Pressure Angle Explained
What pressure angle controls, why 20° is the modern standard, and how it changes a gear's shape and strength.
Pressure angle is the angle between the tooth flank's line of action and the tangent to the pitch circle at the point where two gears touch. Every gear in this catalog uses the modern standard, 20°. Older gearing (particularly pre-war and some clock/instrument gearing) often used 14.5°, which is still occasionally seen for compatibility with legacy parts.
Pressure angle sets the base circle radius directly: base radius = pitch radius × cos(pressure angle). A larger pressure angle pushes the base circle inward (smaller), which does two things to the tooth shape:
- Wider tooth base — the tooth is thicker at the root, which is stronger in bending.
- More separating force — more of the contact force between meshing teeth is directed radially (trying to push the gears apart) rather than tangentially (doing useful work), which increases bearing/bushing load.
20° is the standard compromise between those two effects for general-purpose gearing, which is why it's the default — and the only option — across this entire catalog. It's also why two gears with different pressure angles can't mesh correctly even if their module matches: the flank shapes and base circles don't line up.
Effect on undercutting
A higher pressure angle also reduces the minimum tooth count before root undercutting occurs (roughly 17 teeth at 20°, versus around 32 at 14.5°), which is one more reason 20° suits compact gear trains better. See the design guide for how that interacts with tooth-count choice, or Involute Gears Explained for the base-circle geometry itself.