GB2225817A

GB2225817A - Lubricating a turbojet engine

Info

Publication number: GB2225817A
Application number: GB8925209A
Authority: GB
Inventors: Jerome M Friedel; Michel G Hugues; Michel S P Lemonnier
Original assignee: Societe Nationale dEtude et de Construction de Moteurs dAviation SNECMA; SNECMA SAS
Current assignee: Safran Aircraft Engines SAS
Priority date: 1988-11-09
Filing date: 1989-11-08
Publication date: 1990-06-13
Anticipated expiration: 2009-11-08
Also published as: GB2225817B; GB8925209D0; FR2638782B1; FR2638782A1

Abstract

A lubrication circuit for the bearings of a turbojet aero-engine includes an auxiliary oil feed (11) for lubricating the bearings in negative g flight conditions and preferably also in the event of a breakdown in the main oil feed. The auxiliary oil feed (11) connects the oil tank (1) to an auxiliary spray nozzle (12) in the bearing enclosure (4) and is operative to deliver oil to the bearing enclosure when required by pressurised air introduced and held in the tank by pressurisation means associated with the tank. The pressurised air may be supplied by the oil recovery pumps (5) of the main oil feed when the aircraft is inverted, or by a bleed from the compressor of the turbojet engine. <IMAGE>

Description

BEARING LUBRICATION CIRCUIT FOR A TURBOJET ENGINE The invention relates to a lubrication circuit for the bearings of a turbojet aero-engine.

The arrangement of such a lubrication circuit currently in use is illustrated diagrammatically in Figure 1 of the accompanying drawings. In this circuit a proportioning pump 2 feeds oil from a tank 1 to main spray nozzles 3 situated in a bearing enclosure 4. A recovery pump 5 provides for the return flow of oil from the enclosure 4 to the tank 1. The feed pump 2, which operates at high pressure and low rate of flow, draws oil from the bottom of the tank 1 under normal flight conditions (top and bottom always referring to the position occupied in normal flight conditions) and delivers the oil under pressure to the spray nozzles 3 in the bearing enclosure 4. The recovery pump 5, on the other hand, operates at low pressure and a high rate of flow as it has to recover a more or less emulsified oil at the bottom of the bearing enclosure 4 and return it back to the tank 1.In order to prevent the occurrence of excess pressure in the tank 1 during operation, the tank is normally provided with a ventilation system. This system comprises a pressure relief valve 6 which is set, for example, at 0.2 bax and which vents air from the top of the tank 1, thus maintaining a slight pressure at the inlet of the feed pump 2 and thus preventing any loss of performance which could occur, particularly during flight at high altitudes. The pressure relief valve 6 opens within a casing 7 which supports the tank 1 and other equipment of the engine. The casing 7 is fitted with an oil separator 8 opening to the atmosphere 9, and with a return duct provided with a recovery pump 10 for returning the separated oil to the tank 1. The casing also communicates with the bearing enclosure 4 via the tube 7a.

This lubrication circuit is satisfactory under normal flight conditions, but it does not cope with the problems posed by operation under negative g flight conditions, especially in inverted flight, or when there is a breakdown in the oil supply, such as when the feed pump seizes for example.

Several solutions have been proposed in an attempt to overcome these problems, as it is often essential to maintain at least a minimum lubrication of certain vital parts of the engine under all operation conditions.

Indeed, if lubrication of the bearings should stop, for whatever reason, the inevitable result would be deterioration of parts, and possibly an accident.

For this purpose, US Patent No. 4 424 665 describes a lubrication system combining a main circuit with a secondary circuit which includes a second feed pump. A similar solution is also described, for example, in French Patent No. 2 536 120.

French Patent No. 2 482 658 proposes the addition of an emergency oil mist lubrication arrangement which includes an auxiliary emergency tank.

French Patent No. 2 491 141 proposes the use of an oil suction device as a second feed pump associated with an additional circuit.

Other solutions propose special modifications to the oil tank, such as the provision of inner partitioning intended to prevent the unpriming of the feed pump in negative g flight conditions. French Patent No. 2 466 612, for example, describes a solution of this type.

None of these known solutions, however, has been entirely satisfactory, either from the point of view of mass, or from the point of view of overall size, these being important factors for such aircraft equipment as this.

