ESDU 15004 considers the effect of key and keyway design on the stresses in a keyed shaft loaded in torsion@ bending or under combined torsion and bending. Common types of keys@ such as the Woodruff key and the square key@ are discussed and stress concentration data is given for a range of keyway geometries for both the sled-runner and the end-milled type keyways. A method to optimise keyway design is presented to reduce stress concentrations in the keyway in a shaft. A worked example illustrates the use of the data in this Item. ESDU 15004 is closely related to ESDU 86028 which presents data on the fatigue strength of a shaft with the keyway in bending@ and a shaft loaded through the key in torsion.