Abstract:
A screw includes a head and a shank, and includes a coolant passage which has an inflow opening and an outflow opening. The cooling pasage does not pass axially through the entire head and shank. By avoiding a coolant flow which passes axially through the entire screw, especially effective utilization of a coolant is achieved, in particular by coolant which also flows on the outer surface of the screw.

Description:
[0001]    The present application hereby claims priority under 35 U.S.C. §119 on European patent application number EP 02018492.5 filed Aug. 16, 2002, the entire contents of which are hereby incorporated herein by reference.  
         FIELD OF THE INVENTION  
         [0002]    The invention generally relates to a screw which can be internally cooled by a flowing coolant.  
         BACKGROUND OF THE INVENTION  
         [0003]    Screws subjected to high mechanical and thermal loads are used, for example, in gas turbine construction. An example of application is the fastening of a combustion-chamber inner lining to a wall of a combustion chamber.  
           [0004]    It is normally attempted to operate a gas turbine with as high a temperature as possible in the combustion chamber in order to achieve a high efficiency. Gas temperatures of 1200° C. to 1300° C. are typically achieved at the outlet of the combustion chamber. The combustion-chamber inner lining is often fastened to the combustion-chamber wall with screws inserted from the inside, i.e. from the combustion chamber. The head of the screws is therefore directly exposed to the fuel gas in the combustion chamber. If the screw or a part of the screw, in particular the screw head, due to a failure of said screw, gets into the combustion chamber and is entrained by the gas flow, serious damage to the downstream turbine is the result.  
           [0005]    In order to reliably prevent failure of the fastening screws arranged in the combustion chamber, the screws can be designed to be coolable. To this end, the fastening screw has, for example, an axial bore through which a cooling fluid, in particular cooling air, is directed into the combustion chamber from the outside of the latter. The cooling fluid mixes with the fuel gases in the combustion chamber. As a result, the temperature in the combustion chamber is undesirably reduced. The loss of efficiency associated therewith is tolerated in order to ensure sufficient strength of the fastening screws by means of the cooling. However, ensuring sufficient mechanical loading capacity of the fastening screws means a considerable coolant requirement.  
         SUMMARY OF THE INVENTION  
         [0006]    An object of an embodiment of the invention is to specify a screw which can be internally cooled by a flowing coolant. A further object of an embodiment of the invention is to specify a screw which has an especially low coolant requirement.  
           [0007]    An object may be achieved by a screw having a head, a shank, and a coolant passage having an inflow opening and an outflow opening. The coolant passage does not pass axially through the entire head. The screw has “closed cooling”.  
           [0008]    It is out of the question for a coolant inlet or outlet to be arranged in each case on both end faces of the screw, i.e. both on the end face of the shank and on the top surface of the head. At least one coolant inlet or outlet is arranged laterally, i.e. between the top surface of the head and the end face of the shank. In particular in cases in which an exclusively axial coolant inlet or outlet is not possible or is not desirable from the processing point of view, a coolant flow directed radially toward the longitudinal axis of the screw can therefore be produced—in the case of a lateral coolant inlet. Due to this radial coolant flow, the screw is cooled from outside by the same coolant which cools it from inside. This makes possible especially effective use of coolant, i.e. very effective utilization of the heat capacity of the coolant, and thus especially economical use of coolant. The temperature difference between the coolant and the screw to be cooled is utilized to an especially high degree.  
           [0009]    According to a preferred configuration, the coolant outlet is arranged laterally on the screw in such a way that the coolant flowing out of the screw flows in the axial direction along the shank. In this way, in addition to the inner cooling of the screw, especially effective cooling of the outer surface of the screw is also ensured. In order to cool the shank over its entire length not only from inside but also from outside, the coolant outlet opening is advantageously arranged as far as possible directly next to the head or on the head itself or in the transition region between head and shank. The coolant flows out of the coolant outlet opening axially in the direction of the end face of the shank. On the other hand, the coolant inlet opening is preferably located on the end face of the shank, i.e. on the shank end. The coolant inflow direction is thus opposed to the coolant outflow direction.  
           [0010]    In order to cool the entire circumference of the shank as uniformly as possible from outside, a plurality of outlet openings, preferably distributed at least approximately in a rotationally symmetrical manner about the axis of the screw, are preferably arranged on the shank circumference and/or on the head. In addition, the uniformly distributed outlet openings have the advantage that asymmetrical weakening of the cross section of the shank and thus an unnecessary reduction in the mechanical loading capacity are avoided. The coolant outlet opening or openings is/are preferably arranged in a screw collar defining the shank toward the head as a thickened region of the shank. By the arrangement of the coolant outlet openings in the screw collar, weakening of the cross section of the shank on account of the coolant outlet openings is avoided. Furthermore, the screw collar can serve to deflect the direction of flow of the coolant.  
