving force transmission mechanism having a function in which it is able to stop transmitting a driving force may be provided between the driving motor and the sheet conveyance roller. A toothed clutch is one example of such a driving force transmission mechanism. A toothed clutch includes a driving-side ratchet and a driven-side ratchet that are biased in the direction in which they are coupled. Thus, when the driving motor is rotated in the sheet conveying direction, the driving-side ratchet and the driven-side ratchet engage each other to transmit the driving force produced by the driving motor to the sheet conveyance roller. On the other hand, when a jam is being cleared and the sheet conveyance roller is rotated in the direction opposite to that in which a sheet is conveyed, the pawl of the driven-side ratchet slides over the pawl of the driving-side ratchet, so that the driven-side ratchet is not rotated and transmission of the driving force from the sheet conveyance roller to the driving motor is prevented. In this case, since the load imposed on a jammed sheet is comparatively small, a user can easily remove the sheet from the sheet nip portion.
However, for the conventional art described above, there are the following problems.
A user must rotate the driven-side ratchet forward a number of times when clearing a jam where a sheet is caught at the conveyance nip portion of the sheet conveyance roller. Therefore, the pawl of the driven-side ratchet must slide over the pawl of the driving-side ratchet many times. And since the driven-side ratchet and the driving-side ratchet are biased forward by a spring, in the direction in which they are coupled, the ratchet pawls forcefully strike each other each time the pawl of the driven-side ratchet slides over the pawl of the driving-side ratchet, and an impulsive sound (an action sound) is produced. It is assumed that this impulsive sound will frequently cause user discomfort.
As means for preventing the generation of such an impulsive sound, there is an arrangement whereby, when a user opens the cover of an apparatus to clear a jam, a lever that interlocks with the cover reversely impels the pawl of a driven-side ratchet separating, against the biasing force exerted by an biasing spring, away from the pawl of a driving-side ratchet (see Japanese Patent Application Laid-Open No. H10-142996). According to this arrangement, since the ratchet pawls do not forcefully strike each other while the user is removing the jam, the occurrence of the impulsive sound can be prevented. However, since a lever and an interlocking mechanism for securing the main body of the apparatus to the lever must be additionally provided, the number of parts is increased.
A pressure release mechanism is also proposed whereby, when a sheet jam has occurred, a drive motor is rotated in reverse, in the direction opposite that in which a sheet is normally conveyed, to remove the pressure imposed on a conveyance nip portion (see Japanese Patent Application Laid-Open No. 2000-029347). According to this arrangement, since no pressure is imposed on the conveyance nip portion while the jam is being cleared, the process can be easily performed.
However, the pressure release mechanism, for example, may not work, as in a case wherein power to the image forming apparatus is turned off during the image forming processing. At this time, while a user is clearing the jam, the ratchet pawls would strike each other and produce the above described impulse sound.
Further, according to the arrangement whereby the pressure imposed on the conveyance nip portion is released by rotating the driving motor in reverse, in the direction opposite to that in which a sheet is conveyed, the driving-side ratchet pawl will slide over the driven-side ratchet pawl each time the pressure release procedure is performed. And thus, an impulse sound will still be generated by the ratchet pawls.