Secondary oil system

A secondary oil system for a gas turbine engine includes a reservoir in a bearing sump of the engine and a mist tube extending across the top of the reservoir and connected at one end to a compressed air source and at the other end to a nozzle in a bearing sump. A shut-off valve responsive to primary system oil pressure normally blocks the mist tube but unblocks the latter when primary system pressure is low. The compressed air pressurizes the reservoir when the shut-off valve is open and forces oil through a pick-up tube to another orifice in the mist tube located at a venturi passage. Oil issuing into the mist tube mixes with high velocity air in the venturi passage to form a mist which is conducted by the mist tube to the nozzle from which it is directed at a rotor shaft bearing. The orifice at the venturi passage is at a lower elevation than the pressurizing orifice relative to the bottom of the reservoir and defines the normally filled oil level in the reservoir.

This invention was made in the course of work under a contract or 
subcontract with the U.S. Department of Defense. 
FIELD OF THE INVENTION 
This invention relates to secondary oil systems for aircraft propulsion gas 
turbine engines. 
BACKGROUND OF THE INVENTION 
For the purpose of providing limited flight capability after loss of oil 
pressure in a flight propulsion gas turbine engine, secondary oil systems 
have been proposed which release reserve oil supplies in response to loss 
or substantial reduction of oil pressure in a primary or main oil system 
of the engine. Also, secondary oil systems have been proposed in which 
reserve oil is dispersed as a mist to maximize lubrication and cooling. A 
secondary oil system according to this invention is a misting-type system 
having a particularly simple, self-contained structure for maximum 
reliability and minimum cost and weight. 
SUMMARY OF THE INVENTION 
This invention is a new and improved secondary oil system particularly for 
a flight propulsion gas turbine engine on an aircraft. The secondary oil 
system according to this invention includes a reservoir located in a 
bearing sump of the engine and filled with oil from the primary oil system 
when the primary system is active, a mist tube traversing the reservoir 
between a junction with a source of compressed air and a nozzle in the 
sump, a first orifice in the mist tube at a first elevation above the 
bottom of the reservoir, and a shut-off valve between the first orifice 
and the compressed air junction. When the primary oil system is active, 
the shut-off valve closes the mist tube and oil spills out of the 
reservoir through the first orifice so that the latter defines the 
normally filled oil level in the reservoir. The secondary oil system 
according to this invention further includes a second orifice in the mist 
tube at an elevation relative to the bottom of the reservoir above the 
first orifice and open to the interior of the reservoir, a venturi passage 
at the first orifice, and a check valve between the reservoir inlet and 
the primary oil system. When primary oil pressure is low, the shut-off 
valve opens and compressed air is admitted to the mist tube. The 
compressed air in the mist tube pressurizes the reservoir through the 
second orifice and mixes with oil from the first orifice at the venturi 
passage so that a mist is formed and transported by the mist tube to the 
nozzle. The nozzle directs a jet of the air/oil mist at the bearing for 
secondary lubrication and cooling.

DESCRIPTION OF A PREFERRED EMBODIMENT 
FIG. 1 is a schematic representation of a secondary oil system 10 according 
to this invention for a flight propulsion gas turbine engine on an 
aircraft. The secondary oil system includes a tank or reservoir 12 ideally 
located in a bearing sump near a bearing of the engine which is normally 
lubricated by the primary oil system of the engine and which, for the 
engine to sustain flight propulsion, requires secondary lubrication when 
primary lubrication is interrupted. The reservoir 12 is illustrated in an 
upright position corresponding to level flight of the aircraft and 
includes bottom 14 and a top 16. A duct 18 of the engine carries 
compressed air from a compressor of the engine, not shown, to a remote 
location and is representative of any convenient source of compressed air 
on the engine. 
The secondary oil system 10 further includes a mist tube 20 which traverses 
the reservoir near the top 16. A first end of the mist tube merges with 
the compressed air duct 18 at a compressed air junction 22. A second end, 
not shown, of the mist tube 20 is connected to a nozzle in a sump of the 
engine near a bearing for which secondary lubrication is required. 
The mist tube 20 has a restriction or venturi passage 24 therein and a 
first orifice 26 at the smallest diameter or throat of the venturi 
passage. The first orifice 26 affords communication between the venturi 
passage 24 and an upper end 28 of a pick-up tube 30. A lower end 32 of the 
pick-up tube is located near the bottom 14 of the reservoir. The mist tube 
has an arch 34 upstream of the first orifice 26, i.e. between the first 
orifice and the compressed air junction 22. A second orifice 36 in the 
mist tube is located on the downstream side of the arch 34 relative to the 
compressed air junction 22 and affords communication between the mist tube 
and the reservoir 12. The elevation of the nozzle is lower relative to the 
bottom 14 of the reservoir than the first orifice 26. 
