Flexible driving shaft
Field of Invention
The present invention relates to power transmission systems for wind or water
turbines. More particularly, the present invention relates to an arrangement for
transmitting torque from the rotor of a wind turbine to the gearbox while minimising
the negative effects of structural deflections and manufacturing tolerances.
Background Art
Wind turbine drivetrains commonly consist of an aerodynamic rotor, which is
supported on a rotor shaft, itself supported on one or more bearings, which support
the shaft relative to the turbine structure while permitting rotation around the rotor
axis. The rotor shaft is connected to a speed increasing gearbox, commonly
comprising a number of planetary and/or parallel gear stages. The output of this
gearbox is then connected by a further shaft to a generator which produces the
electrical output.
In addition to the torque load causing rotation around the main axis, the rotor
shaft is also subject to off-axis loads, chiefly moments around axes perpendicular to
the main axis. These loads cause deflections of the rotor shaft and supporting
structure. If these loads and deflections are transmitted into the gearbox, they can
have damaging effects on the gears and bearings, leading to premature failure.
Historically, this has led to turbine designs where the weight of the gearbox is
supported primarily by a rigid connection between the rotor shaft and the gearbox
input member, with a second connection between the gearbox housing and the
turbine structure which is designed to resist only torque around the main axis, while
permitting the entire gearbox to move freely in other directions in response to
deflections of the rotor shaft, thus preventing the generation of harmful forces within
the gearbox itself.
Thus US475721 1A discloses a hub shaft connecting the turbine blades to a
gear box. The hub has a funnel-shaped body, which is suspended at the back in a
machine housing and carries main bearings on its outside; the blades rest on the
outer ring of the main bearings. This construction aims to expose the hub shaft to
torsional forces only. Such a shaft is a typical, stiff-bodied drive shaft.
As turbine power outputs increase, this approach becomes less effective. The
demands of increased input torque and increased gearbox mass require a joint
between rotor shaft and gearbox input member which is very large, and time
consuming to assemble, due to very high numbers of bolts or heavy interference fits.
Additionally, the increased gearbox mass (particularly when it is considered
advantageous to directly mount the generator to the downwind end of the gearbox)
introduces very high loads into the bearings supporting the gearbox input member in
the gearbox housing.
It may therefore be considered advantageous to mount the gearbox directly
and substantially rigidly to the bedplate. This reduces the problems described above,
but requires the incorporation of some other means of accommodating structural
deflections, manufacturing tolerances, and other effects which may cause the
gearbox input member to be misaligned with respect to the rotor shaft.
This compensation is achieved in some existing designs by the inclusion of
elastomeric elements in the joint between the rotor shaft and gearbox input member.
As these elements are of much lower stiffness than the rest of the drivetrain, the
misalignment can be accommodated without producing high reaction loads.
However, current elastomer technology cannot match the (typically) 20 year design
life required for wind turbines, due to degradation from repeated loading,
atmospheric and fluid contaminants, and exposure to ultraviolet light. This
necessitates repeated replacement within the turbine lifetime.
WO2008/1 24674A1 teaches that there is a problem with power transmission
coupling elements which accommodate axial, bending, and transverse
displacements, because they must do so while simultaneously carrying relatively
large torsional large torsional loads. A flexible composite driveshaft is disclosed
which overcomes problems associated with metallic flex elements which are that it is
difficult for a such elements simultaneously to carry very large torsional shear and
remain conveniently compliant to imposed out-of-axis distortions.
US201 1/0309631 A 1 discloses a composite shaft of this type to transfer torque from
the wind turbine rotor to the generator, the shaft having high torsional strength but
being flexible in bending. However, a number of problems with a composite main
shaft have been identified (OceanWise magazine, 2012, Volume 1,
http://www.lorc.dk/oceanwise-magazine/archive/2012-1/experts-disagree-can-thewind-
turbine's-cast-iron-main-shaft-be-replaced-by-carbon-fiber), including a need
for a larger diameter, a bigger front bearing and expense. Steel on the other hand
has the necessary strength in all directions, because it is a crystalline material
Various other existing designs compensate for misalignments and deflections
through the use of couplings consisting of a set of radially protruding teeth
associated with each component, and an intermediate component with two sets of
radially protruding teeth, one set of which engage with the teeth associated with the
rotor shaft, and the other set engage with the teeth associated with the gearbox input
member. By the application of particular geometry to each mating pair of teeth, it
can be arranged that each set of teeth permits rotation around axes perpendicular to
the main axis, and as a complete coupling, the effect of these rotations combine to
permit translation along axes perpendicular to the main axis. This type of coupling is
commonly referred to as a 'gear coupling', and in addition to their use in existing
wind turbines, they are a common method of accommodating misalignment in a wide
range of industrial applications. However they are considerably more complex than
other known methods for rotor shaft to gearbox connection, introducing a number of
additional components, and requiring good lubrication. As their mode of operation
involves sliding between two metal surfaces, they will be subject to wear. Their use
in this application is, therefore, a risk to the reliability of the turbine over its design
lifetime.
