Abstract:
A drive mechanism comprising a first sprocket encircled by a tensioned chain and an orbit sprocket arranged to orbit the first sprocket in response to a resynchronisation signal. The orbit sprocket is arranged to displace the tensioned chain relative to the first sprocket. The first sprocket is rotatable and is coupled to an output drive sprocket by the tensioned chain such that the first and output drive sprockets are rotatable in synchronicity, whereby the orbit of the orbit sprocket alters the relative angular orientation of the first and output drive sprockets.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is entitled to the benefit of British Patent Application No. GB 0906736.4, filed on Apr. 21, 2009. 
     FIELD OF THE INVENTION 
     The present invention relates to a drive mechanism. It finds utility in a wide variety of fields including varying the pitch of the rotor blades of a rotor stage of a gas turbine engine. 
     BACKGROUND OF THE INVENTION 
     Chain precession drive mechanisms are known in the art and are used to achieve higher gear ratios than are possible with conventional gear trains. A typical chain precession drive mechanism is described with reference to  FIG. 1  and  FIG. 2 . A first sprocket is connected via an output shaft to a driven device which may be, for example, a pitch change arrangement for a set of rotor blades. Thus rotation of the first sprocket causes rotation of the driven device. There may be a gear train (not shown) to adjust the gear ratio between the first sprocket and the driven device. Encircling the first sprocket is a tensioned chain that is longer than the circumference of the first sprocket. The tensioned chain is held in tension by a tensioning arrangement which exerts a force, downward as illustrated, to take up the slack in the chain. The tensioning arrangement may purely rely on gravity but is more preferably a spring or similar that is constrained to move in a straight line towards or away from the first sprocket, for example in a slide track. 
     An orbit sprocket is arranged to be coplanar with the first sprocket and to be located within the length of the tensioned chain. A motor drives an input shaft that is coaxial with the output shaft. There is an arm extending perpendicularly from the input shaft and connected by one of its ends thereto. The orbit sprocket is connected to the other end of the arm so that rotation of the input shaft causes the orbit sprocket to orbit the first sprocket at a radius greater than the radius of the first sprocket. The track of the orbit sprocket around the first sprocket is shown in  FIG. 2 . There may be static structures to give rigidity to the mechanism and for mounting the shafts and sprockets. 
     The driven device is held stationary when the input shaft is not rotating. When the motor drives the input shaft in response to a resynchronisation signal, the orbit sprocket orbits the first sprocket which displaces the tensioned chain and causes the tensioning arrangement to release at least some of the slack to accommodate encircling the orbit sprocket as well as the first sprocket. Precession of the orbit sprocket first decouples a given link of the tensioned chain from the first sprocket and then permits it to recouple to a different tooth of the first sprocket. Once the orbit sprocket has completed one revolution around the first sprocket and returned to the position shown in  FIG. 1  and  FIG. 2 , the tensioned chain has been displaced relative to the first sprocket by a known amount. Since the tensioned chain does not rotate, but only moves radially, the first sprocket has therefore been rotated by that known amount. Hence the driven device is stepped by this amount, or a scaled amount if a gear train providing a gear ratio is used. For example, for a mechanism in which the tensioned chain has N chain links and the first sprocket has M teeth, the output shaft rotates in the opposite direction at (N−M)/M times the input shaft speed. 
     The motor may drive the input shaft continuously to effect a high ratio gearing between the input and output shafts. The output shaft can be held at any position by arresting movement of the orbit sprocket but least wear is offered by arresting the movement where the orbit sprocket is decoupled from the tensioned chain  36 , as shown in  FIG. 2 . 
     One disadvantage of this mechanism is that in order to drive a device located in a rotating frame of reference, for example to control the pitch of a set of rotating rotor blades, it is necessary to rotate the input motor. This may be complex to implement and may require electrical and hydraulic signals to traverse the stationary-rotating boundary in order to provide the resynchronisation signal. Where the mechanism is used to control the pitch of a rear set of rotor blades in a dual row rotor system, for example in a gas turbine engine having contra-rotating propeller stages in a pusher configuration, the complexity is significantly increased because the control signals are typically generated in the stationary frame of reference and must pass through the front rotating frame of reference to control the rotor blades in the rear rotating frame of reference. The same problem occurs where the mechanism is used to control the pitch of a front set of rotor blades in a dual rotor system in a puller configuration. 
