RULES AND TABLES.
To find the steam pressure on slide valve, multiply the unbalanced area of valve in inches by pounds pressure of steam per square inch, add weight of valve in pounds, and multiply the sum by 0.15.
Safety boiler pressure according to the United States Government rule is as follows: Multiply ⅛ of the lowest tensile strength found on any plate in the cylindrical shell by the thickness expressed in inches or part of an inch of the thinnest plate in the same cylindrical shell, and divide by the radius or half the diameter, also expressed in inches, and the sum will be the pressure allowable per square inch of surface for single riveting, to which add 20 per centum for double riveting.
To find the water pressure on steam pipes leading from boiler to steam gauge, divide the difference in height between the highest point of pipe and the center of steam gauge by 2³/₁₀; the result will be the pressure exerted by the water in the pipe in pounds upon the gauge.
Area of a Circle.—To find the area of a circle when the diameter is given, multiply the diameter by itself, or in other words square the diameter and multiply the result by .7854.
Ex. Diameter 5 inches, 5 × 5 = 25 × .7854 = 29.635 area.
Circumference of a Circle.—To find the circumference of a circle when the diameter is given, multiply the diameter by 3.1416.
Ex. Diameter is 5 inches. 5 × 3.1416 = 15.708 inches circumference.
Diameter of a Circle.—To find the diameter of a circle when the circumference is given, multiply the circumference by .31831.
Ex. Circumference 20 inches. 20 × .31831 = 6.362 diameter.
To find the pressure of pounds per square inch of a column of water, multiply the height of the column in feet by .434. Approximately, we generally call every foot elevation equal to one-half pound pressure per inch, this allows for ordinary friction.
To find the diameter of a pump cylinder to move a given quantity of water per minute (100 feet of piston being the standard of speed), divide the number of gallons by 4, then extract the square root and the product will be the diameter in inches.
To find the horse-power necessary to elevate water to a given height, multiply the total weight of water in pounds by the height in feet and divide the product by 33000 (an allowance of 25% should be added for friction, etc.).
The area of the steam piston multiplied by the steam pressure, gives the total amount of pressure that can be exerted. The area of the water piston multiplied by the pressure of water per square inch gives the resistance. A margin must be made between the power and the resistance to move the piston at the required speed—say 50%.
To find the capacity of a cylinder in gallons, multiply the area in inches by the height of stroke in inches, which gives the total number of cubic inches; divide this amount by 231 (which is the cubic contents of a gallon in inches), the product is the capacity in gallons.
RULES FOR CALCULATING THE SPEED
OF GEARS AND PULLEYS.
In calculating for pulleys, multiply or divide by their diameter in inches.
In calculating for gears, multiply or divide by the number of teeth required.
The driving wheel is called the “driver,” and the driven wheel the “driven.”
Problem 1.—To find the diameter of the driver when the revolutions of the driver and driven and the diameter of the driven are given.
Rule.—Multiply the diameter of driven by its number of revolutions, and divide by the number of revolutions of the driver.
Problem 2.—To find the diameter of the driven to make the same number of revolutions in the same time as the driver, when the diameter and revolutions of driver are given.
Rule.—Multiply the diameter of the driver by its number of revolutions, and divide the product by the required number of revolutions.
Problem 3.—To find the number of revolutions of the driven when the diameter or number of teeth and number of revolutions of the driver and the diameter or number of teeth of the driven are given.
Rule.—Multiply the diameter or number of teeth of the driver by its number of revolutions, and divide by the diameter or number of teeth of the driven.
Problem 4.—To find the number of revolutions of the driver when the diameter of the driven and the number or revolutions of driven are given.
Rule.—Multiply the diameter of driven by its number of revolutions, and divide by the diameter of the driver.