A Catechism of the Steam Engine Part 26

508. _Q._--As the question of the locomotive boiler has been already disposed of in discussing the question of boilers in general, it now only remains to inquire into the subject of the engine, and we may commence with the cylinders. Will you state the arrangement and construction of the cylinders of a locomotive and their connections?

_A._--The cylinders are placed in the same horizontal plane as the axle of the driving wheels, and the connecting rod which is attached to the piston rod engages either a crank in the driving axle or a pin in the driving wheel, according as the cylinders are inside or outside of the framework.

The cylinders are generally made an inch longer than the stroke, or there is half an inch of clearance at each end of the cylinder, to permit the springs of the vehicle to act without causing the piston to strike the top or bottom of the cylinder. The thickness of metal of the cylinder ends is usually about a third more than the thickness of the cylinder itself, and both ends are generally made removable. The priming of the boiler, when it occurs, is very injurious to the cylinders and valves of locomotives, especially if the water be sandy, as the grit carried over by the steam wears the rubbing surfaces rapidly away. The face of the cylinder on which the valve works is raised a little above the metal around it, both to facilitate the operation of forming the face and with the view of enabling any foreign substance deposited on the face to be pushed aside by the valve into the less elevated part, where it may lie without occasioning any further disturbance. The valve casing is sometimes cast upon the cylinder, and it is generally covered with a door which may be removed to permit the inspection of the faces. In some valve casings the top as well as the back is removable, which admits of the valve and valve bridle being removed with greater facility. A c.o.c.k is placed at each end of locomotive cylinders, to allow the water to be discharged which acc.u.mulates in the cylinder from priming or condensation; and the four c.o.c.ks of the two cylinders are usually connected together, so that by turning a handle the whole are opened at once. In Stephenson's engines, however, with variable expansion, there is but one c.o.c.k provided for this purpose, which is on the bottom of the valve chest.

509. _Q._--What kind of piston is used in locomotives?

_A._--The variety of pistons employed in locomotives is very great, and sometimes even the more complicated kinds are found to work very satisfactorily; but, in general, those pistons which consist of a single ring and tongue piece, or of two single rings set one above the other, so as to break joint, are preferable to those which consist of many pieces. In Stephenson's pistons the screws were at one time liable to work slack, and the springs to break.

510. _Q._--Will you explain the connection of the piston rod with the connecting rod?

_A._--The piston rods of all engines are now generally either case hardened very deeply, or are made of steel; and in locomotive engines the diameter of the piston rod is about one seventh of the diameter of the cylinder, and it is formed of tilted steel. The cone of the piston rod, by which it is attached to the piston, is turned the reverse way to that which is adopted in common engines, with the view of making the cutter more accessible from the bottom of the cylinder, which is made to come off like a door. The top of the piston rod is secured with a cutter into a socket with jaws, through the holes of which a cross head pa.s.ses, which is embraced between the jaws by the small end of the connecting rod, while the ends of the cross head move in guides. Between the piston rod clutch and the guide blocks, the feed pump rod joins the cross head in some engines.

511. _Q._--What kind of guides is employed for the end of the piston rod?

_A._--The guides are formed of steel plates attached to the framing, between which work the guide blocks, fixed on the ends of the cross head, which have f.l.a.n.g.es bearing against the inner edges of the guides. Steel or bra.s.s guides are better than iron ones: Stephenson and Hawthorn attach their guides at one end to a cross stay, at the other to lugs on the cylinder cover; and they are made stronger in the middle than at the ends.

Stout guide rods of steel, encircled by stuffing boxes on the ends of the cross head, would probably be found superior to any other arrangement. The stuffing boxes might contain conical bushes, cut spirally, in addition to the packing, and a ring, cut spirally, might be sprung upon the rod and fixed in advance of the stuffing box, with lateral play to wipe the rod before entering the stuffing box, to prevent it from being scratched by the adhesion of dust.

512. _Q._--Is any provision made for keeping the connecting rod always of the same length?

