[0001] The present invention relates to axial air movement fans comprising a plurality of
elongate fan blades disposed about a hub so as to extend in a generally radial direction.
[0002] Each fan blade therefore constitutes a cantilever secured to the hub and extending
outwardly to the outer free end of the blade. The fan blades are elongate and typically
placed at an angle to the circumferential plane of the axis of rotation and may have
an aerofoil type cross-section to drive air across the fan. In cross-section, the
fan blades have a major axis and a minor axis. In operation, when driving air, the
fan blades are subject to air pressure and further forces caused by buffeting and
turbulence of the air passing through the faun. These forces are transmitted to the
hub as a cyclical bending moment through the root of the fan blade which is secured
to the hub. The blades thus tend to bend about the major axis and the stress concentration
at this point can lead to cracking and failure of the blade.
[0003] The normal operating forces are exacerbated if the vibratory loads on the blade are
generated at the natural frequency of oscillation of the cantilevered blade. This
can cause a permanent oscillation or vibration about the major axis of the blade at
its root which can lead to premature and catastrophic failure of the blade..
[0004] Previous proposals have been made to provide an internal damping mass to counterbalance
such oscillations in rotating blades but these have had limitations. Examples of prior
art proposals are shown in
US Patent No. 2828941,
EP1754866 A1 and
US 2009/0324406. It has been found, particularly in large fans, that the natural frequencies of the
individual blades varies. This is commonly due to slight variations between the blades
which are inevitably caused by production tolerances in the construction and materials
of the blade. A consequence of this is that the fan as a whole can be operating outside
its natural frequency but an individual blade can be operating, undetected, at its
own natural frequency. This leads to unexpected catastrophic failure of the blade
which can destroy the whole fan.
[0005] The present invention seeks to provide a solution to this problem.
[0006] According to the present invention there is provided an axial air movement fan comprising
a hub with a plurality of elongate fan blades extending generally radially outwardly
therefrom, wherein each fan blade is at least partially hollow and at least one damping
mass is located within the hollow part, the mass being at least partially provided
by a resilient plate, one edge of which is secured to the fan blade to form a cantilever,
and being secured to the fan blade in such a way as oscillate relative to the fan
blade in response to oscillatory flexing of the fan blade in a first plane, thereby
to damp the flexing of the fan blade, the end of the plate remote from said secured
end being held in a guide device to constrain its movement in a controlled manner
when the plate is oscillating, and wherein a further guide device is located between
the secured end of the plate and the first guide device and engages the plate in such
a way as to enable the natural frequency of oscillation of the mass to be adjusted
to match the natural frequency of the blade.
[0007] In this way, the plate is constrained against radial movement caused by centrifugal
force, but is permitted a degree of lateral resilient movement to enable the plate
to oscillate.
[0008] Although only one mass is shown in this embodiment, it will be understood that two
separate masses may be located in the hollow space in the blade and these two masses
may be tuned to be responsive to different frequencies.
[0009] Preferably, the guide device comprises two members between faces of which said outermost
end is clamped so as to be constrained against lateral movement but is able to pivot
relative to the members, in which case said faces may be curved so that the contact
between the members and the plate is a line contact.
[0010] In one form, a resilient damping mass is located between each of the two members
and the plate.
[0011] Preferably, the further guide device is located closer to the secured end than the
outermost end.
[0012] Furthermore, a resilient damping mass may be located between each of the two members
and the plate.
[0013] In a preferred embodiment, the cross-sectional area of the plate varies along its
length to give a desired form of flexure.
[0014] A plurality of said plates may be provided in each fan blade.
[0015] A preferred embodiment of the present invention will now be described by way of example
with reference to the accompanying informal sketch in which:
Figure 1 shows a schematic view of a blade of an air movement fan illustrating the
principle of the invention,
Figure 2 shows a schematic cross-sectional view showing the hollow interior of the
fan blade of Figure 1
Figures 3a and 3b show a schematic arrangement of a preferred embodiment of the invention,
and
Figure 4 shows an embodiment in which a plurality of plates are provided.
[0016] Figure 1 shows a hub 1 of an axial fan mounted for rotation about an axis 2. A plurality
of elongate fan blades, only one of which is shown as reference 3, equidistantly disposed
about the periphery of the hub and secured thereto so as to extend in a generally
radial direction. As shown, the fan blade consists of an elongate member extending
generally radially and is secured at its root to the hub at a slight angle to the
plane of rotation of the hub. The fan has in cross-section a major axis 4 and a minor
axis 5. It also has a radially extending longitudinal main axis 6.