It can be seen therefore, that an invention which permitted the direct mounting
of the gearbox to the turbine structure yet still permitted the connection between
rotor shaft and gearbox input member to be made without either elastomeric
elements or sliding/wearing parts, in which the main shaft is flexible but constructed
from iron or steel would be advantageous in designing a reliable wind turbine for
larger power ratings.
Disclosure of Invention
According to a first aspect of the invention, there is provided a power
transmission system for increasing the rotational speed from a rotor hub of a wind
turbine, which includes: a rotor shaft configured to be driven by the rotor hub about a
main axis; a speed-increasing gearbox having a gearbox input member; a support
structure supporting the rotor shaft and gearbox; a rotor shaft housing connected to
the supporting structure; and a main shaft comprising steel or iron and having an
upwind end connected to the rotor shaft and a downwind end connected to the
gearbox input member, the main shaft extending downwind along the main axis
inside the rotor shaft. The invention is characterised by the main shaft having a
slender elongate portion disposed between the upwind end and the downwind end,
the upwind and downwind ends having a larger diameter than the elongate portion,
whereby the main shaft has a high torsional strength but is flexible in bending so that
radial and angular misalignment between the rotor shaft and the gearbox input
member is accommodated.
Preferably, the rotor shaft is supported within the rotor shaft housing by an
upwind rotor bearing and a downwind rotor bearing, the bearings configured to
permit rotation of the rotor shaft about the main axis while limiting movement in other
directions.
Preferably, the support structure comprises a bearing arrangement supporting
the rotor shaft for rotation about the main axis, the bearing arrangement restraining
movement of the rotor hub and rotor shaft when subjected to loads other than torque
around the main axis of the drivetrain. This means that the bearing arrangement
permits rotation of the rotor shaft about the main axis while limiting movement in
other directions. This means that all loads on the rotor hub, with the exception of
moments about the main axis, are transmitted through rotor shaft, rotor bearings,
and rotor shaft housing into the supporting structure.
Preferably, the support structure comprises a rotor housing and in which the
bearing arrangement is located between the rotor housing and the rotor shaft.
Preferably, the bearing arrangement comprises at least one bearing.
Preferably, the main shaft has a smaller diameter than the rotor shaft, the
gearbox input member, the rotor shaft housing or the gearbox housing. This means
that the stiffness of the main shaft is correspondingly lower, and thus where radial
and angular misalignments exist between the rotor shaft and the gearbox input
member as a result of manufacturing tolerances, differential thermal expansion of
the drivetrain, deflection of the entire structure under load, or any other factors, this
misalignment will largely be accommodated by deflection of themain shaft, and only
a small part will result in deflection of the gearbox input member and associated
gears and bearings. This benefit can be maximised by increasing the length of main
shaft to the maximum permitted by the available package space.
Preferably, the gear box comprises a gearbox housing rigidly connected to the
support structure. Preferably, the gearbox comprises a gearbox housing rigidly
connected to the rotor shaft housing.
Preferably, the gearbox housing comprises one or more bearings adapted to
support the gearbox input member for rotation about the main axis. This means that
rotation around the main axis is permitted while motions in other directions are
restricted. Preferably, a degree of translation of the gearbox input member along the
main axis is permitted to compensate for thermal expansion of the rotating parts
relative to the gear box housing / rotor shaft housing.
Preferably, the upwind end of the main shaft is rigidly connected to the rotor
shaft and the downwind end of the main shaft is rigidly connected to the gearbox
input member, whereby a moment about the main axis of the drivetrain is transmitted
from the rotor shaft to the gearbox input member by the driving shaft.
Preferably, the main shaft is connected to the rotor shaft via an intermediate
component. Preferably, the intermediate component has a greater flexibility than the
rotor shaft and the supporting structure.
Preferably, the main shaft is connected to the gearbox input member via an
5 intermediate component. Preferably, the second intermediate component comprises
a face spline connection.
According to further aspects of the invention, there is provided a wind turbine.
The present invention will now be described, by way of example only, with
references to the accompanying drawings, in which:
i o Brief Description of the Drawings
Fig. 1 is a perspective view of an example of a wind turbine;
Fig. 2 shows a power transmission system of the present invention; and
Fig. 3 shows a flexible main shaft of the present invention.
Best Mode for Carrying Out the Invention
15 Fig. 1 is a perspective view of an example of a wind turbine. Although an
offshore wind turbine is shown, it should be noted that the description below may be
applicable to other types of wind turbines. The wind turbine 102 includes rotor blades
104 mounted to a hub 106, which is supported by a nacelle 108 on a tower 112.
