sistance, as shown in the annexed diagram to illustrate this particular case. Thus, suppose the weight to be placed on any one of the divisions, it is still the same weight, or 1; but because of the principle of the lever, the resistance is increased equal to the number of times the weight is distant from the fulcrum; consequently the action of the lever tends to press down the valve equal the sum of the weight and resistance, or the number of times the weight is distant from the resistance. 2. THE WHEEL AND PINION, OR CRANE. The mechanical advantage of the wheel and axle, or crane, is as the velocity of the weight to the velocity of the power; and being only a modification of the first kind of lever, it of course partakes of the same principles. 1. To determine the amount of effective power produced from a given power by means of a crane with known peculiarities. Rule.-Multiply together the diameter of the circle described by the winch, or handle, and the number of revolutions of the pinion to 1 of the wheel; divide the product by the barrel's diameter in equal terms Point of resistance. of dimensions, and the quotient is the effective power to 1 of exertive force. Ex. Let there be a crane, the winch of which describes a circle of 30 inches in diameter; the pinion makes 8 revolutions for 1 of the wheel, and the barrel is 11 inches in diameter; required the effective power in principle, also the weight that 36ħbs. would raise, friction not being taken into account. 30 x 8 11 =21·8 to 1 of exertive force; and 21·8 × 36=784.8 lbs. 2. Given any two parts of a crane, to find the third, that shall produce any required proportion of mechanical effect. Rule. Multiply the two given parts together, and divide the product by the required proportion of effect; the quotient is the dimensions of the other parts in equal terms of unity. Ex. Suppose that a crane is required, the ratio of power to effect being as 40 to 1, and that a wheel and pinion 11 to 1 is unavoidably compelled to be employed, also the throw of each handle to be 16 inches; what must be the barrel's diameter on which the rope or chain must coil? 16 x 2=32 inches diameter described by the handle. And 40 =8.8 inches, the barrel's diameter. 3. THE PULLEY. The principle of the pulley, or more practically the block and tackle, is the distribution of weight on various points of support; the mechanical advantage derived depending entirely upon the flexibility and tension of the rope, and the number of pulleys or sheives in the lower or rising block: hence, by blocks and tackle of the usual kind, as shown in fig. 3, Pl. D, the power is to the weight as the number of cords attached to the lower block; whence the following rules. 1. Divide the weight to be raised by the number of cords leading to, from, or attached to the lower block; and the quotient is the power required to produce an equilibrium, provided friction did not exist. 2. Divide the weight to be raised by the power to be applied; the quotient is the number of sheives in, or cords attached to the rising block. Ex. Required the power necessary to raise a weight of 3000 lbs. by a four and five-sheived block and tackle, the four being the moveable or rising block. Necessarily there are nine cords leading to and from the rising block. 3000 Consequently =333 lbs., the power required. Ex. 2. I require to raise a weight of 1 ton 18 cwt., or 4256 lbs.; the amount of my power to effect this object being 500 lbs., what kind of block and tackle must I of necessity employ? 4256 500 =8.51 cords; of necessity there must be 4 sheives or 9 cords in the rising block. As the effective power of the crane may, by additional wheels and pinions, be increased to any required extent, so may the pulley and tackle be similarly augmented by purchase upon purchase; two of the most useful of such applications being represented in figs. 3 and 4, Plate D, the first of which is known by the term runner and tackle, and the second by that of Spanish burton. |