FR2694602A1 - Mechanical safety system for the interruption of oil circulation in a screw compressor. - Google Patents

Abstract

Gas compressor with two worm gear rotors (22) which are engaged. A valve (41) mounted in the discharge pipe cuts off the back flow of gas when the compressor is stopped and then simultaneously a distributor (56) mounted between a zone (62) which is at the discharge pressure and a zone (60) which is at the suction pressure cuts off the oil injection on the rotors using the ambient conditions prevailing inside the compressor and which inherently exist at this moment. The same conditions are used when restarting the compressor to restore oil circulation. Application in particular to compressors of refrigeration circuits.

Description

The present invention relates generally to the field of

compression of a gas in a

  rotary screw compressor with oil injection.

  The invention relates more particularly to a mechanism for isolating the lubrication passages of the rotor bearings and the injection orifice from their supply.

  of oil, which opens into the working chamber of the compres-

at the judgment of the latter.

  Worm compressors used in refrigeration systems consist of complementary male and female worm rotors arranged in a working chamber in a cage. The working chamber can be characterized as a volume whose The general form is that of parallel cylindrical bores which intersect and whose tolerances are close to the outer dimensions of the length and the diameter of the two worm rotors meshing with each other. low-pressure end and a high pressure end which have valveless suction and discharge ports which

  are in direct communication with the working room.

  During operation, refrigerant gas that is at the suction pressure enters the working chamber through the corresponding orifice and is wrapped in a chevron-shaped pouch formed between the worm rotors rotating in opposite directions. pocket closes, its volume contracts and is displaced towards the high-pressure end of the compressor while the rotors

  rotate while meshing inside the working chamber.

  The gas in such a pocket is compressed by decreasing the volume in which it is located until the pocket opens on the discharge port at the high pressure end of the working chamber. at

  which it is expelled by this orifice.

  Given the extremely close tolerances between the two rotors and the walls of the working chamber, the arrangement of the bearings in which the rotors are mounted is decisive for the operation and life of the compressor this is particularly true because the bearings and Rotors of a worm gear compressor are subject to high and variable axial and radial loads The protection and lubrication of rotor bearings is therefore an essential part of the design and operation of rotary worm compressors. The oil is not only sent into the bearings of the rotors, but moreover, in many cases, it is injected into the working chamber of a worm compressor through a corresponding orifice in order to assume several functions. , the oil injected into the working chamber assumes the function of a seal between the rotors and the surfaces of the chamber

  in which they are arranged.

  The oil also forms a lubricant between the rotor leading screwless screw and the one being driven In this respect, one of the rotors is controlled by an external source such as an electric motor, while the other rotor is

  driven as a result of its meshing with the previous one.

  The oil injected into the working chamber of the compressor therefore has the function of preventing excessive wear between

rotors leading and led.

  Finally, the injected oil is used to

  cool the refrigerant undergoing compression to the

  of the working chamber, so as to reduce the thermal expansion of the rotors which would otherwise occur

  following the heat generated by the compression process.

  This cooling by injection therefore allows from the

  start to tighten the clearance between the rotor and the cage.

  When the compressor stops, when the control motor is shut down, if the backflow of gas at the discharge pressure is allowed to occur, on the high pressure side (downstream side) of the refrigeration system in which a compressor The worm gear is used, through the discharge port of the latter, to cause the rotation of the rotors which are no longer controlled inside the working chamber in the opposite direction and at a high speed, and has the consequence that the compressor operates as a regulator by

  relative to the gas downstream of the discharge orifice

  This freewheel rotation of the rotors in the opposite direction can occur at speeds higher than their speed.

  nominal value in normal operation.

  In addition, to the extent that the gas reflux is shut off, for example by a check valve, the initial flow of the pressurized gas discharged downstream and returning through the compressor to the side

  low pressure refrigeration system may nevertheless be

  less than sufficient to ensure that the pressure at the suction end of the compressor exceeds that immediately downstream of the discharge port. This situation can occur when the compressor is operating as a regulator while it is rotating in the opposite direction pumping the closed shut-off valve on the discharge side and may result in large axial forces on the worm rotor unit and its bearings in the opposite direction to that which normally exists and

  is compensated during compressor operation.

