The invention relates to devices for coupling
shafts, such as devices for coupling between a turbine
shaft and a compressor shaft.
It is particularly applicable to devices for
coupling turbine and compressor shafts of turbin5 e
engines propelling aeronautical vehicles.
State of the related art
Low-pressure turbine and low-pressure compressor
10 shafts are generally coupled by a gear coupling device.
In such a device, an example whereof is shown in
figures 1a and 1b, a part ending the compressor
shaft 10 and a part ending the turbine shaft 20 are
mutually engaged coaxially, the two shafts 10, 20
15 extending along the same axis of rotation X-X.
Each of the parts comprises a plurality of
splines 12, 22, the splines of one engaging with the
splines of the other. Each part further has so-called
centring zones, respectively downstream 14a, 24a and
20 upstream 14b, 24b from the splines 12, 22 wherein the

complementary surfaces enable axial centring of the two
shafts in respect to each other.
The splines generally have a constant profile
along the entire length thereof.
It is desirable, in the field of turbine engines5 ,
to reduce the diameter of rotary shafts and increase
the torques transmitted by the coupling devices.
The result is an increase of the ratio of the
torque to be transmitted against the geometrical radius
10 of the shaft splines and particularly significant
forces on the downstream part of the splines, which is
subject to the greatest stress, the transmission of
motion being, in the case of a turbine-compressor
coupling, performed from the downstream turbine towards
15 the upstream compressor in relation to the air flow in
the turbine engine.
In the document FR 2 918 726, a coupling device
wherein the splines of one of the shafts have a
variable profile was proposed.
20 More specifically, the splines 22 of the coupling
part of the low-pressure turbine shaft 20, represented
schematically in figure 2, have at the downstream end
thereof a zone along which the thickness e2 of the
splines is smaller in relation to the thickness e1 of
25 the rest of the spline. The thickness of a spline is
measured between the flanks 23 of the spline, at the
top 26 thereof since the flanks 23 of the splines, have
a profile, in a cross-sectional view, in the form of an
involute to a circle, and the thickness of a spline
30 thus varies according to the level at which it is
measured.

The splines 12 of the low-pressure compressor
shaft 10 in turn have a constant thickness between the
flanks thereof, along the entire length thereof.
In this way, when cold, only the zones of the
splines wherein the thickness between the flanks i5 s
maximum touch the splines of the other shaft. In
operation, heating and the torque transmitted deform
the splines, closing the initial gap.
This deformation helps improve contact pressure
10 distribution between the splines during operation, and
thus reduce stress.
However, this solution generates two critical
zones in operation. The first is situated at the
downstream end of the splines of the low-pressure
15 turbine shaft, on the free edges of the flanks of the
splines and at the junction between the flanks of the
splines and the bottoms of the splines. This zone is a
zone wherein tensile stress accumulates, having a
lesser service life compared to the rest of the shaft.
Consequently, the existence of this zone involves a
reduction in the service life of the shaft.
Furthermore, in order to lighten the shaft, a
cavity 25, i.e. a zone of the shaft wherein the
thickness is reduced in a transverse plane, has been
25 provided between the downstream end of the spline and
the downstream centring zone 24a (see figure 1b), in
relation to the thickness of the shaft at the
downstream centring zone.
This cavity 25 forms a second critical zone where
30 inertia is reduced, rendering said zone weaker and
decreasing the strength thereof in the event of extreme

loading, for example in the event of the expulsion of
the fan blade in the event of rupture thereof (known as
"Fan Blade Off").
Description of the inventio5 n
The aim of the present invention is that of
remedying the problems described above, by developing
spline profiles suitable for satisfactory torque
transmission and ensuring strength in the event of
10 extreme loading.
For this purpose, the invention relates to a
device for coupling shafts by means of a gear mechanism,
comprising a coupling part ending one of the two shafts
15 and intended to be fitted concentrically into a
complementary coupling part ending the other shaft,
said coupling part comprising two centring zones
between which a plurality of splines extend, wherein
the splines of said coupling part have a profile:
20 - which is constant along a median zone of the
coupling part and
- which, in a first zone running in the
continuation of said median zone, has a smaller
thickness, between the flanks of a spline, by
25 comparison with that of the same spline in the
median zone,
and wherein the splines of said coupling part continue,
beyond the first zone wherein they have a smaller
thickness, as far as the centring zone which is on the
30 same side as said first zone in relation to the median
zone, said splines, and the spline bottoms, having, in

the coupling zone between said centring zone and said
first zone where they have a smaller thickness, a
profile that continuously couples said splines and the
spline bottoms to the surface of said centring zone.

