OF THE

FRANKLIN INSTITUTE

OF THE

State of Pennsylvania,

AND

MECHANICS' REGISTER,

DEVOTED TO

Mechanical and Physical Science,

CIVIL ENGINEERING, THE ARTS AND MANUFACTURES,

AND THE RECORDING OF

AMERICAN AND OTHER PATENTED INVENTIONS.

FEBRUARY, 1837.

Practical and Theoretical Mechanics.

Report of the Committee of the Franklin Institute of the State of Pennsylvania on the Explosions of Steam Boilers, of Experiments made at the request of the Treasury Department of the United States. PART II. Containing the report of the sub-committee to whom was referred the examination of the strength of the materials employed in the construction of Steam Boilers.

To the Committee of the Franklin Institute of the State of Pennsylvania, on the Explosions of Steam Boilers:

GENTLEMEN-The sub-committee, to whom was referred the examination of the STRENGTH OF MATERIALS employed in the construction of Steam Boilers, beg leave to submit the following REPORT:

WHILE it is important to know the causes which may produce a dangerous developement of elastic forces in the interior of steam boilers, it is obviously not less so, to understand aright the efficacy of those means on which we rely for confining or controlling their energies. Hence, in investigating the causes of explosions, it is both natural and expedient, to examine separately those facts and principles which concern the divellent and the quiescent forces respectively. The number and variety of circumstances, which affect the character and durability of materials of which steam boilers are formed, are probably not less than of those which tend to modify the action of the fluids which they contain. In this view of the importance to be attached to the subject of the strength of materials, it may be considered remarkable, that while numerous investigations have been made as to the causes of danger, so little should have been attempted in regard to the most direct and obvious means of security. Before the series of experiments here detailed had been commenced, the necessity for such an investigation had been repeatedly pointed out, in public and private lectures, on the steam engine; the reasons assigned for instituting the inquiry, being the very general and unsatisfactory nature of those results, VOL. XIX.-No. 2.-FEBRUARY, 1837.

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which are given in practical treatises, respecting the strength of metals, as dependant on the mode of manufacture, and on the different temperatures and other circumstances to which they are exposed. We had, it is true, a considerable number of results, obtained at different periods, by experiments on the direct cohesion of wrought iron.*

They were, however, in general, undertaken for purposes very different from those which prompted the present investigation.

Few of the experimenters had in view the influence of temperatures on tenacity; and even those data which they furnish for calculating the proper thickness of metal to be employed at ordinary temperatures, in constructing steam boilers are liable to much uncertainty, owing to the diversity in the results themselves. Laborious and protracted as has been this investigation, still the practical importance of the subject has appeared to warrant a careful survey, and a diligent comparison of the various facts which might influence the practice of those who desire to attain a secure action in the steam boiler.

Without entering therefore into all the delicate questions, which, had a mere scientific view been indulged, we might have been prompted to examine, it has been the aim of the committee to obtain and present such classes of facts as both scientific and practical men may make subservient to their respective purposes.

The questions, which in the course of this inquiry, it has been found necessary to investigate, may be classed under three general divisions.

1. PRINCIPAL,

2. INCIDENTAL,

3. SUBSIDIARY.

I. Principal. 1. What is the absolute tenacity per square-inch bar of rolled boiler iron, at ordinary temperatures, and to what irregularities is it liable?

2. The same for rolled copper?

3. What is the effect of increased temperature on the tenacity of iron and copper?

4. What is the tenacity of wrought iron, manufactured by other means than rolling into plates ;-as by rolling it into bars or rods, by hammering and wire-drawing?

5. What are the relative advantages of iron made by refining from different sorts of pig metal and their mixtures?

6. What is the comparative value of sheet iron manufactured by the processes of puddling, blooming and piling respectively, and in the last case, what influence have repetitions of the process?

The following brief table contains some of the general results, obtained by different authors, as the strength of wrought iron.

7. What is the effect of piling into the same slab, iron of different degrees of fineness?

8. What is the comparative tenacity of rolled iron, in the longitudinal, diagonal and transverse directions of the rolling respectively?

