Abstract:
The invention relates to a twin spool turbine engine comprising a high-pressure rotor and a low-pressure rotor, at least one accessory gearbox, a drive means, driving coaxial transmission shafts that transmit movement to the accessory gearbox, characterized in that the drive means comprises a high-pressure drive pinion secured to the high-pressure rotor near its upstream end, a low-pressure drive pinion secured to the low-pressure rotor upstream of the high-pressure rotor, and a power take-off module in direct mesh with the drive pinions, driving the transmission shafts. 
     By virtue of the invention, the transmission shafts run coaxial with one another and therefore pass through a single arm. The use of a power take-off module simplifies the mechanism. The turbine engine is simpler to assemble.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a twin spool turbine engine with power take-off means on the low-pressure and high-pressure rotors, to a power take-off module for the turbine engine and to a method for assembling the turbine engine. 
   2. Discussion of Background 
   Some of the power generated by an aeronautical turbine engine is used to power various components, both of the turbine engine and also of the aircraft to the propulsion of which the turbine engine contributes. 
   Some of this power is actually tapped off the high-pressure (HP) compressor, the compressed air of which is used in particular to pressurize and air-condition the cabin of the aircraft, or for de-icing purposes. Some more of this power is tapped mechanically off the shaft of the HP stage of the turbine engine, to drive the input shaft of an accessory gearbox positioned on a casing of the turbine engine. This input shaft is rotationally driven by a transmission shaft running along a structural arm of the casing and itself driven by a pinion secured to the HP shaft. 
   The accessory gearbox, well known to those skilled in the art by the abbreviation AGB, comprises various machines or accessories, for example a generator, a starter, an alternator, hydraulic fuel or oil pumps, etc. These various accessories are mechanically driven by the HP shaft. 
   The current trend is to increase, on the one hand, the tapping of mechanical power because of the increasing role played by electrical means which are reputed to be more flexible in their use. 
   However, tapping off excessive amounts of mechanical power has a negative effect on the operation of the HP spool because it is liable to cause compressor surge, particularly when the engine is running at low speed. 
   The teachings of the prior art are to tap some of the mechanical power from the low-pressure (LP) spool. Various solutions have been imagined. Document FR 2 606 077 proposes to connect the rotors of the turbine engine directly to the two inputs of a differential positioned near them. Such an arrangement is not, however, always possible because of the space that this arrangement requires within the heart of the turbine engine. Document GB 973 388 describes, for an industrial turbine, driving the accessory gearbox, situated on the exterior casing of the turbine, using power take-offs from the HP shaft and the LP shaft: however, the power take-offs from each of the shafts are connected to the accessory gearbox by bulky gear sets. Such cannot be done in a modern turbine engine in which these gear sets have to pass through the structural arms of the casing. 
   The applicant company has filed the French Patent Application No. FR 03 14 429 regarding a twin spool turbine engine, the accessory gearbox of which is driven by the LP and HP shafts, which are connected to transmission shafts, each passing through one structural arm and which are connected to the two inputs of a differential connected at output to the drive shaft of the accessory gearbox. While such a solution is advantageous in numerous respects, it is, however, sometimes necessary not to clutter two structural arms, this being so as to leave the greatest possible amount of space for running the auxiliaries through. The auxiliaries are fluid-carrying pipework or trunking for electrical cables carrying power or information (speed sensors for example). 
   Document U.S. Pat. No 4,776,163 describes a jet engine in which the HP and LP shafts can alternately drive the accessory gearbox, to which they are connected by coaxial transmission shafts. Because they are coaxial, these shafts can run along a common structural arm. However, as can be seen from the FIGURE in that document, the power take-off from the rotors at the end of the shafts is very bulky and requires a significant amount of space over a broad region of the shafts. 
   SUMMARY OF THE INVENTION 
   Modern turbine engines do not have such amounts of space for the power take-off from the rotors at the ends of the transmission shafts. Furthermore, present-day turbine engines are designed for a power take-off from the HP rotor only and it would be desirable to propose a power take-off and transmission device for the LP and HP rotors which could be installed without significant changes to the structure of the turbine engine. It is also necessary, in the case of the pinions secured to the rotors from which the power is taken off, to comply with maximum diameters, defined on the basis of the maximum pinion peripheral speed that is acceptable in order for gears to mesh with a defined amount of slip. For example, the maximum permissible speed at the periphery of the power take-off pinion taking power from the rotor may be of the order of 160 to 170 m/sec. It is also desirable to install a power take-off and transmission device that is of small size and weight. 
