Abstract:
A lubrication and scavenge system for a gas turbine, capable continued operation in nose-down or nose-up orientations, has a number of oil drainage passages to return oil to one or more collection chambers adjacent a rolling element bearing. A rotary impellor in the chamber forces the oil into a scavenge off-take passageway in the chamber wall leading to a scavenge pump. However, in some orientations windage effects in the chamber can return the oil to the drainage passage rather than permitting the impellor to centrifuge it into the off-take passageway. As a result oil starvation may occur. To avoid this and improve scavenging a shield is located between the drainage path and the off-take passageway adjacent to a face of the impellor. The shield may comprise an additional member, but preferably is formed integrally with a bearing race so as to ease assembly.

Description:
BACKGROUND 
     The invention relates to a lubrication and scavenge system. In particular it concerns a lubrication and scavenge system for a rolling element bearing arrangement in a gas turbine engine. 
     An oil system provides lubrication, cooling and corrosion protection for numerous internal components. In general, gas turbine engines employ a self-contained recirculatory oil system that distributes oil from an oil tank under pressure to bearing chambers and other components throughout the engine. Once the oil has performed its immediate function it falls into a collection volume and is returned to the oil tank by scavenge pumps. Gravity plays some part in the collection process and engine designs must ensure avoid, irrespective of engine orientation, spaces which prevent oil being picked-up by the scavenge pumps. In extreme circumstances this can interrupt recirculation of the oil and lead to oil starvation. The present invention has for an objection to maintain an oil recirculation path in all circumstances. 
     The use of lubricant scavenge systems in gas turbine engines in which lubricating oil is collected and pumped back to an oil reservoir has been long established practice. GB Patent No 774,197 issued to power jets (Research and Development) Limited published in 1957 described a gas turbine lubrication system including an oil scavenge pump having a rotor 
     SUMMARY 
     According to one aspect of the invention there is provided a lubricant scavenge system comprising a scavenge pump, a collection chamber, at least one drainage path leading into the collection chamber, a rotary impellor located in the chamber, at least one off-take passageway in a chamber wall leading from the collection chamber to the scavenge pump, and a shield located between the drainage path and the off-take passageway adjacent a face of the impellor and acting in operation to shield the face of the impellor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention and how it may be carried into practice will now be described in more detail with reference to the accompanying drawings in which: 
         FIG. 1  shows a schematic diagram of an oil lubrication and scavenge system; 
         FIG. 2  is a schematic illustration of a rolling element bearing showing the oil scavenge arrangement; 
         FIG. 3  is a schematic illustration of the scavenge arrangement of  FIG. 2  in nose-down attitude; 
         FIGS. 4   a  and  4   b  show a modified arrangement incorporating the invention in orientations corresponding to  FIGS. 2 and 3 , and 
         FIGS. 5   a  and  5   b  show a further embodiment of the invention in different orientations. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Referring now to  FIG. 1 , there is shown a simplified schematic of a typical oil lubrication and scavenge system for a gas turbine engine is illustrated. The system comprises an oil tank or reservoir  2  from which lubrication oil is drawn by a pump indicated at  4 . The particular system on which the illustration is based is of the pressure relief type in which the pressure of the oil flow to bearing chambers (not shown) is controlled by a pressure relief valve  6 . The pressurised oil passes through a filter  8  and then a heat exchanger  10  before being distributed to oil supply jets, generally indicated at  12 , located in a plurality of bearing chambers, the engine gearbox, etc. The oil feed pump  4  is a positive displacement pump that delivers a known flow, proportional to pump speed. Oil pressure is generated by resistance to oil flow in the oil supply pipes backed by the bearing chamber pressures. The desired flow of oil to a component can be achieved by use of a suitably sized restriction known as an oil jet  12  at the end of the oil line. The design of the jet can provide either a ray or a targeted, coherent stream of oil, directed to a component or to a catching feature that will then feed the component. 
     Lubrication oil supplied to the bearings etc. is evacuated from bearing chambers and the like and returned to the oil tank  2  by a scavenge system. In the first step of this return cycle oil is drained from bearing chambers, or wherever it has been utilised, to one or more collection chambers where it is directed into a scavenge-offtake  14  where the oil is picked-up by a scavenge pump  16  and returned to the oil tank  2  through a scavenge filter  18 . The return path includes a de-aerator to remove entrained air from the oil. Oil tank  2  provides a reservoir of oil to supply the oil system. The de-aerating device may be incorporated within the oil tank  2  or the return passageway for example adjacent the scavenge filter to remove air from the returning scavenged oil. 
     Scavenge pumps  16  generally follow the same construction as the oil feed pump  4 . Each bearing chamber is serviced by a dedicated scavenge pump  16  except where bearing chamber pressure or gravity can be used to drive the oil to a shared sump. The capacity of a scavenge pump  16  is usually much greater than the oil flow it is required to return to the tank  2 , in order to accommodate non-linear flow/speed relationships and aeration of the oil. Engines designed to operate for extended periods in zero or negative gravity flight conditions will have oil tanks that incorporate features ensuring a continuous supply of oil. 
       FIG. 2  shows a schematic diagram of a bearing chamber and its associated scavenge off take arrangement. A supported shaft  20  is journaled in a rolling element bearing indicated generally at  22 , the inner race  24  of the bearing is fixed to the shaft  20  and the outer race  26  of the bearing is fixed in a bearing housing. A small sump region or collection chamber  30  surrounding the shaft  20  is formed between the bearing  22 , a shaft seal  32  and bounded by the bearing chamber wall  28  on its radially outer surface. Set into the chamber wall  28  is a scavenge off take port  34  and passageway  36  leading to a scavenge pump (not shown). Although only one scavenge port  34  is shown there may be a plurality of such orifices spaced apart around the chamber wall  28  leading into the scavenge passageway  36 . Similarly, there may be more than a single passageway. Facing scavenge port  34  is a scavenge pumping element  38  in the form of a disc carried on the shaft  20 . The periphery of element  38  is aligned with the scavenge port or ports  34  so that centrifugal force arising from rotation of the shaft  20  and element  38  forces oil contacting the disc into the scavenge port(s)  34 . 
