 # Tài liệu Diezel 1410 P2

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30 ## Tài liệu Diezel 1410 P2

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In order to understand and operate an engine efficiently it is necessary for the operator to be familiar with various units of measurement and the instruments by which they are recorded

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1. 2 MEASUREMENTS AND INSTRUMENTS A. MEASUREMENTS Such units as length, volume, and mass 2A1. Fundamental and standard units. In order to understand and are easily converted to the next higher operate an engine efficiently it is denomination by using the simple necessary for the operator to be familiar multiplier, 10. For example: with various units of measurement and Units of Length the instruments by which they are 10 millimeters = 1 centimeter recorded. As soon as any branch of 10 centimeters = 1 decimeter science is developed to any extent, 10 decimeters = 1 meter attempts are made to measure and evaluate the quantities and conditions 1000 meters = 1 kilometer found to exist. To do this a unit must be Units of Weight selected for each measurable quantity. 10 milligrams = 1 centigram These units are derived from a set of 10 centigrams = 1 decigram basic units known as fundamental units. 10 decigrams = 1 gram The fundamental units are units of force, length, and time. 1000 grams = 1 kilogram 1000 kilograms = 1 metric ton Fundamental units should not be confused with standard units. Standard The metric system has been legalized for units of measurement are units that are use in the United States and is frequently established and legalized by the used in scientific and laboratory work, government of a country. Whenever because the smaller units facilitate work standardized units are established, the of extreme accuracy and the use of the fundamental units are expressed in simple multiplier, 10, makes computation terms of the standard units to secure of work quick and easy. uniformity of procedure and comparison. 2A3. The English system of measurement. The English system of 2A2. The metric system of measurement is by far the most measurement. The metric system of commonly used in engineering work in measurement is used generally the United States. The system is given throughout the world, particularly in wide usage primarily because of Europe. It is not in general use in the precedent rather than because of any United States. Because the metric recommending features such as those system is a decimal system, it is less encountered in the metric system. subject to arithmetical error than the other common system, the English In the English system the fundamental system of measurement. Since the units of force, length, and time are metric system uses the simple expressed in the standard units of foot, multiplier, 10, it is easy to establish the pound, and second. Unlike the metric value of the unit of measure as denoted system, the English system has no by the prefix in the name of the unit. common multiplier and the subdivisions The table below explains how the of the units of measurement bear no prefix denotes the value of the unit of common relation to each other. For measure and gives examples of the use example, below are given the units of of the prefix. length and weight and the relationship of the various subdivisions of each. Prefix Example micro (meaning micron, Units of Length
2. millionth) micrometer 12 inches = 1 foot milli (meaning millimeter, 3 feet 1 yard thousandth) milligram 5 1/2 yards = 1 rod (16 1/2 feet) centi (meaning centimeter, Units of Weight hundredth) centigram 16 ounces = 1 pound deci (meaning tenth) decimeter, 2000 pounds = 1 ton (short) decigram 2240 pounds = 1 ton (long) deka (meaning ten) dekameter hecto (meaning hectometer hundred) kilo (meaning kilometer thousand) In the metric system the fundamental units of force, length, and time are expressed in the standard units of kilograms, meters, and seconds. 21 Since all forms of matter are by 2.54, and centimeters converted to measurable in terms of the fundamental inches by dividing centimeters by 2.54, units of force, length, and time, it is possible to combine the units of 2A6. Unit of force. Force is the push, measurement to express measurement pull, or action upon a body or matter at of quantities encountered in various rest which tends to give it motion. In the engineering and scientific work. In the English system, the unit of force is the following sections, the English and pound. In the metric system, the unit of metric units of measurement in force is the kilogram. engineering work are discussed. In the description of each, it is easy to see 2A7. Unit of work. The work done upon how each of these units of a body is equal to the average force measurement may be basically reduced acting upon the body multiplied by the to fundamental units. distance through which the body is moved as a result of the force. In the 2A4. Unit of length. Length is usually English system, the unit of work is the defined as the distance between two foot-pound. For example, if a force of points. In the English system it is 500 pounds acts upon a body to move it expressed in inches, feet, yards, rods, 10 feet, 5000 foot-pounds of work have miles, or fractions thereof. The been done upon this body. accuracy required in engineering work makes it a general practice for 2A8. Units of mass and weight. The engineers to measure length in mass of a body may be defined as the thousandths of an inch. Thus, various quantity of matter in a body without tolerances, clearances, and minute regard to its volume or the pull of gravity measurements are expressed by upon it. The term mass must be decimal divisions of an inch in distinguished from the term weight which thousandths, such as .125 (one hundred is the measurement of the force of twenty five thousandths). gravity acting upon body at any given point upon the earth's surface. Weight In a problem involving measurement of varies with locality, but mass is area, the area of a regular shape may be considered constant. The student must expressed by the product of two not confuse mass with weight although measurements of length. Thus, a square the units are the same for both. The