  In addition, many worm screw compressors equipped with lubrication circuits are designed for the production and downstream maintenance of a relatively high pressure which is used to circulate lube oil from a crankcase or a reservoir towards the bearings of the rotors and / or an injection orifice The high speed of rotation in the opposite direction of the rotors when the compressor stops and the momentary appearance of a relatively high pressure upstream or at the Low-pressure end of the working chamber could under certain circumstances, if allowed to occur, induce oil suction on the bearings or could prevent the supply of these in sufficient quantity with a risk of catastrophic results. Finally, if the oil injection port opening into the working chamber of a worm compressor is not isolated from its nominal pressure supply when the compressor stops, the oil continues to pass. through the injection port to enter the working chamber after shutdown until the system pressures are balanced, due to the pressure difference that exists between the oil supply and the oil chamber. compressor shutdown The absence of reliable isolation between the oil injection port and its supply means that under these circumstances, this working chamber can be flooded with oil. Compressor lubrication system may be oil-starved due to power cut-off from the crank chamber and the oil supply available for distribution to the required locations inside the compressor may insufficient after the next restart of the compressor, with the risk of

catastrophic results.

  It is therefore necessary to implement a safety system to prevent the continuation of the flow of oil to the bearings and the injection port in the working chamber of a worm refrigeration compressor at the stop of this last and to allow this flow

  of oil at the start of the compressor.

  The object of the invention is therefore to isolate from their feed the lubrication passages of the bearings and the oil injection orifice which opens into the working chamber of a screw compressor at the end of this process. last in a manner that is triggered by the existence of the pressurized gas discharge immediately downstream of the

  working chamber when the compressor is running.

  Another subject of the invention is the provision of an arrangement which immediately and mechanically places the lubrication passage of the bearings and the oil injection orifice in communication with their feed by the effect of the compression of the gas and It is also an object of the invention to provide a mechanism for closing the lubrication passages of the bearings and the oil injection port of a compressor. endlessly at the stop of the latter and

  the opening of these passages and this orifice from the beginning

  rage in a way that ensures safety through the use

  ambient conditions that are inherent in the functioning of

  compressor at these times and eliminates the need for

  of external shut-off valves, solenoid valves or sensors to detect the flow of oil within the

compressor.

  The mechanism disposed within the worm compressor cuts the flow of injection and lubrication of the bearings when the compressor stops and allows this flow to exist at compressor startup by the use of the internal differentials of the compressor. pressure and gas flows that are inherent to the compressor and its mode of operation

at these moments.

  The pressure that exists immediately downstream of the compressor discharge port when it is operating is used to position a spool valve drawer against the internal suction pressure so that it allows the lubricating oil flow to flow. a supply to the bearings and to the injection port

  entering the working chamber When the compressor is stopped

  The reflux of the gas, which is at the discharge pressure, to the working chamber closes an internal shut-off valve, located on the discharge side, causing the immediate appearance of a pressure differential on the other side. other of the dispenser drawer The pressure differential has the effect of positioning the drawer so that it isolates

  the oil supply of the bearings and the injection port

  At the start of the compressor, the discharge pressure appears downstream of the working chamber and is exerted on the slide by positioning it so that it allows the flow of oil inside the compressor, so that oil is directed immediately onto the bearings and the injection port. The invention will be described in more detail by way of example with reference to the accompanying drawings, in which: FIG. 1 is a cross-section of FIG.

  compressor of the present invention and schematically represents

  its installation in the refrigeration system; and FIG. 2 is an enlarged partial sectional view of the flow cutoff valve assembly.

  of oil in the compressor of Figure 1.

  In Figures 1 and 2, which will be described together, the refrigeration system 10 consists of an assembly 12 housing a compressor, a condenser 14, an expansion valve 16 and an evaporator 18, all these members being connected in series so as to form a hermetically closed loop refrigeration system The housing of the rotor of the compressor 12 contains two worm rotors of which only one, the rotor 22, is shown. The group of rotors is arranged in a working chamber 24 which further comprises a suction port 26 and a discharge port 28 which constitute the inlet and outlet locations of the refrigerant gas passing through the

  working chamber during operation of the compressor.

  The rotor 22 of the embodiment of FIG. 1 is that of the two worm rotors which is controlled and is rotatably mounted in its housing in bearings 30 and 32. The rotor 22 comprises at one end a shaft 34 which is controlled The housing 38 of a bearing is attached to the discharge end of the housing 20 of the rotor and serves to accommodate the bearing 32 and to seal the end of the housing.

discharge from the working room.