     A further disadvantage is that the chain precession drive mechanism can be back-driven by the load if the motor is not able to hold the variable load of the driven device. This is the case if the mechanism is used to control the pitch of rotor blades in a rotor stage where the aerodynamic loading on the rotor blades may drive the pitch change mechanism against the motor. Therefore a “no-back” device is required, adding weight and complexity to the mechanism. The present invention seeks to provide a drive mechanism that seeks to address the aforementioned problems. 
     SUMMARY OF THE INVENTION 
     Accordingly the present invention provides a drive mechanism that has a first sprocket encircled by a tensioned chain; an orbit sprocket arranged to orbit the first sprocket in response to a resynchronisation signal, the orbit sprocket arranged to displace the tensioned chain relative to the first sprocket characterised in that the first sprocket is rotatable and is coupled to an output drive sprocket by the tensioned chain such that the first and output drive sprockets are rotatable in synchronicity, whereby the orbit of the orbit sprocket alters the relative angular orientation of the first and output drive sprockets. This arrangement has the advantage that the resynchronisation is decoupled from the output drive so that the mechanism cannot be back-driven by the driven load. Other advantages will become apparent from the subsequent detailed description. 
     The first sprocket may be coupled to a reference rotation source and the output drive sprocket may be coupled to a driven device. The output drive sprocket may be coupled to the driven device via a gear train, which may be a reduction gear train. 
     The tensioned chain may be tensioned by a biased sprocket. 
     The resynchronisation signal may activate a motor that causes the orbit sprocket to orbit the first sprocket. 
     The first sprocket may be in one frame of reference and the driven device in another frame of reference. The frames of reference may rotate with respect to each other. One frame of reference may be stationary. 
     A further orbit sprocket may be arranged to orbit the output drive sprocket to further alter the relative angular orientation of the first and output drive sprockets. Two or more output drive sprockets may be coupled to the first sprocket by the tensioned chain, each having a further orbit sprocket arranged to orbit it to alter the relative angular orientation of the first sprocket and the output drive sprocket that is orbited. 
     A further aspect of the present invention provides a drive arrangement comprising two or more drive mechanisms as described above, the two or more drive mechanisms being coupled together. 
     The reference rotation source may be a rotor stage of a gas turbine engine. The driven device may be a rotor pitch mechanism. 
     Another aspect provides a contra-rotating propeller gas turbine engine comprising a drive mechanism as described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  and  FIG. 2  are, respectively, a schematic and an axial view of a chain precession drive mechanism according to the prior art as described above. 
         FIG. 3  is a sectional side view of a gas turbine engine having contra-rotating propeller stages. 
         FIG. 4  is a schematic view of a drive mechanism according to the present invention. 
         FIG. 5  is an axial view of a drive mechanism according to the present invention. 
         FIG. 6  is a schematic view, similar to  FIG. 4 , showing an embodiment of the present invention including a gear train. 
         FIG. 7 , is an axial view similar to  FIG. 5  and showing a position of an orbit sprocket about a first sprocket in the resynchronisation of the drive mechanism according to the present invention. 
         FIG. 8  is an axial view of the sprockets of  FIG. 7  progressed to a second position subsequent to that of  FIG. 7 . 
         FIG. 9  is an axial view of the sprockets of  FIG. 7  progressed to a position subsequent to that of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 3 , a twin-spooled, contra-rotating propeller gas turbine engine is generally indicated at  10  and has a principal and rotational axis  9 . The engine  10  comprises a core engine  11  having, in axial flow series, an air intake  12 , an intermediate pressure compressor  14 , a high-pressure compressor  15 , combustion equipment  16 , a high-pressure turbine  17 , a low pressure turbine  18 , a free power turbine  19  and a core exhaust nozzle  20 . A nacelle  21  generally surrounds the core engine  11  and defines the intake  12  and nozzle  20  and a core exhaust duct  22 . The engine  10  also comprises two contra-rotating propellers  23 ,  24  attached to and driven by the free power turbine  19 , which comprises contra-rotating blade arrays  25 ,  26 . The pitch of the propellers  23 ,  24  is controlled by a drive mechanism according to the present invention, as described below. 