Wind causes the rotor blades 104 and hub 106 to rotate about a main axis 11 (Fig.
2 0 2). This rotational energy is delivered to a power transmission system of the type
shown in Fig. 2 housed within nacelle 108.
Fig. 2 shows a wind turbine drivetrain comprising rotor shaft 1, which supports
rotor hub 106 (Fig. 1) by means of flange 13. Rotor shaft 1 is supported within rotor
shaft housing 3 by upwind rotor bearing 10 and downwind rotor bearing 9 , which
together permit rotation of rotor shaft 1about main axis 11 while limiting movement in
other directions. The terms "upwind" and "downwind" in the context of the present
invention assume that main axis 11 is orientated with the wind direction; thus rotor
hub 106 and flange 13 are at the "upwind" end of the drivetrain, and generator 5 is at
the "downwind" end of the drivetrain. Rotor shaft housing 3 is connected to
supporting structure 6 , and supporting structure 6 is connected to a tower via a yaw
mechanism (tower and yaw mechanism are not shown). The result of this
arrangement is that all loads on rotor hub 102, with the exception of moments about
main axis 11, are transmitted through rotor shaft 1, rotor bearings 9,1 0 , and rotor
shaft housing 3 into the supporting structure. Other arrangements which achieve this
result will be apparent to the skilled person; for example, numbers of rotor bearings
other than two may be used, or rotor shaft housing 3 and supporting structure 6 may
be combined into a single component.
Rotor shaft housing 3 further supports gearbox housing 4 via a bolted
connection between a downwind end of rotor shaft housing 3 and an upwind part of
gearbox housing 4 . Gearbox housing 4 supports generator 5 via a similar bolted
connection between a downwind part of gearbox housing 4 and an upwind part of
the generator housing. In other embodiments of the invention, the gearbox,
generator 5 , or both may be supported instead by direct connection to the supporting
structure.
The gearbox includes gearbox input member 7 , which is supported within
gearbox housing 4 by one or more bearings 14 in such a way as to permit rotation
around the main axis while restricting motions in other directions, or optionally, to
permit some degree of translation along main axis 11 to compensate for thermal
expansion of the rotating parts relative to the gear box housing 4 / rotor shaft
housing 3 . In the embodiment shown in Fig. 2 , gearbox input member 7 is a planet
carrier of a planetary gear stage.
Transmission of torque from rotor shaft 1 to gearbox input member 7 is via
main shaft 2 , which is connected in a substantially rigid fashion to rotor shaft 1 and
gearbox input member 7 . Main shaft 2 is made of steel or iron. In the embodiment
shown in Fig. 2 , at an upwind end of main shaft 2 , the connection is made directly to
rotor shaft 1, via a disc or cone shaped section 12 integral to the rotor shaft. At a
downwind end of main shaft 2 , the connection is achieved via an interference fit on a
cylindrical interface between gearbox input member 7 and main shaft 2 . The
interference fit described connects main shaft 2 to an intermediate component 8 ,
which in turn has a further substantially rigid connection to gearbox input member 7 .
This further connection might be made to transmit torque using a system of
protrusions and recesses in the mating faces of the intermediate component 8 and
the input member 7 . Such an arrangement is defined in the present invention as a
'face spline' connection.
It will be apparent that these connections can be achieved by other methods
while maintaining the function of the invention. For example, the disc or cone
shaped section 12 associated with the upwind connection may be manufactured
integral to main shaft 2 rather than rotor shaft 1, or may be a separate component
attached to rotor shaft 1 by a further connection. Similarly, intermediate
component 8 associated with the downwind connection may be manufactured
integral to either main shaft 2 or gearbox input member 7 . The connections between
components may be made by means other than a cylindrical interference fit, such as
bolts, keyways, or interference fits on a tapered interface, or any other substantially
rigid means of connection.
Main shaft 2 is of substantially smaller diameter than rotor shaft 1, gearbox
input member 7 , rotor shaft housing 3 or gearbox housing 4 . As a result, its stiffness
is correspondingly lower, and thus where radial and angular misalignments exist
between rotor shaft 1 and the gearbox input member 7 as a result of manufacturing
tolerances, differential thermal expansion of the drivetrain, deflection of the entire
structure under load, or any other factors, this misalignment will largely be
accommodated by deflection of main shaft 2 , and only a small part will result in
deflection of gearbox input member 7 and associated gears and bearings. This
benefit can be maximised by increasing the length of main shaft 2 to the maximum
permitted by the available package space.
Referring now to Fig. 3 , which shows the design of tubular main shaft 2 having
the required flexible properties, a slender elongate portion 2a is disposed between
an upwind end 2b and a downwind end 2c. Upwind end 2b is connected directly to
rotor shaft 1, via a disc or cone shaped section 12 integral to the rotor shaft.