The mist tube 20 traverses a housing 38 of a shut-off valve 40 of the 
secondary oil system. The housing 38 has a seat 42 which receives a 
plunger 44 of a valve element 46 in a closed condition of the shut-off 
valve. The mist tube is blocked by the plunger 44 in the closed condition 
of the shut-off valve. A spring 48 bears against a piston head 50 of the 
valve element and urges the plunger away from the seat 42. Separation of 
the plunger 44 from the seat 42 corresponds to an open condition of the 
shut-off valve wherein compressed air is admitted to the mist tube 20 from 
the duct 18. 
A duct 52 of the primary oil system of the engine which loops through the 
reservoir is connected to a chamber 54 in the housing 38 of the shut-off 
valve on the opposite side of the piston head 50 from the spring 48. When 
the engine is operating and the primary oil system is active, normal 
primary system oil pressure in the chamber 54 overcomes the spring 48 and 
maintains the shut-off valve in the closed condition. When the engine is 
not operating or the condition of the primary oil system is impaired to 
the extent that primary system oil pressure is low, the spring 48 actuates 
the shut-off valve to the open condition. 
A branch 56 of the duct 52 conducts primary system oil to an inlet check 
valve 58 having a housing 60 and a spool 62 slidable in the housing 
between an open position exposing an inlet orifice 64 in the housing and a 
closed position covering the inlet orifice. The spool 62 is exposed on one 
side to primary system oil pressure and on the other side to the pressure 
prevailing in the reservoir 12. When primary system oil pressure exceeds 
reservoir pressure, the spool 62 is in the open position and oil from the 
primary system fills the reservoir through the inlet orifice 64. When 
reservoir pressure exceeds primary system oil pressure, the spool is in 
the closed position preventing backflow of oil from the reservoir into the 
branch 56. 
The size of the inlet orifice 64 is coordinated with the size of the first 
orifice 26 such that when the primary oil system is active in normal 
fashion, oil spills from the reservoir through first orifice at the same 
rate that oil enters the reservoir through the inlet orifice. The 
elevation of the first orifice thus establishes the normal oil level in 
the reservoir. Oil spilling from first orifice drips into the bearing sump 
through the mist tube and is scavenged in the usual way. The peak of the 
arch 34 is above the first orifice to minimize the likelihood of oil 
collecting in the mist tube 20 near the shut-off valve 40. 
If primary system oil pressure becomes abnormally low while the engine is 
operating, the spring 48 actuates the shut-off valve to the open condition 
admitting compressed air to the mist tube 20. Compressed air flows through 
the mist tube to the nozzle at the second end of the latter. At the same 
time, the reservoir 12 is pressurized by compressed air through the second 
orifice 36. The pressure in the reservoir shifts the spool 62 in the inlet 
check valve 58 to the closed position and forces oil up through the 
pick-up tube and the first orifice 26 into the mist tube. The oil from the 
reservoir mixes with the compressed air in the mist tube to form an 
air/oil mist which is conducted by the mist tube to the nozzle and then 
directed at the bearing to sustain the latter until the reservoir 12 is 
drained. 
A physical realization 10, of the secondary oil system 10 is illustrated in 
FIGS. 2-6 and includes a housing 66 located in a bearing sump 68 of a gas 
turbine engine around a rotor shaft 70 of the latter. The housing has a 
wide, U-shaped lower part 72 near a first rotor shaft bearing 74, a 
relatively narrower inverted U-shaped upper part 76 between the first 
bearing 74 and a power take-off 78, and a cover 80 which closes an open 
side 82 of the housing. A circular opening 84 for the rotor shaft 70 is 
defined between the upper and lower parts 76,72. A plurality of mounting 
bosses 86 are defined on the housing and have a corresponding plurality of 
threaded holes 88 therein for bolting the housing to a structural portion, 
not shown, of the engine. The secondary oil system 10' sustains a second 
rotor shaft bearing 90 if the primary oil system of the engine becomes 
inactive while the engine is operating. 
A secondary oil reservoir or chamber 92 is defined in the housing 66 
between the cover 80 and a wall 94 of the housing. A first valve boss 96, 
FIG. 4, on the wall;94 projects into the reservoir 92 and has a bore 98 
therein which slidably receives a check valve spool, not shown. The check 
valve spool has an open position wherein an orifice, not shown, 
corresponding to the inlet orifice 64 in secondary oil system 10 described 
above is open and a closed position wherein the inlet orifice is blocked. 
An oil-in port 100 in the wall 94 of the housing 66 is connected through 
internal passages, not shown, in the housing to the bore 98 in the first 
valve boss 96. The oil-in port 100 is also connected by piping, not shown, 
to the primary oil system of the engine such that whenever the primary 
system is active, oil at primary system pressure biases the check valve 
spool in the bore 98 to its open position and fills the secondary oil 
reservoir 92 through the orifice in the first valve boss. Excess primary 
system oil returns to the primary oil system through an oil-out port 102, 
FIG. 3. 
As seen best in FIGS. 3-5, a second valve boss 104 on the housing 66 has a 
stepped bore 106 therein closed at one end by a plug 108. A seat 110 at 
the other end of the stepped bore separates the stepped bore from an inlet 
chamber 112 to a mist tube 114. The mist tube is cast integrally into the 
housing 66 and includes an arch 116 over the upper part 76 of the housing, 
FIGS. 3-4, and a lower extension 118, FIG. 4, behind the wall 94. 