           [0011]    According to a preferred configuration, the head of the screw is internally cooled, the direction of flow of the coolant preferably being deflected in the head, i.e. the latter has a “deflecting region”. A coolant inlet or outlet opening is preferably not provided on the top surface or end face of the head. In this way, processes, for example chemical reactions, which take place in the space bordering the end face of the head are not affected by coolant or other fluid flowing into or out of this space. On the other hand, the coolant flowing out of the screw laterally in the region of the shank, the head and/or the screw collar is advantageously passed on in a specific manner and/or is advantageously conducted in a closed coolant circuit. In this case, a coolant inlet opening arranged axially in the shank end is especially suitable for a specific cooling feed or return. The coolant can in this case flow into the coolant passage, which to begin with runs axially in the shank, without deflection and thus virtually without pressure loss. Furthermore, a feed line can be connected, for example screwed, in a simple manner to the coolant inlet opening arranged on the shank end.  
           [0012]    Specific cooling of the head is especially important for the safety of the screwed connection during high thermal stressing. In this case, the entire head should be cooled as uniformly as possible. This is preferably achieved by the fact that the coolant flowing through the shank into the screw flows in a straight line at least up to the screw collar, in particular up to the head, and is not deflected until in the head, the coolant being split up in the deflecting region into a plurality of coolant partial flows. In this case, the individual sectional passages are distributed at least approximately uniformly, in particular in a rotationally symmetrical manner, in the head of the screw, also in the screw collar if need be.  
           [0013]    The closed internally cooled screw is especially suitable for fastening a combustion-chamber inner lining to a combustion-chamber wall of a gas turbine.  
           [0014]    The advantages of an embodiment of the invention lie in particular in the fact that, by avoiding a coolant flow which passes axially through the entire screw, firstly especially effective utilization of the coolant can be achieved, in particular by coolant which also flows on the outer surface of the screw. Secondly, this avoids an undesirable inflow and/or outflow of coolant into or out of the screw, as a result of which an unintentional direct effect of coolant on a space arranged axially to the screw is ruled out.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    An exemplary embodiment of the invention is explained in more detail below with reference to the drawings, in which:  
         [0016]    [0016]FIGS. 1 a, b  show an internally coolable screw and a cutaway detail of a coolant passage of the internally coolable screw,  
         [0017]    [0017]FIG. 2 shows the internally coolable screw according to FIG. 1 with an associated bush,  
         [0018]    [0018]FIG. 3 shows a gas turbine with a gas-turbine combustion chamber having a screw according to FIGS. 1 and 2. 
     
    
       [0019]    Parts corresponding to one another are provided with the same designations in all the figures.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    [0020]FIG. 1 a  shows an internally coolable screw  1 , the coolant passage or cooling passage  2  of which is shown as a cutaway detail in FIG. 1 b . The screw  1  has a shank  3  and a head  4  and extends along an axis or axis of symmetry A from a top surface  5 , lying at the bottom in the representation, of the head  4  up to a shank end  6 . The shank  3 , with a shank circumference  7 , has a thread  8  and a screw collar  9  of thickened design defining the shank  3  toward the head  4 . On its top surface  5 , the head  4  has a hexagonal actuating opening  10  for a hexagon socket key.  
         [0021]    A coolant or cooling fluid F flows axially at the shank end  6  in inflow direction R 1  into an inflow opening  6   a  of the screw  1  and discharges from the latter at three coolant-outflow openings or outflow openings  12  in outflow direction R 2 , which is opposed to the inflow direction R 1 . The cooling passage  2  runs first of all coaxially in the shank  3  and, in the head  4 , assumes the course indicated by broken lines in FIG. 1 a  and shown in detail in FIG. 1 b . The cooling passage  2  widens close above the actuating opening  10 , as a result of which an impingement-cooling effect is produced in this region. The region of the cooling passage  2  in the head  4  is referred to as deflecting region  13 .  
         [0022]    After it has widened, the cooling passage  2  splits into three curved sectional passages  14 . The sectional passages or branches  14  have a uniform cross section and run inside the head  4  as far as close to the top surface  5 . There, the sectional passages  14  branch in a smoothly blended manner into twice the number of fine passages  15 . The six fine passages  15  run close to the surface of the head  4  before they are blended smoothly inward, i.e. in the direction of the axis A, and two fine passages  15  each are combined to form one sectional passage  14 .  
         [0023]    Via the three sectional passages  14  thus combined in the vicinity of the shank  3 , the coolant F discharges from the screw  1  through the outflow openings  12  arranged in a rotationally symmetrical manner in the screw collar  9 . The head  4  of the screw  1  is thus cooled intensively without coolant F flowing out in the direction of the top surface  5 . A sealing ring  17  can be put onto the inside  16  of the head  4 . The outflow openings  12  are open radially outward, i.e. toward the sealing ring  17  put onto the screw collar  9 .  
