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1901.] MARINE REVIEW. 25 as in our beam engines, and we experience no trouble. It-is all a question of bearing surface. j : In our original Chicago we had, to the writer's mind, the worst de- signed beam engine ever put on board a ship, with unbalanced walking beam, with grasshopper long and short-ended beams to suit a long-stroked engine and a short-stroked crank. Yet these engines were: worked as high as seventy-two revolutions per minute, or 684 ft. piston speed, with- out any great degree of discomfort, except for the heating of the bearings and beam centers, because they were out of all proportion to the work they had to do. After the experience which the navy had with. such an abortion of a design, I would not hesitate to build and run:a properly proportioned and balanced engine on the MacAlpine design as high as 1,000 ft. piston speed or 100 or 120 revolutions per minute. Here are the pressures from steam on the various bearings in the first and latest designs, Figs. 7 to 18 and 19 to 21, respectively. Maximum total pressure on each piston--67,460 Ibs. = 30 tons. Pounds per square inch. First Latest design design Figs. Figs. 7 to 18... 19-to-21. WHat DEAPITNI BB oo sei ees a a eee 200 173 BET PIT So ois «sisi aS wiases gece gia pide wiess WR NG HER Biv Ley CUTS ER Svs » 440 - 366 High pressure and first intermediate pressure crosshead bearings.. 842 454 High pressure and first intermediate pressure guide blocks 48 48 Bearings of links to levers.............. seco S02 468 Center bearings of main levers 560 344 Second intermediate pressure and low pressure guide blocks....... 15 25 The pressures in the first column should be perfectly satisfactory, but the larger bearings were introduced into the two last designs to anticipate any possible criticism. These pressures will very safely allow for any slight increase of the inertia due to the additional parts (that is, the links and reduced mass of the levers) making connection to the back crossheaaq, and also meet all demands for long wear and smooth running. For it should be recollected that a reduction of bearing pressure, by allowing a 'thicker oil film, very quickly reduces the rate of wear--reduction of press- ure to one-half far more than halves the rate of wear. The thicker oil film also keeps down friction, though the bearing diameter is increased. Mr. MacAlpine also discusses the increased elasticity of the connection over the direct connection and the possible unsteady working, the whole gear being thrown into vibration. Make it stiff enough and it will not vibrate. Our beam engines do not. Leaving the rest of his argument-- which I think perfectly sound--to those who are weak in the faith, I will quote his last sentences in this connection: "Finally, it may be asked, What is the crank but a lever, and that not of the best kind, with its large span, rotating journals, and bearings whose diameters bear such a considerable proportion to the length of crank? The proposal is to replace these by levers of a better description. In the early days the crank was long viewed with distrust--which was unfounded; and, I believe, misgivings which some might entertain here will quickly vanish when the proposal has become familiar, and its advantages have been weiged. Indeed, I have already found this to be the case." Tf the lever is all right, if we adopt it here, we can realize a complete solution of the vibration problem. If we decide against it we can make no further progress in this direction with the reciprocating engine. Its adoption gives us also a lighter engine, a simpler engine, and one which takes up much less room in the ship. The non-vibrating engine is also necessarily the better and safer to run. ; We need not enumerate, as' Mr. MacAlpine does, the parts reduced and simplified, but I will reproduce a comparison showing the saving of space, Fig. 28. This Mr. MacAlpine made at my request when he first brought his system to my attention. The lower engine is placed on the : Se, T seem wae cee meh omece "ye ' + yet. : pos Wa. 28. Oe oe same center line, 11 ft. 0 in. from the center bulkhead, as the engine of one of our-ships. This engine is actually shortened up to thelength required for a crank shaft made. in two pieces instead'of four The upper engine is reduced from:Fig. 16.. We might>bringithe lower engine nearer the center; bulkhead) thus making a slightly narrower engine-room than that orequired foptive uppereéngine, but the length and total space occupied by the uppenehgine is much smaller than for the usual design. . This was well illustrated by plans the engineer-in-chief made proposing triple screws for some of our recent ships: While the triple screw arrangement was hardly m possible with the ordinary four-crank engine, it went in very nicely with the MacAlpine engine. Figs. 22 to 27 give a design of merchant ship engine which scarcely needs explanation further than to point out that the moving parts of the pumps are also balanced. As noted earlier, it is not sought to balaiuce the torsional