   Hence, the invention relates to a twin spool turbine engine comprising a high-pressure rotor and a low-pressure rotor, at least one accessory gearbox, a drive means, driving coaxial transmission shafts that transmit movement to the accessory gearbox, characterized in that the drive means comprises a high-pressure drive pinion secured to the high-pressure rotor near its upstream end, a low-pressure drive pinion secured to the low-pressure rotor upstream of the high-pressure rotor, and a power take-off module in direct mesh with the drive pinions, driving the transmission shafts. 
   By virtue of the invention, the transmission shafts run coaxial to one another and therefore pass through a single arm. The use of a power take-off module simplifies the mechanism, which can easily be installed on a jet engine designed in advance for a power take-off from the HP shaft, simply by adding a pinion to the LP shaft, the power take-off module and the coaxial power take-off shafts. A module is also, by definition, ready assembled before being installed, thus assembled, on the turbine engine, thus making the latter easier to assemble. 
   The invention also relates to a power take-off module for a turbine engine as set out hereinabove comprising a box and two power take-off pinions secured to end pieces contained in the box. 
   The invention also relates to a method for assembling a turbine engine as set out hereinabove, comprising the steps consisting in:
         mounting the high-pressure rotor;   mounting the power take-off module;   fitting the transmission shafts into the power take-off module, and   mounting the low-pressure rotor.       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood with the aid of the following description of its preferred embodiment, with reference to the single attached FIGURE which depicts a schematic sectional view of the power take-off region of the turbine engine of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The turbine engine of the invention is a twin spool turbine engine comprising a low-pressure (LP) rotor  1  and a high-pressure (HP) rotor  2 , which are mounted to rotate about the axis  3  of the turbine engine. This type of turbine engine is well known to those skilled in the art. It may be a jet engine or a turboprop engine, for example. In fact it is any turbine engine comprising a compressor and a turbine, of the twin spool type, having a low-pressure spool and a high-pressure spool. Internal and external or inside and outside is to be understood in the description to be internal or external, or inside or outside pertaining to the turbine engine, in the radial direction, with respect to its axis  3 . 
   More specifically, the turbine engine functionally comprises, from the upstream to the downstream end in the direction in which the gases flow, a fan, a compressor, a combustion engine, a turbine and a jet pipe. As it is a twin spool engine it comprises an LP compressor upstream of an HP compressor and an HP turbine upstream of an LP turbine. The rotor of the fan is secured to the shaft  4  of the LP compressor, itself secured to the shaft  5  of the LP turbine. The LP rotor  1  therefore comprises these three elements secured together. The shaft  5  of the LP turbine runs coaxially inside the HP rotor  2 , which is the shaft of the HP compressor and of the HP turbine, secured together. The downstream end of the shaft  4  of the LP compressor, secured to the shaft  5  of the LP turbine, lies just upstream of the upstream end of the HP rotor  2 . The shaft  4  of the LP compressor is fixed coaxially to the outside of the shaft  5  of the LP turbine. There is a seal  6  between the downstream end of the shaft  4  of the LP compressor and the upstream end of the HP rotor  2 , to provide sealing between the region on the outside, bathed in a mist of lubricating oil, and the region on the inside, in which chiefly air circulates. 
   The shaft  4  of the LP compressor comprises, near its downstream end, a drive pinion  7  that will be termed the LP drive pinion  7 . This LP drive pinion  7  is situated downstream of the downstream bearing  8  of the LP compressor shaft  4 . The HP rotor  2  comprises, near its upstream end, a drive pinion  9  which will be termed the HP drive pinion  9 . This HP drive pinion  9  is situated upstream of the upstream bearing  10  of the HP rotor  2 . The diameter of the LP drive pinion  7  is greater than the diameter of the HP drive pinion  9 . 
   The turbine engine comprises a transmission shaft  11  that transmits the movement of the LP shaft, and that will be termed the LP transmission shaft  11 , and a transmission shaft  12  that transmits the movement of the HP shaft, that will be termed the HP transmission shaft  12 . 
   The LP transmission shaft  11  and HP transmission shaft  12  are rotationally driven by a power take-off module  29  from the LP drive pinion  7  and the HP drive pinion  9 . 
   The HP transmission shaft  12  is connected and secured, at its internal end, to a power take-off pinion  13  of the power take-off module  29 , that will be termed the HP power take-off pinion  13 , designed to mesh with the HP drive pinion  9 . The LP transmission shaft  11  is connected and secured, at its internal end, to a power take-off pinion  14  of the power take-off module  29 , that will be termed the LP power take-off pinion  14 , designed to mesh with the LP drive pinion  7 . The LP power take-off pinion  14  is coaxial with the HP power take-off pinion  13 , of a greater diameter than the latter and situated on the outside with respect to it. 
