[Gary Dillard]

The main problem with welding a shaft to a gear isn't strength, it's distortion; you will end up with a shaft axis that's not aligned with with the gear axis, which will create very high point loads on the gears. Gears are typically strength limited at both the tooth root in bending and contact area in bearing, so you would be increasing the stress there. The weld heat affected zone is usually 2-3 times the thickness so a small pinion might have a strength problem where a larger gear wouldn't. Typical steel gear stock material is about the same strength as the weld rod (60-70 ksi tensile), but the teeth are frequently hardened or case carborized to increase their local strength.