A 20-tooth pinion driving a 60-tooth gear, 20° pressure angle, both gears the same material. σF = (Ft / b·m)·YFa·YSa with YFa·YSa ≈ 4.3 for 20 teeth. σH = ZE·ZH·√(Ft/(b·d₁) · (u+1)/u) with ZH ≈ 2.5 and u = 3. The red mark is a safety factor of 1.5. Real ratings multiply Ft by factors for dynamic load, overload and misalignment, often 1.5–2.5 combined, so treat anything near the line as a fail.