FM 1-506 Fundamentals of Aircraft Power Plants ... - Survival Books

WWW.SURVIVALEBOOKS.COMFM 1-506with debris. Support of the falter element is provided bythe bypass valve on one end and the impending bypassindicator on the other. The indicator end has a springloadedsleeve which restrains the filter axially.Bowl and Bypass Indicator. An aluminum bowlhouses the element and contains the impending bypassindicator at the end. Mounting is horizontal to fit thespace available and provide ready access for servicingImpending bypass indication is provided by a small unitwhich is part of the bowl assembly. The indicator isinstalled from the inside of the bowl. It is retained in placewith an external retaining ring. Basic mechanics ofoperation are as follows:Different pressure between filter inlet and outletacts to move a piston against a spring at 44 to60 psi.Piston contains a magnet which normally attractsa redbutton assembly and holds it seatedagainst its spring. When the piston moves, thebutton is released. It extends 3/16 inch tovisually indicate an impending bypass condition.Button is physically reattained from tripping bya cold lockout bimetallic latch if temperature isless than 100 to 130°F. This prevents a false tripduring cold starts.As the button is released, a small spring-loadedball also moves out of position to latch the buttonand block reset. The internal piston assemblyautomatically resets on shutdown;however, the indicator remains latched out.After removing the filter element and the bowlfrom the gearbox, a spring-loaded sleevearound the indicator moves aft and pulls thepiston assembly to a tripped position. Thiscauses the button to trip if operation is attemptedwith no filter in the bowl.To react the indicator, the bowl is held verticallyso the button latch ball can roll out of the latchedposition. The button is then manually reset.If the bowl is reassembled with no falter, the indicatorwill trip when the temperature exceeds the 100 to 130°Flock-out level. The internal latch mechanism preventsresetting the button without disassembling the bowl.Resetting must be done with the bowl removed from theaccessory gearbox and held verically, button up, torelease the latch.Oil Filter Bypass Sensor. The oil filter bypass sensoris a differential-pressure switch which senses fiilter inletminus outlet pressure.The sensor consists of a spring-loaded piston whichmoves aft at high falter differential pressure (60 to 80 psi)and magnetically releases a microswitch lever. Theswitch is in a sealed cavity separated from the oil and iswired to a hermetically sealed electrical connector. Theswitch connects 28-VDC aircraft power when trippedand reopens the circuit at 15 psi minimum differentialNo latch is used in the sensor so resetting is automatic.Also, there is no cold lockout. The pilot will be informedof falter bypassing during cold start warm-ups. Sensortolerance range is set slightly below the tolerance rangeof bypass valve cracking pressure. Therefore, bypassingwill not occur without pilot warning. The impendingbypass indicator will show need-to-change filter elements.This sensor provides backup warning if maintenanceaction is not taken.Lubrication and Scavenge Pump. The lube andscavenge pump is a Gerotor-type pump of cartridgedesign, located on the forward side of the accessorygearbox (refer back to Figure 5-10). It fits into a precisionbore in the gearbox casing. The Gerotor-type pump waschosen because of its wear resistance and efficiency.Gerotor elements are similar to male gear inside a female(internal) gear with one less tooth on the inner member.The inner Gerotors are keyed to the drive shaft, andthe outer Gerotors are pocketed in individual eccentricrings. AS the assembly rotates, oil is drawn into an expandingcavity between teeth on one side. The oil isexpelled when the cavity contracts approximately 180°away. Inlet and discharge ports are cast into the portplates. They are shaped and positioned to fill and emptyat proper timing for maximum volumetric efficiency andresistance to inlet cavitation.There are seven different elements in the pump fromthe spline end forward. They are the lube supply element,C-sump cover, C-sump aft, C-sump forward, A-sumpforward, A-sump aft, and B-sump Delta scavenge elements.The port plate eccentric rings and Gerotors areassembled into a surrounding concentric aluminumtubular housing The housing maintains all elements inproper alignment. The oil suction and discharge passagesfrom the Gerotors are brought radially through thehousing. They match the appropriate locations of themating passages in the engine gearbox casing. The entirestack of port plates is retained in the housing with theretaining rings at the spline end. The outermost end ofthe housing has an integrally cast cover. The cover boltholes are arranged to orient the pump assembly in thegearbox housing during installation.5-13