Fig. 1424AFig. 1424a.
Fig. 1424a.
Let the distance,Fig. 1424a,atoblie in the plane of motion of the tool feed, and measure, say,1⁄4inch, the tool moving, say,5⁄16inch along the cut per lathe revolution. Suppose the edge frombtodto lie at a minute angle to the line of tool traverse, and the depth of the cut to be such that the part frombtocperforms a slight cutting or scraping duty, then the part frombtocwill leave a slight ridge on the work plainly discernible to the naked eye in what are termed the tool marks.
The obvious means of correcting this is to have the parta bof greater width than the tool will feed along the cut, during one revolution of the work (or the cutter, as the case may be); but there are practicable obstacles to this, especially when applied to wrought iron, steel, or brass, because the broader the cutting edge of a tool the more liable it is to spring, as well as to jar or chatter, leaving a surface showing minute depressions lying parallel to the line of tool feed.
If the cutting tool be made parallel and cylindrical on its edges, and clearance be given on the front end of its diameter only, so as to cut along a certain distance only of its cylindrical edge, the rest being a close fit to the bore of the work, the part having no cutting edge, that is, the part without clearance, will be apt to cause friction by rubbing the bore of the work as the tool edge wears, and the friction will cause heat, which will increase as the cut proceeds, causing the hole to expand as the cut proceeds, and to be taper when cooled to an equal degree all over. This may be partly obviated by giving the tool a slow rate of cutting speed, and a quick rate of feed, which will greatly reduce the friction and consequently the heating of the tool and the work. On cast iron it is possible to have a much broader cutting edge to the tool, without inducing the chattering referred to, than is the case with wrought iron, steel, or brass, especially when the finishing cut is a very light one. If the finishing cut be too deep, the surface of the work, if of cast iron, will be pitted with numerous minute holes, which occur because the metal breaks out from the strain placed on it (and due to the cut) just before it meets the cutting edge of the tool. Especially is this the case if the tool be dull or be ground at an insufficiently acute angle.
When the work shows the tool marks very plainly, or if of cast iron shows the pitting referred to (instead of having a smooth and somewhat glossy appearance), there will be less of its surface in contact with the surface to which it fits, and the fit will soon become destroyed, because the wearing surface or the gripping surface, as the case may be, will the sooner become impaired, causing looseness of the fit. In the one case the abrasion which should be distributed over the whole area of the fitting parts is at first confined to the projections having contact, which, therefore, soon wear away. In the other case the projecting area in contact compresses, causing looseness of the fit.
Hydraulic press or forcing fits.—For securing pieces together by forcing one within the other by means of an hydraulic press, the plug piece is made a certain amount larger than the bore it is to enter, this amount being termed the allowance for forcing. What this allowance should be under any given conditions for a given metal, will depend upon the truth and smoothness of the surfaces, and on this account no universal rule obtains in general practice. From some experiments made by William Sellers & Co., it was determined that if a wheel seat (on an axle) measuring 47⁄8inches in diameter and 7 inches long was turned7⁄1000of an inch larger than the wheel bore, it would require a pressure of about thirty tons to force the wheel home on the axle.
At the Susquehanna shops of the Erie railroad the measurements are determined by judgment, the operatives using ordinary calipers. If an axle 31⁄2diameter and 6 inches long requires less than 25 tons it is rejected, and if more than 35 tons it is corrected by reducing the axle.
In order to insure a proper fit of pieces to be a driven or forced fit it is sometimes the practice to make them taper, and there is a difference of opinion among practical mechanics as to whether taper or parallel fits are the best. Upon this point it may be remarked that it is much easier to measure the parts when they are parallel than when they are taper, and it is easier to make them parallel than taper.
On the elevated railroads in New York city, the wheel bores being 41⁄8inches in diameter and 5 inches long, the measurements are taken by ordinary calipers, the workmen judging how much to allow, and the rule is to reject wheels requiring less than about 26 tons, or more than about 35 tons, to force them on. These wheels form excellent examples, because of the excessive duty to which they are subjected by reason of the frequency of their stoppage under the pressure of the vacuum brake. The practice with these wheels is to bore them parallel, finishing with a feed of1⁄4inch per lathe revolution, and to turn the axle seats taper just discernible by calipers.