  The housing 38 of this bearing comprises a discharge passage 40 which communicates with the discharge orifice 38 which channels the gas at the outlet of the compressor. The discharge passage 40 is also in communication with an oil separator 42 in which the lubricant that was transported out of the compressor 12 into the gas stream of

  discharge is separated from this gas before the latter is used in the refrigeration system. It should be noted that a relative quantity

  A large amount of oil is usually transported to the outside of the working chamber by the flow of discharge gas in a worm and oil-injected compressor and this oil must be removed as much as possible. refrigerant gas in order not to affect the performance of the downstream refrigeration system and to ensure that a sufficient quantity of lubricant continues

  to be available for the compressor.

  A stop valve element 41 is disposed in the discharge passage 40. While the element 41 of this valve which is shown in FIG. 1 is a spherical element trapped in a volume 43 against an open spider 45, it will be understood that A large number of very different stop valves can be used in the context of the invention. The stop valve can be arranged in the bearing housing or in the discharge line which connects the compressor to the oil separator. is however intended to isolate the working chamber of the housing

  oil 44 at the compressor stop.

  The compressor 10 includes a plurality of oil passages including lubrication passages 46 and 48 which communicate with the bearings that support the worm rotors and an oil injection passage 50 which opens into the compressor working chamber. the embodiments shown in Figures 1 and 2, all three passages communicate with a common passage 52

oil supply.

  The oil supply passage 52 communicates with the oil separator housing 42. It should be noted that the oil separator 42 and the housing 44 may be incorporated in the compressor assembly and that the housing 44 may communicate with the compressor. With the feed passage through passages which would be entirely located inside the compressor Moreover, the housing 44 could be removed and placed in a tank independent of the oil separator 42 It must nevertheless be repeated that any means to prevent the reflux of gas to the working chamber at the compressor stop must be arranged between this chamber and the oil separator or its housing, regardless of the location to which they are turned.

  A volume 54 in which a distribution element

  scorer 56 is placed is interposed in the passage of food

  oil 52 inside the housing 20 of the rotors.

  The volume 54 is not only in communication with the oil supply passage 52 and therefore with the passages 46, 48 and 50 located inside the compressor, but it also communicates with an area inside the compressor. and which is under a pressure lower than that of the discharge, as well as with an area inside the compressor and which, when it is running, is

  at high pressure or discharge pressure.

  Indeed, the volume 54 communicates through a passage 58 with a zone 60 which is a volume located inside the housing 20 of the rotors and which is at the suction pressure

  during operation of the compressor Zone 60

  with the suction port 26 inside the compressor and in reality it is upstream of the latter

  in the refrigeration system.

  As mentioned above, zone 60 may be in a region of the compressor that is at a pressure

  intermediate and not at the suction pressure.

  less, the zone 60 is always at a lower pressure than the discharge when the compressor is running The volume 54 also opens into an area 62 inside which is a region which is immediately downstream

  of the discharge port 28 which is at the corresponding pressure

  Zone 62 is located on the high pressure side of the refrigeration system when the compressor is in operation.

  market. The slide 56 is axially sliding in the interior

  54 of the volume between a first position shown in FIG. 1 and in which the oil is allowed to circulate in the passage 52 and the chamber 54 towards the passages 46, 48 and 50 around a portion 64 of reduced cross section. drawer 56 and a second position shown in Figure 2 and in which a solid portion of the slide 56 blocks the passage of oil through the chamber 54 The slide 56 occupies the position shown in Figure 1 when its end surface 66 located on the high side pressure is exposed to the discharge pressure that exists in zone 62 when

the compressor is running.

  On the other hand, the end surface 68 on the low pressure side of the slide 56 is exposed as mentioned above to an area of the compressor that the passage 58 sets to the low pressure or suction pressure. The pressure differential on either side of the slide 56 that exists when the compressor is running ensures that the dispenser 56 is always positioned to allow the flow of oil through the chamber 54, as shown in Figure 1, while the compressor is running This ensures that

  reliable way, which is based on the operating conditions

  which is inherent to a running compressor, that the oil is allowed to pass from the housing 44 to the oil injection port and to the bearings of the compressor at

  whenever it works.

