GEARED-ENGINES. 



80. In geared engines the cranks are formed on one shaft, and the screw 

 fixed upon another, the directions of the two shafts being parallel. On the 

 crank-shaft is fixed a toothed-wheel, which 



works in a smaller one, called a pinion, fixed Fig. 24. 



on the screw-shaft. Thus in fig. 24, A may 

 be regarded as the pinion fixed on the screw- 

 shaft, and B the wheel fixed on the crank- 

 shaft, the teeth of the one being engaged in A "SSP^^KH^MSf^ & 

 those of the other at c. 



It is evident that the velocity of rotation 

 of A will be greater than that of c in the 

 same proportion as that in which the number 



of teeth in c is greater than the number of teeth in A. It is always possible, 

 therefore, with a given speed of the crank-shaft, to impart a speed greater 

 in any required proportion to the screw-shaft by regulating in a cor- 

 responding manner the proportion of the teeth in those geared wheels. 



81. One of the objections to the use of gearing in sea-going vessels is the 

 liability of the teeth to rapid wear, and to fracture from sudden shocks in 

 -a rough sea. In order to diminish the risk of this by distributing the 

 pressure over a greater number of teeth, Mr. Fairbairn has adopted in 

 large screw-engines, constructed by him for the Royal Navy, a system of 

 internal gearing in which the crank-shaft wheel has the teeth on its 

 internal periphery, the screw-shaft pinion revolving within it, as shown 

 in fig. 25. 



In screw- vessels of war, all the machinery should be placed 

 below the water-line, so as to be as effectually protected from 

 shot as the screw itself is. 



82. When direct-acting engines without gearing are applied to 

 screw-propelled vessels, the reciprocating motion of the piston 

 must be equal to the velocity 



of the screw, that is, the 

 number of strokes per minute 

 of the piston must be equal 

 to the number of revolutions 

 per minute of the screw. Now 

 to render this compatible with 

 .a sufficiently moderate recti- 

 linear motion of the piston, the 

 length of the stroke must bear 

 a very small proportion to the 

 diameter of the cylinder. This 

 has, in many cases, rendered 

 it necessary in such vessels to 

 subdivide the power of the 

 engines among four smaller cylinders, all the pistons being directly 

 Attached to cranks on the screw-shaft instead of producing it by 

 two larger cylinders, in which an unmanageable proportion must 

 Toe adopted between the diameter and the stroke. 



Another advantage derived from this subdivision of power is, that 



167 