_A._--In every kind of locomotive it is very desirable that the length of the connecting rod should remain invariable, in spite of the wear of the bra.s.ses, for there is a danger of the piston striking against the cover of the cylinder if it be shortened, as the clearance is left as small as possible in order to economize steam. In some engines the strap encircling the crank pin is fixed immovably to the connecting rod by dovetailed keys, and a bolt pa.s.ses through the keys, rod, and strap, to prevent the dovetailed keys from working out. The bra.s.s is tightened by a gib and cutter, which is kept from working loose by three pinching screws and a cross pin or cutter through the point. The effect of this arrangement is to lengthen the rod, but at the cross head end of the rod the elongation is neutralized by making the strap loose, so that in tightening the bra.s.s the rod is shortened by an amount equal to its elongation at the crank pin end.

The tightening here is also effected by a gib and cutter, which is kept from working loose by two pinching screws pressing on the side of the cutter. Both journals of the connecting rod are furnished with oil cups, having a small tube in the centre with siphon wicks. The connecting rod is a thick flat bar, with its edges rounded.

513. _Q._--How is the cranked axle of locomotives constructed?

_A._--The cranked axle of locomotives is always made of wrought iron, with two cranks forged upon it toward the middle of its length, at a distance from each other answerable to the distance between the cylinders. Bosses are made on the axle for the wheels to be keyed upon, and bearings for the support of the framing. The axle is usually forged in two pieces, which are afterward welded together. Sometimes the pieces for the cranks are put on separately, but the cranks so made are liable to give way. In engines with outside cylinders the axles are made straight-the crank pins being inserted in the naves of the wheels. The bearings to which the connecting rods are attached are made with very large fillets in the corners, so as to strengthen the axle in that part, and to obviate side play in the connecting rod. In engines which, have been in use for some time, however, there is generally a good deal of end play in the bearings of the axles themselves, and this slackness contributes to make the oscillation of the engine more violent; but this evil may be remedied by making the bearings spheroidal, whereby end play becomes impossible.

514. _Q._--How are the bearings of the axles arranged?

_A._--The axles bear only against the top of the axle boxes, which are generally of bra.s.s; but a plate extends underneath the bearing, to prevent sand from being thrown upon it. The upper part of the box in most engines has a reservoir of oil, which is supplied to the journal by tubes with siphon wicks. Stephenson uses cast iron axle boxes with bra.s.ses, and grease instead of oil; and the grease is fed upon the journal by the heat of the bearing melting it, whereby it is made to flow down through a hole in the bra.s.s. Any engines constructed with outside bearings have inside bearings also, which are supported by longitudinal bars, which serve also in some cases to support the piston guides; these bearings are sometimes made so as not to touch the shafts unless they break.

515. _Q._--How are the eccentrics of a locomotive constructed?

_A._--In locomotives the body of the eccentric is of cast iron, in inside cylinder engines the eccentrics are set on the axle between the cranks, and they are put on in two pieces held together by bolts; but in straight axle engines the eccentrics are cast in a piece, and are secured on the shaft by means of a key. The eccentric, when in two pieces, is retained at its proper angle on the shaft by a pinching screw, which is provided with a jam nut to prevent it from working loose. A piece is left out of the eccentric in casting it to allow of the screw being inserted, and the void is afterward filled by inserting a dovetailed piece of metal. Stephenson and Hawthorn leave holes in their eccentrics on each side of the central arm, and they apply pinching screws in each of these holes. The method of fixing the eccentric to the shaft by a pinching screw is scarcely sufficiently substantial; and cases are perpetually occurring, when this method of attachment is adopted, of eccentrics shifting from their place. In the modern engines the eccentrics are forged on the axles.

516. _Q._--How are the eccentric straps constructed?

_A._--The eccentric hoops are generally of wrought iron, as bra.s.s hoops are found liable to break. When formed of malleable iron, one half of the strap is forged with the rod, the other half being secured to it by bolts, nuts, and jam nuts. Pieces of bra.s.s are, in some cases, pinned within the malleable iron hoop; but it appears to be preferable to put bra.s.ses within the hoop to encircle the eccentric, as in the case of any other bearing.