The invention seeks to solve the problems discussed above, without recourse to means that are complex or would unduly weigh down the lubricating system.

Accordingly, the invention provides a lubrication circuit for the bearings of a turbojet aero-engine in which oil is fed by a feed pump from an oil tank to main spray nozzles in a bearing enclosure and is returned from the bearing enclosure to the tank by a recovery pump, the tank having a preset pressure relief valve for venting air from the top of the tank (top and bottom being defined relative to the orientation of the tank during normal flight conditions), characterized in that the lubrication circuit includes an auxiliary oil feed directly connecting the tank to the bearing enclosure and capable of feeding oil to at least one auxiliary spray nozzle in the bearing enclosure by means of pressurised air introduced and held in the tank by pressurising means associated with the tank, without the addition of an extra pump or a pressure source externally of the turbojet engine.

Depending on the particular applications envisaged and particularly according to the degree of permeability to air of the bearing enclosure, the pressurising of the tank for the auxiliary oil feed may be implemented in different ways which take particular needs into account.

In the case of a bearing enclosure which is sufficiently permeable to air, the auxiliary oil feed pressurising means may advantageously comprise the oil recovery pump as the source of the pressurised air. In order to operate under negative g flight conditions, in which the oil in the tank collects at the top of the tank and the air received from the bearing enclosure and compressed by the recovery pump collects at the bottom of the tank, the auxiliary oil feed is arranged to take oil from the top of the tank under the action of the compressed air at the bottom. To prevent the oil contained in the tank escaping too rapidly through the pressure relief valve situated near the top of the tank, a throttling restriction may be connected in series with this valve to slow down the passage of oil therethrough during negative g flight without slowing the passage of air during normal flight.To compensate for this, an auxiliary pressure relief valve is preferably connected to the tank to allow the escape of excess air during negative g flight, the auxiliary relief valve being set at a pressure higher than the main relief valve.

Advantageously a venturi device may be incorporated in the auxiliary oil feed, being arranged to aspirate the oil and atomize it using the pressurised air in the bottom of the tank under negative g flight conditions.

Instead of throttling the pressure relief valve to restrict the escape of oil under negative g flight conditions, the relief valve may be connected in series with a control slide valve which is connected to the main oil feed at a point situated downstream of the feed pump and is operative to shut off the connection between the tank and the pressure relief valve in the event of a pressure drop downstream of the feed pump.

In a circuit required to cope with both the conditions of negative g flight and a breakdown in the main oil feed, the auxiliary oil feed may have two oil inlets, one at the top of the tank for negative g flight and the other at the bottom of the tank in the event of a breakdown in the main oil feed, the auxiliary feed including a threeway solenoid valve for selectively connecting either of the two inlets to the auxiliary spray nozzles in the bearing enclosure. Normally the top inlet will be connected.

In the case of engines in which the bearing enclosure is not very permeable to air, on the other hand, the auxiliary oil feed pressurisation means preferably comprises a bleed from the compressor of the turbojet engine as the source of the pressurised air, and the auxiliary oil feed may be operative to feed oil to the bearing enclosure either in negative g flight conditions or in the event of a breakdown in the main oil feed.

In this case the bleed is preferably controlled by a slide valve in dependence upon the delivery pressure of the feed pump at a point situated downstream of the feed pump.

A number of embodiments of the invention will now be described by way of example, with reference to the accompanying diagrammatic drawings, in which:- Figure 1 (described earlier) depicts a known arrangement of a lubrication circuit for the bearings of a turbojet aero-engine; Figure 2 is a diagram of a first example of a lubrication circuit in accordance with the invention capable of operating under negative g flight conditions; Figure 2a is a diagram showing the operation of the circuit of Figure 2 during inverted flight; Figure 3 is a diagram of a second example of a lubrication circuit in accordance with the invention; Figure 3a is a section illustrating the construction of a slide valve utilised in the circuit shown in Figure 3;; Figure 4 is a diagram of an example of a lubrication circuit similar to that shown in Figure 3 arranged to operate under negative g flight conditions and in the event of a breakdown of the main oil feed; Figure 4a is a scrap view showing a detail of the lubrication circuit shown in Figure 4 in a configuration adopted for emergency operation in the event of a main oil feed breakdown; Figure 4b is a scrap view showing an alternative form of a detail of the lubrication circuit shown in Figure 4; Figures 5a,5b, and Sc are similar diagrams of a third example of a lubrication circuit in accordance with the invention, respectively showing the operation of the circuit in normal flight conditions, in inverted flight conditions, and in emergency conditions in the event of a main oil feed breakdown;; Figure 5d is a section showing the construction of a component of the circuit shown in Figures 5a,5b and 5c; Figure 6 is a diagram of a modified form of the bearing lubrication circuit shown in Figures 5a,5b and 5c; and, Figure 6a is a section showing the construction of a component of the circuit shown in Figure 6.

