1、液压专业英语教程2Unit 2 Pressure, Work, and PowerBasic term that are commonly used in the field of hydraulics and pneumatics must be discussed and understood.PRESSUREThe word pressure is defined as force per unit area. Although other units may be used, pressure is commonly expressed in such units a pounds p
2、er square inch. The abbreviation psi is usually employed to indicate pounds per square inch.Fig.1 shows an arrangement of two cylinders that are connected by a pipe or tube. A close-fitting piston is placed in each cylinder. In each cylinder (under the piston), the liquid and the connecting tube are
3、 shown, If it is assumed that there is no movement of each piston and that there is no leakage past each piston, the liquid and all the parts are at rest- a static condition. It is also assumed that a force F1 of 100 pounds acts on piston acts on piston No. q and that there is no friction between ea
4、ch piston and its cylinder wall. If piston No. 1 had a flat or face area of 2 square inches that is in direct contact with the liquid, the pressure in the liquid under piston No. 1 is equal to the force divided by the area (100 divided by 2), or 50 pounds per square inch. Thus, the liquid pressure a
5、t the face of piston No. 1 is 50 psi.Assuming that piston No.2 is essentially at the same level as piston No.1, the liquid between the pistons serves as a medium to transmit the pressure from one piston face to the other piston face. Thus, the liquid pressure at the face of piston No.2 is 50 psi, If
6、 the area of piston No.2 is 6 square inches, the force F2 on the face of piston No.2 is (650), or 300 pounds, Thus, a force of 100 pounds at piston No.1 develops a force of 300 pounds at piston No.2; this is accomplished by making the area of piston No.2 equal to three times the area of piston No.1.
7、 In a sense, the arrangement (see Fig. 1) is a fluid lever, similar to a mechanical lever using a metal bar and pivot.Equal pressure at every point and in every direction in the body of a static liquid (a liquid at rest) is characteristic of all static fluids, liquids, or gases. This is called Pasca
8、ls law, after an early experimenter in this field of study. This law of pressure is very useful, and can be sued to advantage in countless applications.Atmospheric Pressure A blanket of air surrounds the earth; this is called the atmosphere. At the surface of the earth, atmospheric pressure, which i
9、s due to the weight of the air above the surface of the earth, can be measured. Atmospheric pressure is commonly measured with a mercury barometer. Thus , atmospheric pressure is often called barometric pressure.Fig. 2 illustrates the basic principle of a mercury barometer. The glass tube is open at
10、 the lower end and closed tat the upper end. Initially, the tube is completely filled with pure mercury; then it is inverted, with the open end submerged, in a small vessel or cistern containing mercury. The height of the column of mercury gives the direct reading of the barometer; the weight of the
11、 air above the barometer balances the weight of the mercury column. Barometric pressure is usually expressed in inches of mercury. A barometric height of 29.32 inches of mercury corresponds to an atmospheric pressure of about 14.7 pounds per square inch.Pressure MeasurementMany instruments or gauges
12、 that are used for measuring pressure employ a Bourdon tube is a hollow metal tube that is made of brass or a similar material; it is oval or elliptical in cross section, and is bent in the form of a circle. One end of the bourdon tube is fixed to the frame at point A ( where the fluid enters )the o
13、ther end B (closed) is free to move. The free end B actuates a pointer through a linkage system. As fluid pressure inside the tube changes, the elliptical cross section changes, and the free end B of the Bourdon tube moves inward or outward, depending on the character of the change. A convenient pre
14、ssure scale or dial can be arranged from a calibration of the gauge.The position of the free end B of the Bourdon tube depends on the difference in fluid pressure between the inside and the outside of the tube. If the outside of the Bourdon tube is exposed to atmospheric air pressure, the instrument
15、 reading is a measure of so-called gauge pressure. For example, if the pressure reading at the outlet of an air compressor is 100 psi gauge, this indicates that the outlet air pressure is 100 psi above atmos0pheric pressure. In this book, all pressures referred to are gauge pressures. In some instan
16、ces, the pressure in a piece of equipment may be below atmospheric pressure; this condition is designated as vacuum. For example, if the air in a tank is at a pressure that is below atmospheric pressure, the pressure gauge indicates a certain vacuum, or negative gauge pressure,DEFINITION OF WORK, EN
