Tailshaft keys and keyways

KEYS AND KEYWAYS There are approximately 100 reported cases per year of partial or total tail shaft failure and 200 reported cases of lost props. Causes of this are quoted as inadequate force fit between prop and tailshaft causing loss of peripheral grip which allows prop to move and make contact with key. This causes excessive dynamic load to fall on key and shaft adjacent to keyway. This causes incipient cracks (small and superficial ) which usually begin at high stress concentration areas i.e. around the leading edge of the keyway

These fatigue failures may be corrosion N.B. Temperature variations in sea water can alter the force fits

Keys and Keyways

Abrupt changes of shape of section cause stress concentrations to build up due to interruption of the stress flow lines.

This build up in stress causes cracks to develop and supports crack propagation. With this in mind it can be seen that shapes or sections which may be subject to great stresses; should be well rounded or gradually tapered off to give smooth stress flow.

Round end keys used and the keyway in prop boss and cone of the tailshaft are to be provided with a smooth fillet at bottom of keyways, fillet radius at least 0.0125 of shaft diameter at top of cone. Sharp edges at top of keyway to be removed. Two screw pins secure key in keyway and the for'd pin should be at least 1/3 of key length from for'd end. Pin holes should have a depth not exceeding pin diameter. Hole edges bevelled.