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perfectly cylindrical, to run upon the rails; or, in some cases, the middle portion, between the nave and rim, was cast of one entire piece, with circular holes, to make it lighter.

A very formidable objection to the use of iron wheels, was, that the rails, especially when their surfaces were narrow, tended to form, or wear an indented groove around their rims; which groove, when of moderate depth, not only caused considerable friction, but was liable to break the rails by a side pressure. The edges, also, of the top of the cast-iron rail, suffered much by the action of the sides of the groove thus formed, and were frequently broken off, on the interior side, for the whole length of the rail. To remedy this, the breadth of the surface of the rails was increased, which diminished the evil to a certain extent; but the expense of repairs was still considerable.

§ 3.-Cast-iron Wheels, Case-hardened.

A complete remedy was, however, effected a few years ago, by what is called, " case-hardening" the rim of the wheels. This is done by placing a massive ring of cast iron, around the mould for forming the casting of the wheel; and running the metal, which forms the exterior surface of the rim of the wheel, against this cold cylindrical piece of iron. The rapid abstraction of heat by the cold iron, produces such a degree of compactness, and hardness, to the superficies of the wheel in contact with the cold iron, that the file has no effect upon it, and this hardness effectually prevents the action of the rail from wearing the wheel into grooves.

Previous to this, the cost of wheels was a very serious charge, in the annual repair of the carriages; but the wheels now, when properly case-hardened, and where the speed is moderate, work for many years without

wearing away. Several, which have been in use for eight years, are still in good order; and, from their appearance, are likely to remain so for a considerable time to come. The operation of case-hardening was, at first, attended with great difficulty; the rapidity, with which the cold iron caused the rim to cool, produced an unequal contraction of the metal, in all the several parts of the wheel, and made them frequently fly in pieces. The rim, being first cooled, would not yield to the contraction of the spokes in cooling; and, therefore, when the spokes cooled, if the contraction did not cause them to separate immediately, it left such a tension upon them, that the shocks they received, when brought into use, soon made them crack, and thus rendered the wheel useless. Many plans were devised to remedy this, in some, the rim was made considerably thicker than the spokes, that the spokes might cool more rapidly, and thus compensate for the more rapid cooling of the rim by the iron ring; the spokes, in this case, being more

numerous.

The plan now mostly used, where the wheel is entirely formed of cast iron, is, to cast the nave in two pieces, as is shewn in Fig. 1, Plate VI., e c e, being the division; two hoops of wrought iron, i i, i i, being laid around the nave, to secure it.

In Messrs. Losh and Stephenson's patent, to which we have before alluded, there is described a mode of forming the wheels, with wrought-iron spokes, in such a way as to yield to the unequal contraction, occasioned by the case-hardening of the wheels.

A BC, Fig. 2, Plate VI., represents the form of their wheel; cccccc, are the arms, which are of flat malleable iron, dovetailed at the ends. The iron arms being laid in the mould, the cast iron is run around. them, and thus forms one entire wheel; the contraction

in the cooling, draws the dovetailed spokes firmly into the rim and nave; and, by the use of a little borax, an union is formed, between the wrought and cast iron, which produces a degree of combination, that prevents the possibility of their working loose. The spokes were first made straight, as shewn in the drawing, and were six in number, but experience has since shewn that a greater number is preferable; and they are, also, now made of a slightly serpentine form, so as to yield to the contraction of the rim in cooling. In this wheel, the nave is cast entire.

This system of case-hardening the rim of the wheels, aa before stated, has been found to be of very great utility, reducing the wear and cost to a comparatively trifling amount. The hardness, certainly, renders them more liable to crack, or break, by sudden jerks or blows; but this tendency is partly overcome, by the rims being made a little thicker now than formerly; the malleableiron spokes also tend, in a certain degree, to obviate this objection.

It has been urged, against case-hardened wheels, that their hardness makes them liable to cut the rails; this might apply to narrow rubbing surfaces, but cannot have any application to one surface, rolling over another, when the hard surface is the rolling one, and also the broader. We have often examined, very carefully, their action upon the rails, but could never find any tendency in them to cut the rails; whereas, when the common wheels are indented, on the surface of the rim, they are very liable to injure the rails, from the periphery, thus grooved, breaking the sides of the bearing surface of the rails off, and leaving only the middle section. This may be seen, on all those railroads upon which the common wheels have been long used.

The universal adoption of case-hardened wheels, on

all the railroads where a slow rate of travelling is practised, in preference to the common wheels, is, however, the best criterion, which can be adduced, of the general belief of their superiority.

We are also inclined to think, that casting the wheel against a perfectly cylindrical piece of iron, tends to form it more perfectly cylindrical, than casting in the ordinary way, and this will likewise lessen the resistance, arising from the undulatory motion, produced by the imperfect circular form of the rim.

§ 4.-Cast-iron Wheels, with Wrought-iron Tires.

The very great rapidity of travelling, which is now adopted upon public railways, causes the subject of wheels, to be a most important object in railway travelling. The rapid motion, very materially, increases the liability of case-hardened wheels to break, not only from the brittle nature of the material, but, also, by the friction of the wheels upon the rails, at such great velocities, heating and expanding the rims, and thus causing them to crack, and fly to pieces; and various plans have, therefore, been devised, to obviate this objection.

The wheels of the locomotive engines, up to 1826, were formed of common cast iron, the case-hardening being deemed objectionable, as diminishing the adhesion upon the rails. Finding the wear of those wheels very great, I had a rim, or tire, of wrought iron, put upon one set of wheels, of one of the Killingworth engines. This tire was made by the hammer of the workman, and, not being of uniform thickness, produced considerable resistance to the engine. The experiment was, however, pursued a sufficient length of time to prove, that, with regard to common cast iron, the wear was very much less. The trial being so very satisfactory, the Bedlington

Iron Company were induced to erect a pair of rollers, to roll them by machinery, by which means, a uniformity of thickness was preserved. Since that time, the use of malleable iron tires has gradually increased, and is now adopted upon all the public lines, where rapid travelling is practised, not only for engine wheels, but also for the common carriages.

In forming this wheel, the nave and spokes were the same as the common, or case-hardened wheels, but the rim was cylindrical, without the flanch. The rim was then turned, to clear it of all sand, or loose particles, and to make it perfectly cylindrical. The tire, being previously turned into a cylindrical shape, and welded, is then heated to a certain degree of temperature, and the contraction in cooling, causes it to embrace the rim of the wheel sufficiently tight, to prevent it from coming off; experience having shewn, that, when the carriages do not travel at a velocity, which heats the tire, such a mode of laying them on is sufficient.

At very rapid rates of travelling, for locomotive engine wheels, and for the wheels of passenger carriages; cast-iron spokes and rims, have not been found of that degree of safety, as to ensure perfect confidence in their use. Mr. George Stephenson, therefore, on the Liverpool and Manchester railway, adopted cast-iron naves, with wooden spokes and rim, on which the wrought-iron tire was laid. Square boxes are cast in the nave, to receive one end of the spokes, the other ends being inserted into the fellies, in the same manner as in coach wheels. A thin wrought-iron rim is laid around the fellies, upon which the outside, or flanched tire, is laid. This description of wheel ensures perfect safety, but the expense of construction is very considerable.

Mr. George Stephenson has obtained a patent for a wheel, with cast-iron nave and rim, and hollow cast

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