Effect of Keyways
A little consideration will show that the keyway cut into the shaft reduces the load carrying capacity of the shaft. This is due to the stress concentration near the corners of the keyway and reduction in the cross-sectional area of the shaft. It other words, the torsional strength of the shaft is reduced. The following relation for the weakening effect of the keyway is based on the experimental results by H.F. Moore.
e = 1 – 0.2 (w / d ) – 1.1 (h / d )
where e = Shaft strength factor. It is the ratio of the strength of the shaft with keyway to the strength of the same shaft without keyway,
w = Width of keyway,
d = Diameter of shaft, and
h = Depth of keyway = Thickness of key ( t ) / 2
It is usually assumed that the strength of the keyed shaft is 75% of the solid shaft, which is somewhat higher than the value obtained by the above relation.
In case the keyway is too long and the key is of sliding type, then the angle of twist is increased in the ratio kθ as given by the following relation :
kθ = 1 + 0.4 (w / d ) + 0.7 (h / d )
where kθ = Reduction factor for angular twist.
Reference A Textbook of a Machine Design by R. S. Khurmi and J. K. Gupta