Friday, June 12, 2020

Securing propellers

HYDRAULIC FLOATING OF PROPELLERS

Used for keyless propellers, ensures the correct interference fit using measured oil pressures for expanding the boss and hydraulic jacks or a Pilgrim nut for pushing it up or down the tailshaft taper. No heavy slogging required and low shock loads are applied, quick and safe.
A disadvantage is loss of bearing area due to oil grooves which means that propeller must be longer or greater in Dia to give sufficient area to transmit the torque.
To remove, nut just slackened back. Oil injection applied to expand the boss which allows propeller to move off the taper.
Another disadvantage of wet fitting over dry fitting is that wet boss expansion stress is 30% greater than dry fitted which means that boss must be thicker.






SLEEVED PROPELLER

 Usually fitted on large diameter shafting. Usually hydraulically floated and keyless. Difficult to bed large props to taper, easier to bed sleeve. Also each time a prop is refitted, prop bore becomes larger, this is accentuated in large bore dia props. Hence, after a few refits the prop moves to far up the shaft, more economical to replace the sleeve than the whole prop.
Pearlitic cast iron used to mate with forged mild steel shaft because this combination offers the greatest resistance to fretting which can be caused by prop excited vibration

Molecular fretting can occur internally, generally from the center outward due to molecular rubbing together. Surface fretting occurs at the surface due to two items moving over each other due to vibration.When fitting or removing, heat not to be used since it may effect the mechanical properties of the resin. Wedging or withdrawal systems should not be used since this would cause shearing of the araldite.

TRADITIONAL METHOD

Shaft turned to top centre, i.e. when key is on top, convenient for key way inspection and prop slinging. Shaft locked, and prop nut just slacked.
Coupling bolts at tail end flange removed, if lignum vitae bearing- stuffing bearing removed and tailed flange shored up against aft peak bulkhead.
Secured lifting gear to propeller, then wedge off prop using box wedges between stern tube nut and ford face of prop.
If tail end and bronze liner are to be inspected then it must be brought inboard which requires the removal of one or two lengths of intermediate shafting.






BOLTED

Controllable pitch propellers require a hollow prop shaft for the oil and feed back tubes to pass through. non of the above methods are suited to this.
Instead the propeller is bolted to a flange, the other end of the propshaft must therefore be parallel to allow removal from the stern bearing.
The prop shaft is attached to the intermediate shaft by a 'muff' coupling. Once the bolts have been tightened they are secured by tack welding locking bars across the heads.

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