Keyless propeller (palm/cone/thread type coupling)

In a conventional keyed propeller the boss has tapered bore with keyways and the shaft a taper with a keyway, and a key is driven to transmit torque. This introduces stress raisers (the keyway in the shaft is a potential stress concentration where fatigue cracks start), makes the propeller difficult to fit/remove, and the fit depends heavily on the accuracy of the key. In a keyless propeller the propeller is secured by friction alone: the cone (taper) of the shaft is drawn hard into the conical bore of the propeller by a large nut (locking nut), and the high friction between the mating conical surfaces transmits the full torque without a key. Eliminating the keyway removes the stress concentration in the shaft, giving a stronger shaft, and simplifies fitting. The propeller/shaft taper angle is self-energising - the more torque, the tighter the cone grips.

Angular slip (the propeller rotating on the shaft) is prevented by the self-locking friction of the cone. The tapered cone and bore are machined to a very close tolerance so that when the nut is tightened the propeller is forced down the cone, generating high radial pressure and hence large friction between the two surfaces. Torque in either direction produces a wedging action that increases the friction; hence slip cannot occur, provided the nut is correctly tightened to the prescribed torque and the tapers are clean, undamaged and match correctly. Marking of the cone and alignment flats/line ensures correct angular positioning relative to the shaft keyway/flats.

Because there is no key to positively locate the propeller angularly, the propeller must be lifted and driven onto the taper by its own effort (the propeller is partially supported and allowed to ride up the cone under its own weight as the nut is drawn), and its angular position set by rotating on the greased cone until the alignment marks meet. Similarly, removal: the propeller cannot be knocked straight off as with a key (which would free it immediately); instead a contractor (puller) ring with bolts and a puller - a threaded puller bolted to lugs on the boss with a central bolt bearing on the shaft end - is used to jack it off the taper, or the shaft is driven through, or a hydraulic puller draws it off. Axial location depends on the nut and washer arrangement rather than a keyway shoulder.

Driving wedges and jacks between the boss and the shaft, or hammering directly on the boss, subjects delicate parts to irregular shock loading and can distort or crack the cast boss, damage the taper surfaces and the bearing/liner behind the propeller, and risk damaging the shaft or causing misalignment. The propeller cone needs a clean, controlled axial pull. Using wedges and jacks is therefore not advisable because they can permanently damage the propeller boss, the shaft cone and the almond/bearing, and cause the propeller to wedge on the shaft. The approved method is a mechanical or hydraulic puller that applies an even axial force through the puller lugs, or thermal contraction of the shaft, so the propeller lifts cleanly off its taper.