Advantageously, but optionally, the invention may
further comprise at least one of the following features:
- the splines have, in the coupling zone, a
profile that increases in thickness from the
10 zone where they have a smaller thickness
towards the centring zone.
- in the coupling zone, the spline bottom depth
decreases progressively as far as the surface
of the centring zone.
15 - the bottom of the splines has, in the coupling
zone, a curved profile.
- the splines of said coupling part also continue,
on the side opposite the first zone, with a
second zone wherein they have a profile of
20 smaller thickness.
- the length of the median zone is between 0 and
80% of the total length of the spline.
- the intermediate zone runs along a length
between 0 and 25% of the total length of the
25 spline and the thickness of the spline along
this zone decreases by 0 to 25% of the maximum
thickness of the spline.
- the coupling zone runs along a length between
10 and 40% of the total length of the spline.
30 - the end zone of the spline runs along a length
between 0 and 25% of the total length of the

spline, and the thickness of the spline
decreases by 0 to 15% of the maximum thickness
of the spline.
- the splines of the coupling part of the other
spline have a constant profile5 .
The invention also relates to a rotary shaft comprising
a part for coupling with another shaft, intended to be
fitted concentrically into a complementary coupling
10 part ending the other shaft, said coupling part
comprising two centring zones between which a plurality
of splines extend, wherein the splines of said coupling
part have a profile:
- which is constant along a median zone of the
15 coupling part and
- which, in a first zone running in the
continuation of said median zone, has a smaller
thickness, between the flanks of a spline, by
comparison with that of the same spline in the
20 median zone,
and wherein the splines of said coupling part continue,
beyond the first zone wherein they have a smaller
thickness, as far as the centring zone which is on the
same side as said first zone in relation to the median
25 zone,
said splines, and the spline bottoms, having, in the
coupling zone between said centring zone and said first
zone where they have a smaller thickness, a profile
that continuously couples said splines and the spline
30 bottoms to the surface of said centring zone.

The invention further relates to a turbine engine
comprising a compressor wherein the shaft is rotated by
a turbine shaft, wherein the compressor shaft and the
turbine shaft are coupled by a device according to the
invention, the coupling part having a first zone o5 f
smaller thickness and a coupling zone being that of the
turbine shaft, said first zone and the coupling zone
being situated downstream from the median zone in
relation to the direction of the air flow in the
10 turbine engine.
Description of the figures
Further features, aims and advantages of the
invention will emerge from the description hereinafter,
15 which is merely illustrative and not limiting, and
which should be read with reference to the appended
figures wherein:
- Figures 1a and 1b, described above, are
schematic representations of a device for
20 coupling shafts according to the prior art, in
an exploded perspective and longitudinal
section view respectively.
- Figure 2, described above, represents the
splines of a low-pressure turbine shaft
25 according to the prior art.
- Figure 3 represents schematically, in a
longitudinal section, a turbine engine wherein
a coupling device is fitted.
- Figure 4 represents schematically, in a
30 longitudinal section, a device for coupling
shafts according to the invention.

- Figures 5a and 5b represent the splines of a
shaft of a coupling device, in a perspective
and top view respectively.
Detailed description of at least one embodimen5 t
Figure 3 represents a turbine engine 100
comprising a coupling device 1 according to the
invention. The turbine engine 100 conventionally
comprises upstream, in relation to the air flow in the
10 turbine engine, a low-pressure compressor 110, and a
high-pressure compressor 130, and downstream, a lowpressure
turbine 120 and a high-pressure turbine 140.
The exhaust gases at the outlet of the highpressure
compressor 130 rotate the turbines 120, 140.
15 The coupling device 1 enables the low-pressure
turbine 120 to rotate the compressor 110, the torque
being transmitted by means of a gear mechanism.
Figure 4 represents a device 1 for coupling two
shafts 10, 20, for example respectively of a low20
pressure compressor 110 shaft 10, rotated by a lowpressure
turbine 120 shaft 20 of a turbine engine 100.
The shafts 10, 20 each have a coupling part
intended to be fitted concentrically into a
complementary coupling part ending the other shaft. The
25 shafts, including the coupling parts thereof, are
arranged concentrically about an axis of rotation X-X.
The first shaft, in this instance the compressor
shaft 10, has on the inner surface thereof a plurality
of splines 12. The second shaft, in this instance the
30 turbine shaft 20, has on the outer surface thereof a

plurality of splines 22, suitable for engaging with the
splines 12 of the first shaft 10.
On each shaft 10, 20, the splines 12, 22 extend
between two centring zones, one downstream centring
zone 14a, 24a, and one upstream centring zone 14b, 24b5 .
The upstream and downstream centring zones of the
shafts are complementary, and make it possible to
position the compressor shaft 10 correctly in relation
to the turbine shaft 20.
10 For example, the upstream centring zones take the
form of a circumferential protuberance and a
corresponding circumferential groove.
The downstream centring zones of each shaft each
have a cylindrical surface having a parallel axis with
15 the axis of rotation X-X, which is elevated in relation
to the rest of the shaft, particularly in relation to
the bottom of the splines.
With reference to figures 5a and 5b, splines of a
20 shaft of the coupling device 1 are represented,
preferably of the low-pressure turbine shaft 20.
Each spline 12, 22, has two side flanks 13, 23,
and one top 16, 26, connecting the flanks. Between two
successive splines of the same shaft, a spline
25 bottom 17, 27 is occupied by a spline of the other
shaft to enable gearing.
Preferably, the splines 12 of the first shaft have
a thickness e between the flanks that is substantially
constant along the entire length of the spline. As
30 above, the thickness of a spline 12, 22 is measured at
the top 16, 26 of the spline.

