Patent Publication Number: US-2020284384-A1

Title: Powder rotary feedthrough having a purge chamber

Description:
The invention relates to a rotary feedthrough from a stationary medium into a rotating medium or vice versa. Among other things, a powder-gas mixture is to be fed through. 
     Rotary feedthroughs for the feedthrough of non-lubricating media from a stationary machine part into a rotating machine part are known. In particular, the keyword “safe to run dry rotary feedthroughs” plays an important role here. While a seal in the form of two flat circular-ring-shaped sliding seal surfaces arranged one on the other in a sliding manner can also be realized for higher rotational speeds and transmission pressures up to 400 bar when feeding through lubricating media, this is only possible with non-lubricating media at lower rotational speeds and pressures. In particular, if the medium does not have to be fed continuously, the sliding surfaces should be moved apart in the axial direction from time to time in order to form a gap and to be able to cool down, whereby the rotating machine part can continue to rotate at a certain rotational speed. Otherwise, the sliding seal surfaces must remain in contact, which limits the rotational speeds and pressures accordingly. In this respect, certain leakage losses cannot be completely avoided. 
     In DE19932355 B4, leakage losses are reduced by the fact that cylindrical axial cylinder shell surfaces are designed as seal surfaces instead of flat sliding surfaces. With such cylinder surfaces, narrow sealing gaps with much larger surfaces and gap lengths can be realized without changing the radius, i.e. without large radii and corresponding relative speeds, so that the non-lubricating medium can only pass through the cylindrical sealing gap to a small extent. 
     However, a special case is a so-called powder-gas mixture which is used, for example, in the thermal spraying of cylinder liners and other surfaces. Such a mixture can also be referred to as a non-lubricating medium. However, a cylindrical sealing gap is not an option for this application, as the powder would settle in the sealing gap, which would very quickly lead to seizing of the rotary feedthrough. 
     For thermal spraying of cylinder liners, coating powder must be reliably transported to a rotating injector and, accordingly, the sealing must be guaranteed at the point of the powder line where the interface between the “rigid” and “rotating” powder lines is located. To explain the application,  FIG. 1  shows the gun manipulator RotaPlasma of one of the applicants. The gun manipulator  1  is shown, comprising a powder rotary feedthrough  3 , means for current transmission  5 , a gun holder  7 , a gun connection  9  and finally the gun  11 . 
     A sealing at this interface is of essential importance for the coating process as the powder at the end of the powder line is to be injected into a plasma flame under constant conditions by means of a special powder injector. Under the constant conditions, special attention is paid to the parameters powder speed and powder quantity. These values are negatively influenced in particular by leaks in the powder line. A powder leakage, for example in the region of the powder rotary feedthrough, causes pressure variations which lead to pulsation of the powder. In addition, a leak in the powder line can cause the powder to escape to the outside due to the slight overpressure in the powder line, and thus greatly reducing the quantity of powder transported to the injector. The overpressure within the powder line is 100 to 800 mbar, depending on the parameter. 
     According to the state of the art, a combination of Teflon bush-seal ring is used for the powder rotary feedthroughs. This was previously possible because, according to the state of the art, the gun of such a device is rotated at just under 200 revolutions per minute. At 200 rpm, however, the device already has a very limited service life, so that this process cannot be operated economically. The aim would also be to be able to operate the device at rotational speeds of up to 800 rpm and preferably more. In combination with an increased conveying rate adapted to the increased rotational speed, it is then possible to produce layers with significantly improved quality, as disclosed in a patent application already pending from one of the applicants. Such high rotational speeds can also be achieved for short periods with conventional devices. If, however, you want to produce layers beyond the laboratory scale, a new variant of the powder rotary feedthrough is required. 
     It should be noted that the application at 800 rpm and even above is far removed from the rotational speeds of, for example, 20000 rpm that can be achieved in DE 19932355 B4 with non-lubricating media and that the problem of the rotary feedthrough that is safe to run dry described there still plays a subordinate role. 
     However, with a powder-gas-mixture as medium other and quite serious problems occur. In particular, such a powder has in no case only powder grains of a certain grain size. The grain size can be limited very well towards the top, for example by filtering. However, the presence of smaller particles can only be excluded up to a certain percentage, in particular because particle collisions occur due to the rapid transport of the powder, for example, which lead to fragments of smaller dimensions. 
