Patent Description:
Aircraft are required to ground taxi between locations on airfields. An example is taxiing between a runway and the location (e.g. terminal gate) at which the aircraft's passengers are to board or disembark. Typically, such taxiing is achieved by using the thrust from the aircraft's engines to propel the aircraft forwards so that the landing gear wheels are caused to rotate. Since ground taxi speeds are necessarily relatively low, the engines must be run at a very low power. This means that there is a relatively high fuel consumption as a result of the poor propulsion efficiency at this low power. This leads to an increased level of both atmospheric and noise pollution locally around airports. Moreover, even when the engines are run at low power it is generally necessary to apply the wheel brakes to limit ground taxi speeds, leading to a high degree of brake wear.

Reversing of a civil aircraft, e.g. away from a terminal gate, using its main engines is not permitted. When reversing is necessary, or in other situations where ground taxiing via main engine thrust is not practicable, tow trucks are used to manoeuvre aircraft around. This process is laborious and costly.

There is therefore a need for a drive system to power the wheels of an aircraft landing gear during ground taxi operations.

Several autonomous ground taxi systems for both driving the wheels while the aircraft is on the ground and spinning them up prior to landing have been proposed in recent years. An example is disclosed in <CIT>, which proposes a powered nose aircraft wheel system in which a clutch is used to switch between a mode in which the wheel can spin freely and a mode in which the wheel can be driven by an electric motor. The clutch can also operate to enable the motor to pre-spin the wheel prior to landing.

A prior art arrangement which is not restricted to nose landing gears is described in <CIT>. The disclosed system uses an actuator to move a driven toothed pinion gear in and out of driving engagement with a toothed ring gear on the wheel.

<CIT> discloses a device for rotational coupling of a crown gear to a wheel. The device has connecting elements, each comprising a telescopic rod connected to the wheel and the crown gear by universal joints at the respective ends of the rod. The rod comprises sliding elements.

<CIT> discloses a coupling. The coupling has an outer member, an inner member, a cage disposed between them, and rolling elements located in pockets of the cage.

<CIT> discloses a gear. The gear includes wheels, which have outside pockets in which are mounted cylindrical rollers and tubular rollers. The cylindrical and tubular rollers are fixed axially with side caps.

A first aspect of the invention provides a drive system for an aircraft landing gear, the drive system comprising: a pinion gear; a drive shaft arranged to rotate the pinion gear about a drive axis; a driven gear arranged to mesh with the pinion gear to be rotatable by the pinion gear, the driven gear being connectable to a wheel of the landing gear to be capable of rotating the wheel about a wheel axis; and a flexible interface comprising a plurality of driven gear coupling members, each driven gear coupling member having a first connection portion attached to the driven gear, a second connection portion adapted to be attached to the wheel at an offset distance from the wheel axis, and a joint between the first connection portion and the second connection portion, the joint permitting relative movement between the first connection portion and the second connection portion. The joint is a kinematic joint which permits translation of the first connection portion relative to the second connection portion along a first local axis substantially parallel to the wheel axis, the joint comprising a cylindrical joint or a sliding joint to provide the translation along the first local axis. Additionally or alternatively, the joint permits translation of the first connection portion relative to the second connection portion along a second local axis at an angle to the wheel axis, the joint comprising a cylindrical joint or a spherical joint to provide the translation along the second local axis.

The flexible interface thus isolates the drive system from deformation within the landing gear during use, such as wheel axle bending caused by vertical and braking loads, and deformation of the wheel itself, in order to limit wear and/or stresses within the drive system and the wheel.

The joint preferably comprises a spherical joint to provide rotational movement between the first and second connection portions, preferably rotational movement with three or more degrees of freedom. A suitable spherical joint may comprise a ball and socket joint or a spherical bearing, for example. Such rotational movement may serve to isolate the driven gear from misalignment or deformation of the wheel caused e.g. by axle bending, and may additionally help to facilitate assembly of the driven gear with the wheel.

The spherical joint may be further arranged to be capable of translational movement relative to the first connection portion or second connection portion, e.g. by sliding, to achieve translational movement between the first and second portions. Such translational movement may serve to isolate the driven gear from further deformation or misalignment.

In some embodiments the spherical joint may be arranged to be capable of translational movement (e.g. by sliding) along an axis at an angle to the wheel axis (e.g. substantially radial to the wheel axis) to isolate the driven gear from deformation of the wheel shape (e.g. to a lozenge, or oval, shape) caused by tyre loads, and/or from centre distance variations between the driven gear and the pinion gear as a result of wheel rim deformation.

