Method of designing drive unit

A method of designing a drive unit, the drive unit including

• a sheet metal frame having a plurality of bearing holes;
• a housing mounted on the sheet metal frame, the housing being made of a predetermined resin material, the housing provided with a plurality of shaft portions projecting toward the sheet metal frame, each of the plurality of shaft portions having a distal-end portion pivotally supported in the bearing holes;
• and
• a plurality of gears rotatably mounted respectively on the plurality of shaft portions, wherein the drive unit is linked to a preestablished drive source and transmits, via a drive transmission path constituted by the plurality of the gears, rotational driving power of the drive source to rotated components each having a predetermined load torque;
• wherein the design method comprises:
• deriving a minimum reference contact pressure Fa being a contact pressure acting on the plurality of shaft portions made of the resin material, and being the minimum contact pressure that leads to damage in any shaft portion among the plurality of the shaft portions;
• and
• designing the shaft portions or the gears to be of form such that the plurality of shaft portions and the plurality of gears satisfy the relationship F/D×L<Fa, given that, with the plurality of shaft portions and the plurality of gears linked to the drive source and the rotated components, the load acting on a single of the shaft portions among the plurality of shaft portions is F (N), the diameter of the single of the shaft portions is D (mm), and the length of axial contact between the single of the shaft portions and the gear mounted thereon is L (mm).