In the following description, the terms "top" and "bottom" are used with reference to the orientation of the lubrication circuit under normal flight conditions of the aircraft.

The bearing lubrication circuit for a turbojet engine represented in Figure 2 is similar to the known circuit previously described with reference to Figure 1, in that it comprises a single oil tank 1, a feed pump 2, main spray nozzles 3 in a bearing enclosure 4, an oil recovery pump 5, and a pressure valve 6 situated at the top of the tank 1 and opening into a support casing 7 in which the oil tank 1 is located. As in the known circuit, a communication 7a is established between the casing 7 and the bearing enclosure 4, and the casing 7 is fitted with an oil separator 8 having a connection 9 to the atmosphere and with an oil return duct including a recovery pump 10 between the casing 7 and the tank 1.

The features provided in accordance with the invention in the circuit of Figure 2 comprise an auxiliary oil feed 11 from the tank 1 to at least one auxiliary nozzle 12 in the bearing enclosure 4. At its tank end the auxiliary oil feed 11 includes a .venturi 13 and a suction tube 14.

In addition, an auxiliary pressure valve 15 is provided near the bottom of the tank 1, the auxiliary valve 15 being set at a pressure higher than that of the top valve 6. In a typical embodiment, if the top valve 6 is set at 0.2 bar, the auxiliary valve 15 may be set at 0.7 bar.

These settings are particularly suitable in a situation where the bearing enclosure 4 is not very air- tight and includes, for example, seals of the labyrinth type.

This arrangement of the lubrication circuit ensures lubrication of the bearings in negative g flight conditions, especially in inverted flight. The operation is then as follows. As shown diagrammatically in Figure 2a, which shows the circuit under inverted flight conditions, the oil collects at the top part of the tank 1 and, accordingly, feeds the venturi 13 through the tube 14. In these conditions the venturi 13 is no longer immersed in the oil and takes air from the bottom part of the tank 1, while pressurisation of the tank 1 and replenishment of the volume of air are ensured by the recovery pump 5 which draws in air from the bottom part of the bearing enclosure 4.A pressure P2 is thus formed in the tank 1 which is higher than the pressure P1 in the bearing enclosure 4, enabling the auxiliary oil feed 11 to dispense an oil mist in aerosol form from the venturi 13 to the bearings in the enclosure 4 through the auxiliary nozzle 12. The pressure valve 6 is associated with a restriction 6a of small section which limits the flow of oil from the tank 1 through the valve 6 to the casing 7 during the period of negative g flight.

Any excessive rise of pressure in the tank 1 is prevented by means of the auxliary valve 15.

In normal flight conditions, the venturi 13 is immersed in the oil and the end of the tube 14 is outside the oil as shown in Figure 2. In these conditions the auxiliary oil feed 11 is not operative because the pressure in the tank 1, set for example at 0.2 bar by the pressure valve 6, is lower than that in the bearing enclosure 4.

The lubrication circuit in accordance with the invention which has just been described has the advantage of a particularly simple auxiliary oil feed system which operates entirely automatically and the implementation of which does not require the use of any moving mechanical parts. Furthermore, while providing adequate minimum lubrication under inverted flight conditions, the system also has the advantage of low oil consumption afforded by distribution as an oiled air mist.

In the second example shown in Figure 3, as in the case of the first example described with reference to Figures 2 and 2a, the lubrication circuit is designed for a bearing enclosure 4 which is fairly permeable to air, and the pressurisation of the tank 1 is ensured by the recovery pumps 5 which are capable of delivering a high volume of air.

