bubbles that normally occur as the fluid flows from thereservoir, through the system, and back to the reservoir.The air vent allows the air to be drawn in and pushedout of the reservoir by the ever-changing fluid level. Anair filter is attached to the air vent to prevent drawingatmospheric dust into the system.Because it is essential that the fluid in the reservoirbe kept at the correct level at all times, the sight gaugeis provided to allow the normal fluid level to always beseen The baffle plate segregates the outlet fluid fromthe inlet. This allows the fluid time to dissipate airbubbles, contaminants to settle, and the return fluid tocool before it is picked up by the pump.The proper hydraulic fluid level must bemaintained. In some systems low fluid level causesoverheating because the fluid does not have enough timeto cool in the reservoir before it gas back into the pump.Also, some systems will not work at all because the fluidis so low in the reservoir that air gets into the pump.Before adding hydraulic fluid, know what type touse and make sure it is clean. Clean around the fillercap or tube so there is less chance that dirt can get intothe system.STRAINERS AND FILTERSHydraulic systems have a strainer and one or morefilters that remove the impurities that would eventuallycontaminate the hydraulic fluid. The strainer isnormally located in the reservoir or in the inlet line tothe pump. The filter is normally located so only a smallamount of fluid is lost when the element is changed. Thefalter is equipped with a valve that allows the fluid tobypass the filter element should it become clogged. Thefalter element is usually of the paper cartridge, canister,or edge type and is similar to those used in enginelubrication systems.Regular filter maintenance,performed by the mechanics, is necessary to preventcontaminated fluid from being recirculated in thesystem.HYDRAULIC PUMPSThe hydraulic pump creates the flow of fluid withinthe hydraulic system.The pressure in a hydraulicsystem is caused by a restriction placed in the path ofthe fluid as it leaves the pump. Because of the resultingmechanical drive and positive displacement, the pumpmerely moves the fluid regardless of the restriction.When enough pressure is built up, movement of therestriction occurs or a relief valve placed in the systemopens, allowing the fluid to return to the reservoir or thesuction side of the pump.When the pump operates, hydraulic fluid is trappedbetween the gear teeth and the pump housing and iscarried to the outlet side of the pump. As the teeth mesh,a seal is freed by the mating surfaces that prevent theoil from leaking back to the inlet side of the pump. Thesealing action causes the oil to be forced out of the pumpand into the system.CONTROL VALVESControl valves are valves accessible to the operatorfor directing the flow of fluid within the system tooperate the machine or its attachment. By skillful useof the control valves, the operator can regulate the speedand operation of the hydraulic cylinders.NOTE: Hydraulic controls should be operatedsmoothly to eliminate the jerking motion that causesrapid wear and failure of the mechanical parts of themachine.HYDRAULIC CYLINDERSHydraulic cylinders are used to transmit motion inrelation to the volume of fluid directed into the cylinder.The force created by the cylinder is determined by thepressure of the fluid and the area of the piston contactedby the fluid. Thus the larger the piston, the more forcegenerated.Hydraulic cylinders used on heavy equipment areeither single- or double-acting cylinders.Single-Acting CylindersSingle-acting cylinders, similar to the one shown infigure 4-11, view A, are used to exert force in only onedirection. This means the weight or resistance movedmust be located so it causes the cylinder to return to itsoriginal position when pressure is relieved from thepiston. A common use of this type of cylinder is in ahydraulic jack.Double-Acting CylindersDouble-acting cylinders are used on equipmentwhere force is needed in two directions. Unlike thesingle-acting cylinder, the double-acting cylindercontains seals at both ends of the piston where the pistonrod passes through the end of the cylinder. With the useof this cylinder, fluid can be directed to either side of thepiston and cause the piston rod to extend or retract under4 - 9
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