Tilting transmission gearbox comprising a pivoting connection with plain bearings

A transmission gearbox (1) has a casing (2) that tilts around a tilting axis (B—B) via two bearings (19). Each of the bearings includes: a fixed part (20) having a sleeve (20a); and a trunnion (21a) which belongs to a pivoting part (21) and which pivots around the sleeve, the pivoting part being solidly connected to the casing (2). A shouldered wear ring (22) is disposed between the sleeve (20a) and the trunnion (21a) of each bearing (19). The invention is suitable for use in the pivot mounting of a power transmission gearbox (1) that is used to rotate the tilting rotor (14) of a convertible aircraft.

The invention relates to a tilting transmission gearbox and more specifically to a notched tilting power transmission box swiveling about a tilting axis to drive in rotation at least one device driven about the rotation axis which must be capable of swiveling around the tilting axis, almost perpendicular to the axis of rotation of the driven device, with said transmission box comprising a casing mounted to tilt with respect to and preferably between two fixed and rigid side parts of a support, by two coaxial bearings around said tilting axis and spaced at some distance from one another along this tilting axis.

The tilting power transmission gearbox according to the invention may be used, in a non-limitative manner, as a power transmission box on aircraft having an adjustable propulsion system, such as dirigible balloons or aircraft of the <<convertible>> type or yet again, as a power transmission box on wind generators.

In general, the tilting power transmission gearbox according to the invention may be used in any adjustable propulsion on power generation systems by rotation, which, to operate, require a small axial deflection (in the direction of the tilting axis).

In particular, as an application for which the tilting power transmission gearbox according to the invention is of particular interest to the applicants is a transmission gearbox that can be used for driving in rotation, from at least one source of power, such as a motor-propulsion unit, for instance of the turbomotor type, at least one tilting rotor of a “convertible” type aircraft which can operate in aircraft mode or in helicopter mode and whose drive shaft swivels about a tilting axis so as to be able to change from one position, in which the rotor operates like an aircraft propellor, to fly in aircraft mode, to another position in which the rotor operates like a helicopter rotor for the aircraft to fly in the helicopter mode.

A tilting power gearbox for an aircraft of the convertible type with tilting rotors is described more particularly in FR 2 791 319 and FR 2 791 634, which should be referred to for more details on the subject.

However, it should be noted that these two patents describe a convertible aircraft with tilting rotors of a type in which the drive shaft of each rotor is driven in rotation about its own axis by a front tilting reducing assembly of one respectively of the two power transmission gearboxes each including a fixed rear reducing assembly connected to the corresponding front reducing assembly and, respectively, to one or two motors (turbomotors), each supported respectively one of the two fixed wings of the aircraft and an interconnection shaft connecting the two transmissions to drive the two rotors in rotation by either of the two motors, in the event of the other motor failing.

The shaft of each rotor and the corresponding power transmission as well as the corresponding motor, are housed in respectively one of two articulated casings, each provided with a front part tilting about the tilting axis, and a rear fixed part, attached respectively to one of the two fixed wings of the aircraft and in which are housed the corresponding motor and at least, partly, the rear reducing assembly of the corresponding transmission whose front reducing assembly, constructed like the main helicopter transmission gearbox, and the shaft of the rotor corresponding to it housed in the front tilting part of the casing are assembled to tilt with this front casing part with respect to the fixed rear part of the casing and the corresponding facing wing.

In this application and with this architecture for each power transmission gearbox, the tilting link of front tilting reducer assembly operates only through an angular tilting sector limited to approximately 110° with a low speed of rotation included between approximately 1 and approximately 2 rpm and under a static load.

Accordingly, the two bearings through which the front tilting reducer assembly casing, or the tilting power transmission gearbox is mounted to tilt on the support, i.e. are subject to a <<false Brinnel effect>> which is particularly sensitive when the bearings are of the rolling type, such as needle or roller bearings.