When the bra.s.s straps are used, the lugs have generally nuts on both sides, so that the length of the eccentric rod may be adjusted by their means to the proper length; but it is better for the lugs of the hoops to abut against the necks of the screws, and, if any adjustment be necessary from the wear of the straps, washers can be interposed. In some engines the adjustment is effected by s.c.r.e.w.i.n.g the valve rod, and the cross head through which it pa.s.ses has a nut on either side of it, by which its position upon the valve rod is determined.

517. _Q._--Will you describe the eccentric rod and valve levers?

_A._--In the engines in use before the introduction of the link motion, the forks of the eccentric rod were of steel, and the length of the eccentric rod was the distance between the centre of the crank axle and the centre of the valve shaft; but in modern engines the use of the link motion is universal. The valve lever in locomotives is usually longer than the eccentric lever, to increase the travel of the valve, if levers are employed; but it is better to connect the valve rod to the link of the link motion without the intervention of levers. The pins of the eccentric lever in the old engines used to wear quickly; Stephenson used to put a ferule of bra.s.s on these pins, which being loose, and acting like a roller, facilitated the throwing in and out of gear, and when worn could easily be replaced, so that there was no material derangement of the motion of the valve from play in this situation.

518. _Q._--What is the arrangement of a starting lever?

_A._--The starting lever travels between two iron segments, and can be fixed in any desired position. This is done by a small catch or bell crank, jointed to the bottom of the handle at the end of the lever, and coming up by the side of the handle, but pressed out from it by a spring. The smaller arm of this bell crank is jointed to a bolt, which shoots into notches, made in one of the segments between which the lever moves. By pressing the bell crank against the handle of the lever the bolt is withdrawn, and the lever may be shifted to any other point, when, the spring being released, the bolt flies into the nearest notch.

519. _Q._--In what way does the starting handle act on the machinery of the engine to set it in motion?

_A._--Its whole action lies in raising or depressing the link of the link motion relatively with the valve rod. If the valve rod be attached to the middle of the link, the valve will derive no motion from, it at all, and the engine will stop. If the attachment be slipped to one end of the link the engine will go ahead, and if slipped to the other end it will go astern. The starting handle merely achieves this change of position.

520. _Q._--Will you explain the operation of setting the valve of a locomotive?

_A._--In setting the valves of locomotives, place the crank in the position answerable to the end of the stroke of the piston, and draw a straight line, representing the centre line of the cylinder, through the centres of the crank shaft and crank pin. From the centre of the shaft describe a circle with the diameter equal to the throw of the valve; another circle to represent the crank shaft; and a third circle to represent the path of the crank pin. From the centre of the crank shaft, draw a line perpendicular to the centre line of the cylinder and crank shaft, and draw another perpendicular at a distance from the first equal to the amount of the lap and the lead of the valve: the points in which this line intersects the circle of the eccentric are the points in which the centre of the eccentric should be placed for the forward and reverse motions. When the eccentric rod is attached directly to the valve, the radius of the eccentric, which precedes the crank in its revolution, forms with the crank an obtuse angle; but when, by the intervention of levers, the valve has a motion, opposed to that of the eccentric rod, the angle contained by the crank and the radius of the eccentric must be acute, and the eccentric must follow the crank: in other words, with a direct attachment to the valve the eccentric is set _more_ than one fourth of a revolution in advance of the crank, and with an indirect attachment the eccentric is set _less_ than one fourth of a circle behind the crank. If the valve were without lead or lap the eccentric would be exactly one fourth of a circle in advance of the crank or behind the crank, according to the nature of the valve connection; but as the valve would thus cover the port by the amount of the lap and lead, the eccentric must be set forward so as to open the port to the extent of the lap and lead, and this is effected by the plan just described.

521. _Q._--In the event of the eccentrics slipping round upon the shaft, which you stated sometimes happens, is it necessary to perform the operation of setting the valve as you have just described it?