The pressure obtained is sufficient, taking into account the respective positions of the tank 1 and the bearing enclosure 4, being situated for example one above the other, for simple ducting to form the auxiliary oil feed 11 to the auxiliary nozzles 12 for dispensing oil to the bearings of the enclosure 4 in negative g flight conditions. Compared with the first example shown in Figures 2 and 2a, the venturi 13 has been omitted. In this case, however, the pressure valve 6 at the top of the tank 1 is associated with a slide valve 20 mounted in series and controlled by the delivery pressure of the feed pump 2 by means of a connection at point 20a of the main feed circuit. In operation under inverted flight conditions, this pressure drops, causing the slide valve 20 to close.

Figure 3a illustrates the construction of the slide valve 20 by means of which a connection is normally maintained between the top of the tank 1 and the pressure valve 6 as a result of the balance of forces between that exerted on the smaller diameter end 20b of the slide by the pressure taken at point 20a from the delivery of the feed pump 2 and that exerted by the action of a spring 20d on the larger diameter end 20c of the slide.

An auxiliary pressure valve 15 is provided at the bottom of the tank 1 as in the Figure 2 example, and is set for example at 0.7 bar to ensure sufficient pressure during operation under inverted flight conditions to force oil through the auxiliary feed 11 and nozzles 12, while preventing the development of harmful excess pressures in the tank 1. This example ensures satisfactory operation of the lubrication circuit under negative g flight conditions, especially in inverted flight, and entails only a minimum increase of mass compared to the known lubrication circuit usually used.

In a modification of the second example shown in Figure 4, the auxiliary oil feed passage 11 for the lubrication of the bearing enclosure 4 is fitted with a three-way solenoid valve 21 connected to a line 21a from the top of the tank 1, to a line 21k from the bottom of the tank, and to a line 21c leading to the auxiliary nozzles 12 of the bearing enclosure 4. In addition, the auxiliary pressure valve 15 communicates via two ducts 22 and 23 respectively with the top and bottom of the tank 1, each of these ducts including a throttling restriction 22a,23a.

In normal operation the tank 1 is pressurised by the recovery pumps 5 under the control of the top pressure valve 6, and oil is delivered to the bearings of the enclosure 4 by the feed pump 2 and the main spray nozzles 3. The solenoid valve 21 is de-energised and adopts the position shown in Figure 4 in which the lines 21a and 21c are connected and only air flows through them.

If a breakdown occurs in the normal oil delivery system, for example a seizure of the feed pump 2, the delivery pressure of the pump 2 drops and the slide of the valve 20 moves to assume the position shown in Figure 3a, thus closing the passage to the pressure valve 6. The solenoid valve 21 is energised into the emergency position in which the line 21b is connected to the line 21c as shown in Figure 4a. This permits oil to be delivered from the bottom of the tank 1 to the auxiliary nozzles 12 via the lines 21b and 21c under the action of the air pressure created in the tank 1 by the operation of the recovery pumps 5 and the isolation of the pressure valve 6, thus effecting an adequate minimum lubrication of the bearings in the enclosure 4. The auxiliary pressure valve 15 normally remains closed but will operate to prevent any excess or accidental rise of pressure in the tank 1.The restrictions 22a and 23a are identical and, consequently, the passage of air through the restriction 22a takes precedence when excess pressure causes the valve 15 to open.

When the lubrication circuit experiences negative g flight conditions, and especially inverted flight, the feed pump 2 ceases to be supplied and the delivery pressure at point 20a accordingly drops. The slide of the valve 20 thus assumes the position shown in Figure 3a, closing the passage to the pressure valve 6 and preventing any leakage of the oil which has moved to the top part of the tank 1. The solenoid valve remains in its de-energised position shown in Figure 4, but in this case the connected lines 21a and 21 supply oil from the top of the tank 1 to the auxiliary nozzles 12 under the action of the pressure created in the bottom of the tank 1 by the inflow of air delivered from the bearing enclosure 4 by the recovery pumps 5.Any excess pressure in the tank 1 is again prevented by opening of the auxiliary pressure valve 15 as the need arises. As before, the passage of air takes precedence, but this time through the restriction 23a. It will be appreciated that the role of the restrictions 22a and 23a in the ducts 22 and 23 respectively is to prevent an accidental emptying of the tank 1. For this purpose, these restrictions provide little resistance to the flow of air, but are able to brake the flow of oil strongly, should the need arise.