Moreover, each spline 22 of the second shaft 20
has a variable profile.
With reference to figure 5b, each spline has:
- A median zone ZM, along which the spline has a
constant thickness em between the flank5 s
thereof.
- A first zone, or intermediate zone ZI, running
in the continuation of the median zone ZM, and
situated downstream therefrom in the case of a
10 turbine engine, along which the thickness ei of
the spline is smaller in relation to the
thickness em of the spline in the median zone.
Preferably, this thickness ei decreases from
the median zone ZM towards the downstream
15 centring zone.
- A so-called coupling zone ZR, running in the
continuation of the intermediate zone ZI, on
the same side as the intermediate zone ZI in
relation to the median zone (in this instance
20 downstream from the intermediate zone ZI in the
case of a turbine engine), and forming a
downstream end of the spline 22.
The coupling zone ZR connects the intermediate
zone ZI to the downstream centring zone 24a of the
25 shaft 20. For this purpose, the spline has, on this
zone, a thickness er between the flanks 23 thereof that
increases from the intermediate zone ZI to the centring
zone 24a, until the tops 26 of two successive splines
of the shaft and the surface of the centring zone 24a
30 join and form a continuous surface.

As such, the splines have, at the coupling zone, a
profile continuously coupling said splines and the
bottom of the splines 27 to the surface of said
centring zone 24a.
Thus, the downstream ends of the splines 22 ar5 e
merged together with the downstream centring zone 24a.
In this way, the flanks 23 of the splines 22 have no
free edges downstream where stress would be accumulated.
The splines proposed thus have an increased service
10 life in relation to the splines of coupling devices
according to the prior art.
Moreover, the cavities 25 adjacent to the
downstream ends of the splines according to the prior
art are replaced by coupling zones ZR extending from
15 the splines 22 as far as the centring zone 24a. There
is thus no zone of lower inertia on the turbine
shaft 20, which is thus reinforced, and the service
life is enhanced.
20 Furthermore, as seen more particularly in figure
5a, the spline bottom 27 has, at the coupling zone ZR a
decreasing thickness, from the maximum depth at the
intermediate zone to a zero depth at the centring
zone 24a.
25 The spline bottom 27 preferably has a curved
profile, in a longitudinal sectional view,
advantageously wherein the radius of curvature is
continuous.
In this way, the surface formed by the spline
30 bottom 27 and by the adjacent flanks has no
discontinuity liable to induce stress accumulation.
Again with reference to figure 5b, the splines 22
may also have an upstream end zone ZE, adjacent to the
median zone ZM and extending from same in the upstream
direction. In this zone, each spline has a smalle5 r
thickness ee in relation to the thickness em in the
median zone.
The relative proportions of the various zones of
the splines 22 are as follows:
10 - The upstream end zone ZE has a length between 0
and 25% of the total length of the spline. The
thickness ee of the spline at the upstream end
thereof - and thus at the end of the zone - is
reduced by 0 to 15% of the maximum thickness of
15 the spline, this maximum thickness being the
thickness em at the median zone ZM.
- The median zone ZM has a length between 0 and
80% of the total length of the spline. The
thickness em of the spline is constant, and
20 equal to the maximum value thereof, which is
defined in ANSI standard B92-1-1996 and is
dependent on the turbine engine.
- The intermediate zone ZI has a length between 0
and 80% of the total length of the spline.
25 Along this zone, the thickness ei of the spline
decreases by 0 to 25% in relation to the
maximum thickness thereof. Preferably, the
thickness ei of the spline is minimal at the
downstream end of this zone, at the junction
30 with the coupling zone ZR.

- Finally, the coupling zone ZR has a length
between 10 and 40% of the total length of the
spline. The thickness er of the spline
increases until the flanks of two consecutive
splines are joined5 .
According to one particular embodiment of the
coupling device, the median zone may have a length of
60 mm, the intermediate zone a length of 25 mm, and the
10 coupling zone a length of 30 mm. In this embodiment,
the splines 22 have no upstream end zone ZE, i.e. the
upstream ends of the splines correspond to the upstream
ends of the median zones ZM.
15 A device for coupling shafts wherein the geometry
is suitable for transmitting a greater torque and
provides an enhanced service life in relation to the
prior art is thus proposed.
This device is suitable, without being limited
20 thereto, for installation in turbine engines.

