Transmission Oil
The transmission oil system controls and monitors the flow, pressure, temperature, and contamination of the oil used for lubrication and cooling of the gears and bearings in the transmission, mast and freewheel assemblies.
The transmission oil system has the components that follow:
Oil pump inlet screen
The oil pump inlet screen is installed on the forward right side of the transmission assembly inside the transmission casing. It is located immediately in front of the pump, and traps and contains particulate contaminates. It is made of No. 23 gauge steel screening that is brazed to an inlet sleeve and plug.
Oil pump
The oil pump is installed on the forward left side of the transmission assembly inside the transmission casing. It is a positive displacement and constant volume pump type and is driven by the accessory drive gear at a 1:1.42 ratio. The pump also has a pass through shaft that drives the hydraulic pump. The oil pump has a rated pressure of 80 PSI (552 kPa), a maximum pressure of 150 PSI (1034 kPa), and supplies a 6.0 to 6.7 gallons per minute (22.7 to 25.3 L/min) flow.
Oil filter housing and manifold assembly
The oil filter housing and manifold assembly is a part of the transmission assembly and is installed on its aft left side. It is an assembly of an oil manifold assembly, an oil filter housing, and an oil filter element.
The oil manifold assembly is made from an aluminum alloy casting and has provisions for installation of components of the transmission oil indicating system. There are passages in the oil manifold assembly that let the oil move in and out of the transmission assembly and between the attached components. The oil filter housing is attached to the oil manifold and contains the replaceable oil filter element which is a 3-micron pleated paper element.
Attached parts are as follows:
Oil temperature switch
The oil temperature switch is a normal-open switch that closes at a predefined value of pressurized unfiltered uncooled oil temperature of 230°F (110°C).
Oil temperature bulb
The oil temperature bulb is a temperature sensor that provides continuous temperature measurement readings of pressurized unfiltered uncooled oil.
Oil filter bypass indicator
The oil filter bypass indicator is a differential pressure sensing device. It operates by sensing the differential oil pressure upstream and downstream of the filter element. The extended red indicator gives the ground crew an indication of a clogged filter.
Oil filter bypass valve
The oil filter bypass valve is installed on the oil filter housing and manifold assembly. The oil filter bypass valve will bypass the oil filter if the difference between the oil pressure into the filter and the oil pressure out of the filter reaches the preset value of 30-34 PSID (206.8 to 234 kPa) and 190°F (88°C) which indicates that the filter element is clogged. The valve is spring loaded and will open to let the oil bypass the oil filter.
Oil bypass thermostat
The oil bypass thermostat is installed on the oil filter housing and manifold assembly. It controls the flow of pressurized uncooled oil to the oil cooler in relation to the oil temperature. When the oil is less than 200°F (93.3°C), the thermostat directs the oil away from the oil cooler. As the oil temperature increases, the valve starts to close which directs oil to the oil cooler. The thermostat fully closes at a temperature between 215 and 225°F (101.6 and 107.2°C) at which point all circulating oil is directed to pass through the oil cooler. If there is a high differential pressure across the oil cooler the bypass valve is designed to open and provide a relief flow of 12.8 GPM (59.8 l/min) at 25 PSI (172.4 kPa) and 230°F (110°C) to the transmission gears and bearings.
Pressure line (from transmission to oil cooler)
The pressure line (from transmission to oil cooler) is attached to the truss assembly on the right side of the lower main-rotor drive compartment and center aft fuselage. It connects the oil cooler to the transmission assembly and carries pressurized filtered oil. It is an assembly of aluminum alloy tubes and a reinforced elastomeric flexible hose.
Oil cooler assembly
The oil cooler assembly contains two separate and independent cores welded together. The right side core is for the engine oil system and the left side core is for the transmission oil system. The oil cooler assembly is mounted on the truss assembly below the oil cooler blower assembly and the units are connected by means of a transition air duct.
Oil cooler blower assembly
The oil cooler blower is mounted aft of the aft engine firewall on the truss assembly and supplies cooling air to the engine and transmission oil cooler. The oil cooler blower shaft is part of the tail rotor driveshaft system and is used to power the blower. It is mounted on two sealed bearings and is connected to the forward and the aft short shafts. The oil cooler blower shaft passes through the blower assembly housing, and the blower impeller is bolted to the shaft.
