Abstract:
This invention relates to a unison ring assembly, comprising: a first unison ring and a second unison ring concentrically aligned along a principal axis, the first and second rings being rotatable about the principal axis; and, a gear mechanism for simultaneously rotating the first and second unison rings about the principal axis.

Description:
TECHNICAL FIELD OF INVENTION 
       [0001]    This invention relates generally to an actuation mechanism for variable vanes in an axial flow gas turbine engines. The vanes are variable in that they are rotatable about their major axis such that the delivery angle of fluid flow to a rotor stage of a compressor or the like may be altered to benefit the performance of the engine. Although principally geared towards gas turbine engines, the invention may be applicable to other engines or devices which have arrays of rotatable members. 
       BACKGROUND OF INVENTION 
       [0002]      FIG. 1  shows a ducted fan gas turbine engine  10  comprising, in axial flow series: an air intake  12 , a propulsive fan  14  having a plurality of fan blades  16 , an intermediate pressure compressor  18 , a high-pressure compressor  20 , a combustor  22 , a high-pressure turbine  24 , an intermediate pressure turbine  26 , a low-pressure turbine  28  and a core exhaust nozzle  30 . A nacelle  32  generally surrounds the engine  10  and defines the intake  12 , a bypass duct  34  and a bypass exhaust nozzle  36 . The engine has a principal axis of rotation  44 . 
         [0003]    Air entering the intake  12  is accelerated by the fan  14  to produce a bypass flow and a core flow. The bypass flow travels down the bypass duct  34  and exits the bypass exhaust nozzle  36  to provide the majority of the propulsive thrust produced by the engine  10 . The core flow enters in axial flow series the intermediate pressure compressor  18 , high pressure compressor  20  and the combustor  22 , where fuel is added to the compressed air and the mixture burnt. The hot combustion products expand through and drive the high, intermediate and low-pressure turbines  24 ,  26 ,  28  before being exhausted through the nozzle  30  to provide additional propulsive thrust. The high, intermediate and low-pressure turbines  24 ,  26 ,  28  respectively drive the high and intermediate pressure compressors  20 ,  18  and the fan  14  by interconnecting shafts  38 ,  40 ,  42 . 
         [0004]    It is commonplace in state of the art gas turbine engines  10  to include variable vanes at various locations in the engine to generally help control the air flow passing through the engine core, thus improving the performance of the engine.  FIG. 2  illustrates a typical Variable Stator Vane assembly  210  which is used in various engines for aerospace, industrial and marine applications. 
         [0005]    The assembly includes four unison rings  212  coaxially arranged relative to the principal axis  44  of the engine  10  around the exterior of the intermediate compressor  18  (not shown in  FIG. 2 ). Each unison ring  212  has a plurality of lever arms  214  each of which attach to a vane spindle (not shown) via the mounting holes  216  located in the distal end thereof. The aerofoil portions of the vanes are rotatably mounted within the airflow path of the compressor  18  such that they can rotate about the major axis of the vane which is coincidental with the rotational axis of the spindle. The unison rings  212  are arranged such that rotating them around the principal axis of the engine results in the lever arms  214  pivoting about the vane spindles, thereby rotating them and the aerofoil portions within the airflow channel of the compressor  18 . 
         [0006]    To rotate the unison rings, an input link is provided to engage with a crankshaft which drives the unison rings via link rods. As the levers rotate around the centre of rotation of the vane spindles, the lever arms fixed interface with the unison ring requires that the unison ring translates axially as well as circumferentially as it translates. 
         [0007]    This design requires that all the interfaces between moving components have bushes fitted to accommodate the necessary movement. Additionally, to keep the unison rings circular as they accommodate the loads within the system, the rings are fitted with centralising features that contact the casings to maintain roundness. This can be achieved by using mushroom headed bolts or centralising screws which contact pads that are bonded or bolted to the engine casing. 
         [0008]    This assembly is unnecessarily complex and heavy. Thus, the present invention seeks to provide an alternative arrangement. 
       STATEMENTS OF INVENTION 
       [0009]    In a first aspect, the present invention relates to a unison ring assembly, comprising: a first unison ring and a second unison ring concentrically aligned along a principal axis, the first and second rings being rotatable about the principal axis; and, a gear mechanism for simultaneously rotating the first and second unison rings about the principal axis. 
         [0010]    The gear mechanism includes a sun planet gear arrangement. The first unison ring may be the ring gear and the second unison ring may be the sun gear. 
         [0011]    The gear mechanism may be arranged to rotate the unison rings at different speeds. 
         [0012]    The planet gear may be eccentrically mounted so as to have a rotational axis which is offset by different amounts relative to each of the first and second unison ring gears. 
         [0013]    The planet gear may include a first section which engages with the ring gear and a second section which engages with the sun gear. The gear ratio of the first and second sections may be different. 
         [0014]    The first unison ring may be radially outwards of the second unison ring with respect to the principal axis of the engine. 
