Device for generating a combustible gas mixture from a carbonaceous starting material with a shaft seal for rotating drives for wood gas applications

Abstract

A device for producing a combustible gas mixture from a carbon-containing starting material, in particular from lumpy wood, comprising a feed unit (10) for feeding the starting material to a reactor (19), wherein the reactor (19) comprises at least one oxidation zone (24) for oxidizing the starting material and a reduction zone (25) for reducing at least one intermediate product of the oxidation, wherein a shaft seal is provided for rotating drives, and wherein a rotating shaft (4) is sealed against a dust-, pressure-, and tar-laden atmosphere by at least one sealing bush (7), wherein the sealing bush (7) consists of at least one split ring (3) consisting of an inner ring (33) mounted on the shaft (4) in a rotationally fixed manner, which inner ring is connected via a sealing gap (34, 34a-d) directed in the axial direction to a radial outer ring (32) arranged in a housing (1) of the sealing bush (7), characterized in thatthat at least one further seal (2) is arranged in the housing (1) in the axial direction away from the split ring (3), and that an annular space (31) is arranged between the split ring (3) and the at least one further seal (2), which is filled with a lubricant (38) which can be introduced via a fluid-conducting bore (5) connected to the annular space (31) and which penetrates and lubricates the entire sealing gap (34, 34a-d).

Description

[0001] Wood gasification technology has developed significantly in recent years. While the original idea was to generate gas for vehicle propulsion, many experts now see the future of wood gasification in the decentralized supply of energy in the form of electricity and heat.

[0002] As a result, the requirements for performance and reliability of wood gasification plants have increased enormously. An operating time of 8,000 hours per year is often required and necessary to ensure the economical operation of such a plant.

[0003] The processes for maintaining gas production must therefore be fully automated and operate with maximum reliability. These include material feeding, material mixing, cleaning, parameter recording and many more.

[0004] Many of these processes are carried out using devices that are driven by rotational energy, including screw conveyors, flaps, scrapers, turners and other mechanical elements.

[0005] To protect against dust, tar, gas and other harmful materials and / or substances and / or relatively high temperatures present inside the wood gasifier containers, the drives of these devices are usually driven from outside the container by means of electric motors or other devices for generating rotary energy.

[0006] This requires a virtually gas-tight seal on the shaft of the drives that drive screw conveyors, flaps, scrapers, rotators and other mechanical elements for transmitting rotary energy.

[0007] Sealing these drive shafts from the environment is difficult due to the substances and particles in the gas chamber inside the containers. The gas chamber contains, among other things, gas, tar, dust, wood dust, incrustations, slag, wood fragments, and coal pieces. These substances can even be particle-sized, which places high demands on the service life and tightness of the respective rotary union.

[0008] In particular, the presence of tar reduces the service life of conventional seals such as shaft seals, O-rings, circular cross-section rings made of elastic materials, mechanical seals, stuffing boxes / stuffing glands, packing glands, bellows mechanical seals, and floating ring seals. Such seals are unsuitable for their intended purpose.

[0009] The tar binds the existing sealing elements and can harden over time. Another major problem is the abrasive materials such as wood chips, dust, and slag that adhere to the seal due to the tar. This combination exposes the sealing elements to severe, continuous wear.

[0010] DE 10 2009 038 242 A1 shows a device for producing a combustible gas mixture from a carbon-containing starting material, in particular from lumpy wood, with a feed unit for feeding the starting material to a reactor, wherein the reactor comprises an oxidation zone for oxidizing the starting material and a reduction zone for reducing at least one intermediate product of the oxidation, wherein a shaft seal is provided for rotating drives and wherein a rotating shaft is sealed against a dust-, pressure- and tar-laden atmosphere with a labyrinth-like ceramic seal which consists of an inner ring which is mounted on the shaft in a rotationally fixed manner and which is connected to a radial outer ring via a sealing gap directed in the axial direction.

[0011] However, satisfactory sealing is not possible with the ceramic seal used, as particles, dirt, dust, tar or similar can penetrate through the sealing gap and thus contaminate the downstream drive unit of the shaft.

[0012] State-of-the-art stuffing boxes are generally subject to wear due to their inherent design. Thus, additional wear caused by the atmosphere in containers during wood gasification does not directly lead to seal failure, but merely to shortened maintenance intervals and more frequent seal replacement. The stuffing box can also be regularly filled with grease or other lubricating materials to increase its service life.

