WHEELS AND AXLES.

377. Of the mechanical details of car building it is not necessary here to speak; but of those matters which fit a car for special duty, and depend upon particular characteristics of any road, such as the gauge, something must be said.

The trend of the wheel tire, as remarked in Chapter XIII., is not turned cylindrical, but conical. A perfectly straight road would of course require no cone upon the wheels; the object of the latter being to vary the wheel diameter when upon curves. The general practice is to give a certain standard cone to all wheels, for all gauges. This is quite wrong, as will be seen by the following formula, which is from “Pambour on the Locomotive Engine.”

Let m m′, fig. 156, represent the outer rail, and n n′ the inner one. The circumferences upon the same axles must evidently vary as the length of these curves, which are included between the same radii.

Let D, be the diameter of the first wheel, and d, that of the second; and we shall have,

or otherwise

and

We have also,

Expressing the radius of curvature by r, and the half gauge by e, the above proportion may be expressed by

and also

and finally

This equation shows the difference in diameters that ought to exist between the inner and outer wheels, that the required effect, (no dragging of the outer and no slipping of the inner wheel,) is produced.

And the formula becomes

or .288 inch on both wheels, or 0.144 inch for each wheel; which for four inches breadth, gives a curve of 1
28 of the width, or decimally, 0.144, and vulgarly, ⅐ of an inch. For a three feet wheel, the rule gives a cone of 0.11 inch.

Note.—Messrs Bush and Lobdel cone their wheels 0.08 inches in a four inch tire; or ¼ inch per foot. The formula above for a three feet wheel, and 4′ 8½″ gauge, gives a curve of 0.09 inches.

The wheel most used upon American roads is made of cast-iron, in one piece, and consists either of two side plates, connected by a hub and rim, or of a central plate ribbed on the sides. Messrs Whitney and Son, (Philadelphia,) pass all their wheels through an annealing process, which renders them much less liable to fracture from shocks and from cold than when the wheel is allowed to cool at once, when hot from the foundery.

The wheels used upon English roads are made with a wrought iron rim and spokes, with a cast hub, the tire being, put on separately. Such wheels are less liable to fracture, but cost more than the American wheel.

378. A very frequent cause of accident upon railroads, is the breakage of axles. Experiments made at Wolverhampton, (England,) upon differently formed axles, show very plainly that it is quite wrong to reduce the diameter of the axle at the middle. That if any variation exists it should be in making the middle the largest. That the effect of a shoulder behind the wheel was to decrease very much the strength. Probably the strongest and most economical railroad axle, would be a wrought iron tube. Certainly a hollow axle is much stronger in resisting tension than a solid one containing the same amount of material.

Note 1.—Thomas Thorneycroft, of Wolverhampton, England, an educated man, and a manufacturer of railway axles, observes:—That the various forms of axles, as generally made, possess within themselves the elements of destruction. That there are certain fixed principles to be observed in proportioning axles, and that just as such principles are departed from, just so much is liability to failure increased.

He says:—It is doubtful whether the wheel is the support and the journal the loaded part, or the reverse. If the latter is the case, then the cone of the wheels causes a lateral strain, tending to bend the axle; and should that bending extend no further than one half of the elastic limit, if long continued, fracture must result; and should the elastic limit be exceeded, the plane of the wheel will be removed from that in which it ought to revolve.

The object of the first experiment was to determine the effect of the form of the longitudinal section of the axle upon its elastic limit.

By reducing the diameter of the axle from 45
16 inches at centre, to 3¾ inches; the limit of elasticity was reduced from .343 to .232 inches; and the load, to produce that elasticity, from fourteen to seven tons.

Experiment second was to ascertain the effect of a reduction of diameter at the centre, upon the ability to resist sudden shocks. One half of the axle was made 4½ inches in diameter from middle to end, and the other half was reduced from 4½ to four inches at centre. The wheel being fixed, and a ram allowed to fall upon the journal, when the following result was obtained. Under forty-six blows, the unreduced end was bent to an angle of eighteen degrees. Under sixteen blows, the reduced end was bent to twenty-two degrees.

Experiment third was to ascertain the effect of a shoulder behind the wheel, one end being turned with a shoulder of one eighth of an inch, as a stop to the wheel, the other end turned plain. Tested by hydraulic pressure, the shouldered end broke with sixty tons, the plain end with eighty-four tons.

