U.S. patent application number 12/156198 was filed with the patent office on 2009-12-03 for rotors and manufacturing methods for rotors.
This patent application is currently assigned to Controlled Power Technologies Limited. Invention is credited to Stephen Henry Hill, Richard William Quinn.
Application Number | 20090297344 12/156198 |
Document ID | / |
Family ID | 41380077 |
Filed Date | 2009-12-03 |
United States Patent
Application |
20090297344 |
Kind Code |
A1 |
Hill; Stephen Henry ; et
al. |
December 3, 2009 |
Rotors and manufacturing methods for rotors
Abstract
A rotor for driving, or being driven by, a fluid has rotor
blades that follow a screw thread shape, all portions of all blades
having the same screw pitch. This enables the rotor, or a molding
pattern for use in making a mold, to be withdrawn from a mold part
by a screw motion without damage to the mold part. This helps to
make it economically viable to manufacture the rotor by a molding
or casting process in which the mold parts are not destroyed to
release the rotor, such as injection molding, for example with
reinforced plastics. Additionally, in the case of manufacturing
methods in which the mold is destroyed to release the rotor, the
process of making or assembling the mold may be improved. In some
molding processes, the ability to remove the rotor or the pattern
by screw motion improves the economic viability of the process by
reducing the number of mold parts required.
Inventors: |
Hill; Stephen Henry;
(Wotton-Under-Edge, GB) ; Quinn; Richard William;
(Ashby Parva, GB) |
Correspondence
Address: |
Thomas Langer
Suite 1210, 551 Fifth Avenue
New York
NY
10176
US
|
Assignee: |
Controlled Power Technologies
Limited
Laindon
GB
|
Family ID: |
41380077 |
Appl. No.: |
12/156198 |
Filed: |
May 30, 2008 |
Current U.S.
Class: |
415/208.1 |
Current CPC
Class: |
F01D 5/048 20130101;
F05D 2220/40 20130101; F04D 29/284 20130101; B22C 9/22 20130101;
F04D 29/624 20130101; B22C 9/02 20130101 |
Class at
Publication: |
415/208.1 |
International
Class: |
F01D 1/02 20060101
F01D001/02 |
Claims
1. A rotor, for driving a fluid by rotation of the rotor about an
axis of rotation or for being driven by a fluid so as to rotate
about an axis of rotation, or a pattern for making a mold part for
making the rotor, wherein the rotor or pattern comprises (i) a hub
having a blade-bearing surface and (ii) a plurality of rotor blades
extending from the blade-bearing surface of the hub, the blade
bearing surface of the hub having a first surface portion at a
first radial distance from said axis of rotation and a second
surface portion at a second radial distance, greater than said
first distance, from said axis of rotation, the second surface
portion being spaced from the axis of rotation in the same angular
direction from said axis of rotation as the first surface portion
and being spaced from the first surface portion in a direction
parallel to the axis of rotation, both the first and second surface
portions bearing at least a part of a rotor blade, at least some of
the rotor blades having a respective portion that is spaced from a
respective portion of another rotor blade in a direction parallel
to the axis of rotation but is at the same radial distance from the
axis of rotation and is spaced from the axis of rotation in the
same angular direction, wherein the value of d.theta./dx is
constant and the same for the whole of all blades of the rotor or
pattern (where x represents distance in a direction parallel to the
axis of rotation, and .theta. represents angular direction
perpendicular to the axis of rotation taking the axis of rotation
as the origin) except for differences in the slopes of the blade
surfaces arising solely from variation in the thickness of the
rotor blades.
2. The rotor or pattern according to claim 1, wherein the
blade-bearing surface of the hub has a third surface portion at a
third radial distance from the axis of rotation, the third surface
portion being spaced from the axis of rotation in the same angular
direction from said axis of rotation as the first and second
surface portions and being midway between the first and second
surface portion in a direction parallel to the axis of rotation,
and wherein the third radial distance is less than the average of
the first and second radial distances.
3. The rotor or pattern according to claim 2, wherein the
blade-bearing surface of the hub is substantially parallel to the
axis of rotation at the first surface portion and the blade-bearing
surface of the hub is substantially perpendicular to the axis of
rotation at the second surface portion.
4. The rotor or pattern according to claim 1, wherein said
respective portions of at least one pair of rotor blades extend
circumferentially with respect to the axis of rotation over an
angle of at least 5.degree. subtended at the axis of rotation.
5. The rotor or pattern according to claim 1, wherein said
respective portions of at least one pair of rotor blades extend
circumferentially with respect to the axis of rotation over an
angle of at least 10.degree. subtended at the axis of rotation.
6. The rotor pattern according to claim 1, wherein said respective
portions of at least one pair of rotor blades extend
circumferentially with respect to the axis of rotation over an
angle of at least 15.degree. subtended at the axis of rotation.