Additionally or alternatively, the spherical joint may be arranged to be capable of translational movement (e.g. by sliding) along an axis substantially aligned with the wheel axis to isolate the driven gear from relative tilting between the driven gear and the pinion gear as a result of wheel axle deflections.

The joint preferably permits relative movement within at least two degrees of freedom.

As stated above, the joint may permit translation of the first connection portion relative to the second connection portion along the first local axis substantially parallel to (aligned with) the wheel axis. Such movement enables the driven gear to be tilted relative to the wheel to permit isolation of relative tilting between the driven gear and the pinion gear as a result of wheel axle deflections. In such a case, the joint comprises a kinematic cylindrical joint or a kinematic sliding joint to provide the translation along the first local axis.

As stated above, the joint may permit translation of the first connection portion relative to the second connection portion along the second local axis at an angle to the wheel axis. Such movement enables relative translation between the driven gear and the wheel to permit isolation of centre distance variations between the driven gear and the pinion gear as a result of wheel rim deformation, and/or isolation of wheel shape changes (e.g. from round to an oval or lozenge shape) caused by tyre loads. In such a case, the joint comprises a kinematic cylindrical joint or a kinematic spherical joint, e.g. a slidable kinematic spherical joint, to provide the translation along the second local axis. The second local axis may be substantially radial to the wheel axis.

The joint preferably permits: translation of the first connection portion relative to the second connection portion along a first local axis substantially parallel to the wheel axis, translation of the first connection portion relative to the second connection portion along a second local axis at an angle to the wheel axis, and (limited) relative rotation between the first and second local axes.

The driven gear may be substantially ring-shaped and the plurality (preferably <NUM> or <NUM>, or as many as necessary for successful transmission of torque from the driven gear to the wheel) of driven gear coupling members may be substantially evenly distributed about its circumference.

Each driven gear coupling member preferably comprises a resilient member (such as a spring) arranged to bias the first connection portion towards the wheel axis. In this way the resilient members serve to centre the driven gear with respect to the wheel.

The flexible interface preferably comprises a pinion gear coupling member arranged to connect the pinion gear to the drive shaft to permit tilting of the pinion gear relative to the drive axis. The pinion gear coupling member preferably comprises a crowned spline joint between the drive shaft and the pinion gear. The pinion gear coupling member alternatively comprises a constant velocity joint (CV joint) between the drive shaft and the pinion gear. The pinion gear coupling member may be arranged to permit translation of the pinion gear along the drive axis.

The first connection portion of each driven gear coupling member may comprise a bushing mounted on the driven gear and the joint may comprise a cooperating cylindrical shaft connected to the second connection portion and arranged to slide within the bushing.

The second connection portion of each driven gear coupling member may comprise a socket part and the joint may comprise a ball part connected to the first connection portion (preferably by a rigid connection to the cylindrical shaft) and arranged to rotate and translate within the socket part.

The flexible interface may comprise one or more failsafe catches, each failsafe catch being adapted for attachment to the wheel and arranged to retain the driven gear in the event of a failure of one or more of the driven gear coupling members. The one or more failsafe catches may be attached to the second portion of one or more of the driven gear coupling members.

Preferably, one of the pinion gear and the driven gear comprises a sprocket and the other of the pinion gear and the driven gear comprises a series of rollers arranged to form a ring, each roller being rotatable about a roller axis at a fixed distance from an axis of rotation of the pinion gear or driven gear, respectively. Each of the series of rollers may be rotatable about a pin, the pins each being fixed at at least one end to an annular support member.

An aircraft landing gear experiences many different modes of deformation during use. In particular, each wheel axle <NUM> is deflected relative to the landing gear leg <NUM> as a result of the vertical loads due to the weight of the aircraft (<FIG>; 220A indicates the wheel axle before deflection, and 220B indicates it after deflection) and the horizontal loads applied during braking (<FIG>; 220A indicates the wheel axle before deflection, and 220B indicates it after deflection). In addition, the shape of each wheel rim <NUM> is deformed (to a lozenge, or oval, shape) due to tyre loads (<FIG>; 210A indicates the wheel rim before deflection, and 210B indicates it after deflection). Each deformation mode typically provides deformation within the range of +/-<NUM> at the extremities of the wheel. For example, the vertical height of the wheel may be reduced by <NUM> as a result of wheel distortion by tyre loads, and the wheel may tilt through about <NUM>-<NUM> degrees as a result of axle bending caused by vertical aircraft loads, resulting in a displacement of about <NUM> at the periphery of the wheel rim.

