Abstract:
The invention relates to a method for lubricating and cooling a bearing that is subject to high loads and to a device for carrying out said method. The invention is characterized by dissolving a lubricant in a supercritical gas that is cooled down to a low temperature and feeding the gas/lubricant mixture to the baring. The gas/lubricant mixture is relaxed in the bearing area, thereby releasing the lubricant and lubricating the bearing. The now subcritical gas that is cooled down to a low temperature is used for cooling and is discharged to the exterior. The gas used is carbon dioxide (CO 2 ) and the lubricant is a hydrocarbon-based lubricant.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a method for lubricating and cooling a highly-loaded bearing, in particular a fast-running and/or thermally-loaded bearing, with a lubricant and a gas being mixed, and to a device for carrying out said method. 
       BACKGROUND OF THE INVENTION 
       [0002]    Highly-loaded bearings of the above-described type serve for example for mounting motor spindles in machine tools, compressor and turbine shafts in gas turbines, transmission shafts in fast-running transmissions etc. At high rotational speeds of bearings, heat is generated as a result of the friction, which heat leads to a temperature increase in the bearing. The increased bearing temperature generally adversely affects the geometry of the bearing and therefore the bearing accuracy, and in addition, said increased bearing temperature also adversely affects the quality of the lubricant and therefore the lubricating action. In order to avoid said disadvantages, various measures are already known for cooling highly-loaded bearings, specifically for example by means of a coolant which is conducted through cooling ducts in the bearing surroundings or by means of an increase in the lubricant quantity beyond the minimum lubricant quantity required for lubricating. The first measure is constructively and structurally complex and expensive, and leads in general to high temperature differences in the bearing, with the risk of the bearing being damaged. The latter measure possibly entails high hydraulic losses in the bearing, so that the drive power must be correspondingly increased, resulting in the efficiency being impaired. 
         [0003]    With the Japanese patent publication no. 2000-329140 a bearing unit is known in which, after passing through a radial bearing and an axial bearing, a lubricant is added in a mixing chamber to a gas, in this particular case air, in order to cool the lubricant. The lubricant/gas mixture is subsequently supplied to a so-called occlusion device, in which the gas which has absorbed part of the lubricant heat is separated again from the lubricant and discharged into the environment, and from which the cooled lubricant is extracted and supplied back to the bearings. 
         [0004]    Said arrangement, however, has the disadvantage that the lubricant heats up to its end temperature while still in the bearing region, so that there are relatively high temperature differences between the region in which the lubricant is supplied to the bearing arrangement and the region in which the lubricant leaves the bearing arrangement, and the bearing geometry is therefore still adversely affected. Furthermore, the required quantity of lubricant is very high, so that high hydraulic losses occur in the bearing, and the efficiency of said bearing is reduced. 
       OBJECT OF THE INVENTION 
       [0005]    The invention is based on the object of creating a method for lubricating and cooling a highly-loaded bearing by means of the lubricant supply, and to a device for carrying out said method, by means of which effective and uniform cooling of a bearing can be obtained without additional hydraulic losses in the bearing. 
       DESCRIPTION OF THE INVENTION 
       [0006]    The invention is based on the knowledge that, utilizing a typical property of supercritical or hypercritical gases, specifically that of being able to dissolve other media such as for example lubricants, it should possible to effect a heat transfer from the bearing to the gas directly and from the lubricant to the gas already in the region of the bearing and not only downstream of the bearing, in order to thereby ensure effective and uniform cooling of the entire bearing without additional lubricant quantities. 
         [0007]    According to the features of claim  1  and claim  9 , the invention proceeds from a method and a device for lubricating and cooling a highly-loaded bearing, in particular a fast-running and/or thermally-loaded bearing, in which a lubricant and a gas are mixed in order to cool the bearing. 
         [0008]    The method according to the present invention provides the following method steps:
       a) A gas is placed into a supercritical state.   b) A lubricant is dissolved in the supercritical gas.   c) The gas/lubricant mixture is supplied to the bearing and is relieved of pressure therein with the lubricant being separated from the gas.   d) Gas and lubricant are discharged out of the bearing.       
 
