Popular Science Monthly/Volume 24/February 1884/Fifty Years of Mechanical Engineering
|FIFTY YEARS OF MECHANICAL ENGINEERING.|
I WILL begin by referring to the steam-plant employed for manufacturing purposes. In 1832 the stationary engine was commonly the beam-engine, often condensing but seldom compounded. Steam was supplied by boilers having but little resemblance to the boilers which most of us are familiar with. The name given the boilers explains their form; they were variously called tun, hay-stack, balloon, elephant, chimney, and ring boiler, to each of which they severally bore a striking resemblance. They were built in utter disregard of all laws relating to the strength of material, but were well adapted for the convenience of the firemen, in that the flues were of such size that a man could pass through them to remove accumulated soot.
The result was, that the boilers were incapable of withstanding an internal pressure of more than four or five pounds to the square inch. The low pressure made a large cylinder necessary to secure the required power, and the size of the cylinder restricted the speed, which rarely exceeded 250 feet a minute. The boilers were commonly fed by a tank situated high enough to enable the water to overcome the pressure of the steam. The low pressure and slow piston-speed necessitated very large cylinders relatively to the power obtained. The consumption of fuel was about ten pounds to the one horse-power per hour.
The governing was done by slowly-revolving pendulum-arms scarcely securing centrifugal force enough to raise the balls and actuate the butterfly-valve in the steam supply-pipe, thus making a very poor and inefficient governor. The low speed made a very heavy fly-wheel necessary to secure uniformity of motion, also costly trains of gear-wheels to secure the rotative speed required for factory-work.
In 1882 the boilers are cylindrical, frequently internally fired, and, thanks to Sir William Fairbairn's circumferential bands, the flue, subjected to external pressure, is so strengthened that the danger of collapse is removed even with our present high pressures. The tendency of the day seems to incline toward the water-tube sectional type of boiler and a rational system of inspection and test. The pressures in use to-day vary from 80 to 150 pounds. The piston-speed is nearer 500 feet per minute, often 800 and 1,000. An engine of 1832 capable of exerting 25 one horse-power to-day would indicate about 250 working under fair conditions. The same expenditure of fuel to-day would give nearly four times the power.
The decrease in size of the cylinder due to the higher pressures has made higher rotative speeds possible; hence, the engine requires a much lighter fly-wheel, and the governing is made more effective. The most efficient engines of to-day are found in our city pumping-stations. Here the conditions are favorable for securing the highest economy, a duty of 100,000,000 foot-pounds being frequently secured. The engine of to-day for mill-use is, comparatively speaking, a portable engine requiring nothing but a foundation to bolt it to. The engine of fifty years ago was not self-contained or self-supporting, but required to be built from the ground up, and the support of walls and timbers.
To-day the practice is to make large engines condensing and often compound, expanding the steam in some instances ten volumes. The higher pressures and rotative speeds of to-day have made the use of high expansions possible in comparatively small engines, and economies are secured which, but a few years ago, would have been wonderful for large engines. The governing is done by quick-running governors which either throttle the supply-pipe or alter the point of cut-off, and thus secure uniformity of motion with the highest expansive use of the steam.
In 1832 no steamship had essayed the passage across the Atlantic. The marine boilers of 1832 were unfit for resisting any considerable pressure, in fact, so weak were they that they have been known to collapse when steam had been let down. The engine and boilers took up so much of the tonnage of the vessel, and used such enormous quantities of coal, that it was predicted that it would never be possible to cross the Atlantic unaided by sail. In fact, the prediction held good for a long time, for transatlantic steamship lines were compelled to establish coaling-stations at Halifax and Queenstown in order to reduce the coal carried, and allow of a little cargo being taken on. In 1832 all hulls were wood, and salt-water was invariably used in the boilers, much to their injury. The speed rarely exceeded eight knots an hour.