         [0015]    The unison ring assembly may further comprise a plurality of planet gears distributed around the unison ring gears. The plurality of planet gears may be arranged to concentrically align the unison ring gears about the principal axis. 
         [0016]    The or each planet gear may be arranged to axially locate the sun and planet gears relative to the principal axis. 
         [0017]    The or each planet gear may be arranged to axially and radially align the unison rings. 
         [0018]    The unison ring assembly may further comprise at least one actuator connected to and configured to directly rotate one of the ring gear, planet or sun gear. 
         [0019]    The unison ring assembly is for a gas turbine engine. The first and second unison rings may be connected to variable stator guide vanes. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0020]    The following drawings are used to describe embodiments of the present invention. 
           [0021]      FIG. 1  shows a known ducting fan gas turbine engine. 
           [0022]      FIG. 2  shows a known unison ring assembly. 
           [0023]      FIG. 3  shows a cross section of a unison ring assembly according to the invention. 
           [0024]      FIG. 4  shows an axial view of a planet gear of an embodiment of the invention. 
           [0025]      FIG. 5  shows an alternative embodiment of the present invention. 
           [0026]      FIG. 6  shows a yet further embodiment of the present invention. 
           [0027]      FIG. 7  shows various driving mechanisms for rotating the unison rings. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0028]    In  FIG. 3  there is shown a unison gear ring assembly  310  according to the present invention. The unison gear ring assembly  310  is arranged to provide rotational adjustment to a plurality of rows of variable vanes  312  in an annular compressor of a axial flow gas turbine engine, similar to the one shown in  FIG. 1 . 
         [0029]    In  FIG. 3  there is shown a portion of a compressor  314  having a first stage  316  and a second stage  318  of variable vanes  312  which are arranged in axial flow series, the axial flow being referenced by arrow  320 . In the described embodiment, the variable vanes  312  are stator vanes (VSVs), but the invention may be of broader application within a gas turbine engine, or elsewhere. It is to be noted that although  FIG. 3  only shows one vane per stage, this is one of an annular array of blades. Further, although not shown, the variable vanes  312  will be interspersed with rotors having rotor blades in a working engine, as is well known in the art. Further, there may be more than two rows of variable vanes. Indeed, it is well known to have at least four VSVs in a gas turbine engine. 
         [0030]    Each of the VSVs includes an aerofoil portion  322  which is located within the airflow  320  channel of the compressor  314  and is mounted therein so as to be rotatable about a radially extending major axis  324  of the vane. The radially outer end of the aerofoil portion  322  of the vane  312  adjoins a spindle  326  which is coaxial with the rotational, major axis  324  of the vane  312  and which projects through the compressor wall  328  into the space which occupies the unison ring gear  310 . The spindle  326  engages with the unison ring gear  310  which is actuable so as to rotate the spindle  326  and thus the angle of the VSV  312  relative to the direction of the gas flow in the compressor when in use. Hence, in use, the gas flow delivery angle of the vane  312  can be adjusted as required for a particular performance. 
         [0031]    The unison ring assembly  310  includes a first unison ring  330  and a second unison ring  332 , each being associated with and arranged to rotate a stage of VSVs  316 ,  318 . The unison rings  330 ,  332  are rotatably mounted about the principal axis of the engine and are engaged and simultaneously driven by a single gear mechanism  334  so as to rotate the unison rings. 
         [0032]    The engagement between the unison rings  330 ,  332  and the respective spindles  326  is by way of a rack and pinion type of arrangement in which the spindles  326  of the VSVs  312  carry a flange  336  or lever arm which has a toothed rim  338  so as to effectively provide a cog, or pinion, for engagement with a corresponding geared section  340 , or rack, on its respective unison rings  330 ,  332 . 
         [0033]    An axial view of the gear mechanism  334  which drives the unison rings  330 ,  332  of the described embodiment is shown in  FIG. 4 . The gear mechanism is in the form of a sun and planet gear system having a plurality of rack and pinion type gears to translate the force between the respective members of the arrangement. In the embodiment, the first unison ring  330  provides the ring gear, the second unison ring  332  provides the sun gear, and a driving member  334  provides the planet gear. The driving member  334  includes two opposing arcuate segments  339  of plate having a toothed rim  340  which engages with a corresponding toothed section  341  on the unison ring gears  332 . The driving member  334  is rotatably mounted on a spindle  342  which is cantilevered from the wall of the compressor  328  or other suitable structure. The arrangement is such that rotating the driving member  336  results in a rotation of the unison rings  330 ,  332 , which, in turn, rotates the respective VSVs  312  via the spindles  326 . 