[0013] Maintenance intervals for stuffing boxes used to seal a shaft in a wood gasifier are approximately 1. Re-tightening of the bushing packing: every 250h, 2. Replacing the bushing packing: every 4000h, 3. Relubrication with grease or similar: every 500 hours.

[0014] It should be noted that the current state of the art of stuffing box technology does not provide a virtually gas-tight seal. A certain degree of leakage is always present, which should be viewed critically when used in explosion-protected areas.

[0015] This leak increases as the stuffing box wears. With a lubricated stuffing box, lubricant leaks in both directions along the shaft. This grease contaminates the wood gas system around the shaft and the drive unit and must be cleaned regularly. Another disadvantage of the stuffing box is its size; sealing a 30mm shaft requires a stuffing box length of approximately 10cm.

[0016] The invention is therefore based on the object of developing a device for generating a combustible gas mixture from a carbon-containing starting material with a shaft seal for rotating drives for wood gas applications of the type mentioned at the beginning in such a way that extended maintenance intervals and a more compact size can be achieved with better sealing properties.

[0017] To achieve the stated object, the invention is characterized by the technical teaching of claim 1.

[0018] The seal according to the invention was developed to achieve a virtually wear-free and, in particular, virtually gas-tight seal for rotating shafts used to transmit rotary energy in wood gasification. The rotating shafts to be sealed have a rotational speed ranging from 1 rpm to 100 rpm. They are therefore slow-running shafts.

[0019] Essentially, this seal consists of one or more rings with a circular cross-section made of elastic materials and one or more relatively narrow sealing gaps of approximately 0.05-3 millimeters wide, possibly also provided with deflection sections, between the movable and the fixed part of the seal, through which a lubricant is introduced in the opposite direction to the drive.

[0020] This lubricant can be grease, oil, soap or other carbon-based, higher-viscosity substances or similar and protects the ring(s) made of elastic material from the gas area inside the wood gas plant.

[0021] To increase efficiency, the relatively narrow sealing gap can undergo one or more changes of direction (as viewed in the axial direction in cross-section), which, among other things, effectively increases the pressure loss coefficient. Multiple changes of direction are also referred to as a labyrinth seal.

[0022] Therefore, the narrow gap can be realized using a labyrinth sealing ring if necessary, which offers the advantage of being inexpensive and easy to replace in the event of damage.

[0023] The narrow sealing gap makes it extremely difficult for tar, dust, slag, and wood particles to reach the elastic sealing ring(s). Relubrication through a hole, groove, or similar device located between the narrow sealing gap and the elastic sealing ring(s) pushes any particles, dirt, dust, tar, or similar material that may be present in the sealing gap or in the space between the elastic sealing ring toward the gas area. There, the lubricant is carried in the direction of flow through the wood gasification system and completely burned in the reactor or engine of the system.

[0024] The sealing of the environment against the gas area is achieved exclusively by the almost gas-tight ring(s) made of elastic materials, whereby the sealing gap and the lubricant located there merely represent its protection.

[0025] This method allows for the creation of seals that are compact, cost-effective, and virtually wear-free. Tests have shown that the sealing ring(s) made of elastic material exhibited no wear even after more than 20,000 hours of operation, with regular relubrication of the sealing gap every 2,500 hours.

[0026] Furthermore, this seal offers a much higher level of gas tightness compared to the conventional, state-of-the-art stuffing box.

[0027] An extract of the areas of application of this seal are: 1. Drives of screw conveyors for material transport, 2. Scrapers for moving and mixing material such as wood, coal or dust, 3. Shafts for closing flaps in the gas area of ​​the wood gasifier, 4. Drives for cleaning filter surfaces or movable sensors for detecting fill levels.

[0028] When using elastic sealing rings made of materials with higher temperature resistance such as silicone, PTFE, fluoroelastomers or similar materials and a temperature-resistant lubricant, the seal can withstand temperatures of up to 250°C in continuous operation.

[0029] In a preferred embodiment of a device for wood gas applications, the sealing arrangement according to the invention is arranged at various locations in the system.

[0030] The first point here concerns an area of ​​the feed unit, namely the conveyor screw located closest to the gas generator, which feeds the lumpy wood into the gas generator.

