Note: Descriptions are shown in the official language in which they were submitted.
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EPICYCLIC GEAR SYSTEM
The present invention relates to epicyclic gear systems, and more
particularly, to the reduction of backlash in gear systems.
Backlash is the clearance or space that exists between parts that do not fit
together seamlessly. In the example of gear systems, backlash may occur
between
sets of gear teeth. This can have the effect of creating a delay between one
gear
rotating and that one gear causing another gear to begin to rotate. In systems
concerned with positional accuracy, backlash can lead to inaccurate positional
measurements, which can then lead to inaccurate operation of instruments.
An epicyclic gear stage typically comprises a sun gear which receives an
input torque, two or more planet gears which are intermeshed with and revolve
around the sun gear, receiving torque therefrom, and a carrier. The carrier
generally has two functions: carrying the planet gears as they revolve
collectively
around the sun gear and transferring the torque received from the revolving
movement to an output shaft. The planet gears are free to rotate around their
axes
within the carrier, but the circular movement of their axes around the sun
gear
drives the rotary movement of the carrier. The planet gears are typically
further
intermeshed and contained within a ring gear which may be fixed or may also be
able to rotate.
Multi-stage epicyclic gearboxes are commonly used to provide a high-ratio
reduction from a rotational input to a rotating position transducer. In such
position
sensing systems, the backlash within the gearbox typically dictates the
positional
accuracy of the sensor. In a multi-stage epicyclic gearbox, the contribution
of
backlash in the limiting of sensor accuracy is highest at the stage closest to
the
gearbox output. In a four-stage gearbox, for example, the contribution of
backlash
per stage in governing sensor accuracy is typically 0.5%, 5%, 22%, and 100% as
percentage contribution compared to stage four. Hence, even if backlash can
only
be eliminated in the fourth stage, a significant proportion of the overall
effect of
backlash on positional sensor accuracy can be reduced.
Viewed from a first aspect, the invention provides an epicyclic gear system
having an epicyclic gear stage, the epicyclic gear stage comprising: a sun
gear; a
carrier, wherein the carrier comprises a plurality of planet gear axles; a
plurality of
= planet gears, each planet gear being located on one of the planet gear
axles; a ring
gear; and an output shaft, wherein the output shaft is connected to the
carrier via a
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biasing mechanism positioned between the carrier and the output shaft, the
biasing
mechanism being configured to urge the carrier and the output shaft away from
one
another.
Existing epicyclic gear stages may have methods of backlash reduction that
are bulky, heavy or complex, which can lead to difficulties satisfying
installation
envelope constraints.
The gear system of the first aspect achieves backlash reduction in an
epicyclic gear stage by biasing the carrier and an output shaft away from one
another. This may be done using a force having a major component or entirely
in a
plane normal to an axis aligned through the centre of the sun gear and
bisecting the
centres of the planet gears. There may be a plurality of planetary gears and
at
least two planetary gears may be disposed opposite one another across the sun
gear and proximate to the biasing mechanism acting on the carrier. The biasing
mechanism urges the carrier such that the force of the biasing mechanism is
reacted by the engagements between each of the planetary gears and the ring
gear
and between each of the planetary gears and the sun gear. In other words, the
carrier is preloaded against the output shaft and the force from the biasing
mechanism is such that the carrier and output shaft are in equilibrium.
The simplest system to which this can be applied is an epicyclic gear stage
with two planet gears disposed opposite one another across the sun gear. There
may be epicyclic gear stages to which this invention may be applied that only
have
two planet gears. When a rotational input is provided from the sun gear, the
sun
gear will act to rotate whichever planet gear directly reacts to the direction
of input,
either clockwise or anti-clockwise. This then causes the carrier to rotate and
thus
cause the output shaft to rotate due to the carrier acting on the output shaft
through
the biasing mechanism. A rotational input of the opposite direction will act
to rotate
the other planet gear that did not previously directly react to the first
direction of
input, and therefore the carrier and the output shaft will rotate in the
opposite
direction. In either case, in this example of an epicyclic gear stage having
two
planet gears, only one planet gear is directly driven in any given direction
while the
opposite planet gear effectively free-wheels due to tooth backlash.