         [0013]    A lubricant for lubrication and a supercooled gas for cooling are accordingly advantageously present in the bearing region. 
         [0014]    Since the cooling and transport medium for the lubricant is a gas, no additional bearing losses are generated. By means of a suitable selection of the gas, it is possible to obtain that, during the relief of pressure, said gas is present for a brief time as “solid snow” which can act not only as cooling medium but additionally also directly as lubricant. Since the heat transfer from the bearing and from the lubricant to the gas takes place already in the region of the bearing and not only downstream of the bearing, uniform and effective cooling of the bearing is obtained. By means of the gas cooling, it is possible above all for the mounted rotating machine part to also be cooled without an increase in losses, so that the temperature differences between the inner ring and the outer ring of the bearing can also be kept within low limits. 
         [0015]    According to one embodiment of the invention, it is provided that the gas is extracted from a gas tank and, after flowing through the bearing, is discharged into the atmosphere, with an environment-friendly gas of course being used, as will be explained in more detail further below. 
         [0016]    According to a further embodiment of the invention, with a suitable selection of the gas, said gas can be stored in the gas tank under natural ambient conditions and, only for the purpose of being placed into a supercritical state, can initially be adiabatically compressed, and then isobarically cooled before the lubricant is supplied and dissolved in the supercritical gas. 
         [0017]    In the bearing, the gas is preferably adiabatically relieved of pressure into an intensely supercooled state while releasing the lubricant, and is then isobarically heated, that is to say heated at a constant ambient pressure, while absorbing bearing heat, and discharged into the atmosphere. The lubricant is for example extracted from a lubricant tank, lubricant sump or the like and, after passing through the bearing, is returned into the lubricant tank again. Carbon dioxide (CO 2 ) and a hydrocarbon-based lubricant are provided respectively as a suitable gas and suitable lubricant for the method according to the invention. 
         [0018]    The device according to the invention for carrying out the above-described method has a device for mixing a lubricant with a gas, and is characterized in particular by the following features:
       a gas tank for storing a gas under natural atmospheric conditions;   a compressor (pump), connected downstream of the gas tank, for compressing the gas;   a heat exchanger serving as a cooling device, connected downstream of the compressor, for cooling the gas;   an apparatus, connected downstream of the heat exchanger, for supplying a lubricant to the gas;   a device, connected downstream of said apparatus, for supplying the gas/lubricant mixture to the bearing;   a throttle or apparatus, assigned to the bearing, for relieving the pressure of the gas/lubricant mixture; and   a device, connected downstream of the bearing, for discharging the gas and for collecting the lubricant.       
 
         [0026]    As has already been described further above, the gas is preferably discharged into the atmosphere, so that the gas circuit is an open circuit. 
         [0027]    According to a further embodiment of the device according to the invention, a lubricant tank is provided for the lubricant, from which lubricant tank the lubricant is extracted for the purpose of being supplied to the gas, and into which said lubricant is returned after passing through the bearing. The lubricant circuit is accordingly a closed circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    One preferred embodiment of the invention is explained in more detail below with reference to the appended drawings, in which: 
           [0029]      FIG. 1  schematically and diagrammatically shows the procedure for lubricating and cooling a radial bearing; 
           [0030]      FIG. 2  shows a temperature/time diagram of the gas during the procedure; 
           [0031]      FIG. 3  shows a flow diagram of the method with the different method steps; 
           [0032]      FIG. 4  shows the procedure of the gas in a p-v diagram; and 
           [0033]      FIG. 5  shows the procedure of the gas in a T-s diagram. 
       