_A._--If the eccentrics shift upon the shaft, they may be easily refixed by setting the valve open the amount of the lead, setting the crank at the end of the stroke, and bringing round the eccentric upon the shaft till the eccentric rod gears with the valve. It would often be troublesome in practice to get access to the valve for the purpose of setting it, and this may be dispensed with if the amount of lap on the valve and the length of the eccentric rod be known. To this end draw upon a board two straight lines at right angles to one another, and from their point of intersection as a centre describe two circles, one representing the circle of the eccentric, the other the crank shaft; draw a straight line parallel to one of the diameters, and distant from it the amount of the lap and the lead: the points in which his parallel intersects the circle of the eccentric are the positions of the forward and backward eccentrics. Through these points draw straight lines from the centre of the circle, and mark the intersection of these lines with the circle of the crank shaft; measure with a pair of compa.s.ses the chord of the arc intercepted between either of these points, and the diameter which is at right angles with the crank, and the diameters being first marked on the shaft itself, then by transferring with the compa.s.ses the distance found in the diagram, and marking the point, the eccentric may at any time be adjusted without difficulty.

[Ill.u.s.tration: Fig. 45.]

522. _Q._--Will you describe the structure and arrangement of the feed pumps of locomotive engines?

_A._--The feed pumps of locomotives are generally made of bra.s.s, but the plungers are sometimes made of iron, and are generally attached to the piston, cross head, though in Stephenson's engines they are worked by rods attached to eyes on the eccentric hoops. There is a ball valve, fig. 45, between the pump and the tender, and two usually in the pipe leading from the pump to the boiler, besides a c.o.c.k close to the boiler, by which the pump may be shut off from the boiler in case of any accident to the valves.

The ball valves are guided by four branches, which rise vertically, and join together at the top in a hemispherical form. The shocks of the ball against this cap have in some cases broken it after one week's work, from the top of the cage having been flat, and the branches not having had their junction at the top properly filleted. These valve guards are attached in different ways to the pipes; when one occurs at the junction of two pieces of pipe it has a f.l.a.n.g.e, which along with the f.l.a.n.g.es of the pipes and that of the valve seat are held together by a union joint. It is sometimes formed with a thread at the under end, and screwed into the pipe. The b.a.l.l.s are cast hollow to lessen the shock, as well as to save the metal. In some cases where the feed pump plunger has been attached to the cross head, the piston rod has been bent by the strain; and that must in all cases occur, if the communication between the pump and boiler be closed when the engine is started, and there be no escape valve for the water.

523. _Q._--Are none but ball valves used in the feed pump?

_A._--Spindle valves have in some cases been used instead of ball valves, but they are more subject to derangement; but piston valves, so contrived as to shut a portion of water in the cage when about to close, might be adopted with a great diminution of the shock. Slide valves might be applied, and would probably be found preferable to any of the expedients at present in use. In all spindle valves opened and shut rapidly, it is advisable to have the lower surface conical, to take off the shock of the water; and a large lift of the valve should be prevented, else much of the water during the return stroke of the pump will flow out before the valve shuts.

524. _Q._--At what part of the boiler is the feed water admitted?

_A._--The feed pipe of most locomotive engines enters the boiler near the bottom and about the middle of its length. In Stephenson's engine the water is let in at the smoke box end of the boiler, a little below the water level; by this means the heat is more fully extracted from the escaping smoke, but the arrangement is of questionable applicability to engines of which the steam dome and steam pipe are at the smoke box end, as in that case the entering cold water would condense the steam.

525. _Q._--How are the pipes connecting the tender and locomotive constructed, so as to allow of play between the engine and tender without leakage?

_A._--The pipes connecting the tender with the pumps should allow access to the valves and free motion to the engine and tender. This end is attained by the use of ball and socket joints; and, to allow some end play, one piece of the pipe slides into the other like a telescope, and is kept tight by means of a stuffing box. Any pipe joint between the engine and tender must be made in this fashion.

526. _Q._--Have you any suggestion to make respecting the arrangement of the feed pump?