couple about the crank shaft, as Mr. MacAlpine has shown that the effect in vibrating the ship is excessively small. I will only transcribe the results, referring to the original paper for the full details of the calcu- lation. He first shows that sensible elastic torsional vibration will not occur at all. The ship is composed of parts such as the skin and decks which give torsional rigidity, together with machinery and woodwork, etc., which do not add sensitly to the elastic reaction. These latter parts, together with the cargo, add to the moment of the inertia about the axis of vibration, and thus lengthen the period of torsional vibration. From actual figures supplied by Prof. F. P. Purvis, Mr. MacAlpine concludes that. the total moment of inertia of a ship will range from a higher limit of about three times that due to the parts giving torsional rigidity, to less than twice; the figures for warships being nearer the lower limit. But he calculates the result for an increase of two, three, and four times, showing that even the latter high value would not effect his argument for the absence of elastic torsional vibrations. He then gives the ordinary inves- tigation for the period of torsional vibration of a circular tube or bar, the mode of vibration being that with two nodes, as these are the slowest tor- sional vibrations into which a ship would be thrown by couples applied at the center by an engine placed there. If T = the time of a complete vibration in seconds; 1 = the length of the tube in feet; n= the increase of moment of inertia (2, 3, and 4 times) as ex- plained above. | p = the weight per cubic foot of the elastic material of the tube. -q = modulus of rigidity. a g = the intensity of gravity = 32.2. Then, : T= 1/2" 17 Nee? (17) when there are two nodes. The speed of vibration is independent of the diameter of the tube. I will quote the numerical results from Mr. MacAlpine's paper, mak- ing the requisite changes in the numbering of his equations. "' 9 for steel = 500 Ibs. per cubic foot. le ot = 12 X 108 Ibs. per square inch. . = = 12 x 144 & 106 Ibs. per square foot. g == 20. "For a tube (ship) 400 ft. long we get from equation (17) 500 e " T -- 400 a = 103792 7/2. ve Vass ~exUiee "If N.= the number of vibrations per minute-- es 60 1582 NS = ee (18) .08792 j/n s/n "In a ship 400 ft. long, 112.5 may be taken as near the highest number of revolutiors of any engine which will be put in it, even if it is a warship. The number of the.,synchronizing period then is-- gad : 1,582 : e: (CC - (19) "n= multiplier for moment of inertia 2 3 4 "N = number of vibrations per min., equation (18) 1,119 918 791 "S = number of synchronizing period, eq. (19) 9.9 8.1 ie Mr. MacAlpine finds that for several other sections the results do not change much from the above. What is required to make a torsionally _ stiff structure: is to so stiffen it that the sections can change shape but very slightly when the twisting couple is applied. This is amply done-in a ship by means of the frames, beams, and bulkheads; so we may be sure the above result does not differ greatly from the truth for a ship. Espe- cially is this the case since the time of one vibration lengthens in the same proportion as the square root of the elastic reaction diminishes. We see, then, that the proportionate change in the period is much smaller than that of the elastic reaction. The cylinders being arranged in pairs athwartship, the intensities of the torsion couples due to inertia are changed from those of the ordinary design. Mr. MacAlpine calculates the increase from that of second period upward over what it would be for the same two cylinders placed over the shaft; that is the increase due to the displacement athwart- ship: The results are: Period. F Percentage increase. 20 en ei eae Sots debolneiee era tana. aes 25.7 Sd ye sr Gece ae ia eee 0 4 AU oes lee Oe ee ie Ot eh 6 oe ee a es ee Shia. 7 0 Othe s.5 Sore ee a 3.4 (G0 Ua ee ec es ea ey 0 ete. . ete; : In his paper it is shown that the synchronizing period will be some- where in the neighborhood of the seventh to the tenth, and the results just given show that there will be no increase of the tendency to set up elastic »torsionabvibrations of these periods in this engine over what we have in the ordinary four-crank engine. Hence. we need not fear these vibrations, as they do not occur with the ordinary engine. There is a very large un- balanced first-period inertia torsional couple produced by this engine, which scarcely exists in the direct-connected engine. It will prodwee no élastié torsional vibrations, as it is only applied from a seventh' to, d'ténth as quick asthe lowest of these vibrations, but it will rotate the ship as a rigid body. As, for instance, if we suspend a watch by a'thtédd with its face horizontal, the vibration of the balance wheel would: produce a very small vibration of the whole watch. These are not elastic vibrations, but vibrations of a rigid body...Their extent can readily be determined if we know the angular swing and moment of inertia of the balance wheel, and