The basic problem of the invention is to reduce or to eliminate entirely this <<false Brinnel effect>> of the bearings and more generally, to propose a tilting setup for the power transmission gearbox on its support, better suiting the various demands of the embodiment of the technique and more specifically offering better dynamic behaviour of the propulsion system in which this type of integrated power transmission gearbox is included, in particular by reducing the pressures in the aforementioned bearings, and the friction during the deflection of this propulsion system about the tilting axis.

To achieve this, the tilting power transmission gearbox of the type presented above is characterised in that the two bearings are plain bearings each of which includes:

a fixed part integral with respectively one of the two side parts of said support, and including a sleeve, more or less cylindrical, approximately coaxial with said fixed part of the other bearing, about the tilting axis,

a swiveling part integral with said tilting casing and including a more or less cylindrical annular trunnion, mounted to swivel about said sleeve of the fixed part of said bearing and more or less coaxial with the corresponding trunnion of the swiveling part of the other bearing, and

a wear ring, including a cylindrical tubular part, engaged axially between said sleeve of said fixed part and said trunnion of said mobile part and a radial collar, protruding radially toward the outside of said tubular part and with respect to its axis, integral with an axial end of said tubular part which is turned towards said corresponding lateral part of the support.

In an advantageous embodiment, the casing is mounted to tilt between the two fixed and rigid lateral parts of the support and the sleeves of the fixed parts of the two plain bearings, extending towards one another, between said parts of said support.

In order not to generate any resisting couple when the transmission gearbox tilts, it is an advantage for the wear ring to be constructed on the basis of a material having a low friction coefficient, for instance PTFE,

In addition, for better tilting set up of the casing thanks to the bearings, the wear ring of at least one bearing, and preferably of both, is secured to the swivelling part of said bearing.

The attachment of the wear ring to the swiveling part of the corresponding bearing is ensured, in a manner that benefits from simplicity, by at least three screws inserted in a direction more or less parallel to the tilting access, passing through said radial collar and attached to the swiveling part of said corresponding bearing,

In addition, to decrease the Hertz pressures on the bearings, the tubular parts of the wear ring on at least one bearing, and preferably on each of them, will have a radial internal bearing surface the shape of which is slightly domed and convex toward the tilting axis while the location and amplitude of the domed convex shape will be such that the internal radial bearing surface is more or less cylindrical having a circular section and a load, more particularly static, taken up by said tilting casing.

In addition, to also decrease the pressure peaks at the axial ends of the inside radial bearing surface of the wear ring of at least one bearing, and preferably on each of them, this internal radial bearing surface will have, at least at one of its two axial ends, and preferably at each of the two axial ends, an arrangement forming a chamfer or an inclined face.

Finally, for application to a convertible aircraft with tilting rotors, the bearings will be solicited by an angular sector with a limited tilting movement of approximately 110° with a low speed of rotation between approximately 1 and approximately 2 rpm, with said tilting casing bearing with a static load about the tilting axis.

The invention also refers to an aircraft of the convertible type capable of operating in aircraft mode or helicopter mode, including at least one tilting power transmission gearbox designed to drive in rotation at least one rotor about a rotation axis from at least one power source, such as a motor-propulsion unit, with said rotor mounted to tilt with said transmission gearbox about a tilting axis more or less perpendicular to said axis of rotation to change between aircraft and helicopter mode, with the aircraft characterised in that said tilting power transmission gearbox is a transmission gearbox according to the invention as defined above.

Tilting power transmission gearbox1shown inFIG. 1includes a tilting casing2housing a reducing assembly which, in the aforementioned application, driving in rotation a tilting rotor of a convertible aircraft as described in the two documents of the aforementioned patent documents, establishes the front tilting reducer assembly, arranged like a main helicopter transmission gearbox, of a transmission connecting a motor to this rotor and an interconnection shaft to another similar transmission, said transmission including a non-tilting motor reducing assembly, supported by a structure secured to the aircraft structure, for instance the structure of a rear fixed part of a motor casing, the front tilting part of which encloses power transmission gearbox1.