This may, at first sight, seem strange, but examination makes it reasonable and plain. Let a wheel having a parallel bore be forced upon a parallel axle, and then forced off again, and the bore of the wheel will be found taper to an appreciable amount, but increasing in proportion as the surface of the hole varied from a dead smoothness; in other words, varying with the depth of the tool marks in the bore and the smoothness of the cut.
Let the length of the wheel bore be 7 inches long, and theamount allowed for forcing be .004 inch, and one end of the wheel bore will have been forced (by the time it is home on the axle) over the length of 7 inches of the axle-seat, whose diameter was .004 larger than the bore: a condensation, abrasion, or smoothing of the metal must have ensued.
Now the other end of the same bore, when it takes its bearing on the shaft, is just iron, and iron without having suffered any condensation. If the tool marks be deep, those on one end will be smoothed down while those at the other remain practically intact. Clearly then, for a parallel hole, a shaft having as much taper as the wheel bore will get in being forced over the shaft best meets the requirements; or, for a parallel shaft or seat, and a taper hole (the taper being proportioned as before), the small end of the taper hole should be first entered on the shaft, and then when home both the axle and the wheel-bore will be parallel.
It may be remarked that the wheel seat on the axle will also be affected, which is quite true, but the axle is usually of the hardest metal and has the smoothest surface, hence it suffers but little; not an amount of any practical importance.
In an experiment upon this point made in the presence of the author by Mr. Howard Fry and the master mechanic of the Renovo shops of the Philadelphia and Erie railroad, an axle seat finished by a Whitney “doctor,” and parallel in diameter, was forced into a wheel having a parallel bore, and removed immediately. On again measuring the axle, the wheel-seat was found to be1⁄1000taper in its length.
The wheel-bore was found to be but slightly affected in its diameter, which is explained because it being very smooth, while the turning marks in the axle were plainly visible, the abrasion fell mainly upon the latter.
When the enveloping piece or bore is not solid or continuous, but is open on one side, the degree of the fit may be judged from the amount that it opens under the pressure of the plug piece.
Fig. 1425Fig. 1425.
Fig. 1425.
Thus the axle brasses of American locomotives are often made circular at the back, as shown inFig. 1425, and are forced in endways by hydraulic pressure. The degree of tightness of the brass within the box may, of course, be determined by the amount of pressure it requires to force it in, but another method is to mark a centre punch dot as atj, and before the brass is put in mark from this dot as a centre an arc of a circle asl l. When the brass is home in the box a second arckis marked, the distance betweenlandkshowing how much the brass has sprung the box open widening ath. In an axle box whose bore is about 4 inches to 5 inches in diameter, and 6 inches long,1⁄32inch is the allowance usually made.
Shrinkage fits are employed when a hole or bore requires to be very firmly and permanently fastened to a cylindrical piece as a shaft. The bore is turned of smaller diameter than its shaft, and the amount of difference is termed the allowance for shrinkage. The enveloping piece is heated so as to expand its bore; the shaft is then inserted and the cooling of the bore causes it to close or contract upon the shaft with an amount of force varying of course with the amount allowed for contraction. If this allowance is excessive, sufficient strain will be generated to burst the enveloping piece asunder, while if the allowance for shrinking is insufficient the enveloping piece may become loose.
The amount of allowance for shrinkage varies with the diameter thickness, and kind of the material; but more may be allowed for wrought iron, brass, and copper, than for cast iron or steel.
Again, the smoothness and truth of the surfaces is an important element, because the measurement of a bore will naturally be taken at the tops of the tool marks, and these will compress under the shrinkage strain, hence less allowance for contraction is required in proportion as the bore is smoother.
In ordinary workshop practice, therefore, no special rule for the amount of allowance for shrinkage obtains, the amount for a desultory piece of work generally being left to the judgment of the workman, while in cases where such work is often performed on particular pieces, the amount of allowance is governed by experience, increasing it if the pieces are found in time to become loose, and decreasing it if it is found impossible to get the parts together without making the enveloping piece too hot, or if it is found to be liable to split from the strain.