     The gas turbine engine  10  works in a conventional manner so that air entering the intake  12  is accelerated and compressed by the intermediate pressure compressor  14  and directed into the high pressure compressor  15  where further compression takes place. The compressed air exhausted from the high pressure compressor  15  is directed into the combustion equipment  16  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, low-pressure and free power turbines  17 ,  18 ,  19  before being exhausted through the nozzle  20  to provide some propulsive thrust. The high, low-pressure and free power turbines  17 ,  18 ,  19  respectively drive the high and intermediate pressure compressors  15 ,  14  and the propellers  23 ,  24  by suitable interconnecting shafts. The propellers  23 ,  24  normally provide the majority of the propulsive thrust. 
     A first, exemplary embodiment of the drive mechanism of the present invention is shown in  FIG. 4  and  FIG. 5 . The first sprocket  30  in this case is rotatable, being driven via the first input shaft  52  by a reference rotation source  54 . The reference rotation source  54  may be, for example, a rotor stage of a contra-rotating propeller gas turbine engine  10 . The first sprocket  30  is located in a static frame of reference  58  whilst the reference rotation source  54  is in a rotating frame of reference  56 . As in the prior art arrangement, a motor  42  drives a second input shaft  60  to which is connected an orbit sprocket  40  via an arm  46  that extends perpendicularly from the second input shaft  60 . The orbit sprocket  40  is coplanar with the first sprocket  30  and has an orbit radius that is greater than the radius of the first sprocket  30  so that movement in either direction around its orbit track  48  is accomplished without interference or direct contact between the sprockets  30 ,  40 . 
     A tensioned chain  36  encircles the first sprocket  30  and is of greater length than the circumference of the first sprocket  30 . Also within the perimeter of the tensioned chain  36  are an output drive sprocket  62  and a biased sprocket  64 . The output drive sprocket  62  is connected via an output shaft  32  to a driven device  34 , such as a rotor pitch change mechanism. The biased sprocket  64  is coplanar with the first and output drive sprockets  30 ,  62  but is offset when viewed in the axial direction. It is hinged at a pivot point  66  and biased away from the first and output drive sprockets  30 ,  62 , as shown by arrow  68 , to maintain the tension of the tensioned chain  36 . The first sprocket  30  is rotated by the reference rotation source  54  which causes the output drive sprocket  62 , by the connection between the links of the tensioned chain  36  with the teeth of each of the first sprocket  30  and the output drive sprocket  62 , to rotate in synchronicity with the first sprocket  30 . The biased sprocket  64  pivots to take up any slack in the tensioned chain  36 . 
     When it is desired to change the angular orientation of the output drive sprocket  62 , and therefore the driven device  34 , relative to the first sprocket  30 , and therefore the reference rotation source  54 , a resynchronisation signal is generated that activates the motor  42 . This causes the orbit sprocket  40  to move around the first sprocket  30  which displaces the tensioned chain  36  away from connection with the teeth thereof. Alternatively the orbit sprocket  40  need not have teeth but may interact with the tensioned chain  36  in a different manner to minimise the wear on the chain  36  and the orbit sprocket  40 . The progression of the orbit sprocket  40  around the first sprocket  30  in a clockwise direction is shown in  FIG. 7  to  FIG. 9 . As the orbit sprocket  40  increases the amount of the tensioned chain  36  required in the vicinity of the perimeter of the first sprocket  30 , the biased sprocket  64  is pivoted about point  66  to be pulled closer to the output drive sprocket  62  and/or the first sprocket  30  to release some of the tensioned chain  36 . Similarly, as the orbit sprocket  40  returns to the position shown in  FIG. 5  the biased sprocket  64  takes up the slack in the tensioned chain  36  by pivoting away from the first sprocket  30  and the output drive sprocket  62 . A complete revolution of the orbit sprocket  40  around the first sprocket  30  displaces the tensioned chain  36  circumferentially with respect to the first sprocket  30  and thereby displaces the relative angular orientation of the first sprocket  30  and the output drive sprocket  62  by a known amount. As with the prior art arrangement, the amount of relative displacement is a function of the number of teeth on the first sprocket  30  and the number of links in the tensioned chain  36 . 
