STUFFING BOXES. diameter of rod; α= 2.5 d+.5'; b 1.5 d+.125"; = The stuffingbox of the form shown in the figure is generally used for small work, such as the spindles of valves, etc. The outside of the stuffingbox is threaded to receive a hexagonal nut that fits over the gland. As the nut is screwed down, the gland is pressed downwards and compresses the packing. The proportions used are: d+.125"; 2 d +.25"; 1.5 d+.25"; .25 d+.0625"; k = .5 d. This design may be used for rods up to 14 in. in diameter. Make the number of threads per inch the same as for a bolt whose diameter is equal to the diameter of the rod. GEARING. The circular pitch of a gear-wheel is the distance in inches measured on the pitch circle from the center of one tooth to the center of the next tooth. If the distance of the teeth of a gear thus measured were 2 in., we would say that the circular pitch was 2 in. D= diameter of pitch circle, in inches; C N circumference of pitch circle, in inches; number of teeth; ᅲ = 3.1416. Addendum = .3 P. Root = 4 P. The thickness of the teeth for a cut gear is equal to .5 P, and for a cast gear .48 P. The diametral pitch of a gear-wheel is the name given to the quotient that is obtained by dividing the number of teeth in the wheel by the diameter of the pitch circle in inches; or, the diametral pitch may be defined as the number of teeth on the circumference of the gear-wheel for 1 in. diameter of pitch circle. A gear with a pitch diameter of 5 in., and having 40 teeth is 8 pitch; one with the same pitch diameter and having 70 teeth is 14 pitch. In the gear of 8 pitch there are 8 teeth on the circumference for each inch of the diameter of the pitch circle; and in one of 14 pitch there are 14 teeth on the circumference for each inch of the diameter of the pitch circle. Let P D = diametral pitch; diameter of pitch circle, in inches; N = number of teeth; d = outside diameter; length of tooth; thickness of tooth; t The circular pitch corresponding to any diametral pitch may be found by dividing 3.1416 by the diametral pitch; and the diametral pitch corresponding to any circular pitch may be found by dividing 3.1416 by the circular pitch. (a) If the diametral pitch of a gear is 6, what is the corresponding circular pitch? (b) If the circular pitch is 1.5708 in., what is the corresponding diametral pitch? DIAMETRAL PITCHES WITH THEIR CORRESPONDING CIRCULAR PITCHES. Diametral Correspond Diametral Pitch. Pitch, or Teeth, ing Circular Pitch, or Teeth, per Inch in Diameter. per Inch in Diameter. 1234567 Corresponding Circular Pitch. The volt is the practical unit of electromotive force or electrical pressure. It is that electromotive force which will maintain a current of 1 ampere in a circuit whose resistance is 1 ohm The electromotive force of a Daniell's cell is 1.072 volts. The ampere is the practical unit denoting the strength of an electric current, or the rate of flow of electricity. It is that strength of current or rate of flow which would be maintained in a circuit whose resistance is 1 ohm by an electromotive force of 1 volt. One ampere decomposes .00009342 gram of water (H2O) per second; or deposits .001118 gram of silver per second. The ohm is the practical unit of resistance. It is that resistance which will limit the flow of an electric current under an electromotive force of 1 volt to 1 ampere. The legal ohm is the resistance of a column of mercury 106 centimeters long and 1 square millimeter sectional area at 0° C. One mile of pure copper wire in. in diameter has a resistance of 13.59 ohms at a temperature of 59.9° F. To make the significance of these units clearer, take the analogous case of water flowing through a pipe under a pressure of a column of water. The force that causes the water to flow is due to the pressure or head; the flow or current of water is measured in gallons per minute; and the resistance that opposes or resists the flow of water is caused by the friction of the water against the inside of the pipe. In electrotechnics, the electromotive force or electrical potential expressed in volts corresponds to the pressure or head of water; and the resistance in ohms to the friction in the pipe. The unit that expresses the rate of transmission of electricity per second is called the ampere, while the flow of water is expressed in gallons per minute. In either case the strength of current or rate of flow depends on the ratio between the pressure and the resistance; for, as the pressure increases, the current increases proportionately; and as the resistance increases, the current diminishes. This relation, as applied to electricity, was discovered by Dr. G. S. Ohm, and has since been called Ohm's law. Ohm's Law. The strength of the current in any circuit is directly proportional to the electromotive force in that circuit and inversely proportional to the resistance of that circuit, i. e., is equal to the quotient arising from dividing the electromotive force by the resistance. EXAMPLE.-The electromotive force of a circuit is 110 volts, and its resistance is 55 ohms; what is the strength of current? SOLUTION.- E 110 volts. R - 2 amperes. 55 ohms. C = E 110 The unit by which electrical power is expressed is called the watt. It is that rate of doing work when a current of 1 ampere is passing through a conductor under an electromotive force of 1 volt, and is equal to of a horsepower. |