[0031] The feed screw is located in a receiving chamber that is heavily exposed to the tar-containing gas and wood dust; the pressure there is up to about 100 mbar.

[0032] In this tar-contaminated atmosphere, the sealing arrangement according to the invention must now be arranged to ensure a longer service life.

[0033] In the application shown, the sealing arrangement is located in the area at the rear, front end of the conveyor screw and the associated drive motor.

[0034] In a further preferred embodiment of the present invention, a drive unit is arranged at the bottom end of the gas generator, and a drive motor serves to drive a rotating grate. Since the drive motor is arranged at the bottom end of the gas generator, temperatures in the range of 100 to 300°C with a corresponding pressure and a highly dusty atmosphere are present there.

[0035] In a third application of the present invention, the sealing arrangement according to the invention is used on a further component of the wood gas plant, namely on the filter unit downstream of the gas generator, which is connected to a filter cleaning device whose filter cleaning device is designed, for example, as a vibrator and is driven in rotation by an associated gear motor, wherein the introduction of the drive shaft of the gear motor into the filter unit is again formed by the sealing arrangement according to the invention.

[0036] Here, too, a tar-contaminated atmosphere can exist, with a relatively high temperature of, for example, 130 °C and a pressure of up to 100 mbar.

[0037] In a fourth embodiment, the sealing arrangement according to the invention is used in a device downstream of the filter unit, which serves to transport separated dust and coal pieces. Here, the seal is attached to the front end of a conveyor screw. The atmosphere here is very heavily contaminated with dust.

[0038] In these applications in the area of ​​a wood gas plant, it is necessary to achieve a reliable sealing arrangement for the rotary union of a drive shaft of a drive motor, which ensures longer maintenance intervals with lower wear.

[0039] This is where the invention comes in, which essentially consists of the combination of two different sealing arrangements.

[0040] According to the invention, a first sealing arrangement which is resistant to temperature and pressure as well as tar-contaminated atmosphere is arranged on the housing of a sealing bush, wherein this first sealing arrangement is designed as a split ring which consists of a stationary outer ring which is connected to a rotating inner ring via a radial gap.

[0041] In the area between the outer ring and the inner ring, the narrowly dimensioned sealing gap with a width in the range between approximately 0.2 to 3 mm is arranged, whereby the sealing gap can be designed either as a labyrinth sealing gap or as a zigzag or as an arc or as a shaft sealing gap or as a straight sealing gap running in the axial direction.

[0042] Since the sealing gap is directed at least partially in the radial direction and at least partially in the axial direction, superior sealing properties are achieved because there are both axial and radial sealing components.

[0043] In any case, it is important that the split ring keeps the highly contaminated atmosphere away from the sealing arrangement arranged behind it in the axial direction. This is achieved by the sealing gap being provided with a lubricant filling on the side facing away from the dirt, which penetrates and lubricates the entire sealing gap.

[0044] This creates a lubricant-borne seal against the tar and gas-laden atmosphere and the sealing arrangement behind it is protected against the penetration of tar and other harmful substances, in particular dust and other abrasive and adhesive particles.

[0045] The lubricant acts as a sacrificial fluid, meaning it enters the reservoir after passing through the seal gap, where it is completely burned in the direction of flow. It thus serves to lubricate the seal and also seal the seal gap against the ingress of harmful particles. It consumes itself without causing any damage. With regular relubrication, lubricant consumption is approximately 10 grams per 2,500 hours.

[0046] The second sealing arrangement used at an axial distance behind the first sealing arrangement (formed from the split ring) consists in a preferred embodiment of at least one sealing ring which forms a conical sealing lip which sits with a relatively narrow cross-section on the outer circumference of the rotating shaft and which is pressed inwards in the radial direction onto the sealing point of the shaft with the aid of a force accumulator, e.g. an annular spring which rests on the outer circumference.

[0047] It is essential that this shaft seal with the sealing lip is supported radially outwards by a metal sleeve which is covered with a rubber jacket and arranged in a free space in the fixed housing part of the seal, so that the radially inwardly arranged sealing lip or sealing sleeve is radially supported by the metal sleeve.

[0048] Also known as shaft seal.

[0049] In a second embodiment of the present invention, it can be provided that the second, previously described sealing arrangement shaft sealing ring with the sealing lip and the metal sleeve is also replaced by a simple O-ring.