_A._--It would be a material improvement if a feed pump was to be set in the tender and worked by means of a small engine, such as that now used in steam vessels for feeding the boilers. The present action of the feed pumps of locomotives is precarious, as, if the valves leak in the slightest degree, the steam or boiling water from the boiler will prevent the pumps from drawing. It appears expedient, therefore, that at least one pump should be far from the boiler and should be set among the feed water, so that it will only have to force. If a pump was arranged in the manner suggested, the boiler could still be fed regularly, though the locomotive was standing still; but it would be prudent to have the existing pumps still wrought in the usual way by the engine, in case of derangement of the other, or in case the pump in the tender might freeze.

527. _Q._--Will you explain the construction of locomotive wheels?

_A._--The wheels of a locomotive are always made of malleable iron. The driving wheels are made larger to increase the speed; the bearing wheels also are easier on the road when large. In the goods engines the driving wheels are smaller than in the pa.s.senger engines, and are generally coupled together. Wheels are made with much variety in their constructive details: sometimes they are made with cast iron naves, with the spokes and rim of wrought iron; but in the best modern wheels the nave is formed of the ends of the spokes welded together at the centre. When cast iron naves are adopted, the spokes are forged out of flat bars with T-formed heads, and are arranged radially in the founder's mould, the cast iron, when fluid, being poured among them. The ends of the T heads are then welded together to const.i.tute the periphery of the wheel or inner tire; and little wedge-form pieces are inserted where there is any deficiency of iron. In some cases the arms are hollow, though of wrought iron; the tire of wrought iron, and the nave of cast iron; and the spokes are turned where they are fitted into the nave, and are secured in their sockets by means of cutters.

Hawthorn makes his wheels with cast iron naves and wrought iron rims and arms; but instead of welding the arms together, he makes palms on their outer end, which are attached by rivets to the rim. These rivets, however, unless very carefully formed, are apt to work loose; and it would probably be found an improvement if the palms were to be slightly indented into the rim, in cases in which the palms do not meet each other at the ends. When the rim is turned it is ready for the tire, which is now made of steel.

528. _Q._--How do you find the length of bar necessary for forming a tire?

_A._--To find the proper length of bar requisite for the formation of a hoop of any given diameter, add the thickness of the bar to the required diameter, and the corresponding circ.u.mference in the table of circ.u.mferences of circles is the length of the bar. If the iron be bent edgewise the breadth of the bar must be added to the diameter, for it is the thickness of the bar measured radially that is to be taken into consideration. In the tires of railway wheels, which have a f.l.a.n.g.e on one edge, it is necessary to add not only the thickness of the tire, but also two thirds of the depth of the f.l.a.n.g.e; generally, however, the tire bars are sent from the forge so curved that the plain edge of the tire is concave, and the f.l.a.n.g.e edge convex, while the side which is afterward to be bent into contact with the cylindrical surface of the wheel is a plane.

In this case the addition of the diameter of two thirds of the depth of the f.l.a.n.g.e is unnecessary, for the curving of the f.l.a.n.g.e edge has the effect of increasing the real length of the bar. When the tire is thus curved, it is only necessary to add the thickness of the hoop to the diameter, and then to find the circ.u.mference from a table; or the same result will be obtained by multiplying the diameter thus increased by the thickness of the hoop by 3.1416.

529. _Q._--How are the tires attached to the wheels?

_A._--The materials for wheel tires are first swaged separately, and then welded together under the heavy hammer at the steel works; after which they are bent to the circle, welded, and turned to certain gauges. The tire is now heated to redness in a circular furnace; during the time it is getting hot, the iron wheel, turned to the right diameter, is bolted down upon a face plate or surface; the tire expands with the heat, and when at a cherry red, it is dropped over the wheel, for which it was previously too small, and it is also hastily bolted down to the surface plate; the whole ma.s.s is then quickly immersed by a swing crane in a tank of water five feet deep, and hauled up and down till nearly cold; the tires are not afterward tempered. The tire is attached to the rim with rivets having countersunk heads, and the wheel is then fixed on its axle.

530. _Q._--Is it necessary to have the whole tire of steel?