3. 3 feet by 3 feet has 9 square feet of standard kilogram is defined as the mass area. Likewise, a problem of measuring of a certain piece of platinum iridium in volume, where the shape is adaptable to possession of the International Bureau of linear measurement, may be expressed Weights and Measures. The fundamental by the product of three measurements unit of mass, the gram, is one one- of length. Thus, a cube 3 feet by 3 feet thousandth of the standard kilogram. by 3 feet has 27 cubic feet of volume. English System Metric System 2A5. Conversion factors of length. 1 ounce = 26.35 grams Often when using the English system in 1 pound = 0.454 kilograms engineering work it is necessary to 1 gram = 0.0353 ounces convert measurements to the metric system and vice versa. To change units 1 kilogram = 2.205 pounds of one system to those of another it is necessary to have a conversion factor Kilograms are converted into pounds by that establishes the relation between the multiplying the number of kilograms by two systems for the same quantity. The 2.205, and conversely pounds are most commonly used conversion converted into kilograms by multiplying factors between the English and metric the number of pounds by 0.454. For systems are: example, 1 metric ton (1000 kilograms) equals 1000 x 2.205 or 2205 pounds. English System Metric System 2A9. Unit of pressure. Pressure is 1 inch = 2.54 centimeters defined as force per unit area acting 39.37 inches = 1 meter against a body. In the English system, the unit of pressure may be expressed as All English system measurements of pounds per square inch or pounds per length may be reduced to inches and all square foot. metric system measurements of length to centimeters. Knowing the basic Since all forms of matter have weight, conversion factor, inches can be the air of the earth's atmosphere has converted to centimeters by multiplying weight. At sea inches 22 level, the weight of air exerts a pressure of 180 degrees or graduations between of 14.7 pounds per square inch and has the freezing point and the boiling point of a weight of approximately 0.08 pounds pure water at sea level. On the Fahrenheit per cubic foot. At higher altitudes, thescale the freezing point of water is fixed pressure, and therefore the weight, at 32 degrees and the boiling point of becomes less. water at 212 degrees. The centigrade scale is established with a range of 100 degrees or graduations between the Gage pressure. Pressure gages are freezing point and the boiling point of commonly used to determine the pressure existing or to record the peak water at sea level. On the centigrade scale the freezing point of water is fixed pressure attained within a container. at 0 degrees and the boiling point of Most pressure gages make no allowance for atmospheric pressure and water at 100 degrees. normally register zero at existing atmospheric pressure. a. Absolute zero temperature. Absolute zero temperature is theoretically the lowest temperature that can be obtained. Absolute pressure. In practically all It is that temperature at which all pressure problems, atmospheric molecular motion ceases entirely and at pressure is present and must be
5. of 1 pound of fuel oil is known as the 24 hours = 1 day fuel oils heating value. 2A14. Units of velocity. Velocity may be Since heat is a form of energy, it cannot defined as the rate of movement of a be destroyed but may be converted into body. If a body moves a specified mechanical energy. One Btu of heat is distance during a specified time at a equivalent to 778 foot-pounds of work. uniform speed, the velocity may be Thus, the conversion factor for power determined by dividing the distance by to heat is: the time. There are two types of velocity normally encountered, linear and 1 hp = 33,000 / 778 = 42.42 Btu per angular. If the velocity is linear, the minute movement is in a straight line and the velocity may be expressed in terms such 2A13. Unit of time. The standard unit as feet per second, feet per minute, or of time in both the English system and miles per hour. If the velocity is angular, the metric system is the second. The the movement of the body is rotary or second is defined as 1/86,400 part of a about a central axis, and the velocity may mean solar day. The mean solar day is be expressed in revolutions per minute or obtained by taking the average length revolutions per second. In engineering of all the days of the year, a day being work it is common practice to rate the measured from the noon of one day to velocity of shafts, wheels, gears, and the noon of the next. other rotating parts in revolutions per minute (rpm). B. INSTRUMENTS 2B1. General. In the previous section expansion or contraction of the we have defined and explained the instrument from changes in temperature fundamental units of measurement and can be considerable. the standard units of measurement for both the English and the metric c. Calipers. Engineers and machinists systems. It is the purpose of this section frequently use calipers to secure accurate to enumerate and describe the various measurements of inside and outside instruments by which these diameters. Figure 2-2 shows how various measurements are computed and caliper settings may be taken and how recorded. the registered setting of the calipers may be measured by a ruler or by a micrometer. 2B2. Instruments for measuring length. a. General. In engineering and machine work there are several d. Micrometer calipers. Engineers instruments for measuring length, area, frequently rely on the micrometer caliper and volume. Since the measurement of (Figure 2-3) to obtain measurements area and volume often can be obtained accurate to 1/1000 of an inch. This by compounding simple measurements instrument is particularly useful for of length, instruments used for measuring relatively short lengths and computing area and volume are also the diameter of journals or cylinders. The described here. common commercial micrometer consists of a frame; an anvil, or fixed b. Rulers and tapes. The most common measuring point; a spindle; a sleeve, or method of obtaining simple barrel; and a thimble. The spindle has measurements of length is by the ruler threads cut 40 to the inch on the portion or tape (Figure 2-1). A ruler may be that fits inside the sleeve. The thimble graduated into feet, inches, or fractions fits over the end of the sleeve, and thereof. Rulers and tapes used in rotating the thimble turns the spindle. engineering work are most frequently 