A drive system <NUM> for autonomous taxiing of an aircraft according to an embodiment of the present invention as shown in <FIG> is arranged to drive a wheel <NUM> of the landing gear. The drive system <NUM> comprises a pinion gear <NUM> mounted on a drive shaft <NUM> via a flexible interface comprising a crowned spline joint <NUM>, the drive shaft <NUM> being driven by an appropriately geared motor (not shown). The motor may be arranged to drive only one wheel, or two or more wheels via a differential or similar. Thus, one, some, or all of the wheels of the landing gear may be actively driven by the drive system, and there may be multiple drive systems per landing gear. The pinion gear <NUM> is meshed with a driven gear <NUM> which is in the form of an annular rim gear attached to a wheel rim <NUM> of the wheel <NUM> via a flexible interface comprising three driven gear coupling members <NUM> distributed evenly around the wheel rim <NUM>. The driven gear has a larger diameter than the drive pinion. This arrangement provides for a torque-magnifying gear ratio and an efficient use of space.

The deformation modes discussed above can result in misalignment and/or distortion within the drive system <NUM> since the pinion gear <NUM> is mounted on the leg or axle (not shown) of the landing gear, while the driven gear <NUM> is mounted on the wheel <NUM>, which is rotatable about the axle. In the absence of the flexible interface <NUM>, <NUM>, the axle deflections (<FIG>) can result in a tilt of the driven gear <NUM> with respect to the pinion gear <NUM>, i.e. the rotational axes of these gears are tilted with respect to one another. Similarly, in the absence of the flexible interface <NUM>, <NUM>, the wheel rim deformation (<FIG>) due to tyre loads can result in a translational displacement of the driven gear <NUM> with respect to the pinion gear <NUM>, i.e. the rotational axes of these gears are displaced with respect to one another. Such wheel rim deformation may also cause undesirable distortion of the driven gear <NUM>. Alternatively, a rigid connection between the driven gear <NUM> and the wheel <NUM> may cause further distortion within the wheel rim <NUM>.

The flexible interface <NUM>, <NUM> serves to isolate the drive system <NUM> from these deformations.

The driven gear coupling members <NUM> of the flexible interface each comprise a joint member <NUM> having a shaft portion <NUM> which is received within a bushing <NUM> mounted through a web of the driven gear <NUM>, the shaft portion <NUM> being capable of both limited translational and rotational movement within the bushing <NUM> to provide a kinematic cylindrical joint.

The joint member <NUM> also has a ball portion <NUM> separated from the shaft portion <NUM> by a connecting portion <NUM>, the ball portion <NUM> being received within a socket member <NUM>. The socket member <NUM> is rigidly connected to the wheel rim <NUM> and has a socket chamber <NUM> within which the ball portion <NUM> is located and a slot opening <NUM> through which the connecting portion <NUM> extends and which provides an opening to the socket chamber <NUM>. The socket chamber <NUM> is generally elongate to permit movement of the ball portion <NUM> of the joint member <NUM> along a linear path delimited by the extent of the slot opening <NUM>. The ball portion <NUM> is also able to rotate within the chamber <NUM>. In this way, the ball portion <NUM> and socket member <NUM> provide a kinematic ball and socket joint.

Each socket member <NUM> includes a spring <NUM> which is arranged to urge the ball portion <NUM> towards the wheel axis. In this way, the three springs <NUM> serve to centre the driven gear <NUM> with respect to the wheel rim <NUM>.

Each socket member <NUM> also includes a catch finger <NUM> which is rigidly attached to the socket member <NUM> and extends therefrom through an oversized through hole <NUM> through the web of the driven gear <NUM>. The through hole <NUM> is sized to ensure that there is no contact between the catch finger <NUM> and the driven gear <NUM> during normal operation of the drive system <NUM>, but if the joint member <NUM> of the driven gear coupling member <NUM> were to break, or the coupling member otherwise fail, the catch <NUM> would retain the driven gear <NUM> and maintain a connection with the wheel <NUM>.

<FIG> show possible configurations of the socket member <NUM>, the appropriate configuration being selected according to the specific wheel deformation modes to be accommodated. In <FIG> the chamber <NUM> is arranged to provide a straight linear translation of the ball portion <NUM> in a substantially radial direction of the wheel axis about which the wheel <NUM> rotates. In <FIG> the chamber <NUM> is arranged to provide a straight linear translation of the ball portion <NUM> in a direction which is at an angle to the radial direction of the wheel axis. In <FIG> the chamber is arranged to provide a curved linear translation of the ball portion <NUM> (the line in <FIG> indicating the line of translation) which is at an angle to the radial direction of the wheel axis.