The strength of the enveloping piece is again an element to be considered in determining the amount to be allowed for shrinkage. It is obvious, for example, that a ring of 8 inches thick, and having a bore of, say, 6 inches diameter, would be less liable to crack from the strain due to an allowance of1⁄50inch for contraction, than would a ring of equal bore and one inch thick having the same allowance. The strength or resistance to compression of the piece enveloped in proportion to that enveloping it, is yet another consideration.
The tires for railway wheels are usually contracted on, and Herr Krupp states the allowance for contraction to be for steel tires1⁄100inch for every foot of diameter; in American practice, however, a greater amount is often employed. Thus upon the Erie railroad a 5 foot tire is given1⁄16inch contraction. The allowance for wrought iron or brass should be slightly more than it is for steel or cast iron, on account of the greater elasticity of those metals.
Examples of the practice at the Renovo shops of the Pennsylvania road are as follows:
Class E, diameter of wheel centre, 44 inches; bore of steel tire, 4315⁄16inches.
Class D, diameter of wheel, 50 inches; bore of tire, 499⁄16inches.
It is found that the shrinkage of the tire springs or distorts the wheel centre, hence the tires are always shrunk on before the crank-pin holes are bored.
Much of the work formerly shrunk on is now forced on by an hydraulic press. But in many cases the work cannot be taken to an hydraulic press, and shrinkage becomes the best means. Thus, a new crank pin may be required to be shrunk in while the crank is on the engine shaft, the method of procedure being as follows: In heating the crank, it is necessary to heat it as equally as possible all round the bore, and not to heat it above avery darkred. In heating it some dirt will necessarily get into the hole, and this is best cleaned out with a piece of emery paper, wrapped round a half-round file, carefully blowing out the hole after using the emery paper. Waste or rag, whether oiled or not, is not proper to clean the hole with, as the fibres may burn and lodge in the hole; indeed, nothing is so good as emery paper.
It is desirable to heat the crank as little as will serve the purpose, and it is usual to heat it enough to allow the pin to push home by hand. It is better, however, to overheat the crank than to underheat it, providing that the heat in no case exceeds a barely perceptible red heat. If, however, the crank once grips the pin before it is home, in a few seconds the pin will be held so fast that no sledge hammer will move it. It is well, therefore, to have a man stationed on each side of the crank, each with a sledge hammer, and to push the crank pin in with a slam, giving the man in front orders to strike it as quickly as possible at a given signal; but if the pin does not move home so rapidly at each blow as to make it appear certain that it will go home, the man at the rear, who should have a ten-pound sledge, should be signalled to drive out the crank pin as quickly as he possibly can for every second is of consequence. All this should be done soquickly that the pin has not had time to get heated to say 100° at the part within the crank.
So soon as the pin is home, a large piece of wetted cotton waste should be wrapped round its journal, and a stream of water kept running on it, to keep the crank pin cold. At the other end water should be poured on the pin end in a fine stream, but in neither case should the water run on the crank more than can be avoided. Of course, if the crank is off the shaft, the pin may be turned downward, and let project into water.
The reasons for cooling the pin and not the crank are as follows: If the crank be of cast iron, sudden cooling it would be liable to cause it to split or crack. If the crank pin is allowed to cool of itself, the pin will get as hot as the crank itself, and in so doing will expand, placing a strain on the crank that will to some extent stretch it. Indeed, when the pin has become equally hot with the crank it is as tight a fit as it will ever be, because after that point both pieces will cool together, and shrink or contract together, and hence the fit will be a looser or less tight one to the amount that the pin expanded in heating up to an equal temperature with the crank.
The correct process of shrinking is to keep the plug piece as cold as possible, while the outside is cooled as rapidly as can be without danger of cracking or splitting.
The ends of crank pins are often riveted after being shrunk in, in which case it is best to recess the end, which makes the riveting easier, and causes the water poured upon its face to be thrown outward, thus keeping it from running down the crank face and causing the crank to crack or split.