         [0034]    In the described embodiment, the driving member  334  is asymmetric in that it is eccentrically mounted on the spindle  342 . Thus, the distance from the spindle  342  to the each respective unison ring  330   332  is different. Specifically, the distance between the first unison ring  330  and spindle  342  is greater than that of the spindle  342  to the second unison ring  332 . With this arrangement, the gear ratio between the driving member and each unison ring can be different and thus the range of travel of one unison ring  330   332  can be different to that of the other. This means that the range of angular rotation of each unison ring and the associated deflection experienced by each VSV  312  can be different for a common angle of actuation from the driving member  334  which can be advantageous. As can be seen from  FIG. 4 , the number and pitch of the gear teeth  340  for each of the unison rings  330   332  may be different so as to accommodate this difference in gearing. 
         [0035]    In more detail, the unison rings  330   332  can be described as having an annular radial face  344  and an axially facing surface  346 . In the described embodiment, these are provided by having a unison ring gear  330   332  which is substantially L shaped in cross section which comprises a radially extending annular plate lying perpendicular relative to the principal axis of the engine and, adjoined to one end of the plate, an annular flange which extends in an axial direction so as to form a short cylindrical portion. Thus, a surface of the annular plate provides the axially facing surface  344  and cylindrical section provides the radially facing surface  346 , each having a toothed section for engagement with driving member  334 . 
         [0036]    In the described embodiment, the unison ring members  330   332  are placed in a radially separated manner such that radially facing surfaces of each unison ring member, and the respective toothed sections, oppose one another. In this way, the unison rings are balanced across the driving member  334  and the spacing between the unison ring members  330 ,  332  can be maintained by the driving members  334 . This is particularly advantageous where a plurality of the driving members  336  are spaced around the compressor  18  in a circular array, as it naturally retains the unison ring gears  330 ,  332  in concentric relation to each other and removes, or at least reduces, the need to have other centralising features. Such features can be found in the prior art systems and would include the centralising screws and pads as mentioned above in the background section. 
         [0037]    It will be appreciated that variations in the constructional detail of the described embodiment are possible within the scope of the invention. For example, the unison rings  330 ,  332  may not be L shaped, but may be box sections or some other shape which provides the necessary rigidity. Further, the unison rings  330 , 332  may include weight saving or strengthening features not described here, but which may affect the overall shape of the unison rings  330 ,  332 . 
         [0038]    In another embodiment, as shown in  FIG. 5 , the unison ring gears  530 ,  532  each comprise a radially extending plate having toothed sections on both axially facing surfaces. The two unison rings  530 ,  532  are substantially similar and oppose one another along the longitudinal axis of the engine so as to provide in axial series a toothed flange  536  of the first stage vane spindle, a first unison ring  530 , a driving member  534 , a second unison ring  532  and a second toothed spindle flange  538 , with the driving member  534  sitting in a tangential plane relative to the principal axis of the engine. Thus, the unison rings are maintained in place between the two spindles  526  and driving member  534  so there is no axial displacement of the unison rings during use. 
         [0039]    It is of note that the first and second unison rings  530 ,  532  shown in  FIG. 5  have different radial lengths with the racks of the first unison gear  530  opposing each other at a common radial distance from the principal axis of the engine, but with the racks of the second unison ring gear  532  being radially offset. This allows for a common length of spindle  526  to be accommodated for each stage of the compressor  18  when the compressor has a generally convergent geometry and thus inclined walls relative to the principal axis  44  of the engine. 
         [0040]    A technical advantage of the two embodiments described above is that the rotational forces exerted to on the unison rings  330 ,  332 ,  530 ,  532  and spindles  326 ,  526  are all about constant axes of rotation before, during and after rotation. Hence, no axial or radial translation of the unison ring gears  330 ,  332 ,  530 ,  532  or spindles  326 ,  526  occurs in use. Thus, the requirements from the supporting mechanisms are simplified and the loads can be substantially carried by the spindles and driving members  334 ,  534 . 
         [0041]    In yet a further embodiment as shown in  FIG. 6 , the rotational axis of the driving member  634  is angularly offset with regard to the principal axis  44  of the engine, rather than being axially or radially aligned to the principal axis  44 . In this way, the driving member  634  can be arranged to resolve or balance out the radial and axial forces of the unison rings  630 ,  632  between the spindles  626  and unison rings  630 ,  632 , thereby centralising and retaining the unison rings in place. 
         [0042]    With reference to  FIG. 7  and the above described embodiments, the unison ring gears  730 ,  732  and driving members  734  are interlocked such that rotating one of them results in the other two being rotated by a corresponding and proportional amount. Thus, as shown in  FIG. 7 , any of the gears can be attached to an actuation system  736  to provide the required rotative force. The actuation system  736  may be any suitable system known in the art such as electrical, pneumatic or hydraulic. The attachment between the respective gear and the actuation system can also be any suitable attachment as known in the art. 
         [0043]    The unison ring gear arrangements described above may be made from metals e.g. steels, aluminum, titanium alloys etc. or, where the environment permits, composite materials may be considered for the gears to give a light weight design. 
         [0044]    The invention is defined by the appended claims and the above described embodiments should not be taken as limiting the scope of those claims, they are examples only.