Return line (from oil cooler to transmission)
The return line (from oil cooler to transmission) is attached to the truss assembly on the right side of the lower main-rotor drive compartment. It connects the transmission to the oil cooler in the center aft fuselage and carries pressurized filtered cooled oil. It is an assembly of aluminum alloy tubes and a reinforced elastomeric flexible hose.
Pressure line (from transmission to freewheel)
The pressure line (from transmission to freewheel) is located on the left side of the lower main-rotor drive compartment. It connects the freewheel assembly to the transmission on the left side of the engine compartment and carries pressurized filtered oil. It is an assembly of a reinforced elastomeric flexible hose, an aluminum alloy tube, and fire-resistant flexible hoses made of Teflon.
Oil manifold
The oil manifold is located on the left side of the lower main-rotor drive compartment. It is installed on the pressure line to the freewheel assembly and is made from an aluminum alloy casting. The manifold has provisions for installation of components of the transmission indicating system on it and for installation of the pressure line (to freewheel) connections.
Attached parts are as follows:
Oil pressure switch
The oil pressure switch provides a signal when the oil pressure decreases below 30 ± 2 PSI (206 ± 13.8 kPa).
Oil pressure transmitter
The oil pressure transmitter provides continuous readings of the oil pressure that is being supplied to the lubricated parts.
Return line (from freewheel to transmission)
The return line (from freewheel to transmission) is located on the left side of the lower main-rotor drive compartment and engine compartment. It connects the transmission to the freewheel assembly and carries pressurized filtered oil. It is an assembly of reinforced elastomeric flexible hole, an aluminum allow tube, and fire resistant flexible holes made of Teflon.
Oil pressure regulating valve
The oil pressure regulating valve is installed on the left side of the transmission assembly. It is a spring-loaded assembly that has an adjustment screw. It maintains the pressure between 53 and 59 PSI (365.4 and 406.8 kPa). It relieves any excess oil pressure back into the sump.
Operation
The accessory gear on the first bevel gear reduction stage of the transmission assembly drives the oil pump. As the oil moves through the oil pump, the oil pump inlet screen filters out mechanical contaminants and retains them. From the discharge port of the oil pump, the oil moves through an external line that connects to the oil filter housing and manifold assembly.
In the oil filter housing and manifold assembly, the pressurized oil passes by the oil temperature bulb and the oil temperature switch sensors which provide temperature level readings and preset limits indications. After this, the oil goes to the oil filter bypass valve which directs the oil either to the oil filter, or bypasses the oil filter if the oil filter is clogged. A clogged oil filter could lead to oil starvation, so for safety reasons the oil filter is bypassed. The oil filter bypass valve functions by sensing the pressure drop across the oil filter and redirecting the oil based on this pressure difference. An acceptable pressure drop would indicate that the oil filter is working properly by removing particulates from the oil as it flows through the pleats of the oil filter element, while a large pressure drop would indicate that the oil filter is clogged.
After flowing through or around the oil filter, the oil flows past the oil bypass thermostat which directs the oil to one of two possible paths. If the oil needs cooling (oil is above a pre-determined temperature), the oil bypass thermostat valve closes and the oil is directed to the oil cooler, and if the oil does not need cooling (oil is below a pre-determined value), the oil bypass thermostat valve opens and the oil is directed to the oil pressure regulating valve. The oil cooler is a oil-air heat exchanger in which heat in the oil is removed by forced air blowing over the hear exchanger.
After passing the oil bypass thermostat, the oil goes to the oil pressure regulating valve which adjusts the oil pressure to the operating pressure needed by the lubricated system components. If the oil pressure is too high, the valve will release oil back into the sump which will cause the oil pressure to decrease.
The oil then flows to the external oil manifold which has provisions to mount the oil pressure transmitter and the oil pressure switch. These two sensors provide oil pressure readings and preset pressure level alarms for the working lubrication oil. From the oil manifold, the pressurized oil splits into two separate tubes. One tube is the pressure line (from transmission to freewheel assembly) line to the freewheel shaft assembly and the other one is the pressure line (from transmission to freewheel assembly) that connects to the pressure port of the freewheel support assembly. In the freewheel assembly the spent oil collects in the sump in the freewheel assembly casing. From the freewheel assembly sump, the oil then flows back to the sump of the transmission assembly through the return line (from freewheel to transmission).
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