It sometimes becomes necessary and difficult to take out a piece that has been shrunk in, and in this event, as also in the case of a piece that has become locked before getting fully home in the shrinking process, there is no alternative but to reheat the enveloping piece while keeping the enveloped piece as cold as can be by an application of water.
The whole aim in this case is to heat the enveloping piece as quickly as possible, so that there shall be but little time for its heat to be transmitted to the piece enveloped. To accomplish this end melted metal, as cast iron, is probably the most efficient agent; indeed it has been found to answer when all other means failed.
Fig. 1426Fig. 1426.
Fig. 1426.
Fig. 1427Fig. 1427.
Fig. 1427.
The fine measurements necessary for shrinkage purposes render it necessary, where pieces of the same form and kind are shrunk on, to provide the workmen with standard gauges with which the work may be correctly gauged. These often consist of simple rods or pieces of iron wire of the required length.Figs. 1426and1427, however, represent an adjustable shrinkage gauge designed by H. S. Brown, of Hartford, Connecticut.Fig. 1427is a sectional, andFig. 1426a plan side view of the gauge.ais a frame, containing at its lower end a fixed measuring pieceb, and provided at its upper end with a threaded and taper split hub to receive externally the taper-threaded screw capc, and threaded internally to receive a tubee, which is plugged at the bottom by the fixed plugf. The adjustable measuring leggis threaded with the tubee, so as to be adjustable for various diameters of boxes, but it is locked when adjusted by the jamb-nuth. The operation is as follows: The cap-nutcand jamb-nuthare loosened and screwed back, allowing stemgand tubeeto be adjusted to the exact size of the shaft for which a shrinkage fit is to be bored, as, say, in an engine crank. In setting the gauge to the diameter of the shaft, the cap endcand jamb-nuthare screwed home, so as to obtain a correct measurement while all parts are locked secure. The cap-nutcdraws the split hub upon the tubee, and the jamb-nuthlocks upgtoe, so that the shaft measurement is taken with all lost motion, play and spring of the mechanism taken into account, so that they shall not vitiate the measurement. This being done,cis loosened so thatecan be rotated, and raised up (by rotating) to admit the shrinkage gauge-piecej, whose thickness equals the amount to be allowed for the size of borer to be shrunk on the shaft.jbeing inserted,eis rotated back so as to bindjbetween the end ofeand the foot pieceb, whencis screwed down, clampingeagain. Thus the measuring diameter of the gauge is increased to an amount due to the thickness of the gauge-piecej. At the right ofFig. 1426an edge and side elevation ofjis shown, the12⁄1000indicating its thickness, which is the amount allowed for shrinkage, and the 6-inch indicating that this gauge-piece is to be used for bores of 6 inches in diameter. The dotted circlek k l lrepresents a bore to which the gauge is shown applied.
The system of shrinking employed at the Royal Gun Factory at Woolwich, England, is thus described by Colonel Maitland, superintendent of thatfactory:—
“The inside diameter of the outer tube, when cold, must be rather smaller than the outside diameter of the inner tube: this difference in the diameter is called the ‘shrinkage.’ While the outer coil is cooling and contracting it compresses the inner one: the amount by which the diameter of the inner coil is decreased is termed the ‘compression.’ Again, the outer coil itself is stretched on account of the resistance of the inner one, and its diameter is increased; this increase in the diameter of an outer coil is called ‘extension.’ The shrinkage is equal to compression plus the extension, and the amount must be regulated by the known extension and compression under certain stresses and given circumstances. The compression varies inversely as the density and rigidity of the interior mass; the first layer of coils will therefore undergo more compression than the secondhand the second more than the third, and so on.
“Shrinking is employed not only as an easy and efficient mode of binding the successive coils of a built-up gun firmly together, but also for regulating as far as possible the tension of the several layers, so that each and all may contribute fairly to the strength of the gun.