[0050] Several O-rings can also be used one behind the other.

[0051] Instead of using an O-ring, other combinations of O-rings and sealing washers can also be used. This primarily refers to elastic rings with a circular cross-section.

[0052] At an axial distance, the seal can rest against a counter-holder ring or retaining ring to achieve axial support.

[0053] It is important that the shaft to be sealed is designed to be self-supporting, i.e. no axial forces are transmitted via the shaft to the seal, since the invention provides that the actual shaft bearing is carried out in the drive gear of the rotary drive itself.

[0054] If the shaft is not secured in the geared motor itself against radial and axial displacement forces, it can be provided in a further embodiment that the actual shaft bearing is arranged behind the sealing bush according to the invention outside the container.

[0055] In any case, it is important for the invention that the sealing bushing according to the invention, which consists of the combination of at least two sealing arrangements arranged at an axial distance from one another, is always arranged closest to the harmful atmosphere.

[0056] The subject matter of the present invention results not only from the subject matter of the individual patent claims, but also from the combination of the individual patent claims with one another.

[0057] All information and features disclosed in the documents, including the abstract, in particular the spatial configuration shown in the drawings, are claimed as essential to the invention insofar as they are new, individually or in combination, compared to the prior art.

[0058] The invention is explained in more detail below with reference to drawings illustrating only one embodiment. Further essential features and advantages of the invention will become apparent from the drawings and their description.

[0059] They show: Fig. 1: Schematic view of a plant for producing combustible gas from a carbonaceous lumpy combustion material Fig. 2: a schematic longitudinal section through a shaft seal according to the invention Fig. 3a: a first embodiment of a modified form of the split ring Fig. 3b: a second embodiment of a modified form of the split ring Fig. 3c: a third embodiment of a modified sealing arrangement of the split ring Fig. 3d: a fourth embodiment of the split ring design with its sealing gap

[0060] In Fig. 1 shows a schematic and general view of a plant for producing combustible gas, which essentially consists of a feed unit 10 which feeds the carbonaceous material to be burned in the direction of arrow 12 to a conveyor screw 11 which conveys the material via a vertical drop section 13 via an open closure element 14 into an intermediate storage unit 15.

[0061] The closure element 14 is only temporarily open and is intended to prevent the gas generated in the gas generator 19 from escaping to the rear of the supply unit 10.

[0062] The carbonaceous material to be burned is stored in batches in the intermediate storage 15 and is fed, as required, in the direction of arrow 18 to the inlet side of a gas generator 19 via a rotary-driven conveyor screw 17 arranged in a receiving space 8.

[0063] It is now important that the conveyor screw 17 is driven by a geared motor 16, whose drive shaft 4 passes through a sealing bushing 7 according to the invention. An intermediate flange 6 is arranged beyond the sealing bushing 7.

[0064] In the gas generator 19, the material is gasified in the area of ​​a bed of embers 23, with combustion air being introduced via lateral air inlets 20, 21 in the direction of arrow 22. Accordingly, the bed of embers 23 is divided into an upper area, referred to as the oxidation zone 24, and a lower area, referred to as the reduction zone 25.

[0065] The additional sealing bushing 7 according to the invention is arranged in the base area of ​​the gas generator 19 and drives, for example, a grate 55, which is designed as a rotating grate. The drive shaft shown there is non-rotatably coupled to the gear motor 16.

[0066] The gas generated in the gas generator 19 is led via a gas outlet 26 and a gas line 27 into a downstream filter unit 28 in which one or more filters 54 are arranged.

[0067] The filters 54 can be cleaned by a filter cleaning device 53, which in the embodiment shown is designed as a vibrator, which is driven in rotation by a gear motor 16, wherein the drive shaft is again led into the filter unit 28 via the sealing bushing 7 according to the invention.

[0068] From the embodiment shown it follows that the sealing bushing 7 according to the invention is exposed to a tar-containing and dust-laden atmosphere at the indicated locations and can withstand high temperatures.

[0069] For the reasons mentioned, the invention proposes a sealing bush 7 according to a preferred embodiment Fig. 2 before.

[0070] In Fig. 2 shows the rotating drive shaft 4, whereby it is emphasized again that the sealing bush 7 is preferably attached to the Fig. 1. However, other applications can also be provided, which are shown in the schematic drawing according to Fig. 1 are not shown.