     Unlike the prior art arrangement, however, the first sprocket  30  constantly rotates in synchronicity with the reference rotation source  54 . Thus, the tensioned chain  36  also rotates in synchronicity when the orbit sprocket  40  is disengaged from the first sprocket  30 . When the orbit sprocket  40  is moved around the first sprocket  30  parts of the tensioned chain  36  are sequentially decoupled and then recoupled to the teeth of the first sprocket  30  whilst the tensioned chain  36  as a whole continues to be rotated by the rotation of the first sprocket  30 . A new relative angular orientation can be maintained by arresting the movement of the orbit sprocket  40 . Preferably the orbit sprocket  40  is stopped when it is in approximately the position shown in  FIG. 5  so that it is not engaged with the tensioned chain  36 . This minimises the wear on all the components and ensures that the step change in relative angular orientation is exactly known. However, if a smaller step change is required the orbit sprocket  40  may be stopped at a different position on its orbit track  48  around the first sprocket  30 . 
     A second embodiment of the drive mechanism of the present invention is shown in  FIG. 6  which comprises all the components of the arrangement shown in  FIG. 4 , some of which are differently arranged. The reference rotation source  54  drives the first sprocket  30  as in the first embodiment. The first input shaft  52  is the inner shaft of a pair of coaxial and concentric shafts, the second of which is part of an output drive gear train. Instead of the output drive sprocket  62  being directly connected to the driven device  34  there is a series of gears between the two. The output drive sprocket  62  is connected via an output shaft  32  to a first drive gear  70  which meshes with an intermediate gear  72  that is mounted from static structure  50 . The output shaft  32  is also supported by the static structure  50 . Diametrically opposite to the first drive gear  70  and intermeshing with the intermediate gear  72  is a first ring gear  74  supported by the static structure  50 . This first ring gear  74  is connected via the outer shaft  76  across the interface between the rotating frame of reference  56  and the static frame of reference  58  to a second ring gear  78 . At least one second drive gear  80  is arranged around the circumference of the second ring gear  78  to intermesh therewith. Each of the at least one second drive gears  80  is connected via a second output drive shaft  82  to a driven device  34 . 
     By appropriate relative sizing of the drive gears  70 ,  80 , the intermediate gear  72  and the ring gears  74 ,  78  any gear ratio may be achieved between the one or more driven devices  34 , via second drive gears  80 , and the reference rotation source  54 . For example, in applications where it is required that the second ring gear  78  should rotate in synchronisation with the reference rotation source  54 , except when the orbit sprocket  40  is rotated around the first sprocket  30  to change the phase relationship and resynchronise the relative angular orientation, the gear ratio between the first ring gear  74  and the first drive gear  70  should match the gear ratio between the first sprocket  30  and the output drive sprocket  62 . The radius of the orbit track  48  determines the amount of rotational offset caused by a single revolution of the orbit sprocket  40  around the first sprocket  30 . 
