Abstract:
Provided is a device for supplying seal air to the bearings of a gas turbine engine which allows the seal pressure to be kept even between the front and rear gear boxes without complicating the passages for supplying the seal air to the bearing boxes. Sealing air drawn from a part of high pressure compressor is conducted through passages defined between the inner periphery of the rotors of the high pressure compressor and a high pressure turbine and the outer periphery of the outer shaft, and is distributed to the front and rear bearing boxes. By appropriately providing narrowed parts in these passages, seal air is evenly distributed between the front and rear bearing boxes. Thus, even in case of a failure of one of the oil seals, any concentrated leakage of the lubricating oil can be avoided.

Description:
TECHNICAL FIELD 
     The present invention relates to a device for supplying seal air to bearings of a gas turbine engine. 
     BACKGROUND OF THE INVENTION 
     In a multiple-shaft bypass jet engine, an inner shaft supporting the rotors of a low pressure compressor and a low pressure turbine and an outer shaft supporting the rotors of a high pressure compressor and a high pressure turbine typically consist of hollow shafts which are coaxially nested with each other. The outer and inner shafts are supported by separate bearings at their front and rear ends, and each bearing is lubricated by a forced lubrication system which blows lubricating oil fed by a pump to the bearing. 
     In such a forced lubricating system, to positively prevent the lubricating oil from leaking out of the bearing boxes provided in the front and rear ends of the outer and inner shafts, high pressure air drawn from one of the compressors is conducted to the exterior of the oil seals of the bearing boxes to keep the interior of the bearing boxes at a lower pressure than the supporting part. 
     The high pressure air which is supplied to the exterior of the bearing boxes may consist of the high pressure air produced by the high pressure compressor to be used in the combustion chambers, the passages for the seal air, in particular for the front bearing box, tends to be highly complex because of the presence of an intake duct in front of the high pressure compressor. 
     To deal with such a problem, it has been proposed to provide an outlet to each of an intermediate part of the impeller casing of the centrifugal compressor and an outer peripheral part of a back side of the centrifugal compressor so as to provide separate seal air supply passages for the seal portions of the front and rear gear boxes and minimize the length of the seal air passages. 
     However, because the air pressure from the centrifugal compressor varies depending on the point of drawing the high pressure air, supplying seal air to the seal portions via separate passages and from different points of drawing the high pressure air causes an evenness in the seal pressure between the front and rear gear boxes. Therefore, in case of a failure of a mechanical seal, the lubricating oil may be blown toward the part of the lower seal pressure. 
     BRIEF SUMMARY OF THE INVENTION 
     In view of such problems of the prior art, a primary object of the present invention is to provide a device for supplying seal air to the bearings of a gas turbine engine which allows the seal pressure to be kept even between the front and rear gear boxes without complicating the passages for supplying the seal air to the bearing boxes. 
     A second object of the present invention is to provide a device for supplying seal air to the bearings of a gas turbine engine which would prevent any serious loss of lubricating oil even when one of the seals should fail. 
     A third object of the present invention is to provide a device for supplying seal air to the bearings of a gas turbine engine which would not complicate the structure of the engine. 
     According to the present invention, such objects can be accomplished by providing a device for supplying seal air to bearing boxes of a gas turbine engine, comprising: an inner shaft connected to a low pressure compressor and a low pressure turbine; an outer shaft, coaxially disposed with respect to the inner shaft, connected to a high pressure compressor and a high pressure turbine; a front and a rear inner shaft bearings supporting a front and a rear end of the inner shaft, respectively; a front and a rear outer shaft bearings supporting a front and a rear end of the inner shaft, respectively; a front bearing box substantially enclosing the front inner shaft and front outer shaft bearings, the front bearing box being separated into an outer chamber exposed to a front end of the front inner bearing and a rear end of the front outer bearing, and an inner chamber exposed to a rear end of the front inner bearing and a front end of the front outer bearing; a rear bearing box substantially enclosing the rear outer shaft and rear inner shaft bearings, the rear bearing box being separated into an outer chamber exposed to a front end of the rear outer bearing and a rear end of the rear inner bearing, and an inner chamber exposed to a rear end of the rear outer bearing and a front end of the rear inner bearing; a hollow connecting shaft extending coaxially between rotors of the high pressure compressor and the high pressure turbine; a first passage having a first end communicated with a part of the high pressure compressor and a second end communicated with an annular gap defined between the connecting shaft and outer shaft; a second passage extending along an outer periphery of the outer shaft and having a first end communicated with the annular gap and a second end communicated with the outer chamber of the front bearing box; and a third passage extending along an outer periphery of the outer shaft and having a first end communicated with the annular gap and a second end communicated with the outer chamber of the rear bearing box. 