[0071] The rotating shaft 4 can be driven either anticlockwise or clockwise in the direction of arrow 9.

[0072] The Fig. The sealing bush 7 shown in Figure 2 is used as an application in connection with the drive of the conveyor screw 17 in Fig. 1 explained.

[0073] The tar and dust-laden atmosphere is present in the receiving space 8 and the dirt attack occurs in the direction of arrow 39 on the front side of a housing 1 of the sealing bush 7, with the split ring 3 being arranged on this front side and intended to repel the first dirt attack.

[0074] According to the invention, the split ring 3 consists of an outer ring 32 and an inner ring 33 connected to the housing 1 in a rotationally fixed manner, wherein an intermediate space between the two rings 32, 33 is formed by a sealing gap 34.

[0075] In the embodiment shown, the sealing gap 34 consists of an approximately zigzag-shaped shape, so that straight and short sections 35, 36 each connect to one another at an angle at a reversal point 37 and thus result in a zigzag-shaped sealing gap 34 directed in the axial direction.

[0076] This special design of the sealing gap 34 prevents harmful material from being forced through the sealing gap 34 in the direction of arrow 39 under excess pressure from the dirt and dust-laden atmosphere.

[0077] At the opposite end of the split ring 3, an annular space 31 is arranged, which is preferably filled with a highly viscous lubricant 38. The type and composition of the lubricant 38 have already been mentioned in the general description.

[0078] It is important that the annular space 31 is fluidly connected to a bore 5 through which the highly viscous lubricant is introduced.

[0079] After the lubricant has been introduced, which is done under pressure, the bore 5 is closed again.

[0080] In a preferred embodiment of the invention, it is provided that the annular space 31 is located next to the sealing arrangement, which is designed as a shaft sealing ring 2.

[0081] In another embodiment, however, it may be provided that a separation is present so that the lubricant in the annular space 31 cannot reach the shaft sealing ring 2 located behind it.

[0082] The shaft seal 2 here consists of a radially outer metal sleeve 50, which is vulcanized into a rubber jacket or other elastic material. For the sake of simplicity, it is stated that the elastic material is designed as a rubber sleeve 43. Any other elastic material can be used instead of a rubber sleeve.

[0083] The metal sleeve 50 consists of a first leg aligned in the axial direction, which merges into a further leg (radial shoulder 52) deflected in the radial direction.

[0084] The radial projection 52 engages in an associated annular groove 49 which is open radially outwards and thus supports the multiply angled sealing lip 48.

[0085] The sealing lip 48 is conically shaped in section and forms an elastomeric sleeve which rests sealingly and in a force-locking and form-locking manner on the circumferential circumference of the drive shaft 4 only with an (in the sectional view) oblique and relatively acute-angled section in the region of a sealing point 46.

[0086] The radial contact pressure is achieved by an annular spring 47, which generates a spring-loaded preload force in the radial direction on the sealing lip 48.

[0087] The rubber-coated metal sleeve 50 and the sealing lip 48 are supported in the axial direction on a counter-holding ring 40, which in turn rests against a retaining ring 41, secured against axial displacement.

[0088] The centering ring 42 can be part of the gear motor and center the housing 1 of the sealing bush 7 in the direction of the gear motor.

[0089] In the Fig. 3a-3d show various embodiments for the formation of a sealing gap in more detail.

[0090] In the embodiment according to Fig. 3a, the sealing gap 34a is designed as a symmetrically shaped shaft.

[0091] In the embodiment according to Fig. 3b, the sealing gap 34b is designed as a straight, continuous axial gap, whereby it is crucial that the axial length of the sealing gap 34b is significantly longer than the gap width. In this case—as in all other examples—ratio in the range of 1:50 with regard to the gap length compared to the gap width is preferred.

[0092] In Fig. 3c, a sealing gap 34c is shown as a further embodiment as an arc, while in the Fig. 3d the sealing gap 34d shown there consists of several lines adjoining each other at an angle of 90 degrees.

[0093] The invention can also cover the combinations of different sealing gaps from one or more shapes according to Fig. 2 or one or more shapes according to Fig. 3 to 3d.