An advantage of the present invention is that backlash in an epicyclic gear
stage is eliminated using a biasing mechanism, resulting in the order of an
80%
reduction in the overall backlash of a typical four stage epicyclic reduction
gearbox
into a rotary sensor.
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The biasing mechanism of the first aspect may comprise compressive
springs, leaf springs, elastic material, or any other suitable biasing means.
The carrier of the first aspect may be shaped roughly in a semi-circular
shape, wherein the carrier may have two planet gear axles positioned proximate
to
the vertices of the semi-circular shape and the main curve of the shape
following
the curve of the ring gear. The carrier may comprise one member for holding
the
planet gears or it may comprise two members such that the planet gears are
disposed between the carrier members. A configuration having multiple carrier
members may have an advantage of providing increased stability to the planet
gears and a more equal load distribution across the teeth of the planet gears
in
response to input from the sun gear. There may be a bulge in the straight
portion of
the semi-circular shape so that the carrier can fully accommodate the planet
gear
axles and, in the case of a carrier having multiple carrier members, to allow
the sun
gear to extend through the carriers via a carrier member hole. The output
shaft
may be shaped to extend through the ring gear and adjacent to the carrier so
that
the output shaft is within the space in the ring gear unoccupied by the semi-
circular
shape of the carrier. The output shaft may also have a semi-circular shape, a
crescent moon shape or any other shape suitable to complement the shape of the
carrier. Compressive springs may be disposed proximate to the vertices of the
semi-circular shape of the carrier to connect the carrier to the output shaft.
The epicyclic gear system of the first aspect may be used in a positional
sensor system. In such systems, there is a need to minimise backlash in order
to
ensure the accuracy of the sensor. A positional sensor system may utilise a
transducer having a limited effective range of rotation. That is to say, the
positional
sensor system may have a rotational transducer that operates within a specific
angular range, such angular range having an extent less than 360 . In such
systems, the rotation of the output shaft leads to the rotation of the
transducer. The
transducer may be attached or connected to or integrated within the output
shaft of
the epicyclic gear system. In positional sensor systems having limited angular
range, a multi-stage gearbox may be required to provide a high ratio reduction
between an input and the output. This may be achieved with any number of gear
stages. An example of such a system is in an aircraft having a motor operating
an
aileron and having a rotational displacement of 300 revolutions that is
connected to
a positional sensing system. A high ratio or high displacement reduction gear
stage
would need to be provided between the motor and the position sensing
transducer
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,
due to the limited range in which the transducer may operate. The skilled
person
would be able to select a transducer appropriate to the overall system in
which the
sensor operates.
An advantage of using the present invention in a positional sensor system is
a reduction or elimination of backlash in the sensor system, meaning that the
output
shaft comprising or connected to a transducer is able to more accurately
communicate rotational inputs, particularly if the direction of rotation
changes.
The epicyclic gear system of the present invention may alternatively be used
within a gearbox for low-powered applications, such as in 3D printing
machines.
The springs of the present invention would typically be limited by the amount
of
force applied to the springs and their spring constant to determine whether
the
springs would still be able to perform their function of biasing the carrier
and output
shaft away from one another under high loads. In high-powered applications,
the
load placed upon said springs may be such that they are unable to reduce
backlash
in an epicyclic gear stage.
An advantage of using the present invention in a gearbox for a low-powered
apparatus is increased precision in the driving system. In the example of a 3D
printer, reduced backlash in the gearbox allows for an output that may detect
changes in the rotational direction to a greater-precision, meaning the 3D
printer is
able to print with increased accuracy.
The epicyclic gear system may use simple planet gears or compound planet
gears. Compound gear structures may comprise structures such as meshed-
planet, stepped-planet or multi-stage structures.