The object of the fourth experiment was to find the influence of the position of the wheel, as regards the end of the journal. An axle was fastened into a cast-iron frame, in a line with the neck of the journal, when the latter was broke with seven blows of a ram falling ten feet. The other end was keyed into the frame, with the neck of the journal projecting 1½ inch, and broke at the twenty-fourth blow of the same ram, falling ten feet.

The results of the trials are thus summed up by the experimenter:—That axles should never be smaller at the centre than at the ends, but on the contrary, that if a difference in size is made, the centre should be the largest.

The best authorities on the strength of materials, give the hollow tube as three times stronger in resisting twisting, than the solid bar possessing the same weight. Thus an axle with an external diameter of five inches, and an internal diameter of 3¾ inches, is three times as strong as a solid axle of 3¾ inches diameter.

Note 2.—The following experiments were prepared by M. Bourville, and executed by the Austrian government. The apparatus consisted of a bent axle, which was firmly fixed up to the elbow in timber, and which was subjected to torsion by means of a cog-wheel connected with the end of the horizontal part. At each turn the angle of torsion was twenty-four degrees. A shock was produced each time that the bar left one tooth to be raised by the next. An index adapted to the apparatus, indicated the number of revolutions and shocks. Seven axles, submitted to this trial, presented the following results:—

1st. The movement lasted one hour; 10,800 revolutions and 32,400 shocks were produced. The axle, two and six tenths inches in diameter, was taken from the machine and broken by an hydraulic press. No change in the texture of the iron was visible.

2d. A new axle, having been tried four hours, sustained 129,000 torsions, and was afterwards broken by means of an hydraulic press. No alteration of the iron could be discovered, by the naked eye, on the surface of rupture; but tried with a microscope, the fibres appeared without adhesion, like a bundle of needles.

3d. A third axle was subjected, during twelve hours, to 388,000 torsions, and broken in two. A change in its texture, and an increased size in the grain of the iron, was observed by the naked eye.

4th. After one hundred and twenty hours, and 3,888,000 torsions, the axle was broken in many places; a considerable change in its texture was apparent, which was more striking towards the centre; the size of the grains diminished towards the extremities.

5th. An axle, submitted to 23,328,000 torsions during seven hundred and twenty hours, was completely changed in its texture; the fracture in the middle was crystalline, but not very scaly.

6th. After ten months, during which the axle was submitted to 78,732,000 torsions and shocks, fracture, produced by an hydraulic press, showed clearly an absolute transformation of the structure of the iron; the surface of rupture was scaly like pewter.

7th. Finally, as a last trial, an axle submitted to 128,304,000 torsions, presented a surface of rupture like that in the preceding experiment. The crystals were perfectly well defined, the iron having lost every appearance of wrought iron.

CLASSIFICATION OF CARS.

379. Railroad cars come under three general heads,

380. The American passenger car consists of a body about fifty feet long, ten feet wide, and seven feet high, containing seats for about sixty passengers, being cushioned, warmed, lighted, and ventilated. Except for emigrants, second and third class cars are but little used in America.

House, box, or covered freight cars, differ from the “flat,” or platform car, only in having a simple rectangular house, about six feet high and nine feet wide, built upon the floor. This is used for the protection of such freight as will not bear exposure; as furniture, books, dry goods, hardware, and small machinery. Carriages, boxes, bales, masts, lumber, and fuel are carried by platform cars. Bulky machinery, and first and second class freight too large for the box cars, should be protected by tarpaulins.

381. The general arrangement of wheels, springs, and brakes, is the same for the several classes of cars, the chief difference being in the ease of springs. Each car rests upon two “trucks,” consisting of four, six, or eight wheels, so connected by levers and springs, as best to absorb shocks, and connected with the body by a pin only, by which the passage of curves is made quite easy.

Cars used for the movement of earth are so arranged as to allow the body to be tipped up, that the contents may be quickly “dumped,” either at the sides, ends, or middle, as desired.

382. Upon some roads, a continuous draw bar is passed under the whole train, the several cars being attached to it, and to each other by safety chains only. By adopting this, and at the same time by springing the buffer beams tight upon each other, the whole train becomes one piece; and the jerks at stopping and at starting are in a great measure avoided.

As lightness combined with strength is a desideratum in all cases, it will be found best to truss the longitudinal frame pieces of the car with rods, rather than to use large and heavy beams, as done by many builders.

RETARDING OF TRAINS.