CHAPTER 12BASIC DIAGRAMS AND SYSTEMSIn the preceding chapters, you learned abouthydraulic and pneumatic fluids and componentsof fluid power systems. While having a knowledgeof system components is essential, it is difficultto understand the interrelationship of thesecomponents by simply watching the systemoperate. The knowledge of system interrelationis required to effectively troubleshoot andmaintain a fluid power system. Diagrams pro-vided in applicable technical publications ordrawings are a valuable aid in understanding theoperation of the system and in diagnosing thecauses of malfunctions.This chapter explains the different types ofdiagrams used to illustrate fluid power circuits,including some of the symbols that depict fluidpower components. Included in this chapterare descriptions and illustrations denoting thedifferences between open-center and closed-centerfluid power systems. The last part of the chapterdescribes and illustrates some applications of basicfluid power systems.DIAGRAMSAs mentioned earlier in this chapter, totroubleshoot fluid power systems intelligently, amechanic or technician must be familiar with thesystem on which he or she is working. Themechanic must know the function of eachcomponent in the system and have a mentalpicture of its location in relation to othercomponents. This can best be done by studyingthe diagrams of the system.A diagram may be defined as a graphicrepresentation of an assembly or system thatindicates the various parts and expresses themethods or principles of operations. The abilityto read diagrams is a basic requirement forunderstanding the operation of fluid powersystems. Understanding the diagrams of a systemrequires having a knowledge of the symbols usedin the schematic diagrams.SYMBOLSThe Navy uses two military standards thatlist mechanical symbols that must be used inpreparing drawings that will contain symbolicrepresentation. These standards are as follows:1. Military Standard, Mechanical Symbols(Other than Aeronautical, Aerospacecraft, andSpacecraft Use), Part 1, MIL-STD-17B-1.2. Military Standard, Mechanical Symbols forAeronautical, Aerospacecraft, and SpacecraftUse, Part 2, MIL-STD-17B-2.Some of the symbols frequently used in fluidpower systems have been selected from thesetwo standards and are shown in Appendixes IIand III. Appendix II contains symbols fromMIL-STD-17B-1. Appendix III contains symbolsfrom MIL-STD-17B-2.While the symbols shown in the appendixesare not all encompassing, they do provide a basisfor an individual working with fluid powersystems to build upon. Some rules applicable tographical symbols for fluid diagrams are asfollows:1. Symbols show connections, flow paths,and the function of the component representedonly. They do not indicate conditions occurringduring transition from one flow path to another;nor do they indicate component construction orvalues, such as pressure or flow rate.2. Symbols do not indicate the location ofports, direction of shifting of spools, or positionof control elements on actual components.3. Symbols may be rotated or reversedwithout altering their meaning except in cases oflines to reservoirs and vented manifolds.4. Symbols may be drawn in any size.5. Each symbol is drawn to show the normalor neutral condition of each component unlessmultiple circuit diagrams are furnished showingvarious phases of circuit operation.12-1
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