Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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~his in~ention relates to impellers for varlable
pitch axial flow fans, that is to say impellers i~ which
the pitch ~nKle of the blades can be varied while the
impeller i8 rotating~
In such impellers the blades are commonly
supported in bear~ngs carried by a hollow h~b, so as to
be rotatable about respective axes extendi~g radially
fro~ the axis of rotation of the impeller, the lnner ends
of the blades carrying arms which extend outwardly with
respect to the blade axes, and which are coupled to a~
actuator movable parallel to the impeller axis so as to
effect a rotatisn of the blades w thin the respectiv~
bearings, and khereby vary their pitch angle~ The
invention is concerned especially with impeller~ of this
ki~d, which will hereinafter be referred to as of the
kind specified.
It is well known that if a body~ rotatable
about its own axis, i9 rotated about a perpendicular axis~
the centre of mass of the body will have a strong
te~de~cy to align itself with the plane of rotation o~
the body axis.
- In a variable pitch axial flow fan this is
maDife~ted by a turning force whiah tends to reduce
the blade pitch a~gle, that i~ to say the angle measured
between the chord line a~d said plane of rotatio~.
It i8 common to compensate for this turning
moment i~ impellers of the kind specified by attachi~g
cou~terweigh~s to the blade as~embly within the hub.
The actuator i8 then arranged to rotate the blades
~ i~ either directio~, a relatively strong retuxn spring
being provided for rotating the blades in the opposike
direction. However i~ such arrangeme~ts the hub
has to.be ~uf~icientl~ wide to accommodate the
cou~texweights, and moreover ~ince th~ co~nterweight~
~5 add con~idarably to th~ weight Or the blad~ assemblie~,
it mu~t be made of suitably strong and heav~ material
to with~tand the additio~al radial thru~t developed under
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centrifugal force.
An object of the invention is to provide a form of impel]er in
which the use oE colmterweights can be avoided.
According to the invention there lS provided an impeller or a variable
pitch axial flow fan incorporating a plurality of blades extending outwards
from a hollow hub, and supported against radially ou~ward movement by respective
thrust bearings located on the inner surface of the said hub so that the blades
can be freely individually rotated about their axes with respect to the hub,
the inner ends of which blades carry arms which extend outwardly with respect
to the blade axes and are coupled to an actuator which is movable parallel to
the axis of the impeller to rotate the blades within their bearings and so vary
their pitch angle, wherein the actuator is operable to rotate the blades towards
their maximum pitch angle and the blades are shaped to have an inherent turning
movement due to centrifugal forces, which is greater than any opposing moment
due to aerodynamic forces and which provides substantially the whole of the
restoring force for moving the blades towards their minimum pitch angle.
Although movement of the blades towards their minimum pitch angle
position may be assisted by one or more springs, these can be relatively light
compared with those employed in impellers having counterweighted blades for
providing the restoring force.
Moreover the invention enables a relatively smaller and lighter hub to
be used compared with those employed in counterweighted blade impellers.
In addition it has been found that thrust forces in the planes of the
blade bearings on changing the pltch angle Oe the blades are signific~mtly
reduced. Under these circumstances very simple axial thrust bearings can be
utilised without recourse to the addition of journal bearings for the blades;
the line of action of the resultant forces on the bearing remaining within the
ball tracks.
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The actuator is preferakly arranged to be operatcd ky pneumatic
pressure from a suitable source via a rotary seal.
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~ he i~ention will be fu~ther described by
reference to Figures 1 and 2 of the accompanying drawing9
in which Figure 1 illustrates an axial section through
half of one impeller in accordance with the i~Yentio~ for
an axial flow fan, and P~gure 2 show~ a tr~sverse section
of a part of the împeller.
In the drawing 1 represents the impeller hub
shown bolted to a hollow shaft 2 which is arra~ged to
be secured to the shaft of an elec~ric motor ~not show~).
~round the periphery of the hub there are a
~umber of u~iformly spaced radially directed holes ~
into which are fitted the root poxtions 4 of respective
aerofoil section blades 5 (show~ only in part).
~ ach blade 5 i~ held in position by means of a
circular retaining plate 6 which is secured to the inner
end of the blade root 4 by means o~ a nut 7, which is
screwed on to the protruding end of a threaded insert 8
captive within khe blade root, a flange 10 at the
outer end of the blade root being located in a recess
around the outer end of the respective hoIe 3. A washer
or pad 11 bei~g located betwèen the flange and the bottom
o~ the reces~.