The angle of the teeth of any of the gears used in the inventive epicyclic
gear system may be parallel to the axis aligned through the centre of the sun
gear
or they may be angled off of this axis.
Viewed from a second aspect, the invention provides a method for reducing
backlash in an epicyclic gear system having an epicyclic gear stage, the
epicyclic
gear stage comprising: a sun gear; a carrier, wherein the carrier comprises a
plurality of planet gear axles; a plurality of planet gears, each planet gear
being
located on one of the planet gear axles; a ring gear; and an output shaft, the
method comprising: urging the carrier and the output shaft away from one
another.
The epicyclic gear system of the second aspect may comprise any of the
features of the first aspect of the invention.
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Figure 1 illustrates an embodiment of the invention viewed from a direction
orthogonal to the gear stage.
Figure 2 illustrates an embodiment of the invention viewed face-on to the
gear stage.
Figure 3 illustrates force components acting upon an embodiment of the
invention.
Figure 4 illustrates the effects of rotating the sun gear of an embodiment in
the anti-clockwise direction.
Figure 5 illustrates the effects of rotating the sun gear of an embodiment in
the clockwise direction.
Figure 6 illustrates an alternate embodiment of the invention in a three-
dimensional view.
Figure 1 shows a portion of an exemplary epicyclic gear system 1
comprising an epicyclic gear stage 100. Figure 2 views the epicyclic gear
stage
100 from Figure 1 face-on and as a schematic.
The epicyclic gear stage 100 comprises a sun gear 110, a carrier 120, two
planet gears 130, a ring gear 140, and an output shaft 150. The carrier 120
has
two planet gear axles 122 extending into the carrier 120 and having the planet
gears 130 mounted thereon, one planet gear 130 for each planet gear axle 122.
The planet gears 130 have gear teeth (not shown) that engage with
corresponding
gear teeth on the sun gear 110 and on the ring gear 140. Due to manufacturing
methods and tolerances on the machining of the gear teeth of each gear,
engaging
gear teeth do not fit together perfectly and so gaps are formed between
engaged
gear teeth (not shown), creating backlash in the epicyclic gear stage 100.
The planet gear axles 122 are positioned on the carrier 120 such that they
are axially opposite one another across the sun gear 110.
The output shaft 150 and the carrier interact with one another through a
biasing mechanism 160. In this embodiment, the biasing mechanism 160 is a leaf
spring. The leaf spring 160 has spring elements 162 disposed at each end of
the
leaf spring 160 and in two locations between the output shaft 150 and the
carrier
120. The spring elements 162 urge the carrier 120 and the output shaft 160
away
from one another in directions substantially in the same plane as the
epicyclic gear
stage 100. This force is shown by the arrows near to each spring element in
Figure
2. The urging of the leaf spring 160 urges the carrier to one side of the
epicyclic
gear stage 100 in a direction away from the output shaft 150, which, in turn,
moves
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the planet gear axles 122 and, therefore, the planet gears 130 in the same
direction
as the carrier 120. This has the effect of forcing the gear teeth of the
planet gears
130 to engage fully with the sun gear 110 and the ring gear 140 and without a
gap
or backlash between engaged gear teeth. The skilled person will appreciate
that
the same technical effect achieved by the leaf spring 160 can be achieved
through
other means.
The carrier 120 is shaped roughly in a semi-circular shape. The two planet
gear axles 122 are positioned proximate to the vertices 124 of the semi-
circular
shape and the main curve 126 of the shape follows the curve of the ring gear
140.
Also proximate the vertices 124 of the carrier are lips 129. A lip 129 forms a
surface on which the spring element 162 of the leaf spring 160 contacts the
carrier
120. There is a bulge 128 in the straight portion of the semi-circular shape,
the
bulge 128 protruding outwards and radially towards the output shaft 150 from a
point radially inward of the vertices 124 and the lips 129 of the carrier 120
such that
the spring elements 162 of the leaf spring 160 are fully accommodated by the
lips
129. The bulge 128 allows the carrier 120 to fully accommodate the planet gear
axles 122 within the extent of the carrier. The bulge may also extend around a
central shaft 112 connected to the sun gear 110.