In an alternative arrangement shown in Figure 4b, the restrictions 22a and 23a are replaced by a two-way valve 24, for example of the ball type, disposed between the ducts 22,23 on the one hand, and the auxiliary pressure valve 15 on the other hand. In this case the ball 24a prevents any rise of oil towards the auxiliary valve 15.

If, in contrast to the previously described examples, the bearing enclosure is not very permeable to air, such as when sealing is provided by graphite seals, a third example of the lubrication circuit in accordance with the invention may be used as shown in Figures 5a,5b and 5c.

In this case a different arrangement for pressurising the oil. tank 1 is used, involving a connection between a bleed point 30 in the compressor of the turbojet engine (not shown) and the oil tank 1. Included in the pressurised air supply duct 31, which communicates with the bottom of the tank 1 by means of a duct 31a, is a controlled slide valve 32, shown in greater detail in Figure 5d. The valve 32 is controlled by the delivery pressure of the oil feed pump 2 of the lubrication circuit by means of a passage 33 containing a diaphragm between the valve 32 and the main oil feed path to the bearing enclosure 40. An oil drain device 31b is disposed in the duct 31 between the valve 32 and the air bleed point 30 to prevent the rise of oil to the compressor.

The oil tank is connected to the bearing enclosure 40 by an auxiliary oil feed 11 comprising a three-way solenoid valve 21 connected to a line 21a from the top of the tank 1, to a line 21b from the bottom of the tank 1 and comprising a duct 34 which is connected to the pressurised air duct 31 and which includes a venturi 35 arranged to suck oil from the bottom of the tank through a duct 36, and to a line 21c leading to the auxiliary nozzles 12 in the bearing enclosure 40. As in known circuits, the tank 1 is disposed in a support casing 7, and the top of the tank is provided with a pressure relief valve 6 which opens into the casing 7.

Figure 5a shows the lubrication circuit of this example during operation in normal flight conditions. In these conditions oil is delivered to the bearings of the enclosure 40 by the feed pump 2 and the main spray nozzles 3, and the slide valve 32 controlled by the delivery pressure of the pump 2 is operative to close the compressed air supply duct 31. The solenoid valve 21 is in its de-energised, rest position connecting the lines 21a and 21c and allows the little air introduced into the tank 1 by the recovery pumps 5 to be exhausted to the auxiliary nozzles 12 in the bearing enclosure 40. Any excessive rise in pressure which might accidentally occur in the tank 1 is prevented by the action of the pressure valve 6 at the top of the tank 1.

When the aircraft goes into inverted flight the operation of the lubrication circuit, which adopts the position shown in Figure 5b, is as follows. The oil moves to the top of the tank 1 and occupies the space which was previously occupied by air. The feed pump 2 is no longer able to deliver oil and the pressure downstream of the pump 2 drops, especially in the control duct 33 for the valve 32 which, under the action of a spring 32a on the valve slide 32b, then assumes the position shown in Figure 5d to open the compressed air feed passage 31.

The pressurised air then allowed to flow from the turbojet compressor thus enters the tank 1 via the duct 31a and, as a result of the pressurisation of the tank 1, oil flows through the auxiliary oil feed 11 to the auxiliary nozzles 12 to ensure the lubrication of the bearings in the enclosure 40.

The differential pressure acting on the larger end 32c of the slide of the valve 32 acts to regulate the air pressure admitted from the compressor into the tank 1.

In the event of excess pressure developing in the tank 1, the valve 32 closes.

If a breakdown of the main oil feed occurs under normal flight conditions, for example as a result of seizure of the feed pump 2, breakage of the pump drive, or obstruction of the main feed path, the circuit operates as follows with reference to Figure Sc. The oil pressure downstream of the pump 2 drops, and the valve 32 accordingly opens the pressurised air supply passage 31.

The solenoid valve 21 is energised to move to the emergency position connecting the line 21b and the line 21c of the auxiliary oil feed 11.

