🔍 Key Concepts

• Pitch circle of a gear: the imaginary circle where mating gears effectively touch and roll without slipping • Pitch diameter vs. pitch radius: diameter is full width across the gear through the center; radius is from the center to the pitch circle (half the diameter) • How the illustration shows the gear centerline and circles (root, pitch, and outside diameters) with labeled arrows

💭 Think About

• On the drawing, which dashed or curved line represents the pitch circle, as opposed to the root (bottom of teeth) or outside (top of teeth) circle? • Which lettered dimension is drawn from the gear centerline out to that pitch circle (not all the way to the tooth tip or down to the root)? • Which labeled line would become the pitch diameter if it were drawn all the way across the gear, and thus its half would be the pitch radius?

✅ Before You Answer

• Identify the centerline of the gear first, then trace outward to find the pitch circle line before looking at letters. • Confirm that the dimension you choose does not reach the tooth tip (outside circle) or the tooth root (root circle), but stops at the pitch circle. • Compare the letters: one is clearly the full diameter across the gear (that’s pitch diameter), another is just from the center to that same circle (that’s pitch radius). Choose the one that is only a radius, not a full diameter.