CHAPTER VI

FORM--_Continued_. SHAPES TO LESSEN RESISTANCE TO MOTION

Shot formed to move swiftly through the air . . . Railroad trains and automobiles of somewhat similar shape . . . Toothed wheels, conveyors, propellers and turbines all so curved as to move with utmost freedom.

Projectiles and Vehicles of Like Pattern.

While ships are much the largest structures built for motion, and therefore meet resistances which the designer must lessen as best he may, other moving bodies, small as compared with ships, encounter resistances so extreme that their reduction enlists the utmost skill and the most careful study. Speeds vastly higher than those of ships are given to projectiles. A ball leaving a gun muzzle with a velocity of 3,410 feet a second, as at Sandy Hook in January, 1906, suffers great atmospheric resistance, overcome in part by the shot having a tapering or conoidal form. Indians long ago stuck feathers obliquely into arrows so as to keep flight true to its aim by giving shafts a spiral motion; an attendant advantage being to lengthen flight. The same principle appears in rifling, that is, in cutting spiral grooves in the barrels of firearms large and small, a missile receiving a spinning motion through its base, a thin protruding disk of soft metal, forced into the grooves by the explosive. At first the grooves in firearms were straight with intent to preclude fouling; spiral grooves were introduced by Koster of Birmingham about 1620. Delvigne, a Frenchman, devised a lengthened bullet narrower than the bore so as to enter freely, under the pressure of firing it completely filled the bore, rotating with great velocity as it sped forth.

Now that railroad speeds are approaching those of projectiles, the outlines of trains are resembling those of shot and shell. In the experiments with very fast trains at Zossen, in Germany, October, 1903, each car had a paraboloidal front, much diminishing the resistance of the air. Racing automobiles are usually encased in a pointed shell which parts the air like a wedge; their wheels, too, are supported not by spokes, but by disks having no projections. As electric traction becomes more and more rapid in its interurban services, the cars will undoubtedly be shaped to lessen atmospheric resistance. Especially is this desirable in a tunnel service, such as that of the New York Subway, where the resistances are extreme for the same reason that a boat in a canal is harder to draw than if in water both broad and deep. Just as in ship-design, it is in sharpening the front and rear of a car or a train that most economy is feasible; the friction at the sides cannot be much lessened except, in the case of a train, by joining each car to the next by a vestibule such as that of the Pullman Company.

Electric traction finds gain in a track having in places a decided inclination. In the monorail line between Liverpool and Manchester a downward dip in the line at each terminal quickens departure, and in arrival aids the brakes by checking speed on the up-grade. In the swift motion of ordinary machinery the resistance of the air is a source of considerable loss. By encasing a heavy flywheel in sheet iron so as to present a smooth surface to the atmosphere, M. Ingliss has saved 4.8 per cent. of the energy of a 630 horse power engine.

Gearing: Conveyors.

In the simplest machines motion may be transmitted by wheels in contact, faced with adhesive leather, rubber, or cloth. Teeth, however, are usually employed; as wear takes place they permit a little play, a slight looseness, which contact wheels altogether refuse. Toothed wheels have the further advantage that they do not slip, their motion is positive. How teeth may best be contoured involves nice questions in geometry. They should always push and never grind each other, and should move with the least possible friction. In some ingenious designs the teeth of any one particular wheel of a series will enmesh with the teeth of any other wheel, no matter how much larger or smaller. Bevel gears cut by Mr. Hugo Bilgram, of Philadelphia, turn with hardly any friction whatever, although in some wheels the teeth run askew, or are sections of cones which do not meet at their apices. The Bilgram gear cutter, and the Fellows’ gear shaper which turns out plain gear, exert a to and fro planing action. Ordinary gears are cut on milling machines by rotary cutters, or may be manufactured on a Bliss press without cutting the original lines of fibre. The importance of accurate and easy-running gears increases steadily; they are, for example, applied to steam turbines whose velocity must be reduced in the actuation of ordinary machines. Automobiles and bicycles also demand reducing gear running with the utmost freedom.

The grain elevator, invented many years ago, is the parent of manifold conveyors of coal, lime, ore or aught else. Their receivers have links shaped so as to extend for hundreds of feet as continuous belts. Link belting may be had in detachable sections, fitting each other at secure hinges which allow free motion.

The _Augustin B. Wolvin_, a typical ore-carrier on the great lakes, is 56 feet in depth; its hold is curved to allow a clam-shaped bucket to seize ten tons of ore at each dip. It is probable that at no distant day rapid transit in cities will employ continuous moving platforms, just as conveyors and telpherage systems are taking the place of the discontinuous transport of grain, coal, cotton, ore, and heavy merchandise.

Propellers.

The screw, an inclined plane wound about an axis, forms the propeller for steamships and many steamboats. There is a good deal of debate as to the principles which should decide its best lines. Here evidently is a field which will handsomely repay thorough investigation. The power expended in steamships, whether fast or slow, is prodigious; any marked improvement in the contour of screws will mean either a saving of fuel or an increase of speed. Of equal importance with water-propulsion is the setting in motion of air. In blast furnaces enormous volumes of air are forced at high pressure into the fuel and ore: the fans are carefully molded in screw form, any departure from the best curves entailing serious loss. Fans for less important services are seldom shaped with care and usually waste much energy.

Turbines.

Allied to screws are turbine wheels, much the most efficient of water motors. The shaping of their vanes as volutes minimizes the loss of energy in shock as the water comes in, and lessens to the utmost the velocity of the stream as it leaves the wheel. Now that steam turbines are scoring a success both on land and sea the contouring of their vanes with extreme nicety is an important problem of the engineer. A perfected form means the highest economy.

It is interesting to note how the screw propeller, the fan, and the turbine wheel have each led to a converse invention. Mr. Edwin Reynolds, of Milwaukee, has devised a pump in screw form of capital efficiency under low heads. The fan has long had its converse in the windmill, now more popular throughout America than ever before, mainly because shaped with new excellence. In the best models, built of steel, the sails are each a section of a volute carefully designed to discharge the wind evenly, just as in the parallel case of emission from a water mover, such as the Worthington pump. This capital pump is simply a turbine wheel reversed. Its impeller and diffusion vanes take up water from rest, lift it to a height which may be as much as 2,000 feet, and then deliver it at rest, with little loss from internal eddies or slippage.

The Pelton wheel, pre-eminent among water-motors of the impulse type, owes its economy chiefly to each bucket being divided in halves and curved with the utmost nicety.