In 1882 the ships are almost invariably of iron or mild steel, and this enables the introduction of an element of safety impossible with the use of wood: I refer to the compartment and cellular system of naval construction. The use of iron and steel has made the construction of ships of great length possible.
The boilers are of enormous strength, and carry from 80 to 125 pounds pressure. The cylinder or cylinders are now adapted to the economical utilization of all the expansive force due to the pressure used. To secure this, more than one cylinder, is used; all the expansion could be had in one cylinder, but the difference in temperature of the cylinder, due to the temperature of the steam before and after expansion, would cause undue condensation. The substitution of the propeller for the paddle-wheel for sea-navigation and the high speeds required by the former have done much to reduce the size and weight of the marine engine; and have also had a marked effect on the economy. The paddle-wheel has practically disappeared, except on rivers.
A piston-speed of 800 feet a minute is often attained in daily practice. Hence, enormous powers are secured with comparatively little loss of carrying-space.
The marine governor of to-day is almost endowed with prophecy. It anticipates the pitching of the ship and withdrawal of the screw from the water, and cuts off steam just before its occurrence, thus avoiding the dangerous racing of the engine when the screw leaves its work. This, for a long time, has been almost the only danger in bad weather; the racing of the engines subjected the screw-shaft to strains for resisting which the shaft was inadequate. The twisting off of the propeller-shaft of an Atlantic steamer is not an uncommon occurrence. Condensation is now had almost in all cases by the surface condenser, thus returning all the water to the boiler to be used again. It might be well to speak here of a steamship built in 1882. Steamships are now making long voyages at a high rate of speed, voyages which till a short time ago had been left to sailing-vessels. This steamship has some points of interest, and illustrates the most advanced ideas on steam-engineering as applied to the mercantile marine. The engines of this steamer are triple expansive, having one high-pressure, one intermediate, and one low-pressure cylinder, using steam at 125 pounds pressure, generated by boilers whose only peculiarity consists in the fact that they are capable of withstanding such a pressure. On trial these engines gave one horse-power for 1·28 pound of coal burned per hour. This would, according to the usual analogy, indicate a daily working efficiency of about 1·50 pound to the one horse-power. This steamer can carry coal for a voyage of 12,000 miles, and, with proper use of sails, could probably keep under steam for two months without coaling. The weight of the engine and boilers of 1832 was about 1,000 pounds to the horse-power; to-day it is about 300, and in some instances has been reduced to forty-five pounds to the horse power.
An English firm have recently completed a small light compound engine, which, in point of weight, eclipses anything heretofore built. This engine is made of steel and phosphor-bronze; all parts are built as light as possible, the rods and shafting and all parts possible being bored out to reduce weight. At a speed of only 300 revolutions a minute they indicate over twenty horse-power, and weigh but 105 pounds all told. This engine would give fully thirty horse-power actual at a piston-speed of 500 feet a minute. The size is three and three quarters high pressure, seven and a half low pressure, and five stroke. That thirty horse-power can be had from a proper utilization of steam and proper distribution of 105 pounds of metal is certainly most astonishing, especially so, considering that the engine is compound. A ship of 2,500 tons displacement was almost unknown fifty years ago; to-day the transatlantic steamer, the highest class of the mercantile marine, has from 8,000 to 13,500 tons displacement, and engines of 5,000 to 10,000 one horse-power. Several of the transatlantic liners have shown a mean ocean-speed of twenty miles an hour, and make the passage in less than seven days.
The present generation has grown so accustomed to the results of the progress of mechanical science that it has long ceased to wonder at its greatest works.
It may be well here to speak of the torpedo-boats which have been recently built for the English Government; they indicate the extreme limit of naval construction of this day. These little instruments of destruction are only eighty-seven feet in length, ten and a half feet in beam, forward draught eighteen inches, aft fifty-two inches, total displacement thirty-three tons. The engines are compound condensing, of the intermediate receiver type, high-pressure cylinder twelve and three fourths inches, low-pressure twenty and three fourths, stroke twelve inches, and indicated over 500 horse-power, with a gross weight of only eleven tons, boiler, water, engine, condenser, propeller, and shaft included.
