228 



OF THE PARTS OF 



[SECT. vii. 



is less in proportion as the connecting rod is longer ; provided the stress from 

 weight be not materially increased. l 



CRANKS. 



487. The crank is one of the best contrivances for changing a reciprocating 

 into a rotary motion. There are three different cases : 



1. The moving force may be uniform and in a straight line, 



2. The moving force may be uniform and in a curved line, 



3. In either case the force may be variable. 



I 



A crank increases the velocity of the moving force, and in the usual construction, 

 in the ratio of the circumference of a circle to twice its diameter ; but this ratio 

 is susceptible of variation, as is also the action of the power. This will be evident 

 from the annexed figure ; as if A B be the motion of the piston rod, the crank 



Fig. 22. 



1 The -whole increase of stress required for converting a reciprocating into a rotary motion 

 cannot double the friction on the crank axis in any case, and as double this friction never amounts 

 to a tenth part of the power of an engine, there is no reason to hope for an equal degree either of 

 economy or simplicity by using the rotary action of steam. (See art. 313 317. and the table, 

 art. 487.) 