     The drive mechanism of the present invention enables a motor  42  in the static frame of reference  58  to move a driven device  34  in the rotating frame of reference  56 , which has the advantage of being less complex and prone to failure than prior art arrangements that require the motor to reside in the rotating frame of reference  56 . The present invention also enables multiple driven devices  34  to be driven from a single reference rotation source  54 , with the resultant weight and cost benefits. Advantageously, the driven devices  34  may be driven at different angular velocities by appropriate sizing of the second drive gears  80 . The adjustment of the relative angular orientation is by discreet steps of known size. The gear ratios can be arranged to ensure that the orbit sprocket  40  moves once around the first sprocket  30  to provide the desired discreet step, so that wear on the components caused by irregular tension is minimised. If the situation requires it, the orbit sprocket  40  can be actuated to move around the first sprocket  30  more than once in order to offset the output drive sprocket  62  from the first sprocket  30  by a multiple of the basic discreet step size. In extraordinary circumstances the orbit sprocket  40  may be arrested part way around its orbit track  48  in order to provide a smaller relative angular orientation shift. 
     In normal circumstances the orbit sprocket  40  is stopped in the position shown in  FIG. 5 . Advantageously, in this location the mechanism cannot be back driven by load torque on the driven device  34 , for example aerodynamic load on rotor blades. Thus, the output drive sprocket  62  and driven device  34  remain in rotational synchronicity with the first sprocket  30  and the reference rotation source  54 . 
     The present invention may be applied to any differential rotor system such as a dual row propeller, a contra-rotating propeller gas turbine engine, a helicopter rotor system or a marine propeller. In all these cases there are weight, cost and complexity benefits to mounting the motor that effects the differential in a static frame of reference whilst the effect is apparent in a rotating frame of reference. Although the drive mechanism of the present invention has been described with the reference rotation source  54  and the at least one driven device  34  being in the rotating frame of reference  56  and the sprockets  30 ,  40 ,  62  being in a static frame of reference  58  these may be reversed with equal felicity in other applications so that the sprockets  30 ,  40 ,  62  could be located in a rotating frame of reference and the reference rotation source  54  could be located in a static frame of reference or a different rotating frame of reference to the sprockets  30 ,  40 ,  62 . This may be of particular benefit where the present invention is applied in a dual row propeller application. 
     The output gear train may be angled relative to the plane of the sprockets  30 ,  40 ,  62  in order to minimise the consequences of a failure in one or more of the gears. The components can be arranged so that the at least one driven device  34  moves in a known, failsafe direction in the event of such a failure. This may be achieved by making the gear ratio of the output gear train dissimilar to the gear ratio between the first sprocket  30  and the output drive sprocket  62  to bias the movement of at least one driven device  34  in the event of the mechanism jamming. However, this arrangement would require continual input from the motor  42  to keep the at least one driven device  34  synchronised, within tolerable limits, with the reference rotation source  54 . 
     Although specific embodiments have been described and discussed the invention may be altered by substitution or modification of any of the components without deviating from the inventive concept herein described. For example, the motor  42  may be any source of rotational torque or power. There may be one or more redundant tensioned chains  36  or other components to enhance reliability of the drive mechanism and/or minimise the consequences of at least some failure cases. The tensioned chain  36  may be replaced by a toothed belt with the first sprocket  30 , output drive sprocket  62 , biased sprocket  64  and orbit sprocket  40  having complementary indentations. The sprockets  30 ,  40 ,  62  and tensioned chain  36  or belt may have any suitable coarse or fine tooth pitch, which is preferably complementary to improve the interaction. 
     The biased sprocket  64  may be biased by any known method including, but not limited to, springs and compression pads. The pivot point  66  may be replaced or augmented by a device to constrain the biased sprocket  64  to move in a designated direction, such as a slider and complementary track. Alternatively the biased sprocket  64  may be positioned externally of the tensioned chain  36  and may be biased to push the tensioned chain  36  towards the first sprocket  30  and/or the output drive sprocket  62  in order to maintain the tension in the tensioned chain  36 . Any suitable alternative system of maintaining the tension of the tensioned chain  36  may be substituted for the biased sprocket  64 . There may be one or more additional biased sprockets  64  or other tensioning systems, for example located symmetrically between the first sprocket  30  and the output drive sprocket  62 , on the right of  FIG. 5  as illustrated. Providing two or more tensioning systems may necessitate recalculation of the orbit track  48  to achieve the desired offset step for each revolution of the orbit sprocket  40 . The biased sprocket  64  may be omitted in favour of biasing of the output drive sprocket  62  away from the first sprocket  30 , in which case angled, splined or sliding shafts may be required to replace output shaft  32 . 