“The operation of shrinking is very simple; the outer coil is expanded by heat until it is sufficiently large to fit easily over the inner coil or tube (if a large mass, such as the jacket of a Fraser gun, by means of a wood fire, for which the tube itself forms a flue; if a small mass, such as a coil, in a reverberatory furnace at a low temperature, or by means of gas). It is then raised up by a travelling crane overhead and dropped over the part on to which it is to be shrunk, which is placed vertically in a pit ready to receive it.
“The heat required in shrinking is not very great. Wrought iron, on being heated from 62° Fahr. (the ordinary temperature) to 212°, expands linearly about1⁄1000th part of its length; that is to say, if a ring of iron 1000 inches in circumference were put into a vat of boiling water, it would increase to 1001 inches, and according to Dulong and Petit the coefficient of expansion, whichis constant up to 212°, increases more and more from that point upward, so that if the iron ring were raised 150° higher still (i.e.to 362°) its circumference would be more than 1002 inches. No coil is ever shrunk on with so great a shrinkage as the2⁄1000th part of its circumference or diameter, for it would be strained beyond its elastic limit. Allowing, therefore, a good working margin, it is only necessary to raise a coil to about 500° Fahr.,[22]though in point of fact coils are often raised to a higher degree of temperature than this in some parts, on account of the mode of heating employed. Were a coil plunged in molten lead or boiling oil (600° Fahr.) it would be uniformly and sufficiently expanded for all the practical purposes of shrinking, but as shrinkings do not take place in large numbers or at regular times, the improvised fire or ordinary furnace is the more economical mode, and answers the purpose very well.
[22]The temperature may be judged by color; at 500° F. iron has a blackish appearance; at 575° it is blue; at 775° red in the dark; at 1,500° cherry red, and so on, getting lighter in color, until it becomes white, or fit for welding, at about 3,000°.
[22]The temperature may be judged by color; at 500° F. iron has a blackish appearance; at 575° it is blue; at 775° red in the dark; at 1,500° cherry red, and so on, getting lighter in color, until it becomes white, or fit for welding, at about 3,000°.
“Heating a coil beyond the required amount is of no consequence, provided it is not raised to such a degree of temperature that scales would form; and in all cases the interior must be swept clean of ashes, &c., when it is withdrawn from the fire. With respect to the modes of cooling during the process of shrinking, care must be taken to prevent a long coil or tube cooling simultaneously at both ends, for this would cause the middle portion to be drawn out to an undue state of longitudinal tension. In some cases, therefore, water is projected on one side of a coil so as to cool it first. In the case of a long tube of different thickness, like the tube of a R. M. L. gun, water is not only used at the thick end, but a ring of gas or a heated iron cylinder is applied at the thin or muzzle end, and when the thick end cools the gas or cylinder is withdrawn from the muzzle, and the ring of water raised upward slowly to cool the remainder of the tube gradually.
“As a rule, the water is supplied whenever there is a shoulder, so that that portion may be cooled first and a close joint secured there; and water is invariably allowed to circulate through the interior of the mass to prevent its expanding and obstructing or delaying the operation; for example, when a tube is to be shrunk on a steel barrel, the latter is placed upright on its breech end, and when the tube is dropped down on it, a continual flow of cold water is kept up in the barrel by means of a pipe and syphon at the muzzle. The same effect is produced by a water jet underneath, when it is necessary to place the steel tube muzzle downward for the reception of a breech coil. As to the absolute amount of shrinkage given when building up our guns, let us take the 121⁄2-inch muzzle-loading gun of 38 tons as an example.
The objections to fitting work by contraction where accuracy is required in the work are, that if the enveloping piece is of cast iron its form is apt to change from being heated. Furthermore, if the enveloping piece, which is always the piece to be heated, is of unequal thickness all round the bore, the thin parts are apt to become heated the most, and to therefore give way to the strain induced by contraction when cooling, which, while not, perhaps, impairing the fit, may vitiate the alignment of parts attached to it. Thus, a crank pin may be thrown out of true by the alteration of form induced first by unequal heating of the metal round the crank eye, enveloping the shaft; and secondly, because of the weakest side of the eye giving way, to some extent, to the pressure of the contracting strain. To counteract this, the strongest part of the enveloping piece should be heated the most, or if the enveloping piece be of equal strength all round its bore, it should be heated equally all round. To effect this object heated liquids, as boiling water, or heated fluids, as melted lead, may advantageously be employed.