The output shaft 150 is shaped to extend within the ring gear 140 and is
adjacent to the carrier 120 so that the output shaft 150 is within a space in
the ring
gear 140 unoccupied by the carrier 120. The output shaft 150 also has a semi-
circular shape, though the skilled person will appreciate that the shape of
the output
shaft 150 may be any shape that fits within the ring gear 140 without
interfering with
its operation and complements the shape of the carrier. Such shapes may
include
a crescent moon shape or any other shape suitable. The output shaft 150 has
two
ends 152 positioned at approximately opposite sides across the central shaft
112.
The ends 152 are in contact with the spring elements 162 of the leaf spring
160
and, therefore, the output shaft 150 is connected to the carrier 120 via the
spring
elements 162.
The direction of the biasing force F from the leaf spring 160 acting upon the
carrier 120 is illustrated in Figure 3. The carrier 120 experiences an urging
force F
from the leaf spring 160 in the direction away from the output shaft 150. As
the
planet gears 130 are connected to the planet gear axles 122 which, in turn,
are
connected to the carrier 120, the planet gears 130 are also urged in a
direction
away from the output shaft 150. The planet gears 130 have gear teeth (not
shown)
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that engage with the ring gear 140 and the sun gear 110. The urging force F
has at
least a major component in a plane normal to an axis aligned through the
centre of
the sun gear 110 and bisecting the centres of the planet gears 130.
Typically, due to imperfect machining methods, the gear teeth of engaging
gears do not fit together perfectly. Instead, a gap typically exists between
engaging
gear teeth. When one gear is driven to rotate, there may be some delay between
the driven gear starting to rotate and the teeth of the driven gear causing a
second
engaged gear to rotate due to this gap.
The urging force F causes the gear teeth of the planet gears 130 to engage
with the gear teeth on the ring gear 140 and the sun gear 110 on one side of
the
gear teeth. In other words, at the point where the gear teeth of a planet gear
130
and the ring gear 140 engage, the gear teeth of the planet gears 130 are
forced
against the gear teeth of the ring gear 140 and the sun gear 110. At each of
these
points, a quarter of the urging force F reacts to the planet gears 130 such
that the
planet gears 130 do not rotate and the gear stage 100 is stationary.
Figure 4 illustrates effects of rotating the sun gear 110 in the anti-
clockwise
direction. The terms "clockwise" and "anti-clockwise" are defined with respect
to
the gear stage 100 as viewed in the figures about the central shaft 112.
Rotating
the sun gear 110 in this direction causes the planet gears 130 to rotate in
the
clockwise direction which, in turn, causes the carrier 120 and the output
shaft 150
to rotate in the anti-clockwise direction. In the gear stage 100 shown in
Figure 4,
the planet gears 130 may be labelled as an upper planet gear 130a and a lower
planet gear 130b, for ease of reference.
When the sun gear 110 begins to rotate anti-clockwise, the gear teeth of the
sun gear 110 are fully engaged with the gear teeth of the upper plant gear
130a in
the anti-clockwise direction. Therefore, there is no delay between the sun
gear 110
beginning to rotate and the upper planet gear 130a beginning to rotate. Due to
the
urging force F, the gear teeth of the upper planet gear 130a are also fully
engaged
with the gear teeth of the ring gear 140 in the clockwise direction. When the
upper
planet gear 130a begins to rotate in the clockwise direction in response to
the sun
gear 110 rotating in the anti-clockwise direction, there is no gap between the
gear
teeth of the upper planet gear 130a and the ring gear 140 in the clockwise
direction
and so the upper planet gear 130a does not experience a delay between rotation
and engagement with the ring gear 140. Therefore, there is no delay between
the
sun gear beginning to rotate and the carrier 120 being forced to rotate due to
the
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upper planet gear 130a moving anti-clockwise. In turn, there is then no delay
between the sun gear 110 beginning to rotate and the output shaft 150 being
caused to rotate and so backlash has been eliminated from the gear stage in
the
anti-clockwise direction.