     Thus, the seal air drawn from a single point of the compressor can be supplied to both the front and rear bearing boxes evenly without complicating the passages. The inner chamber of each of the bearing boxes typically communicates with an annular gap between the inner and outer shafts, and is appropriately drained or vented out of the inner chamber after separating oil from the air flow. 
     In particular, if the second and third passages each include a part having a narrow cross section than that of the first passage, the seal pressure between the two seal portions can be made highly uniform. Furthermore, because of the flow restricting action of such narrowed parts, even if one of the seals is damaged, because the seal pressure drops only in the affected bearing box, a concentrated oil leakage from the damaged seal can be avoided while the seal pressure in the undamaged bearing box is maintained. 
     Also, if the second end of the first passage comprises a circumferentially elongated slot formed in the connecting shaft for introducing the high pressure air to the inner periphery of the rotor of the high pressure compressor, the stress acting on the peripheral part of the hole due to the centrifugal force can be reduced, and the admission of the swirling flow due to the rotation of the rotor of the compressor into the first passage is favorably promoted. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Now the present invention is described in the following with reference to the appended drawings, in which: 
     FIG. 1 is schematic sectional view of a bypass jet engine embodying the present invention; 
     FIG. 2 is an enlarged fragmentary sectional view of a front part of the jet engine shown in FIG. 1; 
     FIG. 3 is an enlarged fragmentary sectional view of a rear part of the jet engine shown in FIG. 1; and 
     FIG. 4 is a fragmentary perspective view of the high pressure turbine. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic view of a multiple shaft bypass jet engine embodying the present invention. This engine  1  comprises an outer casing  3  and an inner casing  4  consisting of coaxially arranged cylindrical members joined by straightening vanes  2 , and an outer shaft  7  and an inner shaft  8  consisting of coaxially arranged hollow shafts centrally supported in the casings by independent bearings  5   f ,  5   r ,  6   f  and  6   r.    
     The outer shaft  7  has a front end integrally carrying an impeller wheel  9  of a high pressure centrifugal compressor HC, and a rear end integrally carrying a high pressure turbine wheel  11  of a high pressure turbine HT. 
     The inner shaft  8  has a front end integrally carrying a front fan  12  and a compressor wheel  13  supporting rotor vanes for a low pressure axial flow compressor LC immediately behind the front fan  12 , and a rear end integrally carrying a pair of turbine wheels  15   a  and  15   b  supporting rotor vanes placed in a combustion gas flow in a jet duct  14  so as to form a low pressure turbine LT. 
     A nose cone  16  is centrally provided on the front fan  12 , and stator vanes  17  are provided behind the nose cone  12  with their outer ends attached to the inner circumferential surface of the outer casing  3 . 
     Stator vanes  18  of the low pressure axial flow compressor LC are disposed on the inner circumferential surface of a front end portion of the inner casing  4 . Behind the stator vanes  18  are provided an intake duct  19  for conducting the air drawn by the front fan  12  and pre-compressed by the low pressure axial flow compressor LC to the high pressure centrifugal compressor HC, and an impeller casing  20  of the high pressure centrifugal compressor HC. The inner peripheral part of the intake duct  19  is integrally provided with a bearing box  21  for the bearings  5   f  and  6   f  which support the front ends of the outer shaft  7  and inner shaft  8 , respectively. 
