By Mr. Tredgold's rule, Weight in the middle, 3360 × 13 = 8738739 inches diam. 436800 Weight equally distributed, 33600 × 13 = 436800 7.65 inches. To resist Torsion or Twisting. 10 It is obvious that the strength of revolving shafts is directly as the cubes of their diameters and revolutions; and inversely as the resistance they have to overcome. Mr. Robertson 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 revolu tions per minute, requires to be 7 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 shaft connected with a first For the small shafts of a mill or machinery," ::: ::: 400 200 100 From the foregoing, the following rule is derived, viz.: The number of horse power a shaft is equal to is directly as the cube of the diameter and number of revolutions; and inversely as the above multipliers. Ex. 1. When the fly-wheel shaft of a 45-horse-power steam engine makes 90 revolutions per minute, what is the diameter of the journal? 45 × 400 90 = 200 200 = 5 inches diameter. Ex. 2. The velocity of a shaft is 80 revolutions per minute, and its diameter is 3 inches; what is its power? Ex. 3. What will be the diameter of the shaft in the first example, when used as a shaft of the second mover.† Shafts here are understood as the journals of shafts, the bodies of shafts being gene. rally made square. †The diameters of the second movers wil be found by dividing the numbers in the table by 125, and the diameters of the third movers, by dividing the numbers by 156, Horse Power. 10 15 TABLE of the diameters of shafts, being the first movers, or having 400 for their multipliers. 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 is less than that of east iron to resist torsion, it is stronger than east iron to resist lateral pressure, and that is in proportion as 9 is to 14. From the foregoing, it is easy for the millwright 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. Ex. What will be the diameter of a malleable iron journal to sustain an equal weight with a cast-iron journal of 7 inches diameter? 75 = 343. As 14 3439: 220; now 220-5 = 6·04 inches diameter. Strength of Wheels. The arms of wheels are as levers fixed at one end, and loaded at the other; and, consequently, the greatest strain is upon the end of the arm next the axle. For that reason, all arms of wheels should! be strongest at that part, and tapering toward the rim. The rule for the breadth and thickness of arms, according to their length and number in the wheel, is as follows: Multiply the power or weight acting at the end of the arm by the cube of its length; the product of which, divided by 2656 times the number of arms multiplied by the deflection, will give the breadth and cube of the depth. Ex. Suppose the force acting at the circumference of a spurwheel to be 1600 lbs., the radius of wheel 6 feet. and number of arms 8, and let the deflection not exceed th of an inch. 1600 × 6* 2656 x 8 x 1 10 = 163 = breadth and cube of the depth. Let the breadth be 2-5 inches; therefore 163 =652; which is equal to the cube of the depth. Now the cube root of 652 is nearly 403 inches: this, consequently, is the depth or dimension of each arm in the direction of the force. 1 Note.-When the depth at the rim is intended to be half that of the axes, use 1640 as a divisor instead of 2656. The teeth are as beams, or cantilevers, fixed at one end, and loaded at the other The rule applying directly to them where the length of the beam is the length of the teeth and the depth the thickness of the teeth. For the better explanation of the rule the following example is given. Ex. The greatest power acting at the pitch line of the wheel is 6000 lbs., and the thickness of the teeth 14 inch, the length of the ' teeth being 0-25 feet; it is required to determine the breadth of the teeth. 6000 × 0-25 1500 3.2 inches, the breadth required. In order that the teeth may be capable of offering a sufficient resistance after being worn by friction, the breadth thus found should be doubled; therefore, in the above example, the breadtha should be 64, or say 6 inches. The following data are gleaned from experiments, which are therefore valuable, and of much use to the practical mechanic: Rule. Multiply the breadth of the teeth by the square of the thickness, and divide the product by the length; the quotient will be the proportional strength in horse power, with a velocity of 2:27 feat per second. Ex. What is the power of a wheel, the teeth of which are 6 inches broad, 1.5 inch thick, and 18 inch long, and revolving at the velocity of 3 feet per second? = 75, strength at 2-27 feet per second, then 2.27:753= 7.5 x 3 9-91 horse power: Rule. The pitch is found by multiplying the thickness by 2·1, and the length is found by multiplying the thickness by 1-2. Ex. The thickness being 2 inches, what is the pitch and length? 2 × 21 = 42, pitch. 2× 12= 24, length. For table of the proportions of wheels, see next page. Note.—The breadth of the teeth, as commonly executed by the best mechanics, seems to be from about twice to thrice the pitch. BEAN SHOT COPPER -Take copper, melt it, and pour it in a small stream into boiling water. FEATHER SHOT COPPER-Take copper, melt it, and pour it in a small stream into cold water. TO PRESERVE WALLS FROM DAMPNESS.-When the walls are about two feet high, use for one row of stones or bricks a mixtaire of tar.! pitch, and fine sand, in the same way as mortar. The composition must be previously melted to a proper consistence. TO PREVENT IRON FROM RUSTING.Warm your iron till your cannot bear your hand on it without burning yourself. Then rub it with new and clean white wax. Put it again to the fire till it has soaked in the wax. When done rub it over with a piece of serge. This prevents the iron from austing afterwards. Copper 100 parts; tin 4-17. Cast in moulds formed of cupel bone ash. Lead in Grains. Lead, melt it, and pour it in a small stream from a height of three or four feet into cold water. Copper 72 parts; tin 264; iron 14. This alloy is used for the bells of small ornamental clocks. Bath Metal Brass 32 parts; spelter 9. Mix. Horse power, at 3 feet per Horse power, |