7. A rotor, or a pattern for making a mold part for making a rotor,
having a hub and a plurality of rotor blades extending from the
hub, wherein, for each rotor blade, all portions of the rotor blade
at the same radial distance from the axis of rotation of the rotor
form part of the same screw thread spiral, and the screw pitch of
the screw thread spiral is the same at all radial distances of the
blade and is the same for all of the rotor blades, at least some of
the rotor blades overlap axially at least partially in the sense
that a part of one rotor blade at the same polar co-ordinates from
the axis of rotation as a part of another rotor blade but is spaced
axially therefrom, and at least a part of the hub has a diameter
which varies with distance along the axis of rotation.
8. Apparatus for driving a gas, the apparatus comprising an
impeller that is a rotor according to claim 1.
9. A turbocharger or supercharger for an internal combustion
engine, comprising a rotor according to claim 1.
10. Apparatus for driving a gas, the apparatus comprising an
impeller that is a rotor according to claim 7.
11. A turbocharger or supercharger for an internal combustion
engine, comprising a rotor according to claim 7.
12. A mold part for use in making a rotor, the mold part having a
recess for receiving molding material in use, said recess having
recess portions that define both faces of each blade of the rotor,
all parts of each said recess portion having a common value of
d.theta./dx, where x represents distance in a direction parallel to
the axis of rotation, and .theta. represents angular direction
perpendicular to the axis of rotation taking the axis of rotation
as the origin, except for differences in the slopes of the surfaces
of the recess portions arising solely from variation in the width
of the recess portions in the direction of the thickness of the
rotor blades, and at least some of said recess portions overlapping
another said recess portion in the axial direction of the rotor
13. The mold part according to claim 12, wherein part of the
surface of said recess defines a blade-bearing surface of a hub of
the rotor, and said part of the surface of said recess has
respective portions axially spaced from each other, with reference
to the axis of rotation of the rotor, which portions are at
different radial distances from said axis of rotation.
14. The mold part according to claim 13, wherein said part of the
recess surface defining a blade-bearing surface is convex so that
said part of the recess surface is less parallel to the axis of
rotation of the rotor where it is further from said axis of
rotation
15. A method of making a rotor comprising filling the recess of a
mold part according to claim 12 with a flowable material, causing
or allowing the flowable material to solidify, and removing the
solidified material from the mold part by moving it relative to the
mold part with a screw motion.
16. The method according to claim 15, wherein the step of filling a
mold part with a flowable material comprises injecting the flowable
material under pressure.
17. The method according to claim 15, wherein said flowable
material is a settable plastics resin.
18. The method according to claim 17, wherein the plastics resin
contains re-inforcing material.
19. A method of making a mold part according to claim 12 comprising
immersing or embedding a rotor pattern in a mold-forming material,
allowing or causing the mold-forming material to solidify, and
withdrawing the pattern from the solidified material by movement
with a relative screw motion to leave the recess in the solidified
material.
20. The method according to claim 19, wherein the mold-forming
material is or mostly comprises sand.
21. A method of making a rotor comprising making a mold part by a
method according to claim 19, filling the recess left by the
pattern with a molding material, allowing or causing the molding
material to solidify, and releasing the solidified molding material
by destroying the mold part.
22. The method according to claim 21, wherein the molding material
is molten metal, and the step of allowing or causing the molding
material to solidify comprises allowing or causing the molten metal
to cool.
23. An inter-blade mold part for a mold for use in making a rotor
by molding or casting, the inter-blade mold part having a first
surface that defines a surface on one side of a blade of a rotor
and a second surface that defines a surface on the other side of
another blade of the rotor, wherein for both of the first and
second surfaces of the inter-blade part, all parts of the surface
that define a blade surface of a rotor slope at a common screw
pitch, with reference to an axis of rotation of the rotor, except
for differences in the slopes of the first and second surfaces
arising solely from variation in the thickness of said blades.
24. The inter-blade mold part according to claim 23 which is or
mostly comprises sand.
25. A sand mold forming pattern for defining an inter-blade sand
mold part according to claim 24.
Description
FIELD OF THE INVENTION
[0001] Aspects of the present invention relate to rotors of the
type having a plurality of blades for acting on, or for being acted
on by, a fluid, such as impellers in fans and compressors, and
turbine rotors, in which a hub portion and a plurality of blades
are formed in a single integral piece. Other aspects of the present
invention relate to methods of manufacturing rotors.
BACKGROUND OF THE INVENTION
[0002] Many methods are known of making rotors, including various
molding and casting methods. The method used in any particular case
depends on a variety of factors, including the design of the rotor.