The special feature of the boat is the enormous power developed per hundredweight of propelling machinery. The boilers evaporate eighteen pounds of water per hour per square foot of heating surface, and 1·20 pounds of coal per square foot of grate-surface. This is fully six times the amount of water and coal usually dealt with per square foot of surface in furnace and boiler. Such a forced combustion precludes all thought of economy, yet a one horse-power is secured at full speed with an expenditure of three and a half pounds of coal. The forced draught is secured by maintaining in the stoke-hole an air-pressure corresponding to a column of water six inches high; this renders the stoke-hole quite cool and comfortable.
One ton of coal will last for a run of 100 miles at a ten-knot speed. A speed of twenty-two and a half knots has been secured in trials lasting three hours. This is a speed of 2,250 feet a minute, or thirty-seven and a half feet a second, and seems almost incredible.
But, remarkable and important as these results are in the phase of steam-engineering, these little vessels have revealed in their performances under speed-trials facts of equal importance to another department. The speeds attained are high even for large steam-vessels, but enormously high for such small vessels. It is found that passing the ten and twelve knot point, which bears about the same ratio to these little boats that eighteen knots an hour does to large steamers, the ratio of resistance to the speed decreases, and at the fifteen-knot point it is about the 32-power, at the eighteen-knot point about the 3-power, and sometimes at the twenty-two-knot point is as low as the 12-power of the speed.
Effort has been frequently made to utilize steam at much higher pressures than I have mentioned, but, owing to the solvent nature of steam or water at a high temperature, the results have not been satisfactory; among many difficulties encountered was that of lubricating the cylinders.
Loftus Perkins, an English engine-builder of prominence, is devoting much time to the use of steam at about five hundred pounds pressure, and with some success. Unfortunately, the gain to be anticipated from the use of these exceedingly high pressures does not seem to be very great on trial. The Anthracite, a small steamer fitted with engines and boilers specially adapted to the utilization of steam at five hundred pounds pressure, was more wasteful than many steamers using steam at one hundred pounds. However, here is a wide field and one that promises well.
Should the same change of law as to the resistance increasing as the square of the speed be found to hold good in large steamers as in the little torpedo-boats, we shall most of us live to see locomotive speeds at sea. There is now building in this country an engine which will exert the greatest power as yet secured from one cylinder. The stroke is fourteen feet and the diameter of the cylinder is nine feet two inches, and the engine is expected to develop eight thousand horse-power. As an illustration of the size of the engine, the wrist-pin is almost exactly the size of a flour-barrel.
We now come to the engines and boilers used for railways. The year 1832 was the beginning of our present passenger and railway system on this side of the water, and, if the engines imported in that year to run on American roads are any indication of the state of the science of steam-engineering abroad, they could not have been much in advance. At this time the engine and boiler weighed about eight tons, carried forty pounds pressure, and could make about twenty miles an hour under light load and favorable conditions. The engine of that date could not pull more than three or four times its own weight, and had to stop at stations to fill boilers, as they could not pump while running.
The speed to-day is from forty to sixty miles an hour, and the engines weigh from thirty-five to eighty tons, and draw as high as eight hundred tons of paying freight in addition to the weight of the train. To-day the pressures run from one hundred and thirty-five to two hundred pounds. The latter pressure is used in Switzerland. The automatic and continuous breaks now stop a heavy train within four hundred yards at a speed of sixty miles an hour. Recent trials show that these breaks will absorb twenty miles of speed in one minute.
In 1832 the transmission of power was by flat tumbling-rods and cast-iron shafting of great weight and little strength. To-day we have smooth, light, rapidly revolving steel or iron shafting, supplemented and aided with rubber and leather belting where the latter will serve and the former can not. Where power has to be transmitted at a great distance, wire ropes, moving at a high rate of speed, are used. Wire-rope transmission commences at the point where the belt and shafting become too long or heavy to be useful. It is much cheaper than its equivalent of shafting or belting. In fact, a long line of shafting would cost more for oil in a year than a wire rope would in fifteen.