  When the power supply of the motor 36 is cut off, the compressor stops and the gas

  compressed and that is at the discharge pressure immediately reflows

  The immediate effect of the reflux of this gas is to put the element 41 of the shut-off valve at the position in which it is represented in broken line. 1. As soon as the element 41 of this valve is against its seat, the reflux of the gas previously compressed from the downstream of the compressor to the working chamber stops the immediate initial reflux of the gas to the working chamber before

  the shut-off valve element is against its seat, however,

  As a consequence, the rotors begin to rotate in the opposite direction to that in which they are driven.

in rotation by the motor 36.

  This rotation in the opposite direction of the rotors has the effect of evacuating the gas from the discharge zone 62 as soon as the valve element 41 is against its seat and of lowering the pressure in this zone to a value which is lower than that This results from the fact that the rotors, which operate as gas expansion valves during their rotation in the opposite direction, have the effect of pumping the gas from the discharge zone against the shut-off valve which, at this state, is at the position in

which it prevents reflux.

  When the pressure of the zone 62 falls under these circumstances, the discharge pressure at the high pressure end 66 of the slide 56 drops rapidly to a value which is lower than the low pressure prevailing in the suction zone 60. , the pressure exerted on the surface 68 of the low pressure side of the distributor becomes greater than the pressure on its surface 66 located on the high pressure side and the pressure differential on either side of the slide causes the latter to move. to the position shown in Figure 2 In this case also, it is understood that the ambient conditions which are

  inherent to the compressor to a particular state of function

  are used to cause the closure of the passage

  52 oil flow at a convenient time.

  It can be seen from FIG. 2 that the spool 56 can be subjected to the force of a spring 70 tending to push it back to the position shown in FIG. 2 and in which its solid part intersects the oil feed passage 52 It is also observed that a retaining ring 72 is placed in the volume 54 and protrudes in the latter so as not to allow the slide 56 to move in the

  volume 54 beyond the position shown in FIG. 2.

  Although the spring 70 is not essential, it is preferable to use it so that it helps to move the slide 56 to the position at which the flow of oil is cut off the compressor, this spring contributing also to hold the drawer at this position stably, since the conditions prevailing in the discharge zone 62 are

  of a somewhat transitory nature when the compressor stops.

  When the compressor restarts after being stopped, the compression of the gas between the worm rotors starts immediately and the discharge pressure is rapidly established in the corresponding zone 62 and pushes the slide 56 to the position shown in FIG. wherein the oil supply passage 52 is open. The pressure increases together in the oil separator 42 and causes the oil flow from the housing 44 through the passage

  52 to the compressor bearings and the ori-

injection fice.

  It is understood that, all the provisions of the invention provided for cutting the circulation of oil being mechanical and safety and based on the conditions of

  which are inherent to the compressor in the

  of the latter, to be actuated at the right moment, the need for a control of the position of the cut-off valve or that of the detection of the flow of oil towards the bearings of the compressor and the injection orifice at The provision of the invention likewise eliminates the need for an electrical or electronic detection and / or control of the oil flow during the running of the compressor, unlike some systems

  which require a relatively expensive solenoid valve which is susceptible to electrical failure in the oil supply conduit.

Claims (15)