When the sun gear 110 begins to rotate anti-clockwise, the gear teeth of the
sun gear 110 are not fully engaged with the gear teeth of the lower plant gear
130b
in the anti-clockwise direction. However, due to the urging force F of the
leaf spring
160, the gear teeth of the sun gear 110 remain engaged with the gear teeth of
the
lower planet gear 130b in the clockwise direction. Also due to the urging
force F,
the gear teeth of the lower planet gear 130b are engaged with the gear teeth
of the
ring gear 140 in the anti-clockwise direction.
Figure 5 illustrates effects of rotating the sun gear 110 in the clockwise
direction. Rotating the sun gear 110 in this direction causes the planet gears
130 to
rotate in the anti-clockwise direction which, in turn, causes the carrier 120
and the
output shaft 150 to rotate in the clockwise direction.
When the sun gear 110 begins to rotate clockwise, the gear teeth of the sun
gear 110 are fully engaged with the gear teeth of the lower plant gear 130b in
the
clockwise direction. Therefore, there is no delay between the sun gear 110
beginning to rotate and the lower planet gear 130b beginning to rotate. Due to
the
urging force F, the gear teeth of the lower planet gear 130b are also fully
engaged
with the gear teeth of the ring gear 140 in the anti-clockwise direction. When
the
lower planet gear 130b begins to rotate in the anti-clockwise direction in
response
to the sun gear 110 rotating in the clockwise direction, there is no gap
between the
gear teeth of the lower planet gear 130b and the ring gear 140 in the anti-
clockwise
direction and so the lower planet gear 130b does not experience a delay
between
rotation and engagement with the ring gear 140. Therefore, there is no delay
between the sun gear beginning to rotate and the carrier 120 being forced to
rotate
due to the lower planet gear 130b moving clockwise. In turn, there is then no
delay
between the sun gear 110 beginning to rotate and the output shaft 150 being
caused to rotate and so backlash has been eliminated from the gear stage in
the
clockwise direction.
When the sun gear 110 begins to rotate clockwise, the gear teeth of the sun
gear 110 are not fully engaged with the gear teeth of the upper plant gear
130a in
the clockwise direction. However, due to the urging force F of the leaf spring
160,
the gear teeth of the sun gear 110 remain engaged with the gear teeth of the
upper
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planet gear 130a in the anti-clockwise direction. Also due to the urging force
F, the
gear teeth of the upper planet gear 130a are engaged with the gear teeth of
the ring
gear 140 in the clockwise direction.
Figure 6 illustrates a portion of alternate embodiment of the inventive gear
stage 200 from a three-dimensional perspective. This embodiment differs from
the
previous embodiment in that the urging force F is provided by a set of
compressed
coil springs 260. These coil springs 260 are instead of the leaf spring 160 of
the
previous embodiment. The effect achieved by the coil springs 260 is the same
as
that of the leaf spring 160. That is to say, the carrier 120 experiences an
urging
force F from the coil springs 260 in the direction away from the output shaft
150. As
the planet gears 130 are connected to the planet gear axles 122 which, in
turn, are
connected to the carrier 120, the planet gears 130 are also urged in a
direction
away from the output shaft 150. The urging of the planet gears 130 causes the
gear teeth of the planet gears 130 to be physically urged against the gear
teeth of
the sun gear 110 (not shown in Figure 6) and the ring gear 150 (not shown in
Figure 6). Therefore, the effects of rotating the sun gear 110 are the same as
those
described in relation to Figures 4 and 5 for a gear stage using a leaf spring
160.
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