of 1 inch, the volume of fluid in the left cylinderwill decrease by 2 cubic inches. At the same time,the volume in the right cylinder will increase by2 cubic inches. Since the diameter of the rightcylinder cannot change, the piston must moveupward to allow the volume to increase. Thepiston will move a distance equal to the volumeincrease divided by the surface area of the piston(equal to the surface area of the cylinder). In thisexample, the piston will move one-tenth of an inch(2 cu. in. ÷ 20 sq. in.). This leads to the secondbasic rule for a fluid power system that containstwo pistons: The distances the pistons move areinversely proportional to the areas of the pistons.Or more simply, if one piston is smaller than theother, the smaller piston must move a greaterdistance than the larger piston any time the pistonsmove.LIQUIDS IN MOTIONIn the operation of fluid power systems, theremust be a flow of fluid. The amount of flow willvary from system to system. To understand fluidpower systems in action, it is necessary tounderstand some of the characteristics of liquidsin motion.Liquids in motion have characteristics dif-ferent from liquids at rest. Frictional resistanceswithin a fluid (viscosity) and inertia contribute tothese differences. (Viscosity is discussed in chapter3.) Inertia, which means the resistance a massoffers to being set in motion, will be discussedlater in this section. There are other relationshipsof liquids in motion with which you must becomefamiliar. Among these are volume and velocityof flow, flow rate and speed, laminar andturbulent flow, and more importantly, the forceand energy changes which occur in flow.VOLUME AND VELOCITY OF FLOWThe volume of a liquid passing a point in agiven time is known as its volume of flow or flowrate. The volume of flow is usually expressed ingallons per minute (gpm) and is associated withrelative pressures of the liquid, such as 5 gpm at40 psi.The velocity of flow or velocity of the fluidis defined as the average speed at which the fluidmoves past a given point. It is usually expressedin feet per second (fps) or feet per minute (fpm).Velocity of flow is an important consideration insizing the hydraulic lines. (Hydraulic lines arediscussed in chapter 5.)Volume and velocity of flow are oftenconsidered together.With other conditionsunaltered—that is,with volume of inputunchanged—the velocity of flow increases as thecross section or size of the pipe decreases, and thevelocity of flow decreases as the cross sectionincreases. For example, the velocity of flow is slowat wide parts of a stream and rapid at narrowparts, yet the volume of water passing each partof the stream is the same.In figure 2-13, if the cross-sectional area ofthe pipe is 16 square inches at point A and 4square inches at point B, we can calculate therelative velocity of flow using the flow equationQ = v AEquation 2-7.where Q is the volume of flow, v is the velocityof flow and A is the cross-sectional area of theliquid. Since the volume of flow at point A, Q1,is equal to the volume of flow at point B, Q2, wecan use equation 2-7 to determine the ratio of theFigure 2-13.—Volume and velocity of flow.2-9
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