All About Engines

3°4 All About Engines propeller shaft into the ship. In fact, the whole force tending to propel the ship through the water is trans- mitted through the propeller shaft, and the thrust used to be taken up by a “ thrust block.” This was, and for reciprocating engines still is, composed of a number of flanges on the shaft, fitting in recesses in a long bearing. Such a block takes up a large amount of space, and when large powers are trans- mitted the oil is liable to be squeezed out by the great pressure on the flanges. In the turbine driving a propeller directly this is not serious, because the action of the steam on the blades was opposed to the thrust, but in the case of the geared turbine the thrust is not applied to the turbine shaft at all, but to that carrying the propeller and one of the toothed wheels. In the Mitchel Thrust Block the propeller shaft carries a single flange, which bears upon the face of a flat ring through a hole in the centre of which the shaft passes. On the face of this ring are recesses into which bearing blocks are fitted. These are flat on the face, where they rest against the face of the flange, but rounded on the back so that they can rock a little. Owing to this freedom of movement of the blocks the spaces between their front faces and the face of the flange is always wedge-shaped, and it is practically impossible for oil to be squeezed out. And so long as the oil is not squeezed out the bearing will take the thrust without overheating. There are many engineers who believe that the geared turbine and the Mitchel Thrust Block form