A partially illustrated internal passage 120 in the lower part 72 
intersects the small end of the stepped bore 106 on the other side of the 
seat 110 from the inlet chamber 112. The passage 120 connects the small 
end of the stepped bore 106 to an air-in port 122, FIG. 3, on the housing 
66 which is connected by piping, not shown, to the compressor of the 
engine so that the passage 120 is supplied with compressed air whenever 
the engine is in operation. An air-out port 124, FIG. 4, on the housing is 
connected to the air-in port and to additional piping, not shown, for 
conducting compressed air to other locations in the engine. 
A stem 126 of a shut-off valve 128 is slidably disposed in the small end of 
the stepped bore 106 and a piston 130 integral with the stem 126 is 
slidably disposed in the big end of the stepped bore. The shut-off valve 
has a closed position, FIG. 5, in which the stem is seated on the seat 110 
and an open position, not shown, off the seat in which the passage 120 
communicates with the inlet chamber 112. The shut-off valve is biased to 
its open position by a spring 132 and is urged to its closed position by 
oil at primary system pressure introduced into the big end of the stepped 
bore 106 outboard of the piston 130 through a passage 134 in the housing 
66 connected to the oil-in port 100. 
As seen best in FIGS. 3, 4 and 6, a mist-out port 136 in a depression in 
the wall 94 of the housing 66 defines the opposite terminal end of the 
mist tube 114 from the inlet chamber 112. A schematically illustrated 
manifold 138, FIG. 2, is connected to the mist-out port 136 and terminates 
at a representative secondary oil system nozzle 140 aimed at the second 
bearing 90. 
Between the arch 116 and the lower extension 118 of the mist tube 114, the 
latter traverses vertically a third boss 142 on the housing 66. A bore 144 
extends into the third boss 142 from the open side of the housing and 
intersects the mist tube 114 between the arch 116 and the lower extension 
118. The bore 144 is further intersected by an aperture 146, FIG. 6, in 
which is fitted an upper end 148 of a pick-up tube 150. A lower end 152, 
FIG. 3 of the pick-up tube is located in the reservoir 92 near the bottom 
thereof. 
An insert 154 is mounted in the bore 144 with a pair of seals 156 on 
opposite sides of the mist tube. The insert has a small diameter throat 
which defines a venturi passage 158 in the mist tube. A first orifice 160 
in the insert 154 connects the venturi passage 158 and the aperture 144 so 
that the pick-up tube communicates with the mist tube at the venturi 
passage. A strainer 162 on the insert 154 prevents contamination of the 
first orifice 160. 
As seen best in FIGS. 3-4, a second orifice 164 is defined in the mist tube 
114 on the opposite side of the arch 116 from the inlet chamber 112. The 
second orifice opens into the reservoir 92 between the wall 94 and the 
cover 80. The second orifice 164 is at a higher elevation than the first 
orifice 160 relative to the bottom of the reservoir. 
In operation, when the gas turbine engine is operating normally, oil at 
primary system pressure holds the shut-off valve 128 in its closed 
position preventing compressed air from entering the mist tube inlet 
chamber 112. Concurrently, oil at primary system pressure holds the check 
valve spool in the bore 98 in its open position so that oil from the 
primary system fills the secondary oil reservoir 92. The oil level in the 
reservoir 92 rises until it achieves the level of the first orifice 160. 
Then, oil then spills out of the first orifice 160 into the lower 
extension 118 of the mist tube and out of the mist-out port 136 because 
the latter is at a lower elevation than the first orifice. The spill-over 
drips into the sump 68 from the secondary oil system nozzle at the end of 
the manifold 138 and is scavenged from the sump in the usual manner. The 
elevation of the first orifice, therefore, defines the normal oil level in 
the secondary oil reservoir 92. 
If oil pressure in the primary oil system becomes abnormally low, for 
example when the integrity of the primary oil system is breached, the 
spring 132 shifts the shut-off valve to its open position admitting 
compressed air to the inlet chamber 112 and to the mist tube 114. The 
compressed air flows in the mist tube to the venturi passage 158 and 
through the second orifice 164 into the reservoir 92 above the oil level 
therein. The compressed air pressurizes the reservoir so that a pressure 
gradient develops across the check valve spool in the bore 98 which shifts 
the spool to its closed position preventing backflow of oil out of the 
reservoir into the primary oil system. The air pressure on the oil in the 
reservoir also forces oil up the pick-up tube 150 and through the first 
orifice 160 into the venturi passage 158. 
The oil issuing from the first orifice 160 mixes with high velocity air in 
the venturi passage 158 and to form a mist which is transported by the 
lower extension 118 of the mist tube to the mist-out port 136 and into the 
manifold 138. The secondary oil system nozzle 140 directs the mist onto 
the second bearing 90 for sustaining operation of the latter until the 
supply of oil in the reservoir 92 is depleted.