     As a result, however, it does not matter how small the gap between the two seal ring disks is, there will always be particles that are even smaller and will sit in the gap. On the one hand, this results in friction. On the other hand, these can lead to an enlargement of the gap, which in turn can result in larger particles settling in the enlarged gap, and so on. In the end, the leakage described above may occur. However, it can also lead to pressure fluctuations due to the changed sealing quality, which in combination with the rotation of the gun can lead to a pulsating powder flow. All this has the consequence that there can be fluctuations in the thickness of the coating when the coating is applied, because both phenomena, powder pulsation and powder leakage, result, as already described above, in an uneven powder application and thus in an uneven coating thickness distribution. This is one of the most important quality features, in particular for the internal coating of cylinder bores, as the layer is then finished by a further process step (honing). An uneven coating thickness distribution causes, among other things, defects on the final machined layer surface caused by an insufficiently high machining allowance. Since this final machined surface (e.g. in a cylinder bore) represents a seal surface to a counter body (e.g. piston with piston rings), the tightness can no longer be sufficiently guaranteed by the formation of such a defect. If this quality defect occurs, for example, in a crankcase, it is associated with relatively high reject costs or rework costs. 
     It is pointed out in particular that the solution described in DE 199 32 355 B4 by means of sealing cylinder surfaces does not provide any remedy, but additionally leads to the difficulty that fine-grained powder material adheres between the cylinder surfaces and leads to extreme abrasion and thus to the destruction of the cylinder surfaces due to the fast rotation. 
     However, even in the case of seal rings that slide on each other and have flat sliding surfaces, the penetration of abrasive dust particles or powder components into the sliding seal gap cannot be completely avoided, which in the case of conventional rotary feedthroughs leads to the aforementioned very rapid wear of the sliding seal surfaces. 
     The invention is therefore based on the object of providing a rotary feedthrough for the feedthrough of a non-lubricating powder-gas mixture, which shows only low wear at moderately high rotational speeds above 200 rpm and up to 2,000 rpm and can therefore be used economically. 
     This object is met by a rotary feedthrough with the features of claim  1 . The rotary feedthrough according to the invention is characterized by a purging channel running transversely to the axis of the sliding seal disks. This channel, which is formed, for example, by two diametrically opposed openings in the housing of the rotary feedthrough, preferably at the axial position of the sliding seal disks, allows a purging agent to flow transversely to the axis of the sliding seal disks and past them on the outside. The purging agent flow, in one variant an air flow, frees these sliding seal surfaces from powder particles of the medium to be conveyed, in particular when the sliding seal surfaces are moved axially apart for a short time, thereby preventing rapid wear of the sliding seal surfaces. 
     In order to avoid the extreme abrasion described above, it is advisable to resiliently pretension the flat sliding seal surfaces in contact with each other so that these sliding seal surfaces normally touch each other with a well-defined pressure force. Particle fragments of the powder, which nevertheless get between the sliding surfaces, are carried away by the purging agent flow and do not immediately lead to such extreme damage as described above. In general, the contact pressure of the sliding seal surfaces can be adjusted by the geometry of the transport channel in connection with the pressure prevailing therein, the geometry on the outside of the rotating parts in the purge chamber and the pressure prevailing therein, and finally by selecting the contact spring. 
     According to the invention, a constant gas flow (purge gas), for example compressed air, is conducted through a purge chamber arranged around the seal surfaces. The purge gas flow preferably used in operation is approx. 200 l/min, the operating pressure is preferably selected between 0.5 and 1 bar. In addition, a pressure surge of purge gas can be intermittently applied to clean the leakage chamber. This can preferably be done every 30 s with preferably max. 1 bar. 
     The purge chamber and the purge gas have the following effect: powder particles and/or fragments that have penetrated between the two seal surfaces are removed from the sliding seal surfaces due to the transverse purge gas flow and the superimposed rotary motion of a seal surface. 
     In this respect, a part of the powder can be returned to the conveying flow, another part is captured by the volume flow of the purge gas and removed from the purge air chamber (see  FIG. 6, 7 ). This guarantees that the space surrounding the sliding seal disks, where the spring mechanism is located, which is responsible for the contact force on the seal surfaces, does not gradually fill up with powder and interferes with the functioning of the mechanism. 
     As described above, compressed air can be used as purge gas. However, due to the cleaning process of the seal sliding surfaces described above, a (albeit very small) quantity of purge gas penetrates into the transport channel. For this reason, the carrier gas of the powder-gas mixture or at least a similar gas is used as purge gas in a particularly preferred embodiment of the present invention. 
    