[0094] What is important in the present invention is that in a thermally stressed, dust and tar-laden environment, the sealing arrangement is first protected by a split ring which is exposed to this aggressive atmosphere and is supplied with a highly viscous lubricant, and that a shaft sealing ring which is supplied with the same lubricant is connected to the split ring in the axial direction behind it. Drawing legend 1 housing 2 shaft seal 3 split ring 4 drive shaft 5 Hole 6 Intermediate flange 7 Sealing bushing 8 recording rooms (for 17) 9 Arrow direction 10 Feed unit 11 Conveyor screw 12 Arrow direction 13 Case Section 14 locking element 15 cache 16 Gear motor 17 Conveyor screw 18 Arrow direction 19 gas generators 20 Air inlet 21 Air inlet 22 Arrow direction 23 Ember bed 24 Oxidation zone 25 Reduction zone 26 Gas outlet 27 Gas pipeline 28 Filter unit 29 gas outlets (of 28) 30 Arrow direction 31 Annular space 32 outer ring 33 inner ring 34 Sealing gap Section 35 Section 36 37 Turning point 38 Lubricant 39 Arrow direction (tar impact) 40 Retaining ring 41 Retaining ring 42 Centering 43 rubber sleeve 44 45 46 Sealing point 47 Ringfeder 48 Sealing lip 49 Ring groove 50 metal sleeves 51 52 Radial approach 53 Filter cleaning 54 filters 55 Rust

Claims

[1] A device for producing a combustible gas mixture from a carbonaceous starting material, in particular from lumpy wood, comprising a feed unit (10) for feeding the starting material to a reactor (19), wherein the reactor (19) comprises at least one oxidation zone (24) for oxidizing the starting material and a reduction zone (25) for reducing at least one intermediate product of the oxidation, wherein a shaft seal is provided for rotating drives and wherein a rotating shaft (4) is sealed against a dust-, pressure-, and tar-laden atmosphere by at least one sealing bush (7), wherein the sealing bush (7) consists of at least one split ring (3) consisting of an inner ring (33) mounted on the shaft (4) in a rotationally fixed manner, which inner ring is connected via a sealing gap (34, 34a-d) directed in the axial direction to a radial outer ring (32) arranged in a housing (1) of the sealing bush (7), characterized bythat at least one further seal (2) is arranged in the housing (1) in the axial direction away from the split ring (3) and that an annular space (31) is arranged between the split ring (3) and the at least one further seal (2), which is filled with a lubricant (38) which can be introduced via a fluid-conducting bore (5) connected to the annular space (31) and which penetrates and lubricates the entire sealing gap (34, 34a-d). [2] Device according to claim 1, characterized by that the split ring (3) is arranged closest to the dust, pressure and tar-laden atmosphere and is acted upon by it with one of its end faces, and that the further seal (2) is arranged at an axial distance from the split ring (3) in the direction of its other end face in the housing (1). [3] Device according to claim 1 or 2, characterized bythat the sealing gap (34) of the split ring (3) is zigzag-shaped or wavy or straight or curved or labyrinth-shaped and has a width in the range between 0.05 and 3 millimeters. [4] Device according to one of claims 1 to 3, characterized by that the ratio of the length of the sealing gap (34) to its width is approximately 50:

1. [5] Device according to one of claims 1 to 4, characterized by that the further seal (2) is also lubricated by the lubricant (38). [6] Device according to one of claims 1 to 5, characterized by that the lubricant (38) is introduced under excess pressure compared to the attacking atmosphere from an annular space (31) of the housing (1) and enters the attacking atmosphere upon axial passage through the sealing gap (34, 34a-d) of the split ring (3). [7] Device according to one of claims 1 to 6, characterized bythat the sealing ring (2) is made of an elastic, temperature- and abrasion-resistant plastic material. [8] Device according to one of claims 1 to 7, characterized by that the further seal consists of a shaft sealing ring (2) which sits force-fittingly and positively on the outer circumference of the shaft (4) with a radially spring-loaded sealing lip (48). [9] Device according to claim 8, characterized by that the shaft (4) to be sealed engages cantilevered into the housing (1) of the sealing bush (7). [10] Device according to one of claims 1 to 9, characterized by that the further seal designed as a shaft sealing ring (2) has a rubber-coated metal sleeve (50) and that the sealing lip (48) of the shaft sealing ring (2) is supported in the axial direction on a counter-holding ring (40), which in turn rests on a retaining ring (41) secured against axial displacement, and that the retaining ring (41) is supported externally by a centering ring (42).

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