The pressurised air supplied by the passage 31 pressurises the tank 1 via the duct 31a, and also feeds the duct 34 via the venturi device 35. The venturi sucks oil from the tank 1 through the duct 36 and mixes it with the air flowing in the duct 34 from the passage 31 so that the auxiliary oil feed 11 delivers a mixture of air and oil through the solenoid valve 21 to the auxiliary nozzles 12 to ensure the lubrication of the bearings in the enclosure 40.

In accordance with the invention, common means, both for tank pressurisation (controlled valve, bleeding pressurised air from the compressor) and for feeding the oil (auxiliary oil feed, three-way solenoid valve), ensure both the safety of minimum lubrication of the bearings in the event of breakdown in the main oil feed and sufficient lubrication of the bearings during inverted flight.Features of the embodiments described earlier for cases where the invention is applied to bearing enclosures which are fairly permeable to air, and in which tank pressurisation is ensured by the oil recovery pumps, may also be entertained in situations where, as in the embodiment described with reference to Figures 5a,5b and 5c, the bearing enclosure is not very permeable to air and where, consequently, tank pressurisation is dependent on an additional pressurised air supply bled from the turbojet compressor. In particular, as in the second embodiment shown in Figure 4, the solenoid valve 21 may be connected directly to the top and bottom of the tank 1.Similarly, an auxiliary pressure valve 15 may be incorporated, communicating with the top and bottom of the tank 1 by ducts 22 and 23 respectively, each duct including a throttling restriction 22a,23a as in the Figure 4 embodiment or being assciated with a two-way valve 24 as shown in Figure 4b. An example of a lubrication circuit comprising these elements and suitable for situations where the bearing enclosure 40 is not very permeable to air is shown in Figure 6.

In this example the controlled slide valve 42 of the pressurised air supply is also connected to the pressure valve 6. Thus, in the event of a breakdown in the main oil feed, as well as under negative g flight conditions, the slide 42a of the valve 42 adopts the position shown in detail in Figure 6a in which it closes the connection to the pressure valve 6 and regulates the pressure of air supplied to the tank 1 from the compressor bleed to a pre-defined value which may be, for example, 0.5 bar.

In addition, a venturi 35a is disposed at the connection of the outflow line 21c from the solenoid valve 21 to a duct connected to the pressurised air supply passage 31, the venturi 35a being arranged to assist the feed of oil to the bearing enclosure 40 both in negative g flight conditions and in the event of a breakdown in the main oil feed. Also, the line 21a from the solenoid valve 21 to the top of the tank may be provided with a throttle llb.

Claims

1. A lubrication circuit for the bearings of a turbojet aero-engine in which oil is fed by a feed pump from an oil tank to main spray nozzles in a bearing enclosure and is returned from the bearing enclosure to the tank by a recovery pump, the tank having a preset pressure relief valve for venting air from the top of the tank (top and bottom being defined relative to the orientation of the tank during normal flight conditions), characterised in that the lubrication circuit includes an auxiliary oil feed directly connecting the tank to the bearing enclosure and capable of feeding oil to at least one auxiliary spray nozzle in the bearing enclosure by means of pressurised air introduced and held in the tank by pressurising means associated with the tank, without the addition of an extra pump or a pressure source externally of the turbojet engine.

2. A bearing lubrication circuit according to claim 1, in which the auxiliary oil feed pressurising means comprises the oil recovery pump as the source of the pressurised air, and the auxiliary oil feed is operative to feed oil to the bearing enclosure under negative g flight conditions.

3. A bearing lubrication circuit according to claim 1, in which the auxiliary oil feed pressurising means comprises a bleed from the compressor of the turbojet engine as the source of the pressurised air, and the auxiliary oil feed is operative to feed oil to the bearing enclosure either under negative g flight conditions or in the event of a breakdown in the main oil feed.

4. A bearing lubrication circuit according to claim 3, in which the bleed is controlled by a slide valve in dependence upon the delivery pressure of the feed pump at a point situated downstream of the feed pump.

5. A bearing lubrication circuit according to any one of claims 1 to 4, in which the pressure relief valve is connected in series with a device for limiting flow through the valve so as to retain the pressure in the tank.

