GEAR SHAPES

References 2 and 16 include many recommendations for the geometric design of gears considering
strength, inertia, and molding condifions. Many smaller gears are simply made
with uniform thickness equal to the face width of the gear teeth. Larger gears often have a
rim to support the teeth, a thinned web for lightening and material savings, and a hub to facilitate
mounting on a shaft. Figure 9^1 shows recommended proportions. Symmetrical
cross sections are preferred, along with balanced section thicknesses to promote good flow
of material and to minimize distortion during molding.
Fastening gears to shafts requires careful design. Keys placed in shaft key seats and
keyways in the hub of the gear provide reliable transmission of torque. For light torques,
setscrews can be used, but slippage and damage ofthe shaft surface are possible. The bore
of the gear hub can be lightly press fit onto the shaft with care to ensure that a sufficient
torque can be transmitted while not overstressing the plastic hub. Knurling the shaft before
pressing the gear on increases the torque capability. Some designers prefer to use
metal hubs to facilitate the use of keys. Plastic is then molded onto the hub to form the
rim and gear teeth.