port on the bottom and a pneumatic charging portat the top (fig. 9-8). This type of accumulator isused in some airplane elevator hydraulic systemswhere several thousand gallons of fluid are neededto supplement the output of the hydraulic pumpsfor raising the elevator platform. The directcontact between the air or gas and the hydraulicfluid tends to entrain excessive amounts of gasin the fluid. For this reason, direct contactaccumulators are generally not used for pressuresover 1200 psi. The use of this type of accumulatorwith flammable fluid is dangerous because thereis a possibility of explosion if any oxygen ispresent in the gas, and pressure surges generateexcessive heat. For this reason, safety fluids areused in this type of installation.DIAPHRAGM ACCUMULATORSThe diaphragm-type accumulator is con-structed in two halves which are either screwedor bolted together. A synthetic rubber diaphragmis installed between both halves, making twochambers. Two threaded openings exist in theassembled component. The opening at the top,as shown in figure 9-9, contains a screen discwhich prevents the diaphragm from extrudingthrough the threaded opening when systempressure is depleted, thus rupturing the dia-phragm. On some designs the screen is replacedby a button-type protector fastened to the centerFigure 9-8.—Direct-contact gas-to-fluid accumulator.Figure 9-9.—Diaphragm accumulator.of the diaphragm. An air valve for pressurizingthe accumulator is located in the gas chamber endof the sphere, and the liquid port to the hydraulicsystem is located on the opposite end of thesphere. This accumulator operates in a mannersimilar to that of the bladder-type accumulator.FILTRATIONYou have learned that maintaining hydraulicfluids within allowable limits is crucial tothe care and protection of hydraulic equipment.While every effort must be made to preventcontaminants from entering the system, con-taminants which do find their way into the systemmust be removed. Filtration devices are installedat key points in fluid power systems to removethe contaminants that enter the system alongwith those that are generated during normaloperations.Filtration devices for hydraulic systems differsomewhat from those of pneumatic systems.Therefore, they will be discussed separately.The filtering devices used in hydraulic systemsare commonly referred to as strainers and filters.Since they share a common function, the termsstrainer and filter are often used interchangeably.As a general rule, devices used to remove largeparticles of foreign matter from hydraulic fluidsare referred to as strainers, while those used toremove the smallest particles are referred to asfilters.9-7
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