I/We Claim:
1. Device (1) for coupling two shafts by means of a
gear mechanism, comprising a coupling part ending
one of the two shafts (20) and intended to be fitted
concentrically into a complementary coupling part
ending the other shaft (10), said coupling par5 t
comprising two centring zones (24a, 24b) between
which a plurality of splines (22) extend,
wherein the splines (22) of said coupling part have
a profile:
- which is constant along a median zone (ZM) of
the coupling part and
- which, in a first zone (ZI) running in the
continuation of said median zone (ZM), has a
smaller thickness (ei), between the flanks (23)
15 of a spline (22), by comparison with that (em)
of the same spline (22) in the median zone (ZM),
characterised in that the splines (22) of said coupling
part continue, beyond the first zone (ZI) wherein they
have a smaller thickness (ei), as far as the centring
20 zone (24a) which is on the same side as said first
zone (ZI) in relation to the median zone (ZM),
said splines (22), and the spline bottoms (27), having,
in the coupling zone (ZR) between said centring
zone (24a) and said first zone (ZI) where they have a
smaller thickness (ei), a profile that continuously
couples said splines (22) and the spline bottoms (275 )
to the surface of said centring zone (24a).
2. Device (1) according to claim 1, wherein the
splines (22) have, in the coupling zone (ZR), a profile
10 that increases in thickness from the zone (ZI) where
they have a smaller thickness (ei) towards the centring
zone (24a).
3. Device (1) according to any of claims 1 or 2,
15 wherein, in the coupling zone (ZR), the spline
bottom (27) depth decreases progressively as far as the
surface of the centring zone (24a).
4. Device (1) according to the above claim, wherein the
20 bottom of the splines (27) has, in the coupling
zone (ZR), a curved profile.
5. Device (1) according to any of the above claims,
wherein the splines (22) of said coupling part also
25 continue, on the side opposite the first zone (ZI),
with a second zone (ZE) wherein they have a profile of
smaller thickness (ee).
6. Device (1) according to any of the above claims,
30 wherein the length of the median zone (ZM) is between 0
and 80% of the total length of the spline (22).
7. Device (1) according to any of the above claims,
wherein the first zone (ZI) runs along a length between
0 and 25% of the total length of the spline (22) and
the thickness of the spline (22) along this zone (ZI5 )
decreases by 0 to 25% of the maximum thickness (em) of
the spline (22).
8. Device (1) according to any of the above claims,
10 wherein the coupling zone (ZR) runs along a length
between 10 and 40% of the total length of the
spline (22).
9. Device (1) according to any of the above claims,
15 wherein the second zone (ZE) where the splines (22)
have a smaller thickness (ee) runs along a length
between 0 and 25% of the total length of the
spline (22), and the thickness (ee) of the spline
decreases by 0 to 15% of the maximum thickness (em) of
20 the spline.
10. Device (1) according to any of the above claims,
wherein the splines (12) of the coupling part of the
other spline (10) have a constant profile.
11. Rotary shaft (20) comprising a part for coupling
with another shaft (10), intended to be fitted
concentrically into a complementary coupling part
ending the other shaft (10), said coupling part
30 comprising two centring zones (24a, 24b) between which
a plurality of splines (22) extend,
wherein the splines (22) of said coupling part have a
profile:
- which is constant along a median zone (ZM) of
the coupling part, and
- which, in a first zone (ZI) running in th5 e
continuation of said median zone (ZM), has a
smaller thickness (ei), between the flanks (23)
of a spline (22), by comparison with that of
the same spline (22) in the median zone (ZM),
10 characterised in that the splines (22) of said coupling
part continue, beyond the first zone (ZI) wherein they
have a smaller thickness (ei), as far as the centring
zone (24a) which is on the same side as said first
zone (ZI) in relation to the median zone (ZM),
15 said splines (22), and the spline bottoms (27), having,
in the coupling zone (ZR) between said centring
zone (24a) and said first zone (ZI) where they have a
smaller thickness (ei), a profile that continuously
couples said splines (22) and the spline bottoms (27) to the surface of said centring zone (24a).
12. Turbine engine (100) comprising a compressor (110)
which shaft (10) is rotated by a turbine (120)
shaft (20), characterised in that the compressor (110)
25 shaft (10) and the turbine (120) shaft (20) are coupled
by a device according to claim 10, the coupling part
having a first thickness zone (ZI) wherein they have a
smaller thickness (ei) and a coupling zone (ZR) being
that of the turbine shaft, said first zone (ZI) and the
coupling zone (ZR) being situated downstream from the
median zone (ZM) in relation to the direction of the
air flow in the turbine engine (100).