The bore or spline of a Sun Gear is part of the torque path and part of the rotating datum system. If it is underspecified, a correctly cut gear tooth can still run eccentrically or fit the shaft incorrectly.
Internal spline Sun Gears
Internal involute splines are common where the Sun Gear slides or mounts over a driving shaft. Specify the spline standard or complete geometry, tooth count, module/DP or pitch, pressure angle, fit class, effective length and any lead-in or relief.
Integrated Sun Gear shafts
When the gear and shaft are one piece, bearing journals, shoulders and other interfaces become part of the same datum chain as the gear teeth.
Common interface choices
| Plain bore + keyway | Simple torque interface; bore fit, key dimensions and axial retention still need definition. |
|---|---|
| Internal involute spline | Good torque distribution and assembly flexibility; requires complete spline geometry/fit definition. |
| External spline on Sun Gear shaft | Connects the integrated gear shaft to another transmission member. |
| Integral shaft | Eliminates one joint but makes journal-to-gear concentricity and heat-treatment distortion more critical. |
| Press/shrink fit | Needs fit, material, surface condition and assembly method to be controlled together. |
Concentricity should reflect function
Do not add an arbitrary tight concentricity tolerance everywhere. Identify which bore, spline or journal actually establishes rotation in the gearbox and relate the gear tooth runout to that datum. This produces a measurable requirement that corresponds to assembly function.
What to send in an RFQ
Include the gear drawing and a separate spline standard if it is not fully defined on the print. If gauges are mandatory, state the gauge system or inspection requirement. For an integrated shaft, add bearing and coupling interfaces so the manufacturing plan can protect the important datums through heat treatment and grinding.
