Spur Gear Design Basics

Start with tooth count and module

For a standard metric spur gear, the reference pitch diameter is d = mz, where m is module and z is tooth count. Two gears that mesh must have compatible tooth system geometry, including module and pressure angle.

Ratio and speed

For a simple external pair, i = z₂/z₁ = n₁/n₂. A larger driven gear reduces speed. The two external gears rotate in opposite directions.

Center distance

For the basic unshifted reference geometry, center distance is a = (d₁+d₂)/2. Profile shift, backlash targets and manufacturing details can modify practical center-distance decisions.

Geometry is not a strength rating

A pair that meshes geometrically can still fail by tooth-root bending, contact fatigue, scuffing, wear, noise, overheating or shaft/bearing deflection. Standards-based gear rating and supplier data are appropriate for released power-transmission designs.

CAD modeling guidance

Do not over-constrain a concept by modeling every tooth too early. Keep tooth count, module, center distance and shaft interfaces parametric, and separate layout geometry from final manufacturing tooth definition.