Answer MEP Question 61

Q. A. Sketch a transmission shaft coupling which enables the propeller shaft to be withdrawn outward. B. (i) describe the coupling and method fitting and dismantling; (ii) state how the grip of the coupling can be checked when fitted; (iii) why should coupling bolts be interference fits (iv) State what precaution should be taken when dismantling the coupling. (iv) State reasons why the metal that the intermediate and tail end remission shaft are made of. 

(i) describe the coupling and method fitting and dismantling;

- An alternative to the conventional flange couplings for the tail shaft, the muff coupling allows the shaft to be withdrawn outboard. The SKF coupling, shown in figure, consists basically of two steel sleeves.  

- The thin inner sleeve has a bore slightly larger than the shaft diameter and its outer surface is tapered to match the taper on the bore of the cuter sleeve. The nut and sealing ring close the annular space at the end of the sleeves. When the coupling is in position, the outer sleeve is hydraulically driven on to the-tapered inner sleeve. At the same time oil is injected between he contact surfaces to separate them and thus overcome the friction between them. Oil for the operation is supplied by hand pumps; two for the forced lubrication and another hand or power pump for the driving oil pressure. When the outer sleeve has been driven on to a predetermined position, the forced lubrication pressure is released and drained. Oil pressure is maintained in the hydraulic space until the oil between the sleeves drains and normal friction is restored. • After disconnecting hoses, plugs are fitted and rust preventive applied to protect exposed seating. A sealing strip is pressed into the grove between the end of the sleeve and the nut. To disconnect the coupling, oil pressure is brought to a set pressure in the hydraulic space. Then with the shafts supported, oil is forced beta° he sleeves. The outer sleeve slides off the inner at a rate controlled by release of the hydraulic oil pressure, 

' - When it is required to remove the propeller, the process is equally simple and even quicker with the injection of oil between the surfaces obviating I the need for any form of heating or mechanical withdrawal equipment. Precautions are necessary to prevent the propeller jumping at release. 

(ii) State how the grip of the coupling can be checked when fitted; - The grip of the coupling is checked by measuring the diameter of the outer sleeve before and after tightening. The diameter increase should agree with the figure stamped on the sleeve. 

(iii) Why should coupling bolts be interference fits? 

- Traditionally, fixed pitch propellers have been fitted to the tail shaft with a key and taper being forced on to the taper by the tightening of a nut. - The key was intended as a safeguard either against poor fitting, or against reduced grip due to higher sea-water temperature and differential ( expansion of bronze hub and steel shaft. Keyless fitting where reliance is placed entirely on a good interference fit, has proved effective, however, - and this method removes problems associated with keyways and facilitates propeller mounting and removal. - Many fixed propellers are of course flange mounted, being held by bolts as shown in the section on split stem bearings. For these, outward removal of the tail shaft is made possible with the use of a muff coupling. - 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. - Lloyds 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 temperature is 35°C Lloyds also require that at 0°C the stress at the propeller bore, as given by the Von Mises stress criterion, shall not exceed 60% of the 0.2% proof stress of the propeller material as measured on a test bar. 

(iv) State what precaution should be taken, the coupling? 

- To disconnect the coupling, oil pressure is brought to a set pressure in the hydraulic space. Then with the shafts supported, oil is forced between the sleeves. The outer sleeve slid ,off the inner at a rate controlled by release of the hydraulic oil pressure. - Care must be taken to release the hydraulic pressure very very slow to avoid and prevent the propeller jumping at the release of the hydraulic pressure. - When it is required to remove the propeller, the process is equally simple and even quicker with the injection of oil between the surfaces obviating the need for any form of heating or mechanical withdrawal equipment. Precautions are necessary to prevent the propeller jumping at release.