9. What influence may be produced, by long and repeated use, towards modifying the character of boiler iron?

II. Incidental. 1. What is the specific gravity of the specimens submitted to examination?

2. What elasticity is found in the metals under different circumstances of the trial?

3. What relation exists between the force which will produce a permanent elongation in a bar, and that which will entirely overcome its tenacity? 4. What amount of elongation may the several kinds of metal undergo before fracture?

5. Does the amount of constriction or diminution of area, at the section of fracture, bear any relation to the absolute strength of the metals, to the direction in which the strips are cut from the plate, to the breadth and thickness of the strips themselves, or to the temperature under which the trial is made?

6. What is the effect of the rivets on the total strength of a boiler? III. Subsidiary. 1. What is the friction of the apparatus employed to determine tenacities?

2. What is the amount of its elasticity?

3. What is the latent heat of the vapour of water?

4. What is the specific heat of iron, copper and glass, respectively? 5. What is the rate of heating of a given mass of liquid, when subjected to the direct action of a solid of higher temperature?

6. At what rate will the same mass of liquid change its temperature by the action of air alone?

From the foregoing statement, it will be seen that more than twenty distinct topics have demanded the attention of the committee. They have felt strongly inclined to embrace some other points of great practical and scientific importance, but the time already unavoidably consumed, and the very limited means which the other branches of inquiry and experiment on explosion left to be appropriated to the purposes of this sub-committee, compelled the relinquishment, for the present, of those objects which do not immediately concern the construction and use of steam boilers.

The discussion of the questions above enumerated, will necessarily follow an order somewhat different from that in which they are here stated. A view of the apparatus, employed by the committee, claims the first notice. The origin and preparation of the materials to be tested, will also precede the detail of experiments.

Machine for proving the strength of materials.

The apparatus used, by the committee, for the direct determination of the principal questions regarding the strength of the specimens submitted to examination, is represented in plate I. M is a strong frame of oak timber, the two longer sides five feet in length, fourteen inches deep, and six inches thick.

The two shorter, or end pieces, which project beyond the sides to the distance of three inches, are each two feet eight inches long, seven and a half inches thick, and fourteen inches deep.

Between the two side pieces, (one of which is in the figure removed, to exhibit the interior or working parts,) is a space fourteen and a half inches

wide, affording room for a screw, cross-head, guide-rods, connecting blocks and wedges, to hold the specimens under trial; and also for the heating apparatus in experiments at high temperatures.

These four massive blocks or beams of timber, are held together by strong screw bolts, passing through mortises in the end pieces, along tenons into screw nuts imbedded in the timber of the longitudinal beams.

The frame is supported, as represented in the figure, by four firm trussel legs, six inches square, tied together near the bottom, and fastened as well to the ties as to the frame above, by mortising and bolting. The top of the frame is three feet eight inches above the floor on which the machine rests. Through one end A, of the frame M, about six inches below the top, and centrally between the two side beams of the frame, passes the screw S, 23 inches in diameter, and three feet long, cut into threads of an inch apart. Near the head of the screw, is a neck turned rather deeper than the threads, to allow a clamp collar to embrace it; which, together with a strong cast iron plate, against which the head of the screw works, prevents any longitudinal motion of the screw itself.

N is the box or nut of this screw which by the revolution of S, either approaches to or recedes from the end A of the frame; 8, s, are two guiderods, one on each side of the screw, level with its axis and near the inner faces of the longitudinal beams of the frame, serving to support a cross head that contains in its central ring the nut N, and embraces by loops at its extremities the two guide-rods. The purpose of these loops is to prevent the nut from turning by the revolution of the screw.

The cross head thus secured is united by two strong straps or bars of iron i, i, 2 inches wide by half an inch thick, to a block of iron b, which is also furnished with two projecting arms that rest on the guide-rods already described. This block as well as the two others b' and b' is 4 inches long, 4 inches deep, and 13 inches thick, being perforated centrally in the direction of its thickness with a hole in the form of the frustum of a square pyramid, the purpose of which is to admit of wedges placed within them to hold the bars of metal under trial. A more detailed description of these will be given hereafter.