In some practice, locomotive wheel tires are heated for shrinking in boiling water. The allowance for shrinkage is from .075 millimètre to every mètre in diameter, which is .02952 inch to every 39.37079 inches of diameter.
The employment of hot water, however, necessitates that the tires be bored very smoothly and truly, and that the wheel rim be similarly true and smooth, otherwise the amount of expansion thus obtained will be insufficient to maintain a permanent fit under the duty to which a wheel tire is submitted.
Shrinking is often employed to strengthen a weak place or part, or one that has cracked. The required size is, in this case, a cylindrical surface that is not a true cylinder, obtained by a rolling wheel rotated by friction over the surface to be enveloped by the band. Or if the surface is of a nature not to admit of this, a strip of lead or piece of lead wire may be lapped round it to get the necessary measurements.
The bands for this purpose are usually of wrought iron, and require in the case of irregular surfaces to be driven on by hammer blows, so that the fit may be correct. As the band is forced on a heavy hammer is held against it, to prevent its moving back and off the work as the other parts are forced on.
Fig. 1428Fig. 1428.
Fig. 1428.
Very slight bands may be forced on by levers: thus, wagon makers use a lever or jack, such as inFig. 1428, for forcing the tires on their wheels. The wheel is laid horizontally on a table as shown, and the tireaforced out by the vertical lever, the armbaffording a fulcrum for the lever, and itself resting against the hubcof the wheel.
The following extracts are from a paper read by Thomas Wrightson, before the Iron and Steel Institute of Great Britain.
“The large amount of attention bestowed upon the chemical properties of metals, and the scientific methods adopted for their investigation, have led to the most brilliant results in the history of iron and steel industries. It must not, however, be overlooked that iron and steel have highly important properties other than those which can be examined by chemical methods. The cause for so little having been done in accurate observation of the physical properties of iron is twofold: 1. The molecular changes of the metals are so slow, when at ordinary temperatures and when under ordinary conditions of strain, that reliable observations, necessarily extending over long periods, are difficult to obtain: 2. When the temperatures are high—at which times the greatest and most rapid molecular changes are occurring—the difficulties of observation are multiplied to such an extent that the results have not the scientific accuracy which characterizes the knowledge we have of the chemical properties of metals.
“The object of the present paper is to draw attention to somephenomena connected with the physical properties of iron and steel, and to record some experiments showing the behavior of these metals under certain conditions.
“In experimenting the author has endeavored to adopt methods which would, as far as possible, eliminate the two great difficulties mentioned.
“It is obvious that the possible conditions under which experiments may be made are so numerous that all which any one experimenter can do is to record faithfully and accurately his observations, carefully specifying the exact conditions of each observation, and this must eventually lead to a more complete knowledge of the physical properties of the metals.
“The author’s observations have been led in the followingdirections:—
“1. The changes in wrought and cast iron when subjected to repeated heatings and coolings.
“2. The effect upon bars and rings when different parts are cooled at different rates.
“3. These changes occurring in molten iron when passing from the solid to the liquid state, andvice versâ.
PART I.
“To illustrate the practical importance of knowing the effects of reiterated heating and cooling on iron plates, one of the most obvious examples is the action of heat upon the plates of boilers which are alternately heated and cooled, as in use or otherwise. When in use, the plates above the fire are subjected to the fierce flame of the furnace on one side, and on the other side to a temperature approximating to that of the steam and water in the boiler. Where the conducting surfaces of the metal are thickened at the riveted seams, a source of danger is frequently revealed in the appearance of what are known as ‘seam-rips.’
