Abstract:
Device for connecting two rotating shafts ( 10, 12 ), one driving and the other driven, comprising rectilinear splines ( 20 ) formed on the driving shaft ( 10 ) and engaged in complementary rectilinear splines ( 22 ) of the driven shaft ( 12 ), the splined region of the driven shaft having, in the vicinity of one of its longitudinal ends, at least one cylindrical portion ( 26 ) having a greater torsional flexibility.

Description:
BACKGROUND OF THE INVENTION AND DESCRIPTION OF THE PRIOR ART 
       [0001]    The present invention relates to a device for connecting two rotating shafts, in particular in a turbomachine, between a turbine shaft and a compressor shaft of the turbomachine. 
         [0002]    In a turbomachine such as an aircraft turbofan or turboprop, the shaft of the low-pressure turbine rotationally drives the shaft of the low-pressure compressor, these two shafts being rotationally connected by a system of rectilinear splines which are formed, for example, on the outer cylindrical surface of the turbine shaft and which are engaged in complementary rectilinear splines of an inner cylindrical surface of the compressor shaft. 
         [0003]    In operation, the splines of the two shafts are elastically deformed in torsion, and it is found that the stresses are maximum at the longitudinal ends of the splines of the male shaft, which is the turbine shaft, this concentration of stresses limiting the service life of this shaft. 
       SUMMARY OF THE INVENTION 
       [0004]    The object of the invention is in particular to provide a simple, effective and economic solution to this problem. 
         [0005]    The invention accordingly provides a device for connecting two rotating shafts, one driving and the other driven, one end of the driving shaft having rectilinear splines and being engaged in a female portion of the driven shaft having rectilinear splines complementing those of the driving shaft, wherein the splined region of the driven shaft comprises at least one cylindrical portion which has a greater torsional flexibility and is situated in the vicinity of a longitudinal end of the splined region. 
         [0006]    In this connection device, the greater torsional flexibility of that portion of the driven shaft which is close to one end of the splines of the driving shaft makes it possible to reduce the maximum stresses applied to this end in operation, it being possible for this reduction to be around 30% and to significantly increase the life of the driving shaft. 
         [0007]    According to another feature of the invention, the or each torsionally more flexible cylindrical portion of the driven shaft (and/or the corresponding portion of the driving shaft) is devoid of splines. 
         [0008]    The driven shaft (and/or the driving shaft) thus has two splined portions separated longitudinally from one another by a torsionally more flexible portion, thus making it possible to reduce the stresses applied to the splines of one splined end portion of this shaft while increasing the stresses applied to the ends of the splines of the other splined portion of this shaft. 
         [0009]    In other words, when the driving and driven shafts each have two splined portions separated longitudinally, the maximum stresses are distributed over the ends of the splined portions and are less pronounced than in the case where the shafts have a single splined portion. 
         [0010]    In one embodiment of the invention, the splined region of the driven shaft comprises at least two cylindrical portions having a greater torsional flexibility, these two cylindrical portions being separated from one another. 
         [0011]    When the shafts each comprise three splined portions separated longitudinally, the maximum stresses applied to the splines in operation are distributed over the ends of the three splined portions and are even less pronounced. 
         [0012]    In a variant, the or each torsionally more flexible portion of the driven shaft and/or the corresponding portion of the driving shaft are machined in order to eliminate any contact between them upon the transmission of a torque. 
         [0013]    The torsional flexibility of the driven shaft can be increased, in the or each aforementioned cylindrical portion, by reducing its thickness and/or its diameter, and/or by machining, for example by drilling. 
         [0014]    The length of the or of each more flexible cylindrical portion is approximately 15 to 30% of the total length of that portion of the driven shaft having the splines. 
         [0015]    The invention also relates to a turbomachine, such as an aircraft turbofan or turboprop, comprising a compressor whose shaft is driven rotationally by a turbine shaft, wherein the shaft of the compressor and the turbine shaft are connected by a device as described above. 
         [0016]    The aforementioned more flexible cylindrical portions of the driven and driving shafts may be in the vicinity of the upstream and downstream ends of the splined regions of the shafts. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The invention will be better understood and other features, details and advantages of the invention will become apparent on reading the description below given by way of nonlimiting example with reference to the appended drawings, in which: 
           [0018]      FIG. 1  is a schematic half-view in axial section of a connection device according to the prior art, between a turbine shaft and a compressor shaft of a turbomachine; 
           [0019]      FIG. 2  is a schematic half-view in axial section of a connection device according to the invention, between a turbine shaft and a compressor shaft of a turbomachine; 
           [0020]      FIG. 3  is a schematic view in axial section of the compressor shaft shown in  FIG. 2 , on a larger scale; 
           [0021]      FIG. 4  is a schematic view corresponding to  FIG. 3  and illustrating a variant embodiment of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]      FIG. 1  is a highly schematic representation of a shaft  10  of a low-pressure turbine and a shaft  12  of a low-pressure compressor of a turbomachine such as an aircraft turbofan or turboprop, the turbine shaft  10  driving the shaft  12  of the compressor about the longitudinal axis  14  of the turbomachine by means of a prior art connection device which uses splines to produce the connection. 