The crowned spline joint <NUM> shown in <FIG> serves to permit the pinion gear <NUM> to tilt with respect to the rotational axis of the drive shaft <NUM>. The crowned spline joint <NUM> includes a plurality of male splines <NUM> which are arranged to cooperate with a corresponding plurality of female grooves <NUM> formed in the pinion gear <NUM>. The splines <NUM> and grooves <NUM> are curved so that the joint has a barrel shape. The splines <NUM> are thus able to slide longitudinally within the grooves <NUM> to permit the pinion gear <NUM> to tilt with respect to the drive shaft <NUM>. The joint <NUM> may include ball bearings (not shown) within the grooves <NUM> to facilitate this movement.

In alternative embodiments the crowned spline joint <NUM> may be replaced by a constant velocity (CV) joint.

In other embodiments the crowned spline joint <NUM> may be slidably mounted on the drive shaft <NUM> to permit relative translation between the pinion gear <NUM> and the drive shaft <NUM>. In such embodiments it may not be necessary for the driven gear coupling members <NUM> to include the shaft portion <NUM> and the bushing <NUM>, and instead the connecting portion <NUM> may be rigidly connected to the driven gear <NUM>.

In yet further embodiments the ball portion <NUM> and socket chamber <NUM> may be replaced by a shaft portion and bushing (not shown) to provide a kinematic cylindrical joint.

In the illustrated embodiments the pinion gear <NUM> and driven gear <NUM> comprise a roller gear (pin gear) or sprocket, respectively. In other embodiments the pinion gear <NUM> may comprise a sprocket and the driven gear <NUM> may comprise a roller gear. A roller gear comprises a series of rollers formed by two rigid annular rings connected together by a series of rollers arranged in a ring to form a continuous track. The rollers are each rotatable about a pin which extends between the annular rings to form a rigid connection between the annular rings. In the illustrated embodiments the roller gear is shown as having two adjacent rows of rollers; in other embodiments only a single row of rollers may be necessary.

A key advantage of achieving the motor-wheel connection via a sprocket and roller gear is that such a mechanism is inherently robust and tolerant of environmental contamination. Thus, it may not be necessary to enclose the drive system within a casing to prevent ingress of debris and other contaminants. In contrast, drive system arrangements employing meshing toothed gears, must be suitably protected from contaminants, the required protective casing adding both weight and expense, and making routine inspection difficult.

Another advantage of the sprocket-roller arrangement is that it is more tolerant of wheel deformation and misalignment between pinion and driven gear than meshing toothed gear arrangements.

In other embodiments the roller gear may be replaced by a roller chain (also known as an attachment chain, or attachment roller chain) extending around an outer circumference of a support member and being fixed thereto.

In yet further embodiments the driven gear and pinion gear may comprise toothed gears of the type usually used in drive transmissions.

Claim 1:
A drive system for an aircraft landing gear, the drive system comprising:
a pinion gear (<NUM>);
a drive shaft (<NUM>) arranged to rotate the pinion gear about a drive axis;
a driven gear (<NUM>) arranged to mesh with the pinion gear (<NUM>) to be rotatable by the pinion gear, the driven gear being connectable to a wheel (<NUM>) of the landing gear to be capable of rotating the wheel about a wheel axis;
a flexible interface comprising a plurality of driven gear coupling members (<NUM>), each driven gear coupling member having a first connection portion (<NUM>) attached to the driven gear (<NUM>), a second connection portion (<NUM>) adapted to be attached to the wheel (<NUM>) at an offset distance from the wheel axis, and a joint (<NUM>) between the first connection portion (<NUM>) and the second connection portion (<NUM>), the joint (<NUM>) permitting relative movement between the first connection portion (<NUM>) and the second connection portion (<NUM>),
characterised in that the joint (<NUM>) is a kinematic joint which permits translation of the first connection portion (<NUM>) relative to the second connection portion (<NUM>) (a) along a first local axis substantially parallel to the wheel (<NUM>) axis, the joint (<NUM>) comprising a cylindrical joint or a sliding joint to provide the translation along the first local axis, and/or (b) along a second local axis at an angle to the wheel (<NUM>) axis, the joint (<NUM>) comprising a cylindrical joint or a spherical joint to provide the translation along the second local axis.