     The output gear train of the second embodiment may be modified by the use of a chain drive and sprockets instead of the intermediate gear  72 . More or fewer gear stages may be provided as necessary for the application. The gear ratio between the first ring gear  74  and the first drive gear  70  may be different to that between the first sprocket  30  and the output drive gear  62  in order that the second ring gear  78  either leads or lags the first sprocket  30  when the orbit sprocket  40  is stationary. Advantageously, such a lead or lag can drive the driven device  34  to a failsafe position thereby improving the safety, reliability or service disruption caused by a failure. 
     The motor  42  is shown schematically and may be any suitable type of motor including a hollow motor. The relative positioning of the components is exemplary; other arrangements achieving the same inventive effect are considered to be within the scope of the present invention. 
     The drive mechanism may be extended to control driven devices  34  in two different rotating frames of reference. This may require an additional tensioned chain with a separately activated orbit sprocket to provide a compound offset to a second output drive sprocket. A simple version of this extension provides two or more output drive sprockets  62  with an orbit sprocket and associated components arranged to orbit each in order to offset its angular orientation. The revolution of the orbit sprocket  40  around the first sprocket  30  would therefore act to offset all the output drive sprockets  62 . This arrangement could find utility in a dual row propeller arrangement wherein one row of rotor blades requires a different pitch adjustment to the other row but both rotor stages rotate in synchronicity. The drive mechanism may be further extended to control driven devices  34  in more than two different frames of reference. 
     The length and/or shape of the arm  46  may be altered for a whole revolution of the orbit sprocket  40  to change the radius of the orbit track  48  and thereby alter the size of the relative angular offset step. Alternatively the orbit track  48  may be non-circular, by changing the length and/or shape of the arm  46  over the course of a revolution of the orbit sprocket  40 . This enables the amount of rotational offset achieved at a given angular position of the orbit sprocket  40  around the first sprocket  30  to be varied and has benefits in applications where the movement of the orbit sprocket  40  is designed to be arrested at positions other than that shown in  FIG. 5 . The arm  46  may also have a different shape to improve the couple between the orbit sprocket  40  and the tensioned chain  36 . The reference rotation source  54  and driven device  34  may be switched with the consequence that the torque would flow through the gear train in the opposite direction. 
     Although the present invention has been described with respect to a propeller rotor blade system, particularly for changing the pitch of the rotor blades of a rotor stage of the propeller, it is equally applicable in other applications. For example it can be used to control the gear ratio, braking, camber, or any other parameter of a vehicle wheel in a car, van, farm vehicle, train or aircraft. It may be used in a robot, crane or industrial machine, for example to control the angle and position of a robot or crane arm with the majority of the weighty components situated in the base of the robot or crane. The drive mechanism could also be used in a turret vehicle, such as a tank, excavator or fairground ride, to control a parameter, for example the elevation of a gun or excavating scoop, on the turret from within the vehicle. Alternatively it may be used to alter the gear ratio and synchronisation between multiple riders on a bicycle, for example between two riders of a tandem, or between one or more riders and a motor. It could vary the gear ratio to a regenerative motor or flywheel so that it can be turned quicker to store energy and slower to release it. 
     The present invention is equally applicable in applications where the first sprocket  30  and/or output drive sprocket  62  are non-circular, permitting torque or speed variations or periodic angular offset of the output compared to the input without revolution of the orbit sprocket  40 . 
     Although the orbit sprocket  40  has been described as a sprocket, with or without teeth, it may alternatively be a radially thin device similar in profile to a car engine camshaft timing chain tensioning device. This minimises the wear on the tensioned chain  36  without requiring the orbit sprocket  40  to rotate about its own axis, minimises the required length of the tensioned chain  36  and reduces the travel required by biased sprocket  64  by locating the output drive sprocket  62  and biased sprocket  64  close to the first sprocket  30 .