In the example shown here, the reducer assembly housed in tilting casing2has two reducing stages including one input stage3which is a reducing stage with a pair of spiro-conical driving gears including a primary conical driving gear4driven in rotation about the tilting axis B—B of transmission gearbox1, by a coaxial shaft5, itself driven in rotation from the non-tilting rear reducer assembly (not shown) with the teeth of conical driving gear4meshing with those of a secondary conical driving gear6, integral in rotation, about an axis A—A, more or less perpendicular to the tilting axis B—B of a coaxial shaft7interconnecting with the first reducing stage or input stage3and the second reducing stage or output reducing stage8.

This reducing stage8is an epicyclic stage including a planet pinion9integral with shaft7and conical driving gear6in its rotation about the A—A axis and the toothing of which meshes with the toothing of planet driving gears10mounted to rotate about axes parallel to the A—A axis on a planet-holder11, with the toothing of planet driving gears10also meshing with the inner toothing of a peripheral ring gear12attached to the inside of tilting casing2. Planet holder11is integral in coaxial rotation about the A—A axis with a rotor mast13, guided in rotation within casing2by bearings, not shown, and the other end of which protrudes out of casing2and is integral with hub15supporting the blades16of rotor14, driven in rotation about the A—A axis, which is the axis of rotation of the rotor.

This rotor14and the power transmission gearbox1comprising reducer stages3and8are mounted to tilt around the tilting axis B—B with casing2, mounted to tilt about this axis B—B between two fixed and rigid side parts17of a support, generally designated under reference18, itself fixed and rigid, for instance, in the case of the application in question, a support rigidly linked with the structure of the rear fixed part of the engine casing, itself integral with the structure of an aircraft wing (represented schematically as P inFIG. 1).

The swivelling assembly of casing2about the B—B taxes and between the two lateral parts17of support18is provided by two coaxial plain bearings19turning about the B—B tilting axis and spaced away from one another along this axis B—B so that each bearing19is adjacent to respectively one of the lateral parts17of the fixed support.

As is also shown in detail and at a larger scale in the axial half section ofFIG. 2, each bearing19includes a fixed part20integral with the lateral part of adjacent support17and includes a sleeve20a, more or less cylindrical with a circular section and approximately coaxial with sleeve20aof fixed part20of the other bearing19, about the tilting axis B—B, each sleeve20aextending towards the other sleeve20afrom the corresponding fixed lateral part17. Each fixed part20also has an annular shoulder20b, radial (with respect to the B—B axis) and protruding toward the outside of sleeve20a, and that may be attached to the fixed lateral part of corresponding support17as schematically shown by the dotted lines inFIG. 2, or incorporated into this part17.

Each bearing19also includes a swivelling part21, integral with tilting casing2, in the part of the latter extending under the ring gear12with this swivelling part21including an annular trunnion21awhich is more or less cylindrical and circular in section, at least on its internal radial bearing face, and which is more or less coaxial with trunnion21aof the swivelling part21of the other bearing19.

In each bearing19, trunnion21aof swivelling part21is mounted to swivel about the sleeve20aof fixed part20and each bearing19also includes a wear ring22obtained on the basis of material with a low friction coefficient, for instance PFTE, interposed between the fixed part20and swivelling part21of the corresponding bearing19so as not to generate any resistant couple during the tilting of casing2.

Each wear ring22has a tubular part22a, more or less cylindrical with a circular section, engaged axially between sleeve20aand trunnion21abelonging respectively to fixed part20and swivelling part21of said bearing19. Each wear ring22also has a radial collar22bwhich is an annular collar protruding radially toward the outside of tubular part22acorresponding to it, and with respect to the latter, integral with the axial ends of tubular part22athat is turned towards the corresponding lateral part of support17.

