1. Field of the Invention
The present invention relates to a color image forming apparatus of electrostatic recording type or of electrophotographic recording type, and more particularly, it relates to a color image forming apparatus having an endless belt convey mechanism used with a transfer material convey member or an intermediate transfer member.
2. Related Background Art
In image forming apparatuses such as copying machines, printers and the like, an electrophotographic system has widely been used. An image forming apparatus of electrophotographic recording type includes an image forming portion in which a latent image is formed on a photosensitive drum (image bearing member) by utilizing light, magnetism or charges and the latent image is visualized as a visualized image (toner image), a transfer material convey means for conveying a transfer material to a transfer position of the image forming portion, and a fixing device for fixing the image transferred to the transfer material onto the transfer material.
A belt-shaped convey member or transfer material convey belt is used as the transfer material convey means in view of convenience of conveyance, so that the transfer material can be conveyed to the transfer position and further to the fixing device. In this case, the transfer material is held on the transfer convey belt by an electrostatic absorbing force. The transfer material convey belt is wound around and supported by a drive roller and at least one driven roller so that the belt is moved along an endless path.
When a plurality of image forming portions are provided, the drive roller is disposed at a downstream side of a most downstream image forming portion and at an upstream side of the fixing device, and at least one driven roller is disposed at an upstream side of a most upstream image forming portion. Between the drive roller and the driven roller, the convey belt is maintained in a tension condition, so that the convey belt can be shifted at a constant speed from the most upstream image forming portion to the most downstream image forming portion without loosing between the rollers.
In the image forming portion, a roller or a blade (transfer charger) to which high voltage is applied is opposed to the image bearing member with the interposition of the transfer material convey belt and is urged against the image bearing member with the interposition of the transfer material convey belt by a biasing means. Further, as mentioned above, the transfer material (recording sheet) is electrostatically absorbed to the convey belt so that the transfer material can be conveyed to each image forming portion while being held by the convey belt positively. Then, toner images are successively transferred onto the transfer material, and thereafter are fixed to the transfer material in the fixing device, thereby obtaining a full-color image.
Recently, printers and copying machines have been made compact more and more. To this end, the transfer material convey belt, a sheet supply unit and the fixing unit have been arranged with minimum distances therebetween. However, in an image forming apparatus having such arrangement, the following problems occur.
After the transfer material is supplied from the sheet supply unit, when the transfer material leaves a pair of resist rollers (for feeding out the transfer material to a transfer unit) and reaches the transfer material convey belt, if there is a difference in rotational speed between the transfer material convey belt and the resist roller pair, the transfer material is conveyed while being pulled or pushed between the regist roller pair and the transfer material convey belt. In such a condition, immediately after a trail end of the transfer material leaves the regist roller pair, since the transfer material is conveyed with being balanced between the regist roller pair and the transfer material convey belt is suddenly conveyed only by the transfer material convey belt, abrupt change in speed occurs instantaneously. Further, in the same reason, when a tip end of the transfer material enters into the fixing device (fixing roller), abrupt change in speed occurs in the transfer material.
FIG. 6 is a graph showing a relation between time and a speed variation wave form when the speed variation of the transfer material being conveyed by the transfer material convey belt is measured by a laser Doppler velocimeter. In FIG. 6, a portion "a" shows abrupt speed variation. Such abrupt speed variation of the transfer material leads to poor image such as color deviation or formation of stripes.
Accordingly, in order to prevent the abrupt speed variation from affecting an influence upon the image, in the past, there has been proposed a technique in which a distance between a regist roller pair and a first image forming portion is selected to become greater than a length of the transfer material so that after the transfer material leaves the regist roller pair, the transfer material enters into the first image forming portion for forming a color image. Further, when a distance between a fourth image forming portion (last image forming process in the color image formation) and a fixing roller is also selected to become greater than the length of the transfer material, an influence is not affected to the image.
However, the above technique is contradictory to compactness of the image forming apparatus, and the provision of mechanism for releasing the regist rollers makes the image forming apparatus bulky.
On the other hand, it is considered that the reason why the abrupt speed variation of the transfer material is that a restraining force of a DC motor (drive source) is weak with respect to a shaft of the drive roller for the transfer material convey belt. However, in the past, since the output shaft was restrained by an electromagnetic force in the DC motor, non-steady state disturbance such as the above-mentioned abrupt speed variation could not be restrained adequately (inherent rotational speed cannot be maintained, and the output shaft is rotated by the influence of disturbance).
Further, as shown in FIG. 5, when an output shaft 143a of a DC motor 143 is drivingly connected to a shaft 145 of a drive roller for a transfer material convey belt 124 via timing belt 142, due to tolerance of mechanical elements disposed between the shaft of the drive roller and the output shaft of the DC motor, the restraining force is further weakened. Incidentally, the reference numerals 62, 63 denotes a transfer frame; 61a, 61b denote bearings.