PROPELLER MOUNTING (PILGRIM NUT DRY PUSH UP METHOD)

KEYLESS PROPELLER:

The success of a keyless propeller depends on the accuracy of the hub and shaft tapers and correct grip from the stretched propeller hub on the shaft. The degree of stretch or strain is controlled.by push up. It must ensure adequate grip despite any temperature changes and consequent differential expansion of bronze hub and steel shaft. It must also avoid over stressing of the hub and in particular any permanent deformation. Lloyd's require that the degree of interference be such that the

frictional force at the interface can transmit 2.7 times the nominal torque when the ambient temp, is 35°C. Lloyd's also require that at 0°C , the stress at the propeller bore, as given by 'Van Mises' stress criterian, shall not exceed 60% of 0.2% proof stress of the propeller material as measured on a test bar.

Reasons for fitting Keyless Propeller :-

In keyed propeller, the propeller was pressed on to the tapered tail shaft by "Flogging up" the propeller nut. Taper was 1:15 or 1:12. The interference fit was expected to be capable of transmitting 1.5 times the normal torque, but in practice only the keys were subjected to extreme torque. The incidence of failure of tail shafts due to cracks at the keyway was extremely high, usually this was accompanied by fretting at the interface of propeller boss and shaft. Rounded ends of keyway and "Punt" Shaped key helped by eliminating "Stress Raisers".

Keyless Propeller with cast Iron Sleeve(Drv Push) :

In order to avoid cracking of keyways in keyed propeller due to high torque subjected to the keys and very high boss stress in keyless propeller due to low coefficient of friction at the interface between bronze propeller hub and steel shaft as well as variable expansion due to temperature differences, Cast Iron sleeve is fitted between the propeller and the tail end shaft taper. Cast Iron has similar expansion rates to steel and the increase in coefficient of friction is considerable(cast iron = 0.24, Steel = 0.12, Bronze = 0.08). A keyless propeller transmits torque by friction grip, giving a requirement of more than 80% contact area. This needs a very expensive and critical machining process. A rough machined propeller with a fitted sleeve eliminates this. A pearlitic cast iron sleeve(minimum fretting quality) is machined and than hand bedded to propeller shaft. Finally the sleeve is dry pushed on to the shaft to give final improvement in the contact area. The sleeve is then passed to the propeller manufacturer, where it is fitted inside the boss, secured by a high strength epoxy compound. The propeller is then fitted on to the tail-end shaft by pilgrim nut dry push method. Removal is by oil injection into the oilways between the sleeve and the shaft.

Keyless Propeller (oil fitted, wet push) :

The SKF System - With this system, instead of a dry push, oil is injected by a high pressure pump between the shaft taper and the bore of the propeller boss to a small axial and circumferential grooves or a continuous helical groove machined in the propeller bore. The oil reduces the coefficient of friction between the surfaces to about 0.015. A hydraulic ring jack is arranged between the shaft nut and aft face of the propeller boss, to push the propeller up the shaft taper by the required amount, overcoming the friction force and the axial component of the radial pressure. The amount of push up is pre-determined to achieve the correct interference fit and is controlled by the hydraulic spacer ring. During push up, the boss is floating on a film of oil, this results in a push up force of about l/20th of that needed for a dry process. With this method, the pressure at the interface and the hoof stresses, both in the boss and the shaft are known. When the oil injection pressure is released, the propeller is held in position by the nut for some hours, to allow the oil film to disperse between the shaft/bore surfaces leaving an interference friction grip to attain its maximum value of at least 0.12. For removal, the boss' is expanded hydraulically, and the nut is slackened off an amount equal to the push down distance taking precautions to prevent the propeller jumping at release.

INDEX ADV ENG                                                                                               NEXT