  1 rotary screw compressor for refrigerant gas, comprising a casing (20) enclosing a working chamber (24), said casing further comprising a suction port (26), a discharge port   (28) and an oil supply passage (52) which communicates   all with said working chamber, said casing further comprising a passage (54) for cutting off the oil flow which communicates with said oil supply passage (52) as well as with a zone (62) located in said compressor downstream of said discharge port (28)   and which is at the discharge pressure when said compres-   and two worm rotors (22) which are engaged and rotatably mounted in said working chamber (24), characterized in that valves (41, 56) are provided at the same time. compressor interior can be positioned on the one hand to close said oil supply passage (52) to prevent flow in the latter and on the other hand to open this passage to allow the flow of oil oil in direct response to ambient conditions prevailing in said compressor downstream of said discharge port (28) and existing by inherence from   stopping and starting the compressor.   2 Screw compressor according to the claim   1, characterized in that said valves consist of a means (41) for stopping the reflux of said refrigerant gas to said working chamber (24) from the downstream of the latter immediately following the shutdown of the compressor and an independent distributor element (56) disposed   in said passage (54) for shutting off the oil flow.   3 Screw compressor according to the claim   1, characterized in that said oil flow cut-off passage (54) also communicates with a zone (60) in said compressor which is at a lower pressure than the discharge when the compressor is running. . A worm compressor according to claim 3, characterized in that said distributor element (56) comprises an upstream pressure surface (68) which exists in said zone (60) and which is smaller than that of the discharge when said compressor is in operation and a downstream surface (66) exposed to the pressure prevailing in said zone (62) which is downstream of said discharge port (28), the position of the element (56) of said distributor being function of the pressure differential on either side of this element.   A worm compressor according to claim 4, characterized in that the interruption of the gas reflux following the stopping of the compressor causes the pressure in said zone (62) downstream of said discharge port to be reduced (28) to a value less than that prevailing on said upstream surface (68) of said spool (56), so that it is pushed back by the pressure on said upstream surface so as to be set to said position in which said oil supply passage (52) is closed and in that, at the compressor start, the pressure prevailing in said zone (62) located downstream of said orifice   discharge (28) is increased as a result of the compres-   the gas in said working chamber (24) and reaches a value such that the pressure prevailing in this zone (62) located downstream of the discharge orifice (28) and on said downstream surface (66) of said spool (56) ) surpasses the pressure prevailing on its upstream surface (68), so that said drawer is pushed back to a position at which the oil flow   in said feed passage (52) is allowed.   6 Screw compressor according to the claim   5, wherein said distributor member is a drawer (56) including a portion (64) of reduced section between said upstream surface (68) and said downstream surface (66), the oil flow in said passageway power supply (52) being authorized by alignment of said part   (64) of reduced drawer section with said feed passage   oil delivery (52) within the compressor, while oil flow through said feed passage is prevented by movement of said dispenser to a position within said cutoff path (54) and5 at which said reduced section portion (64) ceases to be   in alignment with said feed passage.   7 Screw compressor according to the claim   6, characterized in that it furthermore comprises a member (70) tending to mechanically push said spool (56) to a position situated inside said cut-off passage (54) and at which the oil flow by said   feed passage (52) is prevented.   8 Screw compressor according to the claim   6, characterized in that said zone (60) which is at a pressure lower than that of the discharge when said compressor is running is at the suction pressure   when said compressor is running.   A worm compressor comprising an enclosure (20) enclosing a working chamber (24), an oil supply passage (52) and a passage (54) for   cut off the circulation of oil said envelope comprising   in addition, a suction port (26) and a discharge port (28) communicating with said working chamber (24), said oil circulation cut-off passage (54) communicating with said feed passage in oil (52) and communicating between an area (62) in said compressor downstream of said discharge port (28) and an area (60) within the compressor that is at a lower pressure than the discharging when said compressor is running, said downstream zone (62) communicating with said discharge port (28), said compressor further comprising two worm rotors (22) which are mounted to be engaged in said chamber working member (24) and means (36) for controlling one of said worm rotors, characterized in that it comprises means (41) for stopping the gas reflux from downstream of said working chamber (24) in the latter immediately to the following the stopping of said means (36) for controlling said controlled rotor, as well as a distributor (56) disposed in said passage (54) for cutting off the oil circulation and having an upstream surface (68) exposed to the pressure prevailing in said area (60) and which   is less than that of the discharge when said compres-   and a downstream surface (66) which is exposed to the pressure in said area (62) downstream of the discharge port (28), said dispenser being positionable to cut the traffic   of oil in said supply passage (52) under the effect of the differential pressure which occurs on either side of the distributor as a result of the operation of said gas reflux stop means (41).   Compressor according to claim 9, characterized in that said worm rotors (22) are mounted in bearings (30, 32) to rotate in said   working chamber (24) and that said feed passage   oil supply (52) communicates with said bearings and with an injection port (50) opening into said chamber work (24).   