Does a keyway leave the shaft's torque capacity unchanged?

The short answer

It should not be assumed unchanged. A keyway removes material and introduces local geometric transitions that can concentrate stress. A wind-generator shaft assessed as a smooth circular section may therefore need additional evaluation at its actual keyed connection, particularly under combined or cyclic loading.

Separate nominal and local stress

A uniform-shaft torque expression describes an ideal section. The keyway changes both the remaining section and the local stress field near its corners. A nominal stress calculated using the original outside diameter does not capture those effects. Material condition, keyway shape and load history influence the relevant assessment, so a generic stress-concentration value cannot be selected from the word keyway alone.

Check the whole keyed connection

Shaft strength, key strength and hub contact are separate possible limits. A key that can transmit the required torque does not prove that the shaft section can carry the combined torsional and bending duty. The hub and axial retention need their own checks. A plain-shaft fatigue result should not be transferred to the keyed part without identifying how the altered geometry was represented.

Request a geometry-specific strength basis

Ask the equipment designer for the actual keyway profile, material and relevant static/fatigue assessment at that location. Ringfeder describes keyless connections as avoiding keyway-related geometry; that product comparison does not quantify any MARSTON shaft penalty. Do not assume removing a key improves an existing assembly, because it also changes torque transmission and retention. The concrete review decision is whether the submitted shaft assessment covers its manufactured keyway and real combined loading, rather than a geometrically simpler reference shaft. If bending is present at the keyway, include its direction and cyclic relationship to shaft rotation. A torque-only section check can miss a different stress component governing the actual keyed location under service loading.