At the Rhine-fall, in Switzerland, eight hundred horse-power is transmitted a distance of two miles to a village where fifty small manufacturing industries, situated in every conceivable position relative to the cable-line, secure power. For ten years the cable street-railway system has been in use in San Francisco. The same system, slightly modified, is being adopted in many Eastern cities.
Fifty years ago compressed air had not been successfully employed in engineering, though its application as a blast to forges is coextant with history. Sir Henry Bessemer's steel process was made possible only upon the ability of engineers to furnish air under pressure in the converter. The importance of compressed air and the part it has taken in recent engineering undertakings can not be overestimated. Without it the boring of most of our tunnels and the placing of masonry foundations under water could not have been accomplished. In 1832 the turbine wheel had just been invented, but not brought into use; in fact, hydro-mechanics has made as great steps forward in the last fifty years as any of her sister sciences.
A recent invention of Sir W. Armstrong deserves mention. A steam-engine actuating a pump is used to secure an artificial head of water, which water is afterward employed in driving various hydraulic motors operating cranes, lifts, driving riveting machinery, and the artificial head is secured by loading a ram of sufficient size with weight enough to place a pressure of seven or eight hundred pounds to the inch in the cylinder. The pumping-engine pumps against this ram, the chamber of which is connected with each of the machines requiring to be driven; whenever the work done in the various motors is less than the work of the engine, the surplus is expended in raising the ram, and when the ram is fully extended an automatic device stops the pump, which again resumes work on the withdrawal of water from the ram by leakage or use in motors. By the aid of this system of storing power, a small steam-pump attached to an accumulator is capable of furnishing three hundred or more horse-power for a short time. This arrangement is adopted in all docks and ship-yards of any pretensions.
Our modern turreted man-of-war handles its eighty and one hundred ton guns, and all the loading machinery, by the aid of similar hydraulic devices. These accumulators give an efficiency of ninety-eight per cent in practice, which amounts to perfection.
In 1832 rolled plates such as are now rolled were unknown, and the rolling of armor-plates twenty-two inches thick, weighing thirty tons, was not thought of.
The process of making wrought-iron by puddling has not changed much, though larger masses are handled. The manufacture of iron by puddling seems doomed; steel is taking its place rapidly; in 1832 masses of steel of over sixty pounds were not made; steel was dealt in by the pound for cutlery-use. Thanks to Sir Henry Bessemer and Dr. Siemens, steel is made on the Bessemer and open-hearth process, and in masses of many tons' weight. The rapid advancement made in engineering skill is due in a great measure to the cheapening of iron and steel making. Never in the history of the iron industry were there so many partially developed processes, the completion of which will revolutionize the industry, and furnish iron and steel at a cost much below present prices.
The unprecedented expanding of our railway interests since 1865 has had much to do with the development of the iron interests. Inventors of prominence promise us steel at one cent a pound, and in the light of the past it is not safe to assert that it will not be done. Steel rails have been sold within a few years at one hundred dollars a ton; to-day they are worth thirty-eight dollars. It is confidently predicted by those who have made it a study, that the downward tendency can not be checked, and that one cent a pound will be reached as soon as the experimenters have worked out plans now in hand.
