How to Design a Helical Gear

A helical gear is a cylindrical gear whose teeth are cut at an angle to the axis of rotation. That single change — angling the teeth — is what separates it from a spur gear, and it drives almost every design decision that follows. This guide walks through the parameters that actually define a helical gear so you can specify one correctly the first time.

Why helical instead of spur?

On a spur gear, each tooth engages its mate all at once, along the full face width. That sudden contact is what makes spur gears noisy at speed. On a helical gear, the angled teeth engage gradually — contact starts at one end of the tooth and rolls across the face. The result is quieter, smoother running and higher load capacity for the same size, which is why gearboxes, automotive transmissions, and most high-speed drives use helical gears.

The trade-off: the angled teeth generate an axial thrust along the shaft, so the design has to account for a bearing that can take that thrust, or you pair two opposite-hand gears (a herringbone arrangement) to cancel it.

The parameters that define a helical gear

1. Module (or diametral pitch)

Module sets tooth size. Because the teeth are angled, you have to be clear about whether you mean the normal module (measured perpendicular to the tooth) or the transverse module (measured in the plane of rotation). Normal module is the usual manufacturing reference. In inch systems this is diametral pitch instead.

2. Number of teeth

Tooth count sets the gear ratio with its mate and, together with the module, the overall diameter. Fewer than about 17 teeth can cause undercutting on a standard profile — helical gears tolerate slightly lower counts than spur because of the helix, but it's still something to watch.

3. Helix angle

Typically between 15° and 30°. A larger helix angle means smoother, quieter engagement and more overlap — but also more axial thrust. Smaller angles reduce thrust but give up some of the smoothness advantage. Both gears in a mesh must share the same helix angle.

4. Hand (left or right)

The helix runs either clockwise (right-hand) or counter-clockwise (left-hand) along the axis. For two parallel-shaft helical gears to mesh, they must be opposite hand — a right-hand gear meshes with a left-hand gear. Getting this wrong is one of the most common helical-gear mistakes.

5. Pressure angle

Almost always 20° (14.5° on some legacy systems). It sets the tooth flank angle and affects strength and smoothness. Keep it consistent across a mesh.

6. Bore, keyway and face width

The bore fits your shaft; a keyway or set-screw flat transmits torque; face width sets how much tooth is in contact (wider = more load capacity, more thrust). These are assembly details, but they belong in the spec from the start.

A quick design sequence

1. Fix the ratio you need, then pick tooth counts for the pinion and gear.
2. Choose a module from the load and a helix angle (start at 20°).
3. Set hand so the pair is opposite-hand, and keep pressure angle consistent.
4. Size the bore, keyway and face width to the shaft and load.
5. Generate the solid, check the mesh, and iterate.

From spec to CAD file

Once you know those six parameters, you don't need to draw involute curves by hand. Cadfai's AI Helical Gear Generator takes them as a plain-English prompt and returns a real parametric solid as STEP and STL — so you can drop it straight into an assembly or send it to be machined. If you're deciding which file to use downstream, see STEP vs STL. And if your drive needs straight-cut gears instead, the Spur Gear Generator works the same way.