         [0024]    The screw  1  is made of a cast material. The shape of the cooling passage  2  is selected in such a way that the screw  1 , including the entire cooling passage  2 , can be produced by a casting process. Further processing steps, in particular machining steps, such as drilling, are not necessary for producing the cooling passage  2 , including the sectional passages  14  and the fine passages  15 . The casting process works without the use of “lost inserts”. To this end, the screw  1  with the cooling passage  2  is formed in such a way that, with an undercut being avoided, the casting process can be carried out using a plurality of mask elements. In this case, provision is made for a first mask element to be positioned in a casting mold, in which first mask element a second mask element is guided like a slide in a displaceable manner. After the casting, the mask elements can easily be removed and can therefore be reused.  
         [0025]    As FIG. 2 shows, the screw  1  can be screwed together with a bush  18 . The bush  18  has an external thread  19  designed as a left-hand thread and also an internal thread  20 , which, in the same way as the thread  8  of the screw  1 , is designed as a right-hand thread and into which the screw  1  can be screwed. The bush  18  is screwed from outside into a wall (not shown here) of a combustion chamber of a gas turbine. A combustion-chamber inner lining is fastened with the screw  1  to the wall of the combustion chamber. Due to the design of the external thread  19  of the bush  18  as a left-hand thread and the design of the internal thread  20  of the bush  18  and of the corresponding thread  8  of the screw  1  as a right-hand thread, the screw  1  is counterscrewed together with the bush  18 . As a safety feature to prevent loosening, the internal thread  20  has an oval cross-sectional region  21 . The screw  1  is secured by clamping the thread  8  in the oval cross-sectional region  21  without further securing elements.  
         [0026]    Enclosed between the combustion-chamber wall, into which the bush  18  is screwed, and the combustion-chamber inner lining, which is held by the screw  1 , is a space in which cooling air F flows, which is then used as preheated combustion air. The coolant F flowing out of the screw  1  flows into this space. This coolant F, which has already been heated in the screw  1 , thus contributes to the increase in the temperature of the combustion air to be fed to the gas turbine and thus to the increase in efficiency. In contrast, in the case of open cooling of the screw  1 , i.e. in the case of a complete axial flow of the coolant F through the entire screw  1 , the coolant F would pass directly into the combustion chamber, reduce the temperature there and thus reduce the efficiency.  
         [0027]    The closed internally cooled screw  1  therefore contributes to the especially effective utilization of the coolant F and at the same time, by increasing the combustion-air temperature, helps to achieve a high efficiency of the gas turbine.  
         [0028]    A gas turbine  22  is schematically shown in cross section in FIG. 3. The gas turbine  22  has a compressor  23  for combustion air, a gas-turbine combustion chamber or combustion chamber  24 , and a turbine  25  for driving the compressor  23  and a generator (not shown) or driven machine. To this end, the turbine  25  and the compressor  23  are arranged on a common turbine shaft  26 , which is also referred to as turbine rotor and to which the generator or the driven machine is also connected, and which is rotatably mounted about its center axis  26   a.    
         [0029]    The combustion chamber  24  is fitted with a number of burners  33  for burning a liquid or gaseous fuel. A combustion-chamber wall  27  is lined with a combustion-chamber inner lining  28 .  
         [0030]    The turbine  25  has a number of rotatable moving blades  29  connected to the turbine shaft  26 . The moving blades  29  are arranged in a ring shape on the turbine shaft  26  and thus form a number of moving blade rows. Furthermore, the turbine  25  comprises a number of fixed guide blades  30 , which are likewise fastened in a ring shape to an internal casing  31  of the turbine  25  while forming guide blade rows. The moving blades  29  thus serve to drive the turbine shaft  26  by impulse transmission of the working medium M flowing through the turbine  25 . The guide blades  30 , on the other hand, serve to guide the flow of the working medium M between in each case two moving blade rows or moving blade rings following one another as viewed in the direction of flow of the working medium M. In this case, a successive pair consisting of a ring of guide blades  30  or a guide blade row and of a ring of moving blades  29  or a moving blade row is referred to as a turbine stage.  
         [0031]    In order to make possible a high efficiency of the gas turbine  22 , the gas turbine  22  is operated at a high temperature of the working medium M. The working medium M discharges from the combustion chamber  24  at a temperature of about 1200 to 1300° C. The compressed combustion air fed for the combustion is preheated in a shell space  23 , which is formed between the inner wall  27  of the combustion chamber  24  and the combustion-chamber inner lining  28 , before entry to the burner  23 . As a result, the inner wall  27  of the combustion chamber  24  is cooled at the same time. The screw  1 , which holds the combustion-chamber inner lining  28  on the inner wall  27  of the combustion chamber  24 , is subjected to high mechanical and thermal loads under these operating conditions. The head  4  of the screw  1  projects into the combustion chamber  24 . The highly efficient cooling of the screw  1 , in particular of the head  4 , provides for a high strength of the screw  1  with sufficient safety reserves under all operating conditions.  
         [0032]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.