6. A bearing lubrication circuit according to claim 5, in which the flow limiting device comprises a throttling section operative to limit the escape of oil from the tank through the pressure relief valve in negative g flight conditions.

7. A bearing lubrication circuit according to claim 5, in which the flow limiting device comprises a control slide valve which is connected to the main oil feed at a point situated downstream of the feed pump and is operative to shut off the connection between the tank and the pressure relief valve in the event of a pressure drop downstream of the feed pump.

8. A bearing lubrication circuit according to claim 4, in which the slide valve is also connected to the pressure relief valve so as to shut off the connection between the tank and the pressure relief valve when the delivery pressure drops at the point downstream of the feed pump.

9. A bearing lubrication circuit according to any one of claims 1 to 8, in which an auxiliary pressure relief valve set at a pressure greater than that of the first pressure relief is also connected to the tank.

10. A bearing lubrication circuit according to claim 9, in which the auxiliary pressure relief valve communicates with the tank at a point situated near the bottom of the tank so as to vent excess air pressure from the tank in negative g flight conditions.

11. A bearing lubrication circuit according to claim 9, in which the auxiliary pressure relief valve communicates with the tank at a point situated near the top of the tank so as to vent excess air pressure from the tank during operation of the auxiliary oil feed in the event of a breakdown in the main oil feed.

12. A bearing lubrication circuit according to claim 9, in which the auxiliary pressure relief valve communicates with the tank both at a point situated near the top of the tank and at a point situated near the bottom of the tank, and is associated with means which privileges the passage of air to the valve rather than the passage of oil.

13. A bearing lubrication circuit according to claim 12, in which the means which privileges the passage of air to the auxiliary pressure relief valve comprises a pair of throttling restrictions disposed one in the communication to the top of the tank and the other in the communication to the bottom of the tank.

14. A bearing lubrication circuit according to claim 12, in which the means which privileges the passage of air to the auxiliary pressure relief valve comprises a two-way ball valve

15. A bearing lubrication circuit according to any one of claims 1 to 14, in which the auxiliary oil feed comprises a duct connected to the tank at a point near the top of the tank so as to receive oil for the lubrication of the bearing enclosure in negative g flight conditions.

16. A bearing lubrication circuit according to any one of the preceding claims, in which the auxiliary oil feed comprises a duct connected to the tank at a point near the bottom of the tank so as to receive oil for the lubrication of the bearing enclosure in the event of a breakdown in the main oil feed.

17. A bearing lubrication circuit according to claim 16 when dependent on claim 15, in which the auxiliary oil feed includes a three-way solenoid valve for selectively connecting either of the two auxiliary oil feed ducts to the auxiliary spray nozzles in the bearing enclosure.

18. A bearing lubrication circuit according to claim 17, in which the auxiliary oil feed duct between the solenoid valve and the top of the tank is fitted with a throttle.

19. A bearing lubrication circuit according to any one of the preceding claims, in which the auxiliary oil feed includes a venturi device whereby oil for lubrication of the bearing enclosure is supplied in the form of an oil mist.

20. A bearing lubrication circuit according to claim 19, in which the venturi device communicates with the tank at a point near the top of the tank so as to receive oil for lubrication in negative g flight conditions.

21. A bearing lubrication circuit according to claim 19, in which the venturi device communicates with the tank at a point near the bottom of the tank so as to receive oil for lubrication in the event of a breakdown in the main oil feed.

22. A bearing lubrication circuit according to claim 19 when dependent on claim 17, in which the venturi device is situated downstream of the three-way solenoid valve and is supplied with pressurised air by the tank pressurising means.

23. A bearing lubrication circuit according to any one of the preceding claims, in which the oil tank is mounted in a casing supporting equipment of the turbojet engine, the casing including a vent to the atmosphere fitted with an cil separator, a duct connecting the casing to the bearing enclosure, and an oil return duct fitted with a recovery pump connecting the casing to the tank, the said pressure relief valve and the auxiliary pressure relief valve (when present) opening into the casing.

24. A bearing lubrication circuit according to claim 1, substantially as described with reference to Figures 2 and 2a, Figure 3 and 3a, Figures 4,4a and 4b, Figures 5a to 5d, or Figures 6 and 6a of the accompanying drawings.