         [0023]    The upstream end  16  of the turbine shaft  10  forms a male cylindrical portion which is engaged in a female cylindrical passage  18  of the shaft  12  of the compressor and which has, on an outer cylindrical surface, a plurality of rectilinear splines  20  of substantially rectangular or trapezoidal cross section, these splines being uniformly distributed around the axis  14  and cooperating with corresponding rectilinear splines  22  of an inner cylindrical surface of the passage  18  of the shaft  12 . 
         [0024]    The turbine shaft  10  is driven rotationally about the axis  14  by means of the energy supplied by the hot gases leaving the combustion chamber of the turbomachine, these hot gases driving the rotors of the low-pressure turbine which are fastened to the downstream end of the turbine shaft  10 . This shaft  10  in turns transmits, by way of the splines  20 ,  22 , a torque to the shaft  12  of the compressor which bears the fan of the turbomachine at its upstream end. 
         [0025]    During the transmission of a torque, the splines  20 ,  22  of the shafts  10 ,  12  are elastically deformed in torsion about the axis  14 , thus generating considerable stresses at the longitudinal ends of the splines  20  of the turbine shaft  10 , in particular at their downstream longitudinal ends, and resulting in a reduction in the service life of this shaft  10 . 
         [0026]    The invention makes it possible to overcome this problem by means of calibrating the torsional flexibility of at least one portion of the splined region of the shaft  12  of the compressor, this portion being situated in the vicinity of the downstream end of the splined region of the shaft  10 . 
         [0027]    In the example represented in  FIGS. 2 and 3 , the splined regions of the two shafts have substantially the same length and the splined region of the shaft  12  of the compressor comprises a cylindrical portion  26  devoid of splines that is situated close to the downstream end of the splined region of the shaft  12  and that has a greater torsional flexibility than the remainder of this splined region. In this way, the downstream end portion of the splined region of the shaft  12  is less resistant to the torsional loads applied by the splined region of the shaft  10 , thus limiting the stresses generated in the downstream end portion of the splines of the shaft  10 , thereby increasing the service life of this shaft. 
         [0028]    The cylindrical portion  26  of the shaft  12  (defined by dashed lines) is situated between two independent splined portions, an upstream splined portion  28  and a downstream splined portion  30 . 
         [0029]    The portion  32  of the turbine shaft, aligned radially with the portion  26  of the shaft  12 , is likewise devoid of splines  22  and is located between two independent splined portions, an upstream splined portion  34  and a downstream splined portion  36 . 
         [0030]    The torsional flexibility of the portion  26  of the shaft  12  has the effect of distributing the stresses in operation over the upstream and downstream longitudinal ends of the two splined portions  34  and  36  of the shaft  10 . The maximum stresses experienced by the splines are thus less pronounced than in the case where the shafts have a single splined portion. 
         [0031]    The torsional flexibility of the portion  26  of the shaft  12  is calibrated by varying its thickness e, its outer radius R and/or its axial dimension L. The smaller the thickness e or the radius R of this portion  26 , the greater its torsional flexibility. The torsional flexibility of the shaft can likewise be increased by machining the cylindrical portion  26  of the shaft  12 . 
         [0032]    In the example represented in  FIG. 3 , the portion  26  of the shaft  12  has two annular rows of radial drilled holes  40 , which are uniformly distributed about the axis  14  of the turbomachine.  FIG. 4  shows a variant embodiment of the invention in which the portion  26  of the shaft  12  has a single annular row of radial drilled holes  40 ′, these drilled holes  40 ′ having a diameter which is greater than that of the drilled holes  40  shown in  FIG. 3 . The number, the arrangement and the dimensions of these drilled holes  40 ,  40 ′ are determined in such a way that the flexibility of this portion  26  can be calibrated with precision. 
         [0033]    The cylindrical portion  26  of the shaft here has a length of approximately 30 mm, and the upstream and downstream splined portions have lengths of approximately 100 mm and 10 mm, respectively. 
         [0034]    In yet another variant (not shown), the portion  26  of the shaft  12  having greater flexibility and the corresponding cylindrical portion  32  of the shaft  10  comprise splines, at least some of these splines being machined in order to eliminate any contact between them upon the transmission of a torque. This torque is transmitted solely by way of the splines  20 ,  22  situated on either side of the cylindrical portions  26  and  32 . The splines  20 ,  22  of the shafts can have a square, rectangular or trapezoidal cross section. 
         [0035]    The splined region of the shaft  12  of the compressor can have two cylindrical portions  26  of greater flexibility, or even more, these portions being separated from one another and being able to be provided in the vicinity of the upstream and downstream ends of the splined region of the shaft  12 .