[0017] Adjacent to its outer end, the fan blade has a hollow section 7 which contains a
mass-spring system. The system consists of a mass 11, which may be formed of lead,
which is suspended on a wire 8 intermediate the length thereof. The wire extends along
the axis 6 of the blade 3 and is secured to the blade at its ends, references 9 and
10. The mass is thus constrained by the wire against movement in the radial direction
caused by centrifugal force apart from a slight resilient extension of the part of
the wire between the mass and the radially innermost fastening, but is permitted a
degree of lateral movement against the resilience of the wire. It is envisaged that
some degree of damping of the movement of the mass may be provided, which may be accomplished
by means of a resilient damper such as a rubber bushing. The weight of the mass, the
resilience of the wire and the initial tension in the wire is calculated to permit
the mass to oscillate in response to vibratory movement of the blade at its first
natural frequency. The fan blade acts as a cantilever and if an exciting force caused
by turbulent airflow over the fan blade is present at the first natural frequency
of the fan blade, permanent oscillation at the natural frequency may occur and this
vibratory movement is opposed and dampened by the corresponding transverse oscillations
of the mass, which responds to the oscillation of the fan blade and serves to damp
out the induced oscillations in the fan blade. In this way, the stresses in the blade,
particularly at the root of the blade, which could lead to early failure of the blade,
are substantially reduced.
[0018] It is desirable for the mass to be located as far towards the outer tip of the fan
blade as possible as this reduces the size of the mass needed. Although shown as a
mass suspended on a wire, it is conceivable that the mass may be supported on a synthetic
rubber bushing system designed to restrain the mass against movement in a radial direction
but to move in a controlled resilient manner in a direction normal to the major axis
of the blade.
[0019] Figures 3a, 3b and 4 show an embodiment of the invention, in which Figure 3a shows
a cross-section through the hollow part of a fan blade viewed in the circumferential
direction showing a flexible plate, Figure 3b shows a view of the flexible plate in
the direction of the arrow A in Figure 3a, and Figure 4 shows a part sectional perspective
view of an arrangement in which two plates are located in the interior of the fan
blade.
[0020] Figure 3a shows a flexible plate 12 located in the interior of a fan blade casing
13. The radially innermost edge 14 of the plate 12 is secured by bolts in a mounting
15 secured to the main body of the fan blade. At its radially outermost end 16, the
plate 12 is located in a guide device 17 which has a pair of opposing clamping members
18 and 19 between which the plate is located. The locating members are secured by
bolts to a mounting bracket 20 secured to the main body of the fan blade13. The surfaces
21, 22 of the clamping members facing and engaging with the plate 12 are curved or
arcuate so that there is only a single line contact between the clamping members and
the plate on a line a short distance from the outer edge 14 of the plate. In this
way, the outer part of the plate 12 is able to pivot slightly when the plate curves
due to oscillatory forces and is also able to accommodate the slight change in length,
that is longitudinal extent, of the plate due to the curvature it adopts when being
deflected. In an alternative embodiment (not shown), resilient means such as rubber
bushing is positioned between the surfaces of the clamping members and the plate,
the resilient means having sufficient flexibility to accommodate the required slight
movement of the end of the plate.
[0021] Additionally, a further guide 23 may be positioned intermediate the ends of the plate
12 to control the amount of movement of the plate in this mid-position and to fine
tune the natural frequency of vibration of the plate to conform to that of the individual
blade. As shown, this further guide' device is located approximately one quarter of
the way from the fixed end of the plate towards the free end, but the position at
which it engages the plate may be varied to give the oscillatory characteristics desired.
In a similar manner to the first guide device, the further guide device has two clamping
members 24, 25 which engage the plate 12 on a line contact. The two clamping members
are again bolted to a mount 26 secured to the main body of the fan blade. It is also
envisaged that resilient damping means may be disposed between the clamping faces
of the members 24 and 25 and the plate 12. In this way, when the plate flexes due
to oscillatory forces, its assumes a shallow S shape with the part below the lower
guide 23 curving in one direction and the part between the two guides 17 and 23 curving
in the opposite direction. By appropriate positioning of the further guide device
it is possible to tune the frequency and amount by which the plate deflects when subjected
to oscillatory forces.
[0022] Referring now to Figure 3b, it can be seen that the cross-sectional area of the plate
can be varied by varying its width, or metal thickness, to give the desired damping
characteristics.
[0023] Turning now to Figure 4, there is shown a part sectional view of a fan blade 13 having
a hollow interior containing two damping plates 12, both in accordance with the present
invention. In this embodiment the two damping plates 12 are identical and only one
will be described for convenience, although it will be appreciated that the two devices
may be of different size, thickness and shape to achieve the damping characteristics
required.
[0024] In this description of Figure 4 like parts will bear like references to those shown
in Figure 3a. The plate 12 is secured in a mounting bracket 15 in turn secured to
the main body of the fan blade 13. At its radially outermost end 16, the plate is
located in a guide device 17 as described in connection with Figure 3a. Just radially
outwardly of the mounting bracket 15, a further guide 23 is located, although, compared
to the embodiment of Figure 3a, the clamping members 24, 25 are directed in the radially
outward direction rather than radially inwardly. The point at which the clamping members
engage the plate is adjustable by means of changing the shape of the members or by
spacers positioned between the clamping members and the mounting 24 through which
the device is secured to the fan blade. In this way the natural frequency of oscillation
can be fine-tuned to match the individual blade.