To facilitate assembly and improve the operation of bearing19, each wear ring22is attached to swivelling part21of corresponding bearing19and, as shown inFIG. 2, this attachment is obtained, for instance, by three screws32that are screwed in more or less parallel to the B—B tilting axis through part of collar22bof said ring22which is radially positioned toward the outside so that the end of the stem of screws32is screwed into the radial shoulder21bconnecting trunnion21aof swivelling part21of bearing19to the remainder of tilting casing2with the heads of screws32embedded in the radial face in the outer axial position (towards the outside of bearing19) of collar22b.

In the application mentioned above of a tilting transmission gearbox1driving the tilting rotor of a convertible aircraft, the swivelling link provided by bearing19between tilting casing2and fixed support18working only in a small angular sector limited to approximately 110° on which bearings19are solicited with a speed of rotation about the B—B tilting axis that is relatively small, between approximately 1 and approximately 2 rpm whereas tilting casing2and therefore bearings19are statically loaded. Since bearings19are affected by a “false Brinnel effect”, their plain structure, according to the invention, is far more advantageous than rolling housing bearings and, in addition, the use of plain bearings19with a wear ring22makes it possible to optimise the shape of the bearing surface in the inside radial position on the tubular part22aof the wear ring22, the internal radial bearing surface on which the swivelling link is made, thus reducing the Hertz pressures and the pressure peak at least at one end of the axial ends of this internal radial bearing surface.

FIGS. 3 to 5are a transversal sectional schematic representation of three internal bearing surface ring shapes superimposed on curves representing, depending on the axial position on ring22, the amplitude of the Hertz pressures and the pressure peaks at the axial ends.FIG. 3corresponds to ring22whose internal radial bearing surfaces and outer bearing surfaces of tubular part22aare perfectly cylindrical with a circular section and corresponding Hertz pressure curve34shows considerable pressures with two pressure peaks at35and36at the axial ends of tubular section22a.

FIG. 4shows an initial variant of the wear ring22′ in which bevels37and38shaped as a chamfer or a truncated inclined section have been machined at each axial end of the internal radial bearing surface of tubular part22′a. The corresponding Hertz pressure curve39represents lower pressure that is less variable according to the axial position than pressure curve34ofFIG. 3, with, in addition, pressure peaks40and41that are highly attenuated at the axial ends.

The second variant of the wear ring22″ ofFIG. 5includes similar bevels37and38as ring22′ ofFIG. 4with, in addition, a domed central part42, convex towards the inside of tubular part22″aof this ring22″ with “doming” amplitude b of approximately several micrometers. It can be seen that pressure curve43is of almost constant amplitude over the entire axial dimension of ring22″ including at the axial ends, said Hertz pressure amplitude also being less than those of curve39inFIG. 4, which themselves are substantially less than the pressures given by curve34ofFIG. 3.

Wear ring22ofFIG. 2is therefore, preferably, a ring like22″ ofFIG. 5in which, in addition, the location and amplitude b depend on the loading applied to corresponding bearing19and the deformations of the surrounding parts with the apex of the domed part possibly located more are less at the middle of the axial length “I” of tubular part22″aof ring22″, as shown schematically inFIG. 7, or yet again, offset toward the axial end on the side of radial collar22b, as shown schematically inFIG. 6or to the contrary, offset toward the other axial end of this tubular part22″a, as shown inFIG. 8. The choice between these different shapes and positions of the convex domed part of the internal radial bearing surface of a ring like22″ is made so that under load, in particular static as supported by tilting casing2and therefore corresponding bearing19, this internal radial part takes on a more are less cylindrical section circular shape which will procure a Hertz pressure curve as shown in curve43, relatively flat with a small amplitude, as shown inFIG. 5, without a pressure peak at the axial ends.

The use of plain bearing19equipped with wear rings like22,22′ and above all22″, will obtain a considerable decrease in friction during swivelling about the B—B tilting axis, while substantially limiting the axial deflections of the propulsion system including a rotor14and an associated tilting power transmission gearbox1, and a considerable decrease in the pressure in the associated bearings19.

This produces better dynamic behaviour of the propulsion system and at bearings19, eliminates the “false Brinnel effect”.