Downwind end 2c is rigidly connected to gearbox input member 7 . The diameter of
the upwind and downwind ends 2b,2c is larger than the diameter of slender portion
2a. This means that torque transfer at the upwind and downwind ends 2b,2c is
facilitated without comprising flexure of the slender portion 2a. An intermediate
component 8 may be disposed between downwind end 2c and gearbox input
member 7 . In the embodiment shown in Fig. 3 , the shaft is hollow.
Further benefit may be achieved by reducing the stiffness of other parts of the
structure. In particular, the section of rotor shaft 12 which supports main shaft 2 may
be designed so as to reduce its stiffness in response to moments around axes
perpendicular to main axis 11. This would permit tilting of main shaft 2 relative to
rotor shaft 1, and increase the capability of the drivetrain to accommodate
misalignment without adverse effects on the gears and bearings in the gearbox. A
similar effect may be achieved by reducing the stiffness of intermediate component 8
in response to moments around axes perpendicular to main shaft 2 .
Industrial Applicability
A flexible shaft design for a wind turbine is achieved which has a high torsional
strength but is flexible in bending, but which does not have a larger diameter nor a
need for correspondingly bigger front bearings, and which avoids the expense of
composite materials. Thus the main shaft has a greater flexibility in response to
loads other than a moment about said main axis than the rotor shaft and the gearbox
housing, and the supporting structure
Claims
1. A power transmission system for increasing a rotational speed of a rotor hub of
a wind turbine, comprising:
a rotor shaft configured to be driven by the rotor hub about a main axis;
a speed-increasing gearbox comprising a gearbox input member;
a support structure supporting the rotor shaft and the gearbox;
a rotor shaft housing connected to the support structure; and
a main shaft comprising steel or iron and having an upwind end connected to
the rotor shaft and a downwind end connected to the gearbox input member,
the main shaft extending downwind along the main axis inside the rotor shaft;
characterised by:
the main shaft having a slender elongate portion disposed between the upwind
end and the downwind end, the upwind and downwind ends having a larger
diameter than the elongate portion, whereby the main shaft has a high torsional
strength but is flexible in bending so that radial and angular misalignment
between the rotor shaft and the gearbox input member is accommodated.
2 . A power transmission system according to claim 1, in which the rotor shaft is
supported within the rotor shaft housing by an upwind rotor bearing and a
downwind rotor bearing, the bearings configured to permit rotation of the rotor
shaft about the main axis while limiting movement in other directions.
3 . A power transmission system according to claim 1, in which the support
structure comprises a bearing arrangement supporting the rotor shaft for
rotation about the main axis, the bearing arrangement restraining movement of
the rotor hub and rotor shaft when subjected to loads other than torque around
the main axis of the drivetrain.
A power transmission system according to claim 2 , in which the support
structure comprises a rotor housing and in which the bearing arrangement i
located between the rotor housing and the rotor shaft.
5 . A power transmission system according to claim 3 , in which the bearing
arrangement comprises at least one bearing.
6 . A power transmission system according to claim 3 or claim 4 , in which the
gearbox comprises a gearbox housing rigidly connected to the rotor shaft
housing.
7 . A power transmission system according to claim 5 , in which the main shaft has
a smaller diameter than the rotor shaft, the gearbox input member, the rotor
shaft housing or the gearbox housing.
8 . A power transmission system according to any of claims 1 to 4 , in which the
gear box comprises a gearbox housing rigidly connected to the support
structure.
9 . A power transmission system according to any of claims 5 to 7 , in which the
gearbox housing comprises one or more bearings adapted to support the
gearbox input member for rotation about the main axis.
10. A power transmission system according to claim 8, in which the one or more
bearings are further adapted to permit a degree of translation of the gearbox
input member along the main axis.
1. A power transmission system according to any preceding claim, in which the
upwind end of the main shaft is rigidly connected to the rotor shaft and the
downwind end of the main shaft is rigidly connected to the gearbox input
member, whereby a moment about the main axis of the drivetrain is transmitted
from the rotor shaft to the gearbox input member by the main shaft.
12. A power transmission system according to any preceding claim, in which the
main shaft is connected to the rotor shaft via a first intermediate component.
13. A power transmission system according to claim 11, in which the first
intermediate component has a greater flexibility than the rotor shaft and the
supporting structure.
14. A power transmission system according to any preceding claim, in which the
main shaft is connected to the gearbox input member via a second
intermediate component.
15. A power transmission system according to claim 14, in which the second
intermediate component comprises a face spline connection.
16. A power transmission system according to any preceding claim, in which the
gear box input member is a planet carrier of a planetary gear box system.
17. A wind turbine comprising a power transmission system according to any of the
preceding claims.
18. A power transmission system substantially as described herein with reference
to the accompanying drawings.