TABLE, Containing the Velocity and Force of the Wind. By MR. ROUSE. 100 146-70 49.200 { A great storm. A hurricane. A hurricane, that tears up trees, carries buildings before it, &c. To find the Force of Wind acting perpendicularly upon a Surface. Rule-Multiply the surface in feet by the square of the velocity in feet, and the product by 002288, the result is the force in avoirdupois pounds. RESISTANCE TO TORSION OR TWISTING. It is obvious that the strength of revolving shafts are directly as the cubes of their diameters and revolutions; and inversely, as the resistance they have to overcome. Mr. Buchanan, in his Essay on the Strength of Shafts, gives the following data, deduced from several experiments, viz.: That the fly-wheel shaft of a 50 horse power engine, at 50 revolutions per minute, requires to be 71⁄2 inches diameter, and therefore, the cube of this diameter, which is =421.875, serves as a multiplier to all other shafts in the same proportion and taking this as a standard, he gives the following multipliers, viz. : For the shaft of a steam engine, water-wheel, or any 400 shaft connected with a first power, For shafts in inside of mills, to drive smaller machinery, } 200 or connected with the shafts above, For the small shafts of a mill or machinery, 100 From the foregoing, we derive the following Rule. The number of horses' power a shaft is equal to, is directly as the cube of the diameter and number of revolutions; and inversely, as the above multipliers. NOTE. Shafts here are understood as the journals of shafts, the bodies of shafts being generally made square. This rule is for cast iron, and it may be used for wrought iron by multiplying the result by 963; or, for oak, by 2238; or, for fir, by 2.06. If the shaft belong to a 7-horse-power engine, and the strap turns 111⁄2 times in a minute, 3 3 ✓ (2407) 5.267 inches diameter for cast iron. For fir, 5.267×2.06=10-85. For oak, 5.267×2-38=12:535. And for wrought iron, 5·267×963=5.0719. NOTE. This rule comes from the best authority, and gives perfectly safe results, though some employ 340, instead of 240, as a multiplier, which gives a greater diameter to the shaft. It is to be remembered that these rules relate to the shafts of first movers, or the shafts immediately connected with the moving power. But these shafts may communicate motion to other shafts, called second movers, and these again to others, called third movers, and so on. The diameters of the second movers may be found from those of the first by multiplying by 8, and those of the third movers by multiplying by 793, respectively. One material may resist, much better than another, one kind of strain; but expose both to a different kind of strain, and that which was weakest before may now be the strongest. This may be illustrated in the case of cast and wrought iron. The cast iron is stronger than the wrought iron when exposed to twisting or torsional strain; but the malleable iron is the stronger of the two when they are exposed to lateral pressure. We shall subjoin a few results of experiments on the weight which was necessary to twist bars close to the bearings: The above rules will often find their application in the practice of the engineer. Much of the beauty of any mechanical structure 13* depends on the proper proportioning of the magnitude of materials to the stress they have to bear; and, what is of far greater moment, its absolute security. It is a well-known fact, that a cast iron rod will sustain more torsional pressure than a malleable iron rod of the same dimensions: that is, a malleable iron rod will be twisted by a less weight than what is required to wrench a cast iron rod of the same dimensions. When the strength of malleable iron is less than that of cast iron to resist torsion, it is stronger than cast iron to resist lateral pressure, and that strength is in proportion as 9 is to 14. From the foregoing, it is easy for the mill-wright to make his shafts of the iron best suited to overcome the resistance to which they will be subject, and the proportion of the diameters of their journals, according to the iron of which they are made. Example. What will be the diameter of a malleable iron journal, to sustain an equal weight with a cast iron journal of 7 inches diam eter 2 73=343; 3 14:343::9:220; now 220.5=6.04 inches diameter. Additional Results of Experiments on Torsional Strain.* Square bars, with a journal 1 inch in diameter, and inch in length. Wrought iron (Ulster Iron Co.), twisted with 326 lbs., and broke with 570 lbs. applied at the end of a lever 30 inches in length. Wrought iron (Swedes), same length of lever, twisted with 367 Ibs., and broke with 615 lbs. Cast iron (foundry), journal 1 inch long, same length of lever, broke with 436 lbs. The diameters for second and third movers are found by multiplying the diameters, ascertained by the above rules, by 8 and 793, respectively. Grier, in his Mechanics' Calculator, gives the following rule for cast iron shafts: 3 ✓ 1240 x number of horses' power =diameter in inches. For wrought iron, multiply result by 963; for oak, by 2-238; and for pine, by 2.06. *Haswell's Engineers' and Mechanics' Pocket-book. |