Question

Sketch a transmission shaft coupling which enables the propeller shaft to be withdrawn

(a) describe the coupling and the method fitting and dismantling (8)

(b) state how the grip of the coupling can be checked when fitted (4)

(c) State what safety precaution should be taken when dismantling the coupling. (4)

Answer 1

Part (a)

Part (b)

(i) An alternative to the conventional flange couplings for the tail shaft, the muff coupling allows the shaft to be withdrawn outboard.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 the above figure consists 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 outer 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 the tapered inner sleeve. At the same time, oil is injected between the 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 riving oil pressure. When the outer sleeve has driven onto 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 drain and normal friction is restored. After disconnection hoses, plugs are fitted and rust prevention is applied to protect exposed seating. A sealing strip is brought to a set pressure in the hydraulic space. Then with the shafts supported, oil is forced into the sleeves. The outer sleeve slides off the inner at a rate controlled by the 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even quicker with the injection of oil between the surfaces obviating the need for any form of heating or mechanical withdrawn equipment. Precautions are necessary to prevent the propeller from jumping at release.

(ii) 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) 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 the release of the hydraulic oil pressure. Care must be taken to release the hydraulic pressure very very slowly to avoid and prevent the propeller from 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 from jumping at release.

✓ Verified Model Answer (Text Solution)

(i) 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 the above figure consists of two steel sleeves.