   Each of the power take-off pinions, the HP one  13  and the LP one  14 , is secured to a respective end piece  15 ,  16  of the power take-off module  29 , which end piece is connected to the corresponding HP shaft  12  or LP shaft  11  to which it is secured. Each end piece  15 ,  16  comprises, from the outside inwards, a cylindrical portion and a frustoconical portion, the diameter of which increases toward the inside. The corresponding HP or LP power take-off pinion  13  or  14  respectively lies at the internal end of the frustoconical portion and here is formed of one piece with the end piece  15 ,  16  supporting it. 
   Mounted between the end pieces  15 ,  16  is a set of bearings allowing these pieces  15 ,  16  and therefore the transmission shafts  12 ,  11  to rotate with respect to one another, either in a co-rotary or in a contrarotary fashion, depending on how the HP rotor  2  and LP rotor  1  are designed to rotate. This set of bearings here comprises, from the outside inwards, a first roller bearing  17 , a ball bearing  18 , which also plays a part in axially positioning the end pieces  15 ,  16  one relative to the other, and a second roller bearing  19 , placed at the internal end of the end piece  16  of the LP transmission shaft  11 . 
   The external portion of the end pieces  15 ,  16 , namely that portion of these pieces that lies on the outside relative to the power take-off pinions  13 ,  14  is enveloped and held in a box  20  of the power take-off module  29  of the turbine engine, that will be named the box  20 . This box  20  is of tubular shape and runs coaxial to the end pieces  15 ,  16  and therefore to the HP and LP transmission shafts  12  and  11  respectively; more specifically, it is of frustoconical overall shape, its diameter increasing towards the inside so as to match the shape of the end pieces  15 ,  16 . Between the end piece  16  of the LP transmission shaft  11  and the box  20  there are two bearings, a ball bearing  21  situated towards the external end of the box  20  and also acting to axially position the end piece  16  with respect to the box  20 , and a roller bearing  22  situated towards the inside end of the box  20 . 
   The box  20  is fixed to the fixed structure of the turbine engine by a flange portion  23  exhibiting an upstream portion  24  hugging the upstream external shape of the box  20 , extending in the downstream direction and exhibiting an elbow  25  from which it exhibits a portion  26  running transversely to the axis  3  of the turbine engine, which portion  26  is fixed in this instance using screws, to a flange  27  of the fixed structure of the turbine engine. The upstream portion  24  is fixed to a flange  28  projecting from the box  20 , on its downstream side, here using screws. 
   Thus, the LP and HP transmission shafts  11  and  12  respectively are kept, via the box  20 , secured to the fixed structure of the jet engine, the end pieces  16 ,  15  of the power take-off module also being protected. They are free to rotate about their axis. 
   The way in which the power take-off module  29  of the invention is assembled and works will now be explained in greater detail. The power take-off module  29  comprises the HP and LP power take-off pinions  13  and  14  respectively, the corresponding end pieces  15 ,  16  and the box  20 , together with the corresponding bearings  17 ,  18 ,  19 ,  21 ,  22 . 
   It will be immediately noted that the overall structure of a turbine engine, which was designed to take a power take-off device on the HP shaft in a longitudinal location similar to that of the invention, is little changed by the fitting of the means for driving the transmission shafts  11 ,  12  of the invention, inasmuch as all that is required is the addition of a drive pinion  7  on the LP rotor  1 , the power take-off module  29  and the two coaxial transmission shafts  11 ,  12 . Since the assembly is compact, its bulk is limited. The means for driving the transmission shafts  11 ,  12  is the LP and HP drive pinions  7  and  9  respectively of the shaft  4  of the LP compressor and the shaft  5  of the LP turbine, and the power take-off module  29 . 
   When assembling the turbine engine, the HP rotor  2  is first of all mounted. The box  20  of the power take-off module  29  is then mounted, which already contains the end pieces  15 ,  16  of the transmission shafts  12 ,  11  and all the required bearings  17 ,  18 ,  19 ,  21 ,  22 , which end pieces  15 ,  16  are not at this stage of assembly secured to their respective shafts  12 ,  11 . In other words, the power take-off module  29  has been preassembled and is mounted directly “as a unit” on the turbine engine. The box  20  is fixed to the flange  27  of the fixed structure of the turbine engine. During the mounting of the module  29 , the HP power take-off pinion  13  of the end piece  15  of the HP transmission shaft  12  will mesh with the HP drive pinion  9  of the HP rotor  2 . 