     The air drawn by the front fan  12  is in part forwarded to the high pressure centrifugal compressor HC via the low pressure axial flow compressor LC as mentioned earlier, and the remaining larger amount of air having a relatively low velocity is expelled rearward through a bypass duct  22  defined between the outer casing  3  and inner casing  4  to provide a primary thrust in the low speed range. 
     To the outer periphery of the high pressure centrifugal compressor HC is connected a diffuser  23  so as to provide a high pressure air to reverse flow combustion chambers  10  provided downstream to the diffuser  23 . 
     In each of the reverse flow combustion chambers  10 , the fuel ejected from a fuel injection nozzle  24  provided on the rear end of the combustion chamber  10  is mixed with the high pressure air supplied from the diffuser  23 , and is combusted. The combustion gas which is expelled from the nozzle N of each combustion chamber  10  which is directed rearward is ejected to the atmosphere via the jet duct  14 , and provides a primary thrust in the high speed range. 
     The inner peripheral part of the jet duct  14  is integrally provided with a bearing box  25  for the bearings  5   r  and  6   r  which support the rear ends of the outer shaft  7  and inner shaft  8 , respectively. 
     The outer shaft  7  of the engine  1  is connected to an output shaft of a starter motor  26  via a gear mechanism now shown in the drawings. As the starter motor  26  is activated, the impeller wheel  9  of the high pressure centrifugal compressor HC is driven, along with the outer shaft  7 , and this causes high pressure air to be supplied to the reverse flow combustion chambers  10 . When fuel mixed with this high pressure air combusts, the resulting pressure of the combustion gas drives the turbine wheel  11  of the high pressure turbine HT and the turbine wheels  15   a  and  15   b  of the low pressure turbine LT. The rotational power of the high pressure turbine wheel  11  drives the impeller wheel  9  of the high pressure centrifugal compressor HC, and the rotational power of the turbine wheels  15   a  and  15   b  of the low pressure turbine LT drives the compressor wheel  13  of the low pressure axial flow compressor LC. As the high pressure turbine wheel  9  and low pressure turbines  15   a  and  15   b  are driven by the jet pressure of the combustion gas, the engine  1  maintains its rotation according to a negative feedback balance between the amount of fuel supply and the amount of intake air. 
     As illustrated in FIG. 2, support parts for the bearing  5   f  supporting the front end of the outer shaft  7  and the bearing  6   f  supporting the front end of the inner shaft  8  are provided at a certain distance from each other in the axial direction. As illustrated in FIG. 3, support parts for the bearing  5   r  supporting the rear end of the outer shaft  7  and the bearing  6   r  supporting the rear end of the inner shaft  8  are provided at a certain distance from each other in the axial direction. 
     Floating ring seals  31   a  and  31   b  are provided ahead of the front bearing  6   f  of the inner shaft  8  and behind the front bearing  5   f  of the outer shaft  7 , respectively, in the front bearing box  21 , and floating ring seals  31   c  and  31   d  are provided ahead of the rear bearing  5   r  of the outer shaft  7  and behind the rear bearing  6   r  of the inner shaft  8 , respectively, in the rear bearing box  25 , to prevent the lubricating oil supplied to the various bearings from leaking out of the bearing boxes  21  and  25 . Labyrinth seals  32   a  to  32   d  are provided between the front and rear ends of the bearing boxes  21  and  25  and the opposing outer circumferential surfaces of the inner and outer shafts  7  and  8 , respectively. 
     The front end of the outer shaft  7  is connected to the inner race of the front bearing  5   f  and a bevel gear  33  for the starter motor, and fits into an axially front end of the impeller wheel  9  via a spline coupling. 
     A front central shaft end of the turbine wheel  11  is connected to the axial center of the back side of the impeller wheel  9  via a hollow connecting shaft  35  having Curvic (tradename) couplings  34   a  and  34   b  on either axial end thereof. The axial center of the back side of the turbine wheel  11  is connected to a collar  36  and serving as an extension of the connecting shaft  35  and fitted with a labyrinth seal  32   c  adjacent to a bearing portion on the rear end of the outer shaft  7  via a Curvic coupling  34   c.    
