knowledge is established, we are warranted in favoring the plate at the expense of the rivet, even more than suggested by Mr. Cooper. Too much attention is demanded by boiler users generally to obtaining an absolute tight-seamed boiler in place of a strongseamed boiler, and while a tight seam is desirable, the fact remains that too large a number of rivets used to secure an absolutely tight seam may reduce the strength. (Sandy Hook and Munhall Exp.; Miss. River Boat Practice.) Mr. W. H. Jenks.-In most formulæ for the proportions of riveted joints, it is assumed that rivet and plate should be of equal strength. If any difference in the relative strength of the two be allowed, it is usually with a half apology to theory for not following more closely its teachings. It may not be amiss to point out, that such difference is in full accord with sound theory, though perhaps better recognized in practice than in theory. Except for one serious defect, the "Wonderful One-Hoss Shay " may be taken to represent a perfectly designed structure. At any rate, it may serve to bring out clearly the fact that a structure should be so designed that every part may be of equal strength, not at the time it is built, but at the time of its failure. This requires the taking into consideration of at least two factors beside that of initial strength. The first and more important of these is wear and tear. A good illustration of the use of this factor is furnished by the brasses and strap of a common connecting-rod. There being no wear on the end of the strap, its dimensions may be determined with reference to stiffness only. The brasses, being subject to wear, must, beside the thickness needed for strength, have an additional allowance of the probable amount of wear. The second factor to be taken into consideration is the probability of failure. It is evident that if, of any two pieces in a structure, one will fail A times for every B time that the second will fail, if A be greater than B, the first piece should be strengthened until what may be called the probability of failure becomes equal for both. Applying these principles to the discussion of riveted seams for boilers, we find that both the wear and tear, and the probability of failure, are greater for the plate than for the rivet. Corrosion, which affects the rivet little if at all, may affect the plate seriously. An old boiler cut apart will often show the plates eaten away between the rivets till quite thin, while a well-defined ring around the rivet-hole shows the plate of full thickness where protected by the rivet-head. The strain brought about by uneven. heating also may tend to cause fatigue in the material of the plates, while affecting the rivets but little. Under the heading of probability would come the chance of crack starting elsewhere in the plate and running into the seam, where it may follow the course of the rivet-holes, tearing the sheet in detail from hole to hole. A flaw in the material also is both more likely to occur, and more serious in its effects, in the plate than in the rivets. Accordingly, the strictly logical method of proportioning rivet to plate would be first, to fix a minimum strength below which neither should go; second, to increase the relative strength of the plate by a sufficient allowance for wear and tear; and third, to still further increase slightly the relative strength of the plate by an amount sufficient to cover the greater probability of failure. This relative probability of failure in plate and in rivet would have to be determined from the recorded failures of boilers in actual use; leaving out of consideration, of course, all failures except those that occur in the riveted seams. It may be bold to criticise established practice, but I have not myself met nor can I recall any case in which a boiler in use has failed through weakness of the rivets, and I believe that both sound theory and good practice would indicate a greater proportionate strength in the plate than in the rivet. A boiler so proportioned would have less strength when new; but taking its whole life into consideration, its average strength would be higher, and its probable life longer. CCCXLIX. STEAM CONSUMPTION OF ENGINES AT VARIOUS SPEEDS. BY JAMES E. DENTON, AND D. S. JACOBUS, HOBOKEN, N. J. (Members of the Society.) INTRODUCTION. THE investigation herein recorded was made upon a 17x30 steam engine driving one of the air compressors of the Rand Drill Co., used in the construction of the new Croton Aqueduct. A general view of the engine is shown in an accompanying plate (Fig. 192). A is the steam cylinder tested, whose exhaust pipe, E, led into the surface condenser, D, whence condensed steam was led by a pipe, K, to weighing barrels, F. C is the companion engine, whose exhaust escaped into the atmosphere. The cut-off in A was adjusted by a hand-wheel, G; H is a speed indicator; I is a revolution-counter; Q is a clamp for hold. ing the air-inlet valves open (Fig. 192). It will be seen that a pair of steam cylinders, A and C, operated two air-compressing cylinders, B and S. The steam cylinders were connected by right-angled cranks, and a liberal fly-wheel was provided. The steam valves were of the riding cut-off or Meyer type, the cut-off being adjustable by the hand-wheel, G, to any point from zero to seven-eighths of the stroke. By running cylinder C, at seven-eighths cut-off, under throttle, and varying the resistance to the motion of the pistons, through the double adjustment afforded by the regulation of the outlet to the compressed air at J, and of the inlet valves of the air cylinder, S, with the clamp, Q, a range of speed from 90 to 9 revolutions per minute could be obtained for any cut-off from seven-eighths to one twenty-fifth stroke at any boiler pressure between 90 and 30 lbs. per square inch. In other words, the resistance to the engine could be varied independently of the speed, and the means of absorbing power were such that any given resistance could be maintained practically constant for any desired period of time. Conditions so favorable for the accurate measurement of steam consumption under various conditions seemed to the writers to form an exceptional opportunity for the collection of data on the effect of considerable variations of piston speed upon cylinder condensation of non-condensing engines, which, so far as they are aware, is a subject which has received very little experimental investigation. Upon the completion of the aqueduct it was found that the compressors at Shaft 17, belonging to Messrs. Breuchaud, Pennell & Co., were in practically perfect order. The valve and cylinder surfaces were absolutely true and free of scratches, and the most rigorous tests failed to show the slightest leakage of steam at either the valve faces or piston rings. Arrangements were accordingly made to have the use of this compressor, and two 75 H.P. boilers for the purpose of the investigation under notice. One steam cylinder, A, was connected to discharge its steam into the surface condenser, D, and the consumption of this cylinder was made the subject of measurement, by weighing the steam in the barrels, F. The exhaust of the cylinder C, was led directly to the atmosphere. Circulating water for the condenser was pumped from the aqueduct shaft. The surface condenser, containing 440 square feet of surface, was kindly loaned by Mr. F. M. Wheeler. Through the courtesy of Messrs. Breuchaud & Pennell, suitable laboring help, vehicles, and pumps, were made available for the rapid preparation and prosecution of the test, and the following gentlemen cordially contributed to a fund which was devoted to meeting certain necessary expenses: * Thos. F. Rowland, Rand Drill Co., A. P. Trautwein, Trustees of the Stevens Institute, H. C. White, F. E. Idell. The Rand Drill Co. also supplied expert labor to operate the compressor. The total cost in money actually paid out to date amounts to about $800, which is distributed according to the following items: [*The authors have forborne to mention their own material contributions.SECRETARY.] |