“The long egg-ended boilers, much used in the North of England, are very subject to this breaking away of the seams. From some tests made by the writer on iron cut from the plates of two different boilers which had ripped at the seams, and one of which seam-rips had led to an explosion resulting in the destruction of much property, though happily of no lives, it was found that the heat acting on the bottom of the boiler had, through time, so affected the iron at the seam as to make it brittle, apparently crystalline in fracture, and of small tensile strength. Farther from the seam the iron appeared in both cases less injuriously affected. But although the alternate heating and cooling of the plates over a long period had produced this change in the molecular condition of the iron, a method of restoration presents itself in the process of annealing. In subjecting the pieces cut from the seam-rips to a dull red heat, and then allowing them to cool slowly in sawdust, the writer found that the fibrous character of the iron appeared again, and renewed testing showed that the ductility and tensile strength were restored.
“The same process of annealing is equally effectual in restoring the tenacity of iron in chains rendered brittle, and apparently crystalline, by long use, and is periodically applied where safety depends upon material in this form. Thus the heating and cooling of iron may be looked upon as the bane or the antidote according to the conditions under which the process is carried out. This affords an example of the importance of the physical effects produced by repeated changes of temperature. The change effected by one heating and cooling is so small that a cumulative method of experiment is the only one by which an observable result can be obtained, and this is the method adopted by the writer in the investigation now to be described.
“It is well known that if a wrought-iron bar be heated to redness, a certain expansion takes place, which is most distinctly observed in the direction of its length. It is also known, although not generally so, that if a bar be thus heated and then suddenly cooled in water, a contraction in length takes place, the amount of this contraction exceeding that of the previous expansion, insomuch that the bar when cooled is permanently shorter than it originally was. If this process of heating and cooling be repeated, a further amount of contraction is found to follow for many successive operations.
“Experiments Nos. 1 and 2 were made to verify this, and to show the increment of contraction after each operation.
“The Table of Experiment No. 5 shows that at the twenty-fifth cooling a contraction of 3.05 per cent. had taken place, or an average of .122 per cent. after each cooling. This is almost identically the same average result as shown in Experiment No. 1 with straight bars.
“The above experiments only having reference to the permanent contraction of the iron in the direction of its length, the author made the following experiments to ascertain the effect in the other dimensions, and to see whether the specific gravity of the iron was affected in the reduction of dimensions.
Fig. 1429Fig. 1429.
Fig. 1429.
“Experiment No. 6.—Wrought-iron plate, .74 inch thick, planed on both surfaces and all edges to a form nearly rectangular, and of the dimensions given inFig. 1429.
“Specific Gravity.—Two small samples were cut out of different parts of the same piece of plate from which the experimental piece was planed, and the specific gravity determined asfollows:—
“Quality.—Subjecting a piece to tensile strain in the direction of the grain, it broke at 21.2 tons per square inch of section, the ductility being such that an elongation of 8.3 per cent. occurred before fracture, with a reduction of 9.6 per cent. of the area of fracture. This may be looked upon as representing a fairly good quality of iron.
“A bar of wrought iron, 11⁄8inches square and 30.00 inches long, was heated to redness, and then allowed to cool gradually in air. Measurements after each of five coolings showed no perceptible change of length.
“Experiment No. 4.—Wrought-iron bar, 11⁄8inches square by 30 inches long, heated to a white heat and cooling gradually in air.
“It may be remarked, that if the bars be heated to white heat a slight contraction does occur, as shown by Experiment No. 4, where a bar of the same dimensions as No. 3 contracted .17 per cent. after the fifth cooling. As, however, the further remarks on this subject have only reference to bars heated to redness and then cooled, the writer would summarize the results of Experiments Nos. 1, 2, and 3, by stating that wrought-iron bars heated to redness permanently contract in their length along the fibre when cooled in water of ordinary temperature; but when cooled in air, they remain unchanged in length.
“To show that this is true as applied to circular hoops, Experiment No. 5 was made upon a wrought-iron bar of 11⁄8inches square in section, welded into a circular hoop, 57.7 inches outside circumference.
“Experiment No. 5.—Wrought-iron hoop, 11⁄8inches square by 57.7 inches outside circumference, heated to a dull red, then cooled suddenly in water.
“This hoop was heated to redness and cooled in water twenty-five times, the circumference of the hoop being accurately measured after each cooling.[23]