     Oil may drain into the collection chamber  30  from the bearing  22  and through at least one drainage port  40  in bearing chamber wall  28  at the exit of drainage passages  42  that communicate with spaces (not shown) within a structure surrounding bearing housing in which lubricating oil may accumulate in some or all orientations of the engine. The passage or passages  42  are formed to drain the contents of such spaces into a convenient space, such as collection chamber  30  from which the oil may be scavenged. 
     In  FIG. 2  the illustrated arrangement has the axis of shaft  20  oriented in a horizontal direction, i.e., across the page in a left-right direction. This is considered to be a normal orientation for an aircraft propulsion engine in cruise operation. A civil aircraft propulsion engine normally spends the majority if not all of its operating life in this orientation with the axis of its main shaft, such as shaft  20 , within a few degrees of horizontal. The greatest excursion from a substantially horizontal orientation occurs during a climb phase when an engine has a nose-up attitude at a greater angle. The capacity of the oil system, in particular the oil tank ensures a sufficient supply of oil. However, certain types of aircraft may be expected to operate at completely different attitudes of nose-up, nose-down or inverted for relatively long periods. In these cases attention is paid to identify potential oil trap spaces and to provide drainage passages through which the spaces may drain into a convenient collection chamber whatever the orientation of the engine axis. However, it has been found in practice that the efficiency of the scavenge collection system is not constant and in some instances scavenge oil is not picked up and can be prevented from draining into the collection chambers. 
       FIG. 3  illustrates the arrangement of  FIG. 2  rotated counter-clockwise through 90 degrees to a nose-up position. In the situation of  FIG. 3  the normal draining of oil into the scavenge collection chamber  30  may be interrupted unless special attention is paid to the positioning and layout passages of drainage passages  42  relative to the element  38 . 
     The oil tank  2  is provided with internal means (not shown) to ensure oil pick-up at all engine orientation so the oil pumping system will continue to operate as normal until the contents of oil tank  2  have been distributed through the oil pump  4 . Unless the scavenge system can continue to return oil to oil tank  2  the lubrication system eventually will be starved of supply. Passages such as indicated at  42  must be provided and located with regard to providing drainage paths from enclosed spaces into collection chamber  30  where oil may be recirculated. However, problems may arise in the orientation of  FIG. 3  because returning oil draining from passage  42  may be forced back through port  40  by windage effects in the collection chamber  30 . As a result oil is effectively prevented from draining into the chamber  30  and being recirculated back into the scavenge offtake port  34  and passageway  36 . 
     The solution provided by the present invention is shown in  FIGS. 4   a  and  4   b  comprises a weir, or shield generally indicated at  44 , placed between the scavenge port  40  and the scavenge pumping element  38 . The effect of shield  44  is to help establish a flow pattern within collection chamber  30  which effectively guides oil droplets and oil mist onto the element  38  at a radial point near to the shaft  20 . This is found to be more effective at maintaining an oil film across the surface of the element  38  which is shed from the periphery of the disc under centrifugal force into the scavenge offtake port  34 . 
     As shown in  FIGS. 4   a  and  4   b  the shield  44  may comprise a shaped and perforated annular member of sheet metal material thickness. The component configuration illustrated has an “L-shaped” cross section consisting of a cylindrical portion  46  that extends in a substantially axial direction parallel to the axis of shaft  20 , and an annular portion  48  lying in a substantially radial plane at one end of the cylindrical portion  46 . For the purpose of mounting the weir the cylindrical portion is formed with a radius matching the radius of the outer surface of the outer race  26  of bearing  22 . Exact dimensions depend upon the details of a chosen mounting arrangement. For example the cylindrical portion  46  could be trapped between the wall of the bearing housing and the bearing outer race  26 , or the housing wall could be at least partly recessed to receive the portion, providing bearing loads can be satisfactorily transferred to the housing wall. A number of apertures or slots  50  are formed in the cylindrical portion  46  of the shield member corresponding to the size and spacing of the scavenge intake ports  40  in the bearing chamber wall. 
     However, such an arrangement involving an additional component, i.e., shield  46  that has to be manufactured accurately and positioned correctly during assembly suffers inherent disadvantages. A solution is illustrated in  FIG. 5  in which the outer race  26  of the bearing  22  is extended laterally at  52  to provide a portion functionally equivalent to the shield  44 . The extension portion  52  of outer bearing race  26  is provided with a number of machined slots  54  spaced apart around the circumference of the bearing race that extend fully through the extended race wall  52  in a substantially radial direction. There are as many such slots  54  as the number of scavenge ports  40  in the bearing chamber wall. Each slot permits scavenged oil from a drainage passage  42  to enter the collection chamber  30  through ports  40 . The thickness of the extended race portion  52 , i.e., the dimension in a radial direction, is sufficient to act as an effective shield over the element  38 . Scavenged oil is thus passed into collection chamber  30  and towards a more advantageous, radially inner position on the element  38 . 
     The arrangement illustrated shows a collection chamber  30  at one side of a bearing  22 . It will be understood that a similar arrangement may be provided at the opposite side of the bearing to function in the same manner when the bearing orientation is inverted relative to the orientation illustrated in  FIGS. 4   b  and  5   b , i.e., rotated through an angle of 180 degrees.