BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an image forming apparatus such as a copying machine or a printer equipped with a function of forming an image on a recording material such as a sheet.

Description of the Related Art

Japanese Patent Laid-Open No. 9-230657 discloses a configuration in which an annular rib is disposed between a central portion and a tooth surface of a gear and the tooth surface and the annular rib are disposed at an interval so as not to come in contact with each other. According to such a configuration, since the tooth surface and the annular rib do not contact with each other, phenomena are suppressed in which a portion of the tooth surface coming in contact with the annular rib is deformed by shrinkage during molding and thus accuracy of the tooth surface deteriorates.

By this configuration, it is considered that variation in a position occurs due to a rotation fluctuation or vibration of an image preparing portion caused by a rotation fluctuation or vibration occurring at a gear engagement cycle and thus a periodic band-like uneven density called a banding image is prevented.

However, the invention disclosed in Japanese Patent Laid-Open No. 9-230657 does not cope with reduction in size of a gear and modules for the purpose of miniaturization of an apparatus body in recent years. It is difficult to make the modules smaller in the case of reducing the size of the gear. This reason is that stress applied to a tooth root of the gear rises when the size of the module becomes smaller.

Under these circumstance, the inventors paid attention to the fact that a portion of an arm formed between the tooth surface and a rotation support portion is disposed at the center of a tooth width direction in the configuration illustrated in FIG. 3 of Japanese Patent Laid-Open No. 9-230657. The inventors found that it is possible to reduce the size of the module and to lower the stress applied to the tooth root of the gear by changing the arrangement of the arm.

SUMMARY OF THE INVENTION

The invention is to provide an image forming apparatus capable of suppressing stress concentration on a tooth root of a gear.

An image forming apparatus that forms an image on a recording material includes: a first helical gear and a second helical gear that are engaged with each other; and a driving portion that applies a driving force to the first helical gear, wherein at least one of the first helical gear and the second helical gear is a helical gear in which torsional rigidity in a tooth width direction of one side end in a width direction of the gear is larger than torsional rigidity in a tooth width direction of the other side end, and wherein a twist direction of helical teeth and a rotational direction of the first helical gear due to the driving portion are set such that the other side end is engaged earlier than the one side end.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