    
     
       The invention is now represented in detail and exemplarily with reference to the figures. 
         FIG. 1  shows a gun manipulator. 
         FIG. 2  shows a powder rotary feedthrough according to the invention. 
     
    
    
       FIG. 2  shows a powder rotary feedthrough  201  according to the invention, which comprises a powder transport channel  205  embedded in a housing  203  with an arrow with a dashed line drawn in the direction of flow of the powder-gas mixture. The small unfilled circles in the powder transport channel should schematically represent the powder particles. The powder transport channel  205  comprises a channel section  205   f , which is stationary in the application and a channel section  205   r , which rotates in the application. The drawn vertical dashed line shall represent the axis of rotation, which is also considered the axis of the rotary feedthrough. 
     If the term rotating components is used here (e.g. channel section  205   r ), the term essentially refers to rotatable supported components, since the component can rest if the device is not used. 
     Where the stationary channel section  205   f  and the rotating channel section  205   r  meet with their ends, a stationary flat circular-ring-shaped sliding disk  207   f  is provided on the stationary channel section  205   f  and a further stationary flat circular-ring-shaped sliding disk  207   r  is provided on the rotating channel section  205   r , which, however, rotates with the channel section  205   r . According to the invention, this region of the powder transport channel  205  is enclosed by a purge chamber  209  to which at least one gas supply  211  and at least one gas discharge  213  are connected. 
     Where the rotating channel  205   r  is embedded within the housing, it is embedded via rotation-permitting bearings  215 , wherein these bearings  215  or further bearings also allow an axial movement relative to the rotation axis. Pressing means  217 , which for example are designed as springs as shown here, press the rotating sliding disk  207   r  against the stationary sliding disk  207   f  with predefined pressure. 
     In the application, while the powder-gas mixture is transported through the powder transport channel, purge gas is fed into the purge chamber  209  through the gas supply  211  and drawn off again through the gas discharge  213 . The purge gas flow can be fed at a higher pressure and passed through the purge chamber than it is inside the transport channel. The corresponding situation is represented schematically in  FIG. 3 . For reasons of clarity, the reference signs are not repeated in the display of the springs. 
     As soon as a small gap is formed between the two seal surfaces and powder penetrates between the two seal surfaces, the powder is sucked together with the purge gas into the rotating channel  205   r  or transported into the purge chamber and powder particles are thus removed from the gap between the sliding disks  207   f  and  207   r , so that the gap closes quickly again essentially due to the spring force ( FIG. 4 ). Due to the overpressure in the purge chamber and the superimposed rotary motion of the sliding disk  207   r , the powder preferably returns into the transport channel and the gap between the sliding disks  207   f  and  207   r  is freed of powder particles. 
     However, if powder particles have penetrated into the purge chamber as shown in  FIG. 5 , they are purged out of the purge chamber with the purge gas. This means that the purge chamber has a self-cleaning mechanism. 
     In an embodiment, the sliding seal disks can be made of ceramic or of hard metal. 
     According to a preferred embodiment, the seal surfaces of the sliding disks  205   f  and  205   r  are coated with a coating, which on the one hand reduces the friction of the disks and on the other hand, preferably also acts as an anti-adhesion coating against adhesion of the powder particles on the seal surfaces. 
     The coating, which can be for example a PVD coating and/or a CVD coating, can be designed for example as a ceramic coating or also as a carbon coating. In the case of the carbon coating, it can be advantageous to realize an increasing sp 2  hybridized portion to the surface of the seal surfaces, starting from a diamond-like coating. In this way, the layer can comprise the outer component as a run-in coating, which in use wears to a certain depth, i.e. to a certain degree of hardness. 
     In general, the coating permits the use of non-ceramic, more easily machinable substrate materials that are used for the sliding disks. The simpler processing of the materials allows narrower tolerances to be achieved. 
     According to a particularly preferred embodiment of the present invention, the mass flow of the purge gas flowing into the purge chamber through the gas inlet is continuously or regularly compared with the mass flow of the gas withdrawn from the purge chamber. If the latter is also only slightly reduced compared to the former, this means that a gap has formed through which the purge gas flows into the transport channel. If necessary, the pretension generated by the springs is then reduced to release powder particles trapped in the gap and thus accelerate the cleaning process. 
     The sliding seal surfaces have so far been described as flat seal surfaces. However, conical or curved seal surfaces are also conceivable. In fact, all rotationally symmetric surfaces that do not have a cylinder shell component can be used. However, the flat seal surfaces are to be preferred since they offer the least resistance to the purge gas flow and are therefore conducive to an effective purge process. 
     A rotary feedthrough was disclosed for the feedthrough of a powder-gas mixture from a stationary machine part into a rotating machine part, with a seal in the form of two flat circular-ring-shaped sliding seal surfaces arranged one on the other in a sliding manner, which sliding seal surfaces are arranged concentric to the axis of rotation of the rotating machine part and can be moved apart from each other in the axial direction, so that they form a gap. The rotary feedthrough is characterized in that the seal is embedded in a purge chamber having at least one gas inlet and at least one gas outlet. 
     The flat seal surfaces are resiliently pretensioned in contact with each another. 
     The flat seal surfaces can be coated with a friction-reducing coating, preferably with a PVD coating. The coating preferably is a coating which is configured as an anti-adhesion coating for the powder material. 
     The coating may comprise a ceramic layer and/or a carbon layer, wherein the carbon layer preferably comprises internally a DLC layer and particularly preferably, starting from the DLC layer towards the surface, comprises a higher sp 2  hybridized portion of carbon compounds. 
     A method for feeding a powder-gas mixture from a stationary machine part into a rotating machine part was disclosed. The method is characterized in that a rotary feedthrough is used as described above and the purge chamber is flowed through by a purge gas at an overpressure compared to the pressure prevailing in the transport channel in such a way that when powder particles penetrate between the sliding surfaces through the gap thereby formed, a flow of purge gas takes place into the transport channel, which flow tears the powder particles out of the gap with it and thus cleans the gap of powder particles. 
     If powder particles are present in the purge chamber, it can be cleaned by means of the flow of the purge gas.