~ t~rust bearing 9 is clamped between the plate 6
and an inwardly fa¢ing surface of the hub 1 and, in use,
when the imp~ller is rotating, the blades are urged
outwardly under centrifugal force, the bearing 9 facilitating
the rotation of the blade a~out its axis relative to the
hub~ -
2he ~hru~t bearing 9 comprises a number of
3 hardened steel balls 12 interposed betwee~ an annularinner race 13 adaace~t the plate 6 and an outer race 14
adaacent the hub ~ur~ace~ ~h~ balls are arra~ged to
roll in shallow anntllar groo~ provided in the ~acin~
~urfa¢es o~ the races 13 and 14 a~d are equidi~tan~ly
~pa¢ed around t~e grooves by a~ ann~lar bearing cage 16
having a re~atively low mass, a~d foI~ed, for example,
from a suitable plastics material such a~ acetal resin
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or from nylon impregnated with graphite. r~he bearing
cage 16 has a substantially channel shaped cross-section
having in its base a plurality of uniformly spaced holes
which accommodate the bearing balls 12, and is arranged
so that in use either the base or the edges of its side
walls bear again,st the surface of the outer race 14.
r~he bearing 9 contains a lubricant having a density
substantially the same as the material of the bearing
cage 16.
The bearing 9 is anclosed by an a~ular housing 20
which is of channel-shaped cross-section having its mouth
directed radially inwards towards the impeller axis, the
depth of the walls of the housi~g being such that the
bearing is accommodated entirely within ths channel which
thus serves to keep the balls a~d the cage of the bearing
covered with lubrica~t despite the effect of centrifugal
force when the impeller is in use. Clearances are
provided between the sides of the housing 20 and the inner
ball race 13 a~d plate 6 ~o permit the blade to rotate
freely.
Adjustment of the pitch angle of the blades 5
is effected by means of an arm 21 extendi~g tran~versely
from the plate 6 and supporting at its end a pin 22
carrying a roller 2~ which }ocates within a channel 24 in
the periphery of a common actuator 25. ~he actuator 25
is in the form of a shell formed in two parts 29,~0 joined
around their periphery~ the rear part 29 surroundinK the
shaft 2 but being free to move axially along it on a
bearing 26.
~ he shaft 2 carries coaxially at its end a circular
plate 27 located within the actuator providing a backing
for a diaph~agm 28 which is clamped around its peripher~
between the two parts 29~30 of the actuator 25 a~ shown.
~he plate 27 ~upports coaxially a stub shaft ~1 which
provides a guide for the front part 30 of' the actuator
which is free to slide axially along the ~haft 31 on a bearing
32. r~he inner edge of the diaphragm 28 is secured to the
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plate 27 around the stub shaft 31, by an annular clamping
plate 50 and screws 510
~ he actuator is made rotationally fast with the
shaft, for exa~ple by means of studs 3~ secured to rear
of the plate 27 and extending into respective guides 34
in the rear part 29 of the actuator 25.
In use of the impeller the inherent blade turning
moment will tend to rotate the blades to their minimum
pitch angle position as shown in ~igure 1 of the drawing.
In order to increase the pitch angle air under pressure
is arranged to be admitted into the space betwee~ the
front part 30 of the actuator and the diaphragm 28, via
a rotatable coupling 35, and a duct formed by a gap or
gaps (not shown) in the bearing 32~ '~his produces a
forward move~ent of the actuator 25 and 50 increases
the pitch angle of the blades by an amouL~ depending
upon the admitted air pressure.
When the pressure is releasad the inherent blade
turning moment, assisted if necessary by light springs
around the studs 33 as at 37, causes the blades to be
returned towards the minimum pitch angle position, which
can be set by an adJustable stop 38~
~ o set the maximum pitch angle air pressure is
admitted~ with the impeller stationary, to produce a
forward moveme~t of the actuator 25 until a forwardl~
extending cylindrical pro~ection 39 on the rear part 29
of the actuator engages the rear surface of the plate 27.
With the nuts 7 securing the plates 6 to the blade roots
slackened the blades 5 are manually set to the required
maximum pit~h angle and the nuts are then tightened. The
air pressure is fully released and the stop 38 adjusted to
give the re~uired mlnimum pitch angle as aforesaid.
'~he impeller is completed by the attachment of
a relatively thin-walled dome-shaped fairing 41 to the
front of the hub 1 as ~hownO
~ he a~sembly of the impeller is facilitated by
constructing the rear wall 42 of the actuator channel 24
in castellated form as shown more clearly in Figuxe 2,
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with the gaps 43 betwee~ the upstanding portions 44 of
the wall of sufficient width to allo~ the passage of the
rollers 23. ~fter the rollers 23 ha~e been located in the
channel 24 the actuator is rotated relative to the ~b
to bring the upstandinK portions opposite the rollers and
i~ then secured against further rotation with respect to
the hub by any co~enient mean~, for example the attachment
of the plate 27 to the shaft 2 so that the studs 33 extend
through the respecti~e guides 34.
