1. Field of the Invention
The present invention relates to a driving force transmission device made of synthetic resin.
2. Description of the Related Art
Because gears made of synthetic resin can be readily formed by injection molding or the like at a low cost, manufacturing costs of drives employing synthetic resin gears can be reduced.
However, synthetic resin gears have drawbacks which metal gears do not, such as sink marks (i.e., defective shallow depressions) resulting from material shrinkage after a thick gear is molded. It is therefore necessary for synthetic resin gears to have a minimum thickness in order to prevent such sink marks from forming, and at the same time have sufficient rigidity and strength.
In a conventional gear 100 shown in FIGS. 5 and 6, an annular rib 106 is disposed between a boss 102 formed at the center of the gear 100 and a rim 104. Ribs 108 are interposed between and connect the boss 102 and the annular rib 106, and ribs 110 are interposed between and connect the annular rib 106 and the rim 104. A disc-shaped member 112 is disposed at a width-direction center of the rim 104 and connects (extends between) the rim 104 and the boss 102. Teeth are disposed along an outer periphery of the rim 104, with bottom lands being formed between respectively adjacent teeth and top lands 114 being formed at crests of the teeth. A radius R is equal to the distance from the center of the boss 102 to an imaginary circle formed by joining the top lands 114 (hereinafter, “top land circle”).
As mentioned above, when the gear 100 is made of synthetic resin, there arises a problem in that portions of top lands 114 corresponding to positions at which the ribs 110 connect to an inner peripheral surface of the rim 104 may be pulled radially inwards due to material shrinkage of the ribs 110 and the like. As a result, radii R drawn to top lands 114 corresponding to the positions at which the ribs 110 connect to the rim 104 become smaller than radii R drawn to top lands 114 not corresponding to positions at which the ribs 110 connect to the rim 104 (i.e., the gear 100 becomes radially nonuniform), whereby the top land circle becomes distorted.
When a synthetic resin gear having a distorted top land circle is used, angle speed of a member (e.g., another gear or a timing belt) driven by the gear varies despite the angle speed of the gear being constant, which results in uneven rotation of the driven member. Consequently, uneven rotation of the driven member can adversely affect the drive in which the gear is employed.