FUNCTIONS OF CONNECTING-RODS.
The functions of rods being to transmit the motion of the pistons to the running-gear, they have very heavy duty to perform. The conflicting strains and shocks to which a locomotive is subjected while running over a rough track at high speed, are, in many instances, sustained by the rods: hence it is of special importance that this portion of the motion should be kept in good order. Main rods convey the power developed in the cylinders to the crank-pins by a succession of pulls and thrusts equal in vigor to the aggregate of steam-pressure exerted on the piston. To endure this alternating tension and compression without injury to the working-parts, it is of the utmost importance that the connections should be close fitted, yet free enough to prevent unnecessary friction. In fitting up main-rod brasses, it does not matter in what position the crank stands, so long as it is convenient for doing the work. But, if the engine has been in service since the pins were turned, they should be calipered through their horizontal diameter when the crank is on the center; since it is well known that the pins have a tendency to wear flat on the sides at right angles to the crank’s length. The back ends of the main-rod brasses should be fitted brass to brass; for that form of doing the work makes the most secure job, and gives the connection all the advantages of a solid box, preventing the straps and brasses from being knocked out of shape by hammering each other,—a result that surely follows the open brasses method of fitting back ends of main-rods. Leaving the forward end brasses a little open is not injurious to that connection, because the line of strain is not so varied as that of the back end.