What effect has a keyway on the strength of a shaft?

1. This is due to the stress concentration at the corners of the key way and reduction in the cross-sectional area of the shaft. It, other Words the torsional strength of the shaft is reduced.

2. In case the key way is too long and the key is of sliding type, then the angle of twist increased.

So it is actually assumed that the strength of the keyed shaft is 75% of the solid shaft.