Locomotive Engine Running and Management

by Angus Sinclair

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.

EFFECTS OF BAD FITTING.

When the work of fitting a set of back-end brasses is completed, they should be put in the strap, and tried on the pin. If, after being keyed close together, they revolve on the pin without pinching, the fit is not too tight. It is of the greatest consequence, in fitting rod-brasses, to ascertain, beyond doubt, that the brasses have been bored out true, and that they fit in the strap so that the line of strain shall be in line with the cross-head and crank-pins. It occasionally happens, through bad workmanship, that when the back end of a rod is keyed up, and the front end not connected, the rod does not point straight to the cross-head pin, but in a line some distance to the right or left. The distance may be very small, yet sufficient to cause no small amount of trouble. By some pinching and jamming, a rod in this condition can be connected up; but it is almost sure to run hot. And a rod in this condition will never run satisfactorily till it is taken down and fitted by a competent machinist. The back end may be all right, and the forward end suffering from oblique fitting. This is even more common than the first case, and the effect is the same. A rod in this condition, besides displaying a tendency to run hot, will keep jerking the cross-head from side to side on the guides, and will probably make the cross-head chafe the guides at certain points. Rods never run cool, and free from jar, unless they are fitted to transmit the power in a direct line between the pins.

STRIKING POINTS AND CLEARANCE.

Before putting up main rods, the striking points of the pistons should be located and marked on the guides. Then, when the rods are put up, the clearance should be divided equally between the two ends. The identification of these points is of greater interest to the engineer who is running the engine than to any other person; for upon their correctness the success of his running may, to some extent, depend. An engine may go out with the clearance badly divided, and run all right for a few days, and the driving of a key may then cause the piston to strike the head. A forcible instance of this kind once came under my observation. A careless machinist, in working on main-rod brasses, had mixed the liners, and shortened the rod, till the piston began to touch the back head. When the engine was working light, there was just a slight jar; but, when the load was heavy, the jar became a distinct pound. The engineer could not locate the knock, and was disposed to think it was in the driving-box. One day that he slipped the engine badly, steam began to issue from the back cylinder-head, which was cracked by a blow from the piston. The cause of the pound was then discovered.

WATCHING RODS ON THE ROAD.

When an engineer starts out with an engine after the rod-brasses have been filed, he should make them a special object of attention. If he can not shake the connection laterally with his hands when there is room for movement within the collars, he should slack up the key till he can do so; for some one has made a mistake in fitting. So long as the rod passes the center without jar when the engine is working hard in full gear, the brasses are tight enough. After running a few miles with newly fitted brasses, the rod will generally need keying up; for liners that were comparatively loose when put up, get driven compactly together, leaving lost motion. Although a connection may be put together brass to brass, there is still some work left for the engineer to do in the way of keying. To do keying correctly needs considerable sagacity, especially in the case of side rods. In the case of back ends of main rods, the key should be got down as soon as possible, to hold the brasses immovably in the strap; but, after this point is reached, there should be no more hammering on the key. Some men persist in pounding down keys that are already snug, and the effect of their blows is to spring the brass out of shape. A key acts as a wedge, which it is; and, when the taper is slight, the blow imparted by a hammer roughly used, exerts an immense force in driving it down. Something must yield; and the brass gets sprung towards the pin, presenting a ridge for a rubbing surface, which heats, and causes delay. After the key is once driven tight home, its work is finished. If the pin then indicates lost motion, the rod should be taken down, and the brasses reduced. In the case of main rods, this should be done at the first signs of pounding; for lost motion entails heavy shocks upon the moving parts. The front end of main rods requires to be very carefully watched, and the connection kept free from jar. Where this part is kept regularly oiled, and free from lost motion, it gives scarcely any trouble; but let the wrist-pin of the common cross-head once get cut through neglect, and it is a difficult matter getting it in good running-order again. The style of cross-head where the pin is part of the casting, although greatly used, is a most awkward article to fit up and keep in shape. The form of cross-head which works between two guide-bars, and has its axis in line with the piston-rod, is becoming deservedly popular.

SIDE RODS.

Many attempts have been made to dispense with side rods, and they certainly are a troublesome part of the machinery to keep right; but no better means of connecting driving-wheels has yet been devised. The first method of coupling driving-wheels together, so that more than one pair might be available for adhesion, was by means of cogs and gearing. This was improved on by an endless chain working over pocketed pulleys; but even this was an extremely crude device,—working with tumultuous jerks, and a noise like a stamping-mill. One of the first real improvements, which George Stephenson effected on the locomotive, was the inventing of side rods. An essential element in locomotive construction needed to make side rods run with safety, is, that all the wheels connected shall be of the same circumference. There is a practice on some roads of putting new tires on wheels just as they come from the rolling-mill, without putting them in the lathe. Such tires are seldom accurate in size; and they cause no end of trouble, especially to side rods. This is one of the economical practices that does not pay.