[0025] In addition, in this embodiment, a further damping means 27 is located generally
intermediate the guide devices 17 and 23. The damping means 27 consists of brackets
28, 29 secured to the main body of the fan blade 13 and each carrying a pair of resilient
damping masses 31, 32 formed by rubber bushes, which abut the faces of the plate 12
and are designed to limit and damp the oscillatory movement of the plate. Although
shown as abutting the plate, it will be understood that in certain circumstances these
resilient masses 31, 32 may be spaced from the surface of the plate in the static
position.
[0026] It can be seen that the fan blade 13 contains two identical plates 12 but it is envisaged
that these plates may differ in size and thickness, and response rate to give the
desired damping characteristics. To increase flexibility, the mountings 24 for the
plates 12 are located in elongate slots 32 in a subframe 33 which is itself secured
to the fan blade. In this way, the whole damping assembly can be built as a subassembly
with varying widths of plate for easy incorporation into the fan blade on final assembly.
[0027] For relatively small sizes of fan blade, it may not be practical to locate the mass
within the blade, in which case the mass may be suspended on the outside of the blade
with the wire being secured to limbs extending outwardly from the blade face. It is
possible that two masses may be provided, one on each face of the blade. It is also
possible for the mass or masses to be secured directly to the blade by means of a
resilient mounting bush formed of synthetic rubber.
[0028] In certain installations, under certain conditions, a higher frequency harmonic may
be generated in the fan blade causing the blades to oscillate about its longitudinal
axis 6. To counter this, a smaller mass may be secured in the hollow section of the
blade in such a way that it is constrained against radial movement but is able to
oscillate about the longitudinal axis 6 of the blade. In such an arrangement, it may
be preferable for the mass to be a disc like structure with its mass concentrated
at the periphery.
[0029] In a further embodiment, the mass may be formed by an outer skin section bonded to
the rest of the blade by means of a resilient bushing and having the same profile
as the fan blade so as not to adversely effect the flow of air past the blade.
[0030] When more than one mass is provided these may be tuned to be responsive to different
frequencies.
1. An axial air movement fan comprising a hub (1) with a plurality of elongate fan blades
(3) extending generally radially outwardly therefrom, wherein each fan blade (3) is
at least partially hollow and at least one damping mass (11) is located within the
hollow part, the mass (11) being at least partially provided by a resilient plate
(12), one edge (14) of which is secured to the fan blade (3) to form a cantilever,
and being secured to the fan blade (3) in such a way as to oscillate relative to the
fan blade (3) in response to oscillatory flexing of the fan blade in a first plane,
thereby to damp the flexing of the fan blade (3), characterised in that,
the end (16) of the plate (12) remote from said secured end being held in a guide
device (17) to constrain its movement in a controlled manner when the plate (12) is
oscillating, and wherein a further guide device (23) is located between the secured
end (14) of the plate (12) and the first guide device (17) and engages the plate in
such a way as to enable the natural frequency of oscillation of the mass to be adjusted
to match the natural frequency of the fan blade (3).
2. A fan as claimed in claim 1, wherein the location at which the further guide device
(23) engages the plate (12) is adjustable relative to the end (!4) of the plate secured
to the fan blade.
3. A fan as clamed in claim 1 or 2, wherein the plate (12) is secured to the radially
outermost face of the hollow space.
4. A fan as claimed in claim 1 or 2, wherein the plate (12) is secured at its radially
innermost edge to the fan blade (3).
5. A fan as claimed in claim 1 wherein the guide device (17) comprises two members between
faces of which said outermost end (16) is clamped so as to be constrained against
lateral movement but is able to pivot relative to the members.
6. A fan according to any one of the preceding claims wherein the further guide device
comprises two members (18, 19) between faces of which an intermediate part of the
plate (12) is clamped so as to be constrained against lateral movement but is able
to pivot relative to the members (18, 19).
7. A fan as claimed in claim 5 or 6, wherein said faces are curved so that the contact
between the members (18, 19) and the plate (12) is a line contact.
8. A fan as claimed in claim 5 or 6 wherein a resilient damping mass is located between
each of the two members (18, 19) and the plate (12).
9. A fan according to any one of the preceding claims wherein the further guide device
(23) is located closer to the secured end (14) than the outermost end (17) of the
plate (12).
10. A fan according to any one of the preceding claims wherein a further damping means
is located between the guide device (17) and the further guide device (23) comprising
damping means to,limit and damp oscillatory movement of the plate.
11. A fan according to claim 10, wherein tHe damping means comprise rubber or synthetic
rubber bushes.
12. A fan as claimed in any one of the preceding claims, wherein the cross-sectional area
of the plate (12) varies along its length to give a desired form of flexure.
13. A fan according to any one of the preceding claims wherein a plurality of said plates
(12) are provided.
14. A fan as claimed in any one of the preceding claims, including a further mass adapted
to dampen a higher frequency harmonic by oscillatory movement about the longitudinal
axis of the blade.