     The impeller wheel  9 , hollow connecting shaft  35 , turbine wheel  11 , collar  36  and inner race of the bearing  5   r  on the rear end of the out shaft  7  are fitted onto the outer shaft  7 , in this order, and a bearing nut  37  threaded onto the outer shaft  7  applies a prescribed initial tension to the outer shaft  7 . 
     Behind the impeller wheel  9  is defined an impeller back chamber  39  by a back plate  38 , and the output pressure of the high pressure centrifugal compressor HC prevails in the impeller back chamber  39 . 
     The hollow connecting shaft  35  is provided with a circumferentially elongated slot  40  to communicate an annular gap  41  defined between the inner circumferential surface of the connecting shaft  35  or the axial center of the impeller wheel  9  and the outer circumferential surface of the outer shaft  7  with the impeller back chamber  39 . Because the opening provided in the hollow connecting shaft  35  consists of the circumferentially elongated slot  40 , the stress acting on the inner periphery of the opening due to the centrifugal force can be minimized, and the swirl resulting from the impeller wheel  9  is more favorably guided into the interior of the hollow connecting shaft  35 . Furthermore, because the axial dimension of the opening is smaller for a given opening area, the distance between the Curvic coupling  34   a  and the edge of the slot  40  can be maximized so that the influence on the mechanical strength of the part associated with the Curvic coupling  34   a  can be minimized. 
     The spline coupling part of the impeller wheel  9  is provided with axial grooves  42 , and the collar  43  fitted with the labyrinth seal  32   b  at the front end of the central shaft of the impeller wheel  9  is provided with an orifice  44  which communicates the spline coupling part with the interior of a rear part of the front bearing box  21 . 
     A plurality of passages  45  communicating a front part of the bearing box  21  with a rear part thereof are provided at a regular interval along a circle. The combined cross sectional area of the passages  45  is sufficiently greater than the cross sectional area of the orifice  44  which was mentioned earlier. 
     The high pressure air compressed by the high pressure centrifugal compressor HC in most part flows into a high pressure chamber  46  accommodating the reverse flow combustion chambers  10 , via the diffuser  23 , and the remaining part of the high pressure air flows into the impeller back chamber  39  from the outer periphery of the impeller wheel  9 . The high pressure air is then flows into the annular gap  41  defined between the inner circumferential surface of the connecting shaft  35  or the axial center of the impeller wheel  9  and the outer circumferential surface of the outer shaft  7  from the slot  40  of the hollow connecting shaft  35  exposed to the impeller back chamber  39 , and reaches a part at which the floating ring seal  31   b  is mounted behind the outer shaft support bearing  5   f  of the front bearing box  21 . Thereafter, the high pressure air reaches a part at which the floating ring seal  31   a  is mounted in front of the inner shaft support bearing  6   f  in the front bearing box  21 , via the passages  45  communicating the front and rear parts of the front bearing box  21  to each other. 
     More specifically, the front bearing box  21  is divided into an outer chamber and an inner chamber. The outer chamber includes the communicating passages  45  and the parts exposed to the front end of the ring seal  31   a  adjoining the front end of the front inner shaft bearing  6   f  and the rear end of the ring seal  31   b  adjoining the rear end of the front outer shaft bearing  5   f . The inner chamber includes the parts exposed to the rear end of the front inner shaft bearing  6   f  and the front end of the front outer shaft bearing  5   f , and communicates with the annular gap  49  defined between the outer and inner shafts  7  and  8 . 
     The partition wall separating the impeller back chamber  39  from the high pressure chamber  46  is provided with an orifice  54  for adjusting the back pressure so that the inner pressure of the impeller back chamber  39  which has a significant influence on the pressure balance of the shafts as a whole can be maintained at an appropriate level by supplying the high pressure air to the impeller back chamber  39  from the high pressure chamber  46 . 