Considering the many improvements which are now proposed and tested, we can safely assume that the steel-plant of the future will differ widely from the plant of to-day. All the available heat and all the useful elements in the ore will be used. Briefly this is as follows: The ores, limestone, and fuel will be placed in the furnace, the molten metal will be run to converters, and there the foreign elements will be removed by a blast, the metal then recarbonized and cast into ingots, the ingots will be rolled into blooms, then the bloom into rails, and the rails will then be placed on small cars, and, while at a temperature of about 1,000° Fahr., will be placed in the flues of steam-boilers until they have given up about 700° Fahr., and then passed on as finished. The slag flowing from the blast-furnace will be placed on cars, and, while at a temperature of 3,000° Fahr., be run into the flues of other boilers used to generate steam for operating the blowers, rolls, etc. This, in brief, is one of the proposed steps in steel-making, viz., the utilization of all the heat in the coal, and afterward all the heat given to the iron and slag by the coal; by so placing the iron and slag as to give up their heat again to boilers used to generate steam for the roller-mills and blowing-engines, which in turn aid the smelting of the iron.
A rail-mill of 500 tons a day, at a low estimate, would secure heat to run a 1,000 horse-power battery of boilers from the cooling rails alone, and 4,000 horse-power in heat from the slag. Hence the steel-plant of the future will have no heating-furnaces, no gas-producers, no coal-consuming boilers, no cupolas, no ash-piles, and no fuel to be consumed except that required to melt the iron. The converter-slag can now be used instead of limestone by the new process. This, in brief, will be, it is confidently predicted, the new rail-mill of the immediate future. Everything is done by the aid of air, steam, and water. Muscle will be in little demand, brains at a premium. In 1832 cast-iron bridges existed of short span, but wrought-iron had not been used. To-day we think little of trusses of 500 feet span, and suspension-bridges of 1,000 feet; while it is proposed to build a steel truss-bridge over a mile long, with two spans of 1,700 feet each. In the power-printing press, an invention of the eighteenth century, we find that the last half-century has wrought wonders. In 1832 the best presses could turn out about 1,000 poorly printed sheets of printed matter; to-day, thanks to Hoe's revolving type and the processes of electroplating and stereotyping, we have presses capable of printing 50,000 impressions an hour; and, what is almost as wonderful, it will number, fold, and stick together the whole. Such a machine costs about $100,000.
We live in an age of progress. The additions to our knowledge made during the last fifty years seem to excel in utility and lasting benefits the knowledge acquired in centuries. Popular belief is that the possibilities of progress in all directions are unlimited. Those who should know, think that in mechanics we have nearly reached the limit which theory, well established, places before us. The steam-engine, using but one tenth of the power to be obtained from the coal, is nearer its limit than most people imagine.
The science of the future is undoubtedly in chemistry, and our great discoveries and greatest progress will be in that science. Mechanics may hereafter expect to take a secondary part. In the iron industry chemistry and mechanics have stood side by side; chemistry generally propounds the problems, pointing the way to the chemical solution, and calling upon mechanics to devise means for carrying out the undertaking.
One of the most notable features of modern industrial progress is the utilization of what has always been considered waste material. This is done by devising and constructing special machinery to meet the case. Sometimes costly experiments are necessary; but, in this age of speculation, those who gain the prizes offered in legitimate business are those who are willing to accept ventures involving large risks. There is no limit to human wants, and the industrial expansion we are engaged in will not be restricted except by the impossible.
Photography and the electric sciences are two arts of which nothing was known fifty years ago: what a gap the removal of one of these would make in our civilization to-day!
Sir Henry Bessemer's steel process has had a very marked influence on the mechanical advancement of the last half-century. Yet so closely allied are all the great steps in progress, that one can not be taken without the other, and Sir Henry was himself compelled to seek or invent numerous devices before his original steel process merited the name.We daily complete engineering works which, in the amount of human labor they represent, far exceed the labor represented by the great Pyramid of Cheops. Undoubtedly the progress of the age, which is so largely engineering progress, does greatly increase the welfare of man. The forces of Nature now do the hard work, and the labor of the toiling millions is lightened many fold. The laboring-man now works with brain and eye, and his occupation is to direct and apply some principle of science. He now has time for improvement, comfort, and refinement; the forces of Nature having become obedient to the will of man, are made to produce for him not only plenty, but conveniences and luxuries formerly undreamed of.
- Read before the Peoria Scientific Society, March 24, 1883.