11 Compressor according to claim 10, characterized in that said distributor (56) comprises a drawer undergoing a mechanical force tending to push it so that it closes said passage (52) for supplying oil, said mechanical force being overcome by the pressure created in said zone (62) located downstream of said discharge orifice (28) and which is sufficient to overcome this mechanical stress and the effect of the pressure prevailing on the distributor side which is exposed to said region which is at a pressure lower than that of the discharge when said compressor is running, so that the dispenser is placed at a position which allows flow into said feed passage (52).   12 Refrigeration system based on a compres-   worm gear comprising an oil supply (44), a condenser (14), an expansion valve (16), an evaporator (18) and a screw compressor (12), said compressor, condenser, expansion valve and evaporator being connected in series so as to form a hermetically sealed refrigeration system and comprising a high pressure side downstream of said compressor and upstream of said expansion valve, as well as a low pressure side downstream of said expansion valve and upstream   said compressor, said compressor comprising   which are at an intermediate pressure between that of discharge and that of suction when said compressor is running, said compressor including a working chamber (24) in which two worm rotors (22) are arranged, a passage ( 52) which communicates with said oil supply (44), a suction port (26) which communicates directly with said working chamber and with said low pressure side of said system, a discharge port (28). which communicates directly with said working chamber and said high pressure side of said system, a passage (54) for shutting off the oil circulation which communicates with the high pressure side of said system and said feed passage (52) oil, characterized in that it comprises distribution members (41, 56) placed in said passage (54) for cutting the circulation of oil, said dispensing members being positioned in such a manner to prevent the circulation of oil in said supply passage (52) under the ambient conditions that inherently exist within said compressor immediately at the   following its judgment and being positioned in such a way as to allow   the circulation of oil in the said feed passages   (52) under the prevailing environmental conditions of   inherent manner inside said compressor immediately following its start.   13 Refrigeration system based on a compres-   Worm gear according to claim 12, characterized in that said distributor members consist of means (41) for stopping the reflux of the refrigerant gas downstream of said working chamber (24) into the latter immediately. following the compressor shutdown, as well as   independent distributor member (56) disposed in said oil circulation cut-off passage.   14 Refrigeration system based on a compres-   Worm gear according to claim 13, characterized in that said oil circulation cut-off passage (54) communicates with one (60) of said locations   in said compressor and at said intermediate pressure   and in that said distributor member (56) comprises an upstream surface (68) exposed to the pressure in said one (60) of said locations in which an intermediate pressure prevails within said compressor, as well as a surface downstream (66) exposed to the pressure in said high-pressure side of said refrigeration system, the position of said distributor member (56) being a function of the pressure differential across the this element.   Refrigeration system based on compres-   Worm gear according to claim 14, characterized in that stopping the gas reflux following the stopping of the compressor causes the pressure at which said downstream surface (66) of said distributor member (56) to be lowered. exposed to a value less than that at which said upstream surface (68) of this distributor element is exposed, so that said distributor element is pushed back by the pressure on said upstream surface (66) to be set at a position at which it prevents the   circulation of oil in said feed passage (52).   16 Refrigeration system based on a compres-   Worm gear according to claim 15, characterized in that said dispenser member is a drawer (56) having a reduced section portion (64) between said   upstream surface (68) and said downstream surface (66), the circula-   oil in said feed passage (52) being permitted by aligning the reduced section portion of said spool with said oil supply passage and the oil flow in said feed passage being prevented by moving said drawer to a position in said oil flow cut-off passage (54) and at which said reduced section portion ceases to align with said feed passage (52).   17 Refrigeration system based on a compres-   Worm gear according to claim 16, characterized in that it further comprises means (70) tending to mechanically push said slide (56) to a position at which it prevents, in said passage of passage of the traffic oil (54), flow through said passage power supply (52).   18 Refrigeration system based on a compres-   Worm gear according to claim 13, characterized in that: said oil circulation cut-off passage (54) communicates with said low pressure side of said refrigeration system and that said dispenser member (56) comprises an upstream surface (68) exposed to the pressure in said low pressure side of said refrigeration system and a downstream surface (66) exposed to the pressure in said high pressure side of said refrigeration system, the position of said distributor element being a function of the pressure differential of   both sides of said element (56).   19 Refrigeration system based on a compres-   A worm gear according to claim 18, characterized in that said dispenser member is a drawer (56) which includes a portion (64) of reduced section between said upstream surface (68) and said downstream surface (66), oil flow in said feed passage (52) being permitted by aligning said reduced section portion of said spool (56) with said feed passage (52) and the oil flow in said feed passage; supply being prevented by movement of said drawer to a position within said circulating cut-off passage (54) and to which said reduced section portion   ceases to align with said oil supply passage (52). 20 Refrigeration system based on a compres-   Worm gear according to claim 19, characterized in that it further comprises means (70) tending to mechanically push said drawer to a position located inside said passage of cut of the circulation and   to which the flow of oil through said feed passage tion (52) is prevented.