   The transmission shafts  11 ,  12  are then fitted into the structural arm of the turbine engine through which they are intended to run. In the particular instance considered here, the turbine engine is a jet engine and the transmission shaft  11 ,  12  run along one arm of what is known as the intermediate casing, well known to those skilled in the art, which is a structural casing forming part of the fixed structure of the jet engine, the external jacket of which lies in the continuation of the fan casing, and to which are generally connected the pylons for securing the engine to the aircraft in the propulsion of which the jet engine is intended to contribute. When they have been fitted, the transmission shafts  11 ,  12  come into mesh with their corresponding end piece  16 ,  15  which in this way becomes secured to it, at least in terms of rotation. In the particular case considered here, the external portion of the end piece  15  of the HP transmission shaft  12  comprises a spline  31 , at its external periphery, with which there meshes a spline  32  on the internal end of the HP transmission shaft  12 , which runs from the internal portion of this tubular internal end portion. These two elements thus rotate as one. Similarly, the external portion of the end piece  16  of the LP transmission shaft  11 , which is tubular, has a spline  33 , extending from its internal periphery, and with which there meshes a spline  34  on the internal end of the HP transmission shaft  12 , which extends from the external portion of this tubular internal end portion. These two elements thus rotate as one. 
   The LP rotor  1  is then fitted into the turbine engine. In doing this, the LP drive pinion  7  of the shaft  4  of the LP compressor will mesh with the LP power take-off pinion  14  of the end piece  16  of the LP transmission shaft  11 . 
   The power take-off module  29 , the LP rotor  1  and the HP rotor  2  and the LP and HP transmission shafts  11  and  12  respectively are thus mounted with respect to one another, the means of driving the transmission shafts  11 ,  12  being correctly positioned. 
   As the turbine engine operates, the LP rotor  1  and the HP rotor  2  rotate, in the same direction or in different directions depending on how the turbine engine is designed. Their LP and HP drive pinions  7  and  9  respectively rotationally drive the LP and HP power take-off pinions  14  and  13  respectively of the power take-off module  29 , and these rotationally drive the BP transmission shaft  11  and HP transmission shaft  12  via the end pieces  16 ,  15  and the splines ( 31 ,  32 ), ( 33 ,  34 ). At their external ends the transmission shafts  11 ,  12  are connected to one or more accessory gearboxes. Thus, each transmission shaft  11 ,  12  can drive a different accessory gearbox or alternatively the shafts  11 ,  12  are coupled in such a way as to drive the same accessory gearbox. For this, the shafts  11 ,  12  may, for example, drive the inputs of a differential gear, the output of which is connected to the drive shaft of the accessory gearbox, in a way well known to those skilled in the art. 
   It may be noted that, in the embodiment of the FIGURE, the axis  30  of the transmission shafts  11 ,  12  is not at right angles to the axis  3  of the turbine engine. The drive pinions  7 ,  9  of the rotors  1 ,  2  and the power take-off pinions  14 ,  13  of the power take-off module  29  are structurally arranged accordingly. Depending on the chosen embodiment, these pinions may be straight-cut, bevel or some other type of pinions, designed by the person skilled in the art to ensure a correct power take-off. The angle formed here by the axis  30  of the transmission shafts  11 ,  12  is dictated by the fact that these are shafts  11 ,  12  passed through a structural arm which is not entirely plumb with the region in which the power is taken off which, once again, corresponds to the region in which the upstream end of the HP rotor  2  and the downstream end of the shaft  4  of the LP compressor are situated. 
   By virtue of the invention, the two transmission shafts  11 ,  12  are coaxial, which means that they can be passed through one single arm of the turbine engine. Furthermore, the power take-off module  29  is very simple to mount, the LP and HP transmission shafts  11  and  12  respectively simply being pushed into their respective end piece  16 ,  15  so as to be rotationally driven by the LP rotor  1  and the HP rotor  2 . The diameter of the HP drive pinion  9  of the HP rotor  2  is the smaller one so as to meet the permissible peripheral speed limit for the teeth of the pinions so that movement can be transmitted with the minimum of slippage. The larger diameter of the LP drive pinion  7  is permissible because the LP rotor  1  rotates at a lower speed. The compactness of the power take-off module  29  and of the drive means in general leads to minimized bulk and mass.