     The high pressure air introduced into the outer chamber of the front gear box  21  or the parts on which the floating ring seals  31   a  and  31   b  are mounted in the front and rear parts of the front bearing box  21  allows the external pressure of the front bearing box  21  to be kept higher than the inner pressure thereof so that the leakage of lubricating oil from the front bearing box  21  can be avoided. This seal pressure is retained by virtue of the labyrinth seals  32   a  and  32   b.    
     The foregoing description covered only the supply passage for the seal air for the front bearing box  21 , but a similar arrangement can be applied to the rear bearing box  25  also. In this case, as shown in FIG. 3, an orifice  47  may be formed in the collar  36  between the floating ring seal  31   c  placed in front of the rear end support bearing  5   r  for the outer shaft  7  and the labyrinth seal  32   c  adjacent thereto, and to passages  48  may be formed in an appropriate part of the rear bearing box  25  for communicating the floating ring seal  31   d  disposed behind the rear end support bearing  6   r  for the inner shaft  8  and the labyrinth seal  32   d  adjacent thereto. 
     More specifically, the rear bearing box  25  is divided into an outer chamber and an inner chamber. The outer chamber includes the communicating passages  48  and the parts exposed to the front end of the ring seal  31   c  adjoining the front end of the rear outer shaft bearing  5   r  and the rear end of the ring seal  31   d  adjoining the rear end of the rear inner shaft bearing  6   r . The inner chamber includes the parts exposed to the rear end of the rear inner outer bearing  5   r  and the front end of the rear inner shaft bearing  6   r , and communicates with the annular gap  49  defined between the outer and inner shafts  7  and  8 . 
     The inner chamber of the front bearing box  21  communicates with the inner chamber of the rear bearing box  25  via the annular gap  49  defined between the inner circumferential surface of the outer shaft  7  and the outer circumferential surface of the inner shaft  8  so that a part of the lubricating oil which has lubricated the bearings  5   f ,  5   r ,  6   f  and  6   r  and the seal air which has passed through the floating ring seals  31   a  to  31   d  are drawn into the gear box GB communicating with the starter motor  26  via a drain hole (not shown in the drawings) provided along the length of the shaft of the drive bevel gear meshing with the bevel gear  33  fixed on an end of the outer shaft  7 , and is expelled to the atmosphere via the bypass duct  22  while the lubricating oil is separated by an oil separator (not shown in the drawings) provided in the gear box GB. 
     A part of the high pressure air introduced into the high pressure chamber  46  is guided by a shroud  50  placed opposite the front surface of the turbine wheel  11  of the high pressure turbine HT, and flows along the front surface of the disk portion  51  of the turbine wheel  11 . As shown in FIG. 4, turbine blades  53  are connected to the peripheral part of the disk portion  51  of the turbine wheel  11  each via a Christmas tree portion  52 , and the aforementioned air flow cools the front face of the portions connecting the turbine blades  53  to the disk portion  51 . 
     The collar  36  is provided with slots  55  similar to those of the hollow connecting shaft  35  so that the high pressure air which has passed into the hollow connecting shaft  35  and the gap  41  between the outer circumferential surface of the outer shaft  7  and the inner circumferential surface of the axial center of the high pressure turbine HT blows through across the turbine wheel  11  of the high pressure turbine HT. Thus, the backside of the connecting portions between the disk portion  51  and turbine blades  53 . 
     The air which has cooled the connecting parts between the disk portion  51  and turbine blades  53  is guided by the shrouds  50  and  56  opposing the front and rear sides of the disk portion  51 , and is ejected from the jet duct  14  drawn by the combustion gas ejected from the nozzles N of the reverse flow combustion chambers  10 . 
     Thus, according to the present invention, because the high pressure air drawn from a part of the compressor is filled into a seal space extending from the front to rear bearing boxes at a uniform pressure, the seal pressure in the front and rear bearing boxes can be established in an effective manner without complicating the passages for supplying seal air. 
     Although the present invention has been described in terms of a preferred embodiment thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims.