U.S. patent application number 14/863905 was filed with the patent office on 2016-01-14 for drive unit for aircraft running gear.
This patent application is currently assigned to L-3 COMMUNICATIONS MAGNET-MOTOR GMBH. The applicant listed for this patent is L-3 Communications Magnet-Motor GmbH. Invention is credited to Manfred Heeg, Johann Oswald.
Application Number | 20160009383 14/863905 |
Document ID | / |
Family ID | 43569481 |
Filed Date | 2016-01-14 |
United States Patent
Application |
20160009383 |
Kind Code |
A1 |
Oswald; Johann ; et
al. |
January 14, 2016 |
DRIVE UNIT FOR AIRCRAFT RUNNING GEAR
Abstract
A drive unit (16) for an aircraft running gear (2) having at
least a first wheel (4) and a second wheel (6) on a common wheel
axis (A) is characterized in that the drive unit (16) is drivingly
coupleable to the first and second wheels (4, 6) such that a
direction of longitudinal extension (C) of the drive unit (16) is
in a plane orthogonal to the common wheel axis (A).
Inventors: |
Oswald; Johann; (Eschenlohe,
DE) ; Heeg; Manfred; (Starnberg, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
L-3 Communications Magnet-Motor GmbH |
Starnberg |
|
DE |
|
|
Assignee: |
L-3 COMMUNICATIONS MAGNET-MOTOR
GMBH
Starnberg
DE
|
Family ID: |
43569481 |
Appl. No.: |
14/863905 |
Filed: |
September 24, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13643855 |
Dec 18, 2012 |
9169005 |
|
|
PCT/EP2010/055688 |
Apr 28, 2010 |
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14863905 |
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Current U.S.
Class: |
244/50 ; 475/150;
475/220; 475/225; 475/230; 475/31; 475/84 |
Current CPC
Class: |
Y02T 50/823 20130101;
B64C 25/405 20130101; F16H 48/34 20130101; Y02T 50/80 20130101;
F16H 48/32 20130101; F16H 48/08 20130101 |
International
Class: |
B64C 25/40 20060101
B64C025/40; F16H 48/32 20060101 F16H048/32; F16H 48/34 20060101
F16H048/34; F16H 48/08 20060101 F16H048/08 |
Claims
1. A drive unit for an aircraft running gear having at least a
first wheel and a second wheel on a common wheel axis, the drive
unit comprising: a differential gear; and a motor drivingly
coupleable to the first and second wheels via the differential
gear, wherein the drive unit is drivingly coupleable to the first
and second wheels such that a direction of longitudinal extension
of the drive unit is in a plane orthogonal to the common wheel
axis.
2. The drive unit according to claim 1, wherein the motor comprises
a bevel gear for engaging with the differential gear.
3. The drive unit according to claim 1, wherein the differential
gear is coupleable to the first and second wheels by first and
second gear structures, respectively.
4. The drive unit according to claim 1, wherein the differential
gear is a bevel differential or planetary differential or ball
differential.
5. The drive unit according to claim 1, wherein the motor is an
electric motor or a hydraulic motor.
6. The drive unit according to claim 1, further comprising: a first
output stage gear engageable with a first wheel axis gear, which is
coupled to the first wheel, for driving the first wheel, and a
second output stage gear engageable with a second wheel axis gear,
which is coupled to the second wheel, for driving the second wheel,
wherein the first and second output stage gears are aligned on a
common output stage axis, which is substantially orthogonal to the
direction of longitudinal extension of the drive unit.
7. The drive unit according to claim 3, wherein the first and
second gear structures comprise a planetary gear, respectively.
8. The drive unit according to claim 6, wherein the first and
second output stage gears are selectively engageable with the first
and second wheel axis gears through moving the first and second
output stage gears in a substantially radial direction of the first
and second wheel axis gears.
9. The drive unit according to claim 8, wherein the moving of the
first and second output stage gears corresponds to a substantially
straight motion of a respective tooth of the first and second
output stage gears towards a respective engagement space between
two respective teeth of the first and second wheel axis gears.
10. The drive unit according to claim 8, wherein the moving of the
first and second output stage gears is effected through pivotally
rotating the drive unit or laterally displacing the drive unit.
11. The drive unit according to claim 1, further comprising an
integrated free-wheel arrangement.
12. The drive unit according to claim 11, wherein a free-wheeling
direction of the free-wheel arrangement is reversible.
13. The drive unit according to claim 1, further comprising a
self-securing engagement/disengagement mechanism.
14. The drive unit according to claim 13, wherein the self-securing
engagement/disengagement mechanism comprises a bell crank.
15. The drive unit according to claim 13, wherein the self-securing
engagement/disengagement mechanism is operated in a pneumatic,
hydraulic or electric manner.
16. The drive unit according to claim 6, further comprising an
engagement/disengagement mechanism adapted to synchronize the
rotating speeds of the first and second output stage gears with the
first and second wheel axis gears by sensing the wheel speed and
adjusting the motor speed.
17. The drive unit according to claim 16, further comprising a
sensing device for sensing the relative positioning of gear teeth
for targeted engaging of the first and second output stage gears
with the first and second wheel axis gears, respectively.
18. An aircraft running gear, comprising: at least a first wheel
and a second wheel on a common wheel axis; and a drive unit that is
drivingly coupleable to the first and second wheels such that a
direction of longitudinal extension of the drive unit is in a plane
orthogonal to the common wheel axis, wherein the drive unit
comprises a motor and a differential gear, with the motor being
drivingly coupleable to the first and second wheels via the
differential gear.
19. The aircraft running gear according to claim 18, further
comprising: a first wheel axis gear, which is coupled to the first
wheel, engageable to a first output stage gear of the drive unit,
and a second wheel axis gear, which is coupled to the second wheel,
engageable to a second output stage gear of the drive unit.
20. The aircraft running gear according to claim 18, further
comprising a running gear leg supporting the first and second
wheels, with the drive unit being mounted to the running gear
leg.
21. The aircraft running gear according to claim 20, wherein the
direction of longitudinal extension of the drive unit is
substantially parallel to the running gear leg.
22. The aircraft running gear according to claim 18, wherein the
aircraft running gear is adapted to be used as a nose running gear
or a main running gear.
23. The aircraft running gear according to claim 19, wherein the
first and second wheel axis gears are mounted on a respective rim
of the first and second wheels.
24. The aircraft running gear according to claim 19, wherein the
first and second wheel axis gears are involute gears or cycloid
gears or Wildhaber-Novikov gears or hypoid gears.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a divisional application of U.S.
Ser. No. 13/643,855, which is a national phase application of
International Patent Application No. PCT/EP2010/055688, having an
international filing date of Apr. 28, 2010. The contents of these
applications are incorporated herein by reference in its
entirety.
FIELD
[0002] The present invention is directed to a drive unit for an
aircraft running gear and to an aircraft running gear comprising a
drive unit.
BACKGROUND
[0003] Conventionally, large commercial aircraft, also referred to
as airplanes hereinafter, use their gas turbine engines to taxi on
an airfield or maneuvering area of an airport. As the gas turbine
engines of airplanes are not designed to operate efficiently in a
low power state, such as needed during the taxiing operation on the
ground, maneuvering of the aircraft on the ground consumes a lot of
fuel. Increasing fuel prices have made this fuel consumption during
taxiing more and more worrisome. Moreover, the fuel efficiency for
the whole flight decreases due to the large amount of fuel that has
to be carried during the flight for taxiing at the destination
airport. Alternatively, special vehicles are used to drag or push
airplanes on an airfield. However, since such special vehicles are
expensive themselves and not available in large numbers at most
airports, they are commonly only used for short distances, such as
the push-back operation from the gate. This again leads to the gas
turbine engines being used for most of the taxiing, which causes
above described disadvantages.
[0004] Alternative solutions for taxiing of aircraft have been
suggested in the prior art. DE 10 2008 006 295 A1 discloses an
electric motor mounted onto a running gear leg of an aircraft. The
electric motor comprises a motor shaft parallel to the axis of the
wheels of the running gear. The motor shaft can be moved axially
between different positions in order to engage/disengage with the
wheel structure of the aircraft running gear and drive the
wheel.
[0005] WO 2009/086804 A1 discloses a motor for driving the wheels
of an airplane landing gear, which is disposed in the base of the
landing gear strut or is installed as a wheel hub motor in the
wheel hub or rim.
[0006] Although improvements could be achieved with these
approaches, it has been found that--especially for large commercial
aircraft--these approaches did not yield satisfying results in
terms of providing the necessary power to drive the aircraft
without the help of the turbine engines, while making efficient use
of the highly limited space available for such drives.
SUMMARY
[0007] Accordingly, the problem underlying the present invention is
to provide a drive for an aircraft running gear that allows for
providing the necessary power to taxi large commercial aircraft,
such as common passenger aircraft, while imposing minimal space
requirements to the overall design of the aircraft running
gear.
[0008] This problem is solved by a drive unit in accordance with
claim 1.
[0009] The claimed drive unit for an aircraft running gear having
at least a first wheel and a second wheel on a common wheel axis is
characterized in that the drive unit is drivingly coupleable to the
first and second wheels such that a direction of longitudinal
extension of the drive unit is in a plane orthogonal to the common
wheel axis.
[0010] Providing the drive unit in such a way that its design
requires it to be oriented in a plane orthogonal to the common
wheel axis for operational coupling with the first and second
wheels allows for a number of advantages. The motor(s) of the drive
unit is/are no longer confined to the distance between the two
wheels in its/their longitudinal direction. Without such a strict
limit on the longitudinal extension of the motor(s), the speed
and/or torque and/or speed torque product achieved by the motor(s)
can be increased as compared to the prior art. Accordingly, more
power for taxiing the aircraft can be generated by the motor(s). It
is pointed out that the direction of longitudinal extension of the
drive unit refers to the axis of the motor shaft of the motor
comprised in the drive unit, which is also referred to as the
longitudinal extension of the motor. Accordingly, the orientation
of the motor allows for a more flexible length of the motor, such
that improved motor characteristics can be realized. The particular
orientation of the drive unit also allows for the distance between
the first and the second wheel to be chosen more flexibly, since
only the lateral dimension of the drive unit restricts this
distance. A decreased distance between the first and second wheel
may result in the overall space requirements for the wheel
arrangement to be decreased, such that the whole aircraft running
gear may be stowed in a more space-efficient manner during the
flight. Commonly, the direction of longitudinal extension of the
drive unit corresponds to the direction of the largest geometric
extension of the drive unit.
[0011] The term common wheel axis refers to the geometrical axis
running through the centers of the first and second wheels.
[0012] The plane orthogonal to the common wheel axis may be
situated between the first and second wheel. In this way, the space
between the first and second wheels may be used much more
efficiently than in the prior art. The drive unit may be positioned
substantially parallel to the running gear leg supporting the first
and second wheels, e.g. in front of the running gear leg.
Accordingly, a large portion of the drive unit lies within the
space between the two wheels. The space between the two wheels
refers to the total room enclosed by the projection of the
circumference of the first wheel onto the circumference of the
second wheel. This space is largely unused in prior art
arrangements, but has to be accounted for when stowing the wheel
arrangement during the flight. Accordingly, the invention allows
for providing a more powerful, possibly larger motor than the prior
art, while decreasing the space requirements through making it
possible to reduce the distance between the wheels and to use the
remaining space between the wheels efficiently.
[0013] According to a further embodiment, the drive unit comprises
a first motor drivingly coupleable to the first wheel via a first
gear structure and a second motor drivingly coupleable to the
second wheel via a second gear structure, wherein the first and
second motors are arranged in tandem along the direction of
longitudinal extension of the drive unit. The arrangement in tandem
refers to a one-behind-the-other arrangement in the direction of
longitudinal extension of the drive unit. Providing a respective
motor for driving each of the two wheels allows for the drive unit
to be capable of driving the first and second wheels independently
and to provide a desired wheel speed difference when the aircraft
is turning a corner. For example, the running gear leg may be
turned by a steering motor in order for the aircraft to steer to
the right or to the left. The steering signal provided to the
steering motor may also be provided to the first and second motors,
such that these motors can drive the first and second wheel in
accordance with the desired turning radius. Accordingly, a turning
of the airplane is made possible that reduces the wear and tear of
the tires and other components of the wheel arrangement. It is also
possible to cause the turning of the aircraft by driving the first
and second wheels at different speeds. The arrangement of the first
and second motors in tandem allows for a space efficient
positioning of the two motors, with the provision of two motors
only adding to the longitudinal extension of the drive unit, but
not to the lateral extension. Therefore, the provision of two
motors does not have an impact on the distance between the first
and second wheels required to accommodate the drive unit.
Consequently, an improved driving of the first and second wheels is
achieved, while ensuring a space-efficient arrangement of the whole
aircraft running gear.
[0014] According to a further embodiment, the first motor in
operation drives a first bevel gear, with the first bevel gear
being drivingly coupleable to the first wheel via the first gear
structure, and the second motor in operation drives a second bevel
gear, with the second bevel gear being drivingly coupleable to the
second wheel via the second gear structure. The first and second
bevel gears allow for a change of the direction of the rotation
axis of the components driven by the first and second motors.
Particularly, the rotation of the shafts of the first and second
motors can cause the rotation of other gear elements that are not
aligned or parallel with the motor shafts and are comprised in the
first and second gear structures, respectively. More particularly,
a turning of the rotation axis of the driven components of
90.degree. can be achieved. Accordingly, gear structure components
whose rotation axis is identical with or parallel to the common
wheel axis can be driven via the first and second bevel gears. This
rotation can then be transmitted to the first and second wheels in
a convenient manner.
[0015] According to a further embodiment, the first and second
motors are arranged in a coaxial manner. This arrangement allows
for a highly efficient use of space, as only one common axis of
rotation is present in the drive unit, around which the first and
second motors are arranged. The lateral extension of the drive unit
can be kept to a minimum, because no two laterally offset motor
shafts are required for driving the wheels.
[0016] According to a further embodiment, the first motor has a
first motor shaft and the second motor has a second motor shaft,
with the first motor shaft being hollow and being arranged around
the second motor shaft. The arrangement of one hollow motor shaft
around the other motor shaft ensures that the first and second
motors can be arranged in a coaxial manner, while complete
independence of the driving of the first and second wheels is
achieved.
[0017] In a particular embodiment, the first and second motors are
electric motors or hydraulic motors.
[0018] In a further embodiment, the first gear structure comprises
a first gear element, having a third bevel gear and a first gear
element shaft, and the second gear structure comprises a second
gear element, having a fourth bevel gear and a second gear element
shaft, with one of the first and second gear element shafts having
a hollow portion and the other one of the first and second gear
element shafts being supported in the hollow portion. The rotation
axes of the first and second gear element shafts may be aligned.
The supporting of one gear element shaft within the other allows
for a highly compact and stable arrangement of the two independent
power transmissions from the first motor to the first wheel and
from the second motor to the second wheel. The first bevel gear may
be in engagement with the third bevel gear, and the second bevel
gear may be in engagement with the fourth bevel gear. In this way,
a first gear ratio stage is realized. The gear ratio between the
first and third bevel gears may be the same as the gear ration
between the second and fourth bevel gears. The power generated by
the first and second motors is transmitted via two coaxial motor
shafts to two gear elements, which are aligned on a common axis,
but are laterally displaced with respect to each other. A compact
power transmission is achieved that provides--at its output--two
laterally displaced gear elements with independent speeds of
rotation. With one gear element supported within the other, the
lateral dimension of the drive unit is kept to a minimum.
[0019] According to another embodiment, the drive unit comprises a
motor and a differential gear, with the motor being drivingly
coupleable to the first and second wheels via the differential
gear. The provision of the differential gear allows for a
mechanical adjustment of the wheel speeds when the aircraft is
turning a corner. Accordingly, the two wheels can be driven with
one motor, while the differential gear ensures the reduction of
wear and tear on the tires and other wheel structure components by
mechanically adjusting the wheel speeds to a given turning radius.
The differential gear may be an integrated differential gear,
meaning that it is integrated into a gearbox. The motor may
comprise a bevel gear for engaging with the differential gear. In
this way, an efficient rotation of the power transmission axis from
the direction of longitudinal extension of the drive unit to a
direction parallel or coaxial with the common wheel axis is
achieved. The differential gear may be coupleable to the first and
second wheels by first and second gear structures, respectively.
Also, the differential gear may be a bevel differential or
planetary differential or ball differential. The motor may be an
electric motor or a hydraulic motor.
[0020] According to another embodiment, the drive unit comprises a
first output stage gear engageable with a first wheel axis gear,
which is coupled to the first wheel, for driving the first wheel,
and a second output stage gear engageable with a second wheel axis
gear, which is coupled to the second wheel, for driving the second
wheel, wherein the first and second output stage gears are aligned
on a common output stage axis, which is substantially orthogonal to
the direction of longitudinal extension of the drive unit. The
common output stage axis may be parallel to the common wheel axis.
In this way, a drive unit may be provided that has two output stage
gears, which may be circular external gears, that may
simultaneously be brought into engagement with the two wheel axis
gears coupled to the first and second wheels. The drive unit as a
whole has above discussed advantages of having its direction of
longitudinal extension in a plane orthogonal to the common wheel
axis, while the provision of output stage gears orthogonal to the
direction of longitudinal extension ensures that a straightforward
selective engagement between the drive unit and the wheel structure
can be realized. The combination of the first and second output
stage gears and the first and second wheel axis gears, which may be
circular external gears, also allows for establishing a gear ratio
stage that is outside the drive unit. As the output stage gears may
have a small diameter and the wheel axis gears may have a large
diameter, a reduction gear stage having large transmission ratio
can be achieved, which helps to produce sufficient torque using a
compact motor. Accordingly, this gear ratio stage is in addition to
all gear ratios that may be implemented within the drive unit,
which helps in keeping the drive unit compact. It is pointed out
that the term coupled, which is used with regard to the attachment
between the first and second wheel axis gears and the first and
second wheels, refers to a rotatably fixed attachment between these
elements. It is intended to encompass all attachments that allow
for a transfer of torque from the first and second output stage
gears to the first and second wheel axis gears and ultimately to
the first and second wheels, respectively. Arrangements that
account for exceptional situations, such as the provision of a play
in the rotatably fixed arrangement or an intended failure of the
rotational fixation in case the torque exceeds a predetermined
threshold, are intended to not be excluded by the term coupled.
[0021] In a further embodiment, the first and second gear
structures comprise a planetary gear, respectively. The planetary
gears allow for a reduction of the rotation speeds and an according
increase of the torques in a very compact manner. With little space
required, a gear ratio stage can be implemented in the drive unit
via the planetary gears. Together with the gear ratio stage
associated with the bevel gears and the gear ratio stage associated
with the output stage gears and the wheel axis gears, three
reduction stages may be realized in a very compact manner. The
bevel gear stage allows for a 90.degree. change of the rotation
axis from the direction of the motor shaft(s) to a direction
aligned with or parallel to the common wheel axis. The reduction
stage at the drive unit output allows for a convenient
implementation of a simultaneous engagement of the two output stage
gears of the drive unit with the wheel axis gears coupled to the
first and second wheels, respectively.
[0022] In a further embodiment, the first and second output stage
gears are selectively engageable with the first and second wheel
axis gears through moving the first and second output stage gears
in a substantially radial direction of the first and second wheel
axis gears. The term selective engagement refers to time-selective
engagement. In other words, the output stage gears may be in
engagement with the wheel axis gears at some points in time,
whereas disengagement between the output stage gears and the wheel
axis gears may be present at other points in time. Accordingly,
selective engagement refers to a connection between two entities
that can be in an engaged or in a disengaged state. Motion in a
substantially radial direction of the first and second wheel axis
gears means that, during the disengagement operation, the common
output stage gear axis substantially stays in a radial motion plane
defined by the common wheel axis and the common output stage axis
in the engaged position.
[0023] The moving of the first and second output stage gears may be
effected through pivotally rotating the drive unit or laterally
displacing the drive unit. Inherently, the pivotally rotating of
the drive unit prevents the common output stage axis from staying
exactly in the radial motion plane. However, by choosing the
distance between the engagement points (of the output stage gears
and the wheel axis gears) and a pivot bearing, e.g. a pivotal
mounting structure for attaching the drive unit to the running gear
leg, to be comparatively large, the disengagement may be effected
in an almost radial direction of the first and second wheel axis
gears. Lateral displacement means that the drive unit is moved,
with the motion not including any rotational component of the drive
unit with respect to the remaining aircraft running gear. The
radial engagement/disengagement direction allows for gentle
engagement operations that keep the wear and tear of the first and
second output stage gears and of the first and second wheel axis
gears low. Typically, the common axis of the output stage gears
remains parallel with the common axis of the wheel axis gears
during engaging/disengaging of the gears, however the distance
between these axes decreases/increases.
[0024] Particularly, the moving of the first and second output
stage gears may correspond to a substantially straight motion of a
respective tooth of the first and second output stage gears towards
a respective engagement space between two respective teeth of the
first and second wheel axis gears. The term straight motion is
meant to describe a motion of the respective tooth along a line
connecting the center of the wheel axis gear, the foot arch of the
wheel axis gear, the tip arch of the output stage gear and the
center of the output stage gear. Engaging refers to a motion of the
tip arch of the output stage gear towards the foot arch of the
wheel axis gear, whereas disengaging refers to a motion of the tip
arch of the output stage gear away from the foot arch of the wheel
axis gear and potentially passing the tip arch of the wheel axis
gear. This sort of engaging motion allows for a minimization of
wear and tear on the gear teeth. Of course, when both the output
stage gears and the wheel axis gears are in rotational motion, the
straight motion of the tooth towards the engagement space only
takes place for an instance in time, with the adjacent tooth and
space performing the straight motion the next instance.
[0025] The drive unit may also comprise an integrated free-wheel
arrangement. A free-wheel arrangement prevents a rotation of the
wheel axis gears to be transmitted to the motor(s) of the drive
unit, even when the drive unit is in an engaged position.
Accordingly, at one point in the power transmission path from the
motor(s) to the output stage gears, a stage may be equipped with an
overrunning clutch or the like that prevents power transmission
from a downstream element to an upstream element, when looking at
the normal operational power flow from the motor(s) to the wheels.
Such a free-wheel arrangement allows the airplane to keep on
rolling, should the motor(s) of the drive unit fail. The failed
motor(s) cannot block the rotation of the wheels. Also, for the
process of engaging the drive unit with the first and second wheel
axis gears, the free-wheel arrangement ensures a synchronization of
the wheel axis gear speeds with the output stage gear speeds, such
that severe damage to the gears due to an un-synchronized
engagement attempt can be prevented during the engaging operation.
The free-wheel arrangement may be incorporated into any rotatably
fixed coupling present in the gear arrangement described. For
example, the coupling of the first and second output stage gears
with respect to the first and second gear structures may have an
integrated free-wheel arrangement. Alternatively, first and second
ring gears of the first and second planetary gears may have an
integrated free-wheel arrangement. The free-wheel arrangement may
be realized mechanically. The free-wheeling direction of the
free-wheel arrangement may be reversible. This allows for the
advantages of the free-wheeling arrangement to be present both when
driving the aircraft forward and backwards with the drive unit.
[0026] In a further embodiment, the drive unit comprises a
self-securing engagement/disengagement mechanism. Such a
self-securing engagement/disengagement mechanism prevents an
inadvertent engagement of the drive unit with the wheel structure,
which could result in unexpected behavior of the aircraft landing
gear, which is potentially highly dangerous, especially during
take-off and landing. The self-securing engagement/disengagement
mechanism may comprise a bell crank. Also, the self-securing
engagement/disengagement mechanism may be operated in a pneumatic,
hydraulic or electric manner.
[0027] In a further embodiment, the drive unit comprises an
engagement/disengagement mechanism adapted to synchronize the
rotating speeds of the first and second output stage gears with the
first and second wheel axis gears by sensing the wheel speed and
adjusting the motor speed. Accordingly, a synchronized angular
velocity of the first and second output stage gears and the first
and second wheel axis gears can be reached, which allows for a
precise engaging of these gears, such that the wear and tear of the
gears can be kept low. The drive unit may comprise a control unit,
which is in communication with a sensor measuring the wheel speed
and generates control commands for the motor of the drive unit. In
the case of two independent motors being provided for driving the
first and second wheels, two sensors may be provided for measuring
the wheel speeds and two control commands may be generated by the
control unit to control the two motors independently.
[0028] In another embodiment, the drive unit may comprise a sensing
device for sensing the relative positioning of gear teeth for
targeted engaging of the first and second output stage gears with
the first and second wheel axis gears, respectively. Using a direct
measuring of the positions of gear teeth allows for a highly
accurate engaging of the gears, as the variable that is decisive
for the wear and tear of the gears, namely their relative
positioning, is directly available for the control of the motor(s)
of the drive unit. The rotational position of an output stage gear
may be determined via a separate sensor, such as an incremental
encoder, a resolver, or another positional sensor at the location
of the output stage gear. In case the motor is an electric motor,
it commonly comprises a positional sensor for determining the
position of the motor, whose output may be used for determining the
position of the output stage gear, with the determination taking
the gearbox gear ratio into account. The position of the wheel axis
gear may also be determined by a positional sensor that may be
integrated into the running gear leg. The aircraft running gear may
comprise an ABS breaking system, in which case an output of a
positional sensor of the ABS breaking system may be used for
determining the position of the wheel axis gear. The positional
sensor for determining the position of the wheel axis gear may be
mounted to the drive unit. The positional sensor may be an optical
or inductive sensor measuring the distance to a tooth of the wheel
axis gear or being triggered by the teeth of the wheel axis gear.
The location of spaces between teeth can be determined very
accurately in this way.
[0029] According to another embodiment, an aircraft running gear
comprises at least a first wheel and a second wheel on a common
wheel axis and a drive unit as described in any of the embodiments
above. The aircraft running gear may comprise a first wheel axis
gear, which is coupled to the first wheel, engageable to the first
output stage gear of the drive unit, and a second wheel axis gear,
which is coupled to the second wheel, engageable to the second
output stage gear of the drive unit.
[0030] The aircraft running gear may also have a running gear leg
supporting the first and second wheels, with the drive unit being
mounted to the running gear leg. The wheels may be supported by the
running gear leg via a wheel shaft assembly. The attachment to the
running gear leg allows for a stable attachment of the drive unit
to the aircraft running gear. The direction of longitudinal
extension of the drive unit may be substantially parallel to the
running gear leg. This arrangement allows for using the space
between the first and second wheels for the positioning of the
drive unit, such that an overall space-efficient aircraft running
gear is formed. Particularly, the stowing space for the aircraft
running gear during the flight is kept low. Also, the positioning
of the drive unit in parallel to the running gear leg ensures that
only minimal additional aerodynamic resistance is introduced by the
drive unit.
[0031] In a further embodiment, the aircraft running gear is
adapted to be used as a nose running gear or a main running gear.
Also, the first and second wheel axis gears may be mounted on a
respective rim of the first and second wheels. The first and second
rims are very suitable structures for mounting the first and second
wheel axis gears, as they are inherently stable structures that are
adapted to carry the weight of the whole aircraft and that are
designed to withstand extreme environmental conditions, during the
flight as well as on the ground. The first and second wheel axis
gears may be involute gears or cycloid gears or Wildhaber-Novikov
gears or hypoid gears. Involute gears and cycloid gears may be
particularly resistant to wear and tear in the detrimental
environment of the aircraft running gear, where large amounts of
dirt commonly accumulate. Wildhaber-Novikov gears may have a
particularly high load bearing capacity. Particularly in
combination with the radial engagement/disengagement of the gears,
the Wildhaber-Novikov gears also allow for excellent durability of
the wheel axis gears.
DRAWINGS
[0032] The invention is described in more detail below with regard
to the exemplary embodiments shown in the accompanying Figures, in
which:
[0033] FIG. 1 shows a three-dimensional representation of an
aircraft running gear according to a first exemplary embodiment of
the invention.
[0034] FIG. 2 shows a cross-sectional view through the aircraft
running gear according to the first exemplary embodiment of the
invention.
[0035] FIG. 3 shows an enlarged portion of the cross-sectional view
shown in FIG. 2.
[0036] FIG. 4 shows a further cross-sectional view through the
aircraft running gear according to the first exemplary embodiment
of the invention.
[0037] FIG. 5a shows an enlarged portion of the cross-sectional
view shown in FIG. 4.
[0038] FIG. 5b shows the enlarged portion shown in FIG. 5a, with
the drive unit being in a disengaged position.
[0039] FIG. 6 shows a cross-sectional view through an aircraft
running gear according to a second exemplary embodiment of the
invention.
DETAILED DESCRIPTION
[0040] FIG. 1 shows a three-dimensional representation of an
aircraft running gear 2 according to a first exemplary embodiment
of the present invention. The aircraft running gear 2 comprises a
first wheel 4 and a second wheel 6, which are connected by a shaft
assembly 12. The first and second wheels 4, 6 are aligned on a
common wheel axis A in the geometrical sense. The first wheel 4
comprises a first rim 32, which is provided with a first wheel axis
gear 8. The second wheel 6 comprises a second rim 34, which is
provided with a second wheel axis gear 10. The first and second
wheel axis gears 8, 10 can be mounted onto the first and second
rims 32, 34 of the first and second wheels 4, 6 in any suitable
manner that allows for a rotatably fixed attachment between the
rims and the wheel axis gears. The rims and the wheel axis gears
may also be made of one piece, respectively, i.e. the first rim 32
and the first wheel axis gear 8 may be made of one piece and the
second rim 34 and the second wheel axis gear 10 may be made of one
piece. In these ways, a fixed coupling between the first and second
wheel axis gears 8, 10 and the first and second wheels 4, 6 is
achieved, such that the rotational motion transmitted to the first
and the second wheel axis gears 8, 10 is transmitted to the first
and second wheels 4, 6. The first and second wheel axis gears 8, 10
are circular external gears, with their teeth being arranged
straight between and perpendicular to the axial edges of the
external gear.
[0041] The aircraft running gear 2 further comprises a running gear
leg 14 running along a leg axis D and a drive unit 16, which is
attached to the running gear leg 14. The drive unit 16 comprises a
first motor 18 and a second motor 20, a gearbox 26, a first output
stage gear 22 and a second output stage gear 24. The first and
second motors 18, 20 are arranged along a common longitudinal axis
C, also referred to as the direction of longitudinal extension of
the drive unit 16. The first and second output stage gears 22, 24
are arranged along a common output stage axis B. The drive unit 16
is moveably mounted to the running gear leg 14, so that the first
and second output stage gears 22, 24 can be selectively brought
into engagement with the first and second wheel axis gears 8, 10.
An engagement operation brings the first and second output stage
gears 22, 24 simultaneously into engagement with the first and
second wheel axis gears 8, 10. The first motor 18 is drivably
coupled to the first output stage gear 22, and the second motor 20
is drivably coupled to the second output stage gear 24. In this
way, the first and second wheels 4, 6 can be driven with different
speeds by the first and second motor 18, 20, such that an aircraft
that is equipped with the aircraft running gear 2 can easily turn
corners in the airfield or maneuvering area of an airport. The
gearbox 26 provides a gearbox gear ratio. Also, the output stage
gears 22, 24 and the wheel axis gears 8, 10 provide an output gear
ratio. The product of the gearbox gear ratio and the output stage
gear ratio allows for a driving of large planes with two comparably
very small motors that can be placed in front of the running gear
leg 14 and extend into the space between the first and second
wheels 4, 6. The gear ratios transform the high motor speeds of the
first and second motors 18, 20 into large amounts of torque
required for driving the aircraft during a taxiing operation.
[0042] In the exemplary embodiment shown in FIG. 1, the first and
second motors 18, 20 are electric motors. However, the drive unit
16 can also be equipped with hydraulic motors.
[0043] FIG. 2 is a cross-sectional view of the aircraft running
gear 2 of FIG. 1. The cross-sectional plane is parallel to the
wheel axis in front of the running gear leg 14. The output stage
axis, on which the first and second output stage gears 22, 24 are
aligned, lies within the cross-sectional plane, such that the drive
unit 16 is cut in half along the longitudinal extension of the
drive unit by the cross-sectional plane, i.e. the cross-section of
FIG. 2 shows the interior of the drive unit 16. As the aircraft
running gear 2 of FIG. 2 corresponds to the aircraft running gear
of FIG. 1, like reference numerals are used for like elements. FIG.
2 illustrates well that the first and second wheel axis gears 8, 10
are mounted onto the first and second rims 32, 34.
[0044] The second motor 20 comprises a second motor shaft 30 that
extends through the first motor 18. The first motor 18 comprises a
first motor shaft 28 that is hollow and is arranged around the
second motor shaft 30. In the exemplary embodiment shown in FIG. 2,
the first motor shaft 28 extends along a small portion of the
second motor shaft 30. The first and second motor 18, 20 are
arranged in a coaxial manner, i.e. the center axes of the first
motor shaft 28 and the second motor shaft 30 are identical and
identical to the axis C defining the longitudinal extension of the
first and second motors. Again, the term axis is used in its
geometrical meaning.
[0045] The first motor 18 and the second motor 20 are arranged in
tandem, i.e. they are arranged in a one behind the other
relationship as seen from the gearbox or in an one above the other
relationship as seen in the cross-sectional plane of FIG. 2. This
viewing direction roughly corresponds to an observer's viewing
direction when positioned in front of the aircraft running gear 2
of an aircraft. The coaxial arrangement of the first and second
motors 18, 20 allows for the provision of two motors that are
co-extensive along the longitudinal extension of the drive unit 16.
In other words, the two motors extend substantially equally from
their common axis in all directions orthogonal to the common axis,
in particular in the lateral direction being defined as the
direction of the common wheel axis.
[0046] FIG. 3 is an enlarged version of the gearbox portion shown
in the center of FIG. 2. FIG. 3 shows the first motor shaft 28 of
the first motor 18 as well as the end portion of the second motor
shaft 30 of the second motor 20. The first motor shaft 28 comprises
a first bevel gear 38 at its end portion. The second motor shaft 30
comprises a second bevel gear 40 at its end portion. The gearbox 26
further comprises a first gear element 42 and a second gear element
44. The first gear element 42 comprises a third bevel gear 54,
which is in engagement with the first bevel gear 38. The second
gear element 44 comprises a fourth bevel gear 56, which is in
engagement with the second bevel gear 40. The first gear element 42
further comprises a first gear element shaft 66, and the second
gear element 44 comprises a second gear element shaft 68. The first
gear element shaft 66 and the second gear element shaft 68 are
aligned along a common axis. In the exemplary embodiment of FIG. 3,
this axis of the first and second gear element shafts 66, 68
coincides with the output stage axis, on which the first and second
output stage gears 22, 24 are aligned. The first and second gear
element shafts respectively extend from a center portion of the
gearbox 26 towards the first and second output stage gears 22, 24
arranged on the lateral ends of the gearbox 26, which can be best
seen in FIG. 2. Through the first to fourth bevel gears, the
rotation of the first and second motor shafts 28, 30 causes the
rotation of the first and second gear elements 42, 44. In this
manner, the rotation axis of the first and second gear elements 42,
44 is orthogonal to the rotation axis of the first and second motor
shafts 28, 30.
[0047] The portion of the first gear element shaft 66 towards the
center portion of the gearbox 26 is hollow. The portion of the
second gear element shaft 68 towards the center portion of the
gearbox 26 is supported within the first gear element shaft 66.
This supporting of the second gear element shaft 68 within the
first gear element shaft 66 allows for an accurate and stable
alignment of the first and second gear element shafts 66, 68 and
also of the first and second gear elements 42, 44 as a whole. The
second gear element shaft 68 is supported within the first gear
element shaft 66 by a first combined axial and radial bearing 70
and a radial bearing 72.
[0048] The gearbox 26 further comprises a first planetary gear 46
and a second planetary gear 48. It also comprises a third gear
element 62 and a fourth gear element 64. The first planetary gear
46 couples the first gear element 42 to the third gear element 62,
and the second planetary gear 48 couples the second gear element 44
to the fourth gear element 64.
[0049] The gearbox 26 comprises a first internal gear 50, which
serves as the ring gear for the first planetary gear 46. The first
gear element 42 comprises a first external gear portion 58, which
serves as the sun gear of the first planetary gear 46. The third
gear element 62 comprises a first plurality of planet gears 74. The
first plurality of planet gears 74 are in engagement with the first
internal gear 50 and the first external gear portion 58. In this
way, the first external gear portion 58, the first plurality of
planet gears 74 and the first internal gear 50 form the first
planetary gear 46.
[0050] The gearbox 26 further comprises a second internal gear 52,
which serves as the ring gear for the second planetary gear 48. The
second gear element 44 comprises a second external gear portion 60,
which serves as the sun gear of the second planetary gear 48. The
fourth gear element 64 comprises a second plurality of planet gears
76. The second plurality of planet gears 76 are in engagement with
the second internal gear 52 and the second external gear portion
60. In this way, the second external gear portion 60, the second
plurality of planet gears 76 and the second internal gear 52 form
the second planetary gear 48.
[0051] The outer portion of the first gear element shaft 66, i.e.
the portion of the first gear element shaft 66 towards the first
output stage gear 22, is supported within a recess of the third
gear element 62 via a second combined axial and radial bearing 78.
In this way, a stable alignment between the first gear element 42
and the third gear element 62 is achieved, which allows for a
reliable functioning of the first planetary gear 46. The outer
portion of the second gear element shaft 68, i.e. the portion of
the second gear element shaft 68 towards the second output stage
gear 24, is supported within a recess of the fourth gear element 64
via a third combined axial and radial bearing 80. In this way, a
stable alignment between the second gear element 44 and the fourth
gear element 64 is achieved, which allows for a reliable
functioning of the second planetary gear 48.
[0052] The third gear element 62 is supported against the housing
of the gearbox 26 via a fourth combined axial and radial bearing
82. Equally, the fourth gear element 64 is supported against the
housing of the gearbox 26 via a fifth combined axial and radial
bearing 84. The first output stage gear 22 is mounted to the third
gear element 62, and the second output stage gear 24 is mounted to
the fourth gear element 64. This mounting can be done in any
suitable way that allows for a rotatably fixed connection between
the third and fourth gear elements 62 and 64 and the first and
second output stage gears 22, 24.
[0053] By supporting the first and second gear elements 42, 44 with
respect to each other and by supporting the third and fourth gear
elements 62, 62 with respect to the first and second gear elements
42, 44 and with respect to the housing of the gearbox 26, an
alignment of the first through fourth gear elements 42, 44, 62, 64
is realized, which allows for a compact and stable gear structure
for transmission of the rotational energy from the first and second
motor shafts 28, 30 to the first and second output stage gears 22,
24. The described gear structure also allows for an independent
drivable coupling of the first motor shaft 28 to the first output
stage gear 22 and the second motor shaft 30 to the second output
stage gear 24 in an extremely compact manner. This allows for
placing the drive unit 16 in the highly space-critical environment
of an aircraft running gear.
[0054] With regard to FIGS. 2 and 3, the overall gear ratio that is
achieved by the exemplary gear structure is discussed. The
described system comprises three reduction stages. The first
reduction stage takes place between the first and second bevel
gears 38, 40 and the third and fourth bevel gears 54, 56,
respectively. The second reduction stage is realized by the first
and second planetary gears 46, 48, respectively. The third
reduction stage takes place between the first and second output
stage gears 22, 24 and the first and second wheel axis gears 8, 10,
respectively. The first and second reduction stages are embedded
into the gearbox 26, whereas the third reduction stage is realized
outside the gearbox through the engagement of the gearbox output
stage with the gears associated with the first and second wheels 4,
6.
[0055] The selective driving of the first and second wheels 4, 6 by
the drive unit 16 is achieved by selective engagement between the
drive unit and the first and second wheel axis gears 8, 10. A
mechanism of selective engagement is referred to as a mechanism
that allows for engagement and disengagement of two elements,
particularly of two gears. The point of engagement/disengagement,
i.e. the point of selective engagement, lies behind the gearbox 26
in terms of the transmission direction of rotational energy. In
other words, the first and second motor shafts 28, 30 are always in
engagement with the gear arrangement within the gearbox 26, i.e.
with the gear arrangement of the first and second reduction stages.
The selective driving between the drive unit 16 and the first and
second wheels 4, 6 is achieved via selective engagement on the
output side of the drive unit.
[0056] In the exemplary embodiment described, the first reduction
stage has a gear ratio of between 1.5 and 2.5. The second reduction
stage has a gear ratio of between 3 and 4. The third reduction
stage has a gear ratio of between 3.5 and 4.5. In this way, it is
possible to drive an aircraft with a maximum take-off weight
between 70,000 kg and 80,000 kg needing a torque of between 10,000
and 18,000 Nm at the nose wheel for taxiing by a single drive unit
having a maximum torque of between 500 Nm and 600 Nm and a maximum
speed of between 6,000 and 8,000 revolutions/min. It is explicitly
stated that these numbers are of illustrative nature and are a mere
example of the overall design of the drive unit and the aircraft
running gear.
[0057] The drive unit allows for taxiing an aircraft without the
help of the main turbines. These are used for starting, landing and
flying the aircraft and can be switched off during the manoeuvring
on the airfield in the presence of the drive unit described above.
The power for operating the drive unit may be provided by an
auxiliary power unit commonly present in modern aircrafts. The
auxiliary power unit is a gas turbine engine smaller than the main
turbines. It is commonly run before takeoff for supplying the
airplane with electrical energy, for example for operating the
cabin air conditioning, the passenger entertainment systems and
other airplane appliances. The auxiliary power unit can be adapted
to provide electrical energy and/or hydraulic pressure for a
hydraulic motor. Alternatively, there can be a separate power
source for the drive unit, for example a fuel cell or a
rechargeable battery.
[0058] FIG. 4 is a further cross-sectional view of the aircraft
running gear 2 depicted in FIGS. 1 and 2. The cross-sectional plane
is orthogonal to the wheel axis and cuts the wheel axis and the
running gear leg at substantially their center portions. The
cross-sectional plane of FIG. 4 is marked in FIG. 2, with the
viewing direction indicated by arrows X-X. FIG. 4 shows that the
direction of longitudinal extension of the drive unit lies within a
plane orthogonal to the common wheel axis A.
[0059] FIG. 4 shows the drive unit 16 in an engaged position with
the first and second wheel axis gears 8, 10. More particularly, the
first and second output stage gears 22, 24 are in engagement with
the first and second wheel axis gears 8, 10, such that the first
and second motors 18, 20 are driveably coupled to the first and
second wheels 4, 6, respectively. The longitudinal extension of the
drive unit 16 is substantially parallel to the running gear leg 14
in the engaged position.
[0060] The mounting of the drive unit 16 to the running gear leg 14
is described in more detail. The drive unit 16 comprises a mounting
arm 88. The running gear leg 14 comprises a supporting portion 86
for mounting the drive unit 16. The supporting portion 86 and the
mounting arm 88 are connected in a manner that allows for a
rotation of the drive unit 16 with regard to the running gear leg
14. In other words, a pivot connection is established between the
supporting portion 86 and the mounting arm 88. In the exemplary
embodiment of FIG. 4, the mounting arm 88 is provided with a hole
for receiving a mounting bolt, screw, rod, or the like. The
supporting portion 86 has a recess for receiving the mounting arm
88 of the drive unit, with a plate being provided at each outer
side of the recess of the supporting portion, one of which being
shown in the cross-sectional view of FIG. 4. The two plates of the
supporting portion 86 comprise a hole, which is aligned with the
hole provided in the mounting arm 88, such that the bolt, screw,
rod, or the like mentioned above, is positioned in a way extending
through the hole provided in the mounting arm 88 and the holes
provided in the supporting portion 86. In this way, the supporting
portion 86 and the mounting arm 88 are connected, with the center
axis of the bolt, screw, rod, or the like being the pivoting axis
for the rotation of the drive unit 16 with respect to the running
gear leg 14.
[0061] FIG. 5a is an enlarged version of the mounting arrangement
between the drive unit 16 and the running gear leg 14 shown in FIG.
4. FIG. 5b shows the enlarged version of the mounting arrangement
of FIG. 5a, with the drive unit 16 being in a disengaged position
with respect to the first and second wheel axis gears 8, 10.
[0062] The drive unit 16 comprises an engagement/disengagement
mechanism 90. The drive unit further comprises an engagement
control arm 94, to which the engagement/disengagement mechanism 90
is coupled, for example by a bolt, screw, rod, or the like. The
engagement/disengagement mechanism 90 comprises a bell crank having
an actuator 92 and a connection element 96. The actuator 92 and the
connection element 96 are connected in a way that allows rotation
with respect to each other, for example by a bolt, screw, rod, or
the like. The connection element 96 is the portion of the
engagement/disengagement mechanism 90 that is connected to the
engagement control arm 94. The actuator 92 is fixed to the
supporting portion 86 at its one end. Its other end comprises the
connection to the connection element 96. The actuator 92 has a
variable length in its longitudinal extension between the one end
fixed to the supporting portion 86 and the other end connected to
the connection element 96. Varying the length of the actuator 92
results in the connection between the actuator 92 and the
connection element 96 to be displaced along a bottom plane 98 of
the recess of the supporting portion 86 provided for receiving the
mounting arm 88 of the drive unit 16. This results in an according
motion of the connection element 96, the engagement control arm 94
and the drive unit 16. The actuator 92 may be an electric,
hydraulic or pneumatic actuator. The operation of the actuator 92
results in a change of the length of the actuator 92, which may be
achieved by providing a piston slidingly positioned in the actuator
92.
[0063] In FIG. 5a, the drive unit 16 is shown in a position of
engagement with the first and second wheel axis gears. In the
engagement position, the length of the actuator 92 is minimal. The
connection element 96 is drawn towards the running gear leg 14,
which in turn pulls the engagement control arm 94 towards the
running gear leg 14. This in turn pulls the lower portion of the
drive unit 16, i.e. the part of the drive unit 16 below the
mounting arm 88, towards the running gear leg 16. This results in
the first and second output stage gears engaging with the first and
second wheel axis gears.
[0064] In FIG. 5b, the drive unit 16 is shown in a position of
disengagement with respect to the first and second wheel axis
gears. As compared to FIG. 5a, the actuator 92 is extended in
length. This results in the connection between the actuator 92 and
the connection element 96 to be moved away from the running gear
leg 14 and down the bottom plane 98 of the recess of the supporting
portion 86, as compared to the positioning of FIG. 5a. The
connection element 96 is also in a position further removed from
the running gear leg 14, which results in the engagement control
arm 94 of the drive unit to be further away from the running gear
leg 14 as well, such that the drive unit 16 is disengaged with
respect to the first and second wheel axis gears. Accordingly, the
length of the actuator 92 determines if a state of engagement or
disengagement is present. Accordingly, the drive unit 16 can be
engaged/disengaged by varying the length of the actuator 92.
[0065] The actuator 92 and the connection element 96 form a bell
crank, which allows for the engagement/disengagement mechanism 90
to be self-securing, which will be discussed as follows. In the
disengagement position, shown in FIG. 5b, the orientation of the
connection element 96 is substantially perpendicular to the bottom
plane 98. The weight of the drive unit 16 is partially supported by
the mounting arm 88 and partially by the connection element 96.
Through the connection element 96, a force normal to the bottom
plane 98 is exerted onto the supporting portion 86. With the force
being normal to the bottom plane 98, no force for moving the
connection between the actuator 92 and the connection element 96
along the bottom plane 98 is caused by the drive unit's weight in
the disengagement position. Thus, in the disengagement position, no
force needs to be provided by the actuator to keep the drive unit
16 disengaged. Accordingly, should the actuator fail while the
drive unit is disengaged, there is no danger of the drive unit 16
inadvertently engaging with the first and second wheel axis gears.
An active operation by the actuator 92 is required to bring the
drive unit 16 and the wheel structure into engagement. Hence, no
damage can be caused to the drive unit 16 or the wheel structure
through an unwanted engagement, for example during the landing of
the aircraft, when the wheels rotate at high speeds due to the
airplane's landing speed. Also, it is ensured that the drive unit
16 is no safety hazard, as an unwanted engagement during take-off
or landing could have severe consequences. Therefore, the
engagement/disengagement mechanism 90 is considered
self-securing.
[0066] FIG. 6 shows a portion of an aircraft running gear 2 in
accordance with a second embodiment of the invention. To a large
extent, the second embodiment of FIG. 6 corresponds to the first
embodiment shown in FIGS. 1 through 5, such that like elements are
denoted with like reference numerals. A description of like
elements if omitted for brevity. However, the drive unit 16 of the
second embodiment of the aircraft running gear 2 shown in FIG. 6 is
designed partially differently. The drive unit 16 of FIG. 6 only
has one motor 120. The motor 120 comprises a motor shaft 130, which
comprises a bevel gear 140. The bevel gear 140 is in engagement
with a bevel gear 152 of a differential gear 150. The differential
gear 150 is coupled to the third and fourth gear elements, as
described with respect to FIG. 3, via first and second planetary
gears 46, 48, respectively, as also described with respect to FIG.
3. The differential gear 150 comprises a first shaft portion 166
and a second shaft portion 168. The first shaft portion 166 is
supported within the recess of the third gear element 62, described
with respect to FIG. 3. The second shaft portion 168 is supported
within the recess of the fourth gear element 64, described with
respect to FIG. 3. Through the supporting of the first and second
shaft portions 166, 168 within the third and fourth gear elements
62, 64, a stable alignment between the differential gear 150 and
the third and fourth gear elements 62, 64 is achieved.
[0067] The differential gear 150 allows for the third and fourth
gear elements 62, 64 to be rotated at different speeds. This in
turn allows for a rotation of the first and second output stage
gears 62, 64 as well as the first and second wheels 4, 6 at
different speeds as well. The differential gear has the innate
property that it adjusts the relative speeds of its two outputs,
i.e. of the first and second differential gear shafts 166, 168,
according to the resistance experienced at the outputs. This allows
for the outer wheel to be driven faster than the inner wheel during
a turning maneuver. Accordingly, when the airplane, whose running
gear is equipped with the drive unit 16 of FIG. 6, turns on an
airfield, the differential gear 150 ensures that the first and
second wheels rotate with their respective speeds according to the
desired turning radius. Accordingly, the low wear and tear of the
tires and the whole wheel structure that can be achieved through
the provision of two motors, as described with reference to the
first embodiment (FIGS. 1 through 5), can also be achieved by
providing the differential gear 150. However, the motor 120 has to
provide twice as much power as each of the first and second motors
18, 20 of the first embodiment to achieve the same driving
capability for the first and second wheels 4, 6.
* * * * *