The block b' is connected to b by a separate pair of straps i', i', and has arms reposing on the guide-rods, or when necessary, admitting a vertical semi-revolution, so as to be laid over backward between the straps i, i. This latter disposition of the block b' was made whenever specimens of 20 or 30 inches in length were to be tried; but when those of only a few inches in length were under trial, b' was used in the position represented in the figure.

The block b' is connected by the strong iron straps i", i", which pass freely through a suitable opening in the head B of the frame, to the lever L. One of these straps is seen at e, the other being on the posterior side of the lever, with which they are united by means of a steel bolt turned with care and well polished. The straps are kept in place by a head, screw nut and washers, on the bolt. This lever is of the rectangular kind, the longer arm being horizontal, the shorter vertical, and the angular point being in the axis of a second or lower bolt which serves as a fulcrum.

At the end next the frame, the lever has a breadth or depth of seven inches and a thickness of one inch. Towards the opposite extremity or that on which the weights are placed, it diminishes to a breadth of four inches, and a thickness of of an inch. The upper edge of the beam is straight to within 24 inches of the broader end, where it curves upwards, affording a massive support for the upper bolt already described. In a ver

tical direction beneath that bolt, and in the prolongation of the upper straight edge of the lever, is the position, as already indicated, of the second steel bolt, serving for a gudgeon, on which the lever turns. The distance between the axes of the two bolts is 2.914 inches, which is therefore the length of the shorter arm of the lever. The bolts are very nearly of the same diameter, being each about 1.086 inches. The lower bolt rests against a plate of cast iron, having suitable projecting cheeks, with bearings adapted for its reception.

A strap from the top of each cheek comes down over the bolt, and is fastened with a thumb screw, to prevent the lever being thrown out of place by the recoil of the machine. The two guide-rods 8, 8, pass through this cast iron plate, as well as through that which serves as a collar to the screw head, S, on the opposite end of the frame. The lever is formed of the best wrought iron, and weighs 164 pounds, the matter being so distributed that if not neutralized by counter weights, its effect in straining any bar attached horizontally to the upper bolt would have been equal to 2495 lbs. To obviate this, and to prevent the weight of the lever from adding anything to the friction, it is accurately counterpoised by means of weights C and C,' corresponding to the parts of its mass which they are respectively required to sustain. Thus the weight of C, the larger counterpoise, is 103 pounds 12 ounces, that of C' 60 pounds 7 ounces. The former is, however, increased to counterpoise likewise, one-half the weight of the two straps i", i", the other half resting, as will be seen, on the horizontal guide-rods 8, 's.

The axes of the pullies p, p', over which the cords r, r' pass, are furnished with cavities to receive steel pivot-points, in order to reduce, as far as practicable, the friction of these parts. The diameter of these pullies is 12 inches.

The iron stirrup, to which the cord r is attached, is applied to the lower bolt or fulcrum of the lever, the projecting ends of which roll on straight, horizontal edges, forming the bottom of two loops with which the stirrup is furnished.

By means of the suspending apparatus above described, the lever is enabled to obey any force acting vertically on its longer arm, with the advantage of ample strength and stiffness, combined with the condition of a theoretical lever, in respect to the gravity of parts.

There are two modes of operating by which a bar of metal, placed in the machine between b' and b" might be broken, so as to ascertain the tenacity.

The first is to apply the force of the screw S to strain the bar in raising a weight W suspended at any convenient point on the arm h of the lever; the second is to employ the screw only to regulate the height of that arm, and to restore it when relieved of the weights, to the horizontal position, whenever the extension of the bar had allowed it to fall below that position.

The latter method was with very few exceptions, adopted by the committee, both because it allowed of a more exact determination of the breaking weights, by a small addition at a time, and because it rendered the effect of the friction constant in its kind, being always in opposition to the gravitating force of the weight W, and subtractive, in the calculation. In order to apply this mode of action without requiring correction for the stiffness of the cord r' and the friction of the pulley p', it was only necessary after adjusting the weight C', to remove so much as would allow the arm h of the lever to descend upon the slightest jarring of the machine. The tenacity of the bar