17、ERGY, AND POWERAs shown in Fig.4, body weighing 20 pounds at a given level is indicated in position No.1. If the body is moved vertically through a distance or displacement of 9 feet, the action involves work. The technical term work is defined as the product of force times displacement, with the fo
18、rce in the direction of the displacement. As the body moves from position No. 1 to position No.2, a force of 20 pounds moves the 20-Ib. body through a displacement of 9 feet. This equals (920)，or 180 foot-pounds of work.Energy is defined as the capacity to do work. Energy refers to a possibility. A
19、body resting at position No.2 has certain capacity, or a certain energy. If the body is moved to the level 9 feet below, (920), or 180 foot-pounds is available to do work. The term work in itself does not involve a time element. Rate of movement, or speed, is often important. Power is defined as the
20、 time rate of doing work. If the body weighing 20 pounds were moved at a constant speed and in a vertical direction through a vertical distance of 9 feet in a time of 2 seconds, the “power” can be calculated as follows:Power=Thus, the power required is 90 foot-pounds per second. One horsepower has b
21、een arbitrarily defined as equal to 550 foot-pounds per second.FORCE AND WORK IN A FLUID DEVICEFig. 5 is an illustration of a pump or compressor delivering fluid (either oil compressed air) to the left-hand side of a piston in a cylinder. Let P represent the fluid pressure, in psi, and A represent t
22、he piston area in sq. in. (abbreviation for square inches).Then the force F acting on the left-hand face of the piston is PA. For a pressure P of 50 psi and an area. A of 2 sq. in., the force acting on the left-hand face of the piston is equal to (502), or 100 pounds. Assuming no friction due to the
23、 cylinder wall, the force F at the piston rod is equal to 100 pounds. Fig. 6 is another illustration of a pump or compressor delivering fluid to the left-hand side of a piston is a cylinder. A s in t he previous example, let P represent the fluid pressure (psi) and let A represent the left-hand pist
24、on area (sq. in.). Then the fluid force F acting on the left-hand piston face is F=PA. If this force remains constant while pushing the piston through a displacement of distance L (inches), the work done by the fluid on the left-hand face of the piston is equal to the force F times the displacement
25、L or the work W=PAL=PL. For a fluid pressure of 50 psi, a piston area of 2 sq. in., and displacement of 3 inches, the work W con be determined as follows.W=(5023)=300inch-poundsDISPLACEMENT ACTIONFig.7 is a diagram of a pump delivering hydraulic fluid through a pipe or tube into a cylinder, The pist
26、on is shown at position No.1 at a given time or part of the stroke. If the pump continues to deliver fluid into the cylinder, the fluid pushes the piston to the right-hand side a distance of 3 inches (position No.2 ). The linear displacement or movement of the piston to the right is equal to 3 inche
27、s. For a piston area of 2 square inches and a piston displacement of 3 inches, the volumetric displacement of the piston is equal 20 (2 sp in 3 in). or 6 cubic inches. Assuming no leakage of fluid across the piston from the left-hand side of the piston to the right-hand side of the piston, the total
28、 amount of fluid added to the cylinder is equal to 6 cubic inches; in other words, 6 cubic inches of fluid was admitted to the cylinder and pushed the piston, for a volumetric displacement equal to 6 cubic inches. With no leakage, this action is frequently called a positive-displacement action. The
29、amount of fluid entering the cylinder is equal to the volumetric displacement of the piston. In reference to the system illustrated in Fig.1(two cylinders and two pistons), the area of piston No. 1 is 2 square inches and the area of piston No.2 is 6 square inches (3 times that of piston No.1). If a
30、force of 100 pounds is applied at piston No. 1, there is corresponding force of 300 pounds at piston No.2. For a given fluid pressure, there is a force multiplication because of the difference in piston areas, assuming no leakage. If it is assumed that piston No. 1 moves downward a distance of 0.03
31、inch for a positive-displacement action of the incompressible hydraulic fluid, the volume of fluid displaced by piston No. 1 is equal to the volume of fluid displaced by piston No. 2 . For a positive-displacement action, piston No.2 then moves upward 0.01 inch (piston area times displacement is equa
32、l to the displaced volume). For piston No.1, the total work is (1000.03), or inch-pounds, For piston No.2, the total work is (3000.01)，or 3 inch-pounds.Rate of Flow and Piston TravelThe volume rate of fluid flow through a device can be expressed in various. Fox example, it is common to express volume rate of flow for liquids in gallons per minute (gpm). One gallon is equal to 231 cubic inches. For a practical example, an oil pump may be said to deliver a flow of 10 gpm; this corresponds to a rate of