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
       [0034]      FIG. 1  diagrammatically illustrates the method according to the invention and shows a device for carrying out said method in the region of the bearing  2 , in the present case a radial bearing, which is to be lubricated and cooled. A gas/lubricant mixture  4  which has been placed into a supercritical state in a way described previously and to be described on the basis of  FIG. 3  and in which a lubricant is dissolved, is relieved of pressure in a throttle device  6 , which is a non-constructive component of the bearing, for example to the ambient pressure prevailing in the bearing  2 . The lubricant which is dissolved in the gas is hereby separated again, so that said lubricant can be used to lubricate the bearing  2 . The gas which is intensely supercooled by the relief of pressure absorbs heat from the surroundings, that is to say from the bearing  2  and from the lubricant, so that the bearing  2  is cooled. The heated gas  8  which has been separated from the lubricant is discharged and thereby removes the heat which is absorbed from the bearing  2 . 
         [0035]      FIG. 2  schematically shows the temperature T of a bearing over the time t without effective cooling (curve a) and with effective cooling according to the present invention (curve b). As can be seen from  FIG. 2 , the temperature T of the bearing over the course of its operating time t without effective cooling (curve a) increases continuously in the direction of a limit value which, in the case of highly-loaded bearings, can lie above a value which is compatible with the bearing, as was explained in the introduction. By means of a constant removal of a specific heat quantity, that is to say the heat quantity Q per unit time, the temperature curve is displaced (curve b) such that the limit value which is ultimately reached lies below the compatibility limit for the bearing. 
         [0036]      FIG. 3  shows a flow diagram of the method according to the invention, with regard to the gas used in said method. The gas is extracted from a gas tank  10  in which it is stored for example under normal atmospheric conditions. The gas is supplied (path {circle around ( 1 )}) to a compressor  12  and is adiabatically compressed in said compressor  12  and therefore placed into a supercritical state, with its temperature T and its pressure p increasing. The supercritical gas is subsequently supplied (path {circle around ( 2 )}) to a cooling device  14  where it is isobarically cooled but remains supercritical. The dissipated specific heat quantity is denoted by Q 0 . 
         [0037]    Lubricant which is extracted from a lubricant tank  21  is added to the cooled gas, as illustrated by the arrow  16 . The lubricant is dissolved in the cooled, supercritical gas. The gas/lubricant mixture is subsequently supplied to a throttle device  18  which is assigned constructively to the bearing, in which throttle device  18  said gas/lubricant mixture is adiabatically relieved of pressure to the ambient pressure prevailing in the bearing and placed into a wet vapor phase. The dissolved lubricant is separated (arrow  19 ), so that it can perform its actual task of lubricating the bearing. 
         [0038]    Since the gas is briefly present as “solid snow” as it is relieved of pressure, it can likewise serve directly as lubricant. As it passes through the bearing (path {circle around ( 4 )}), the gas absorbs heat from the bearing (and from the lubricant), so that these are cooled. The bearing region is illustrated symbolically in  FIG. 3  as a heat exchanger  20 . The specific heat quantity absorbed by the gas is denoted by Q 1 . 
         [0039]    The gas which is isobarically heated in the bearing region (heat exchanger  20 ) is discharged (path {circle around ( 5 )}) into the atmosphere, while the lubricant is supplied back to the lubricant tank  21 . 
         [0040]    The procedure illustrated in  FIG. 3  is briefly summarized again below:
       {circle around ( 1 )} (Ideally unthrottled) CO 2  
           (p 1 , T 1 )   
           {circle around ( 2 )} Adiabatically compressed CO 2  
           (supercritical)   (p 2 &gt;p 1 , T 2 &gt;T 1 )   
           {circle around ( 3 )} Isobarically cooled CO 2  
           (supercritical)   (p 3 =p 2 , T 3 &lt;T 2 )   (Addition of lubricant)   
           {circle around ( 4 )} Adiabatically expanded CO 2  
           (Wet vapor)   (p 4 =pu, T 4 &lt;Tu)   (Lubricant separated)   
           {circle around ( 5 )} Isobarically heated CO 2  (gas)
           (p 5 =pu, T 5 =Tu)   
               
 
         [0056]      FIG. 4  shows the described procedure, with regard to the gas, in a p-v diagram. The gas states denoted by {circle around ( 1 )}, {circle around ( 2 )} etc. correspond to the gas states of  FIG. 3  which are denoted by the same symbols. 
         [0057]    The gas ({circle around ( 1 )}) which is extracted from the gas tank is adiabatically compressed along the curve  22 , isobarically cooled along the curve  24 , adiabatically relieved of pressure along the curve  26 , and isobarically heated along the curve  28 . The bearing heat which is absorbed in the process is dissipated outwards into the atmosphere. As can be seen from  FIG. 4  and as has already been discussed further above, the illustrated process is not a cycle but rather an “open” process, that is to say the gas is expended. 
         [0058]      FIG. 5  shows the same procedure, with regard to the gas, again in a T-s diagram, with the symbols {circle around ( 1 )}, {circle around ( 2 )} etc. which are used again corresponding to the symbols used in  FIGS. 3 and 4 . As is generally known, the areas under an isobaric curve correspond to an enthalpy difference, that is to say a supplied or dissipated specific heat quantity, so that the specific heat quantity Q 0  extracted from the gas can be read from below the isobaric curve  24 ′, and the specific heat quantity Q 1  absorbed by the gas can be read from below the isobaric curve  28 ′. 
       LIST OF REFERENCE SYMBOLS 
       [0000]    
       
           2  Bearing 
           4  Gas/lubricant mixture 
           6  Throttle device 
           8  Gas 
           10  Gas tank 
           12  Compressor 
           14  Cooling device 
           16  Arrow 
           18  Throttle device 
           19  Arrow 
           20  Heat exchanger, bearing 
           21  Lubricant tank 
           22  Curve 
           24  Curve 
           24 ′ Curve 
           26  Curve 
           28  Curve 
           28 ′ Curve 
         a Curve 
         b Curve 
         p Pressure 
         Q Specific heat quantity 
         Q 0  Dissipated heat quantity 
         Q 1  Supplied heat quantity 
         T Temperature 
         t Time