In some cases, the rotor design and size may make molding and
casting processes so impractical or expensive that it is preferred
to machine the rotor from a solid block of material, or to assemble
the rotor from parts.
[0003] Injection molding using a settable material such as
plastics, resins, ceramics or glasses (possibly with reinforcing
material), with a simple two-piece mold which is separated by
movement along the axis of rotation of the rotor, may be used in
some cases provided that there is no overlap in the axial direction
between adjacent rotor blades. However, if adjacent blades overlap
axially then a two-part mold cannot be opened by axial movement
because of obstruction between the rotor blades and the mold parts
at the positions where the blades overlap. At the positions where
part of one blade comes in front of part of another blade, the part
of the mold used to form the back face of the front blade cannot be
removed backwards because it is obstructed by the blade behind it,
and the part of the mold used to form the front face of the rear
blade cannot be removed forwards because of obstruction from the
part of the blade in front of it. If the angle of overlap in the
circumferential direction is small and the axial separation of
adjacent rotor blades is large, it may perhaps be possible to use a
two piece mold with one mold piece defining the front face of each
blade and the other mold piece defining the rear face of each
blade, by arranging the mold pieces so that they are twisted
slightly as they are pulled apart. However, if the angular extent
of overlap is appreciable, such two-piece molding methods are not
usable.
[0004] A further problem for the use of simple two-piece molds
arises in rotor designs where the central hub or core of the rotor
does not have a constant radius along its axial length. Provided
that the hub is cylindrical, the mold pieces will slide over the
hub surface without obstruction as the mold is opened axially.
However, if the hub radius varies, for example it is conical or
flared, the wider portions of the hub will foul the mold portions
used to define the blade roots at hub positions having a smaller
radius, of the mold part which is to be withdrawn rearwardly (i.e.
in the axial direction which corresponds to increasing hub radius).
It may be noted that rotors with flared hubs and overlapping
blades, such as to prevent simple two-piece injection molding, are
widely used, for example in the impellers of centrifugal flow
compressors and fans, and also in radial flow turbines.
[0005] When two-piece injection molding is not possible, it is
nevertheless possible to mold the rotor using a multi-piece mold,
which typically will have a separate mold piece for each space
between a pair of adjacent rotor blades. However, the large number
of mold pieces, which need to be separately manufactured and
assembled precisely, results in a considerable increase in the cost
and complexity both of the process of making the mold and the
process of manufacturing the rotors.
[0006] Interference between overlapping rotors can also cause
problems in other casting and molding manufacturing processes as
well as injection molding. For example, larger rotors are often
made using sand casting. Sometimes a separate sand mold part is
made for each space between a pair of rotor blades. However, it is
known that in some cases the shape of the rotor blades and the
degree of axial overlap is such that, as the mold parts are
assembled, the last one of the inter-blade sand blocks to be
inserted is obstructed by the previously-inserted blocks on either
side of its intended position, such that the final inter-blade
space has to be filled using several separate sand blocks each
representing a separate part of the inter-blade space, each of
which have to be inserted in turn and positioned correctly relative
to each other. This considerably complicates the construction and
assembly of the sand mold.
[0007] In general, axial overlap between adjacent rotor blades and
non-cylindrical rotor hubs tend to complicate any molding or
casting process for making a rotor. For relatively large rotors,
either such complications have to be accepted or a different type
of manufacturing technique has to be used, such as machining from
solid or making the rotors as a plurality of separate parts which
are fitted or fixed together. For smaller rotors, a further
alternative process is known in which a flexible (e.g. silicone
rubber) pattern for the rotor shape is used in the creation of an
expendable mold from a settable material such as plaster of Paris.
The pattern is dipped in the mold-forming material so as to create
a mold shape defining both the front and rear faces of each blade
in a single mold piece. Once the mold material has set, the pattern
is pulled out of the mold and its flexibility allows it to deform
so that it can be withdrawn without damaging the mold.
Subsequently, the material for forming the rotor (typically, molten
aluminum) is poured into the mold and once this is set the mold is
broken in order to release the molded rotor. This process works
satisfactorily in practice, but restricts the material from which a
rotor may be made to aluminum and other low-melting point
metals.
SUMMARY OF THE INVENTION
[0008] In one aspect, the present invention provides a rotor for
driving a fluid, or for being driven by a fluid, by rotation of the
rotor in use about an axis of rotation, the rotor comprising a hub
having a blade-bearing surface and a plurality of rotor blades
extending from the blade-bearing surface of the hub, the
blade-bearing surface of the hub having a radial distance from the
axis of rotation, at its intersection with a first plane
perpendicular with the axis of rotation which first plane also
intersects at least some of the rotor blades, which is less than
the radius of the hub where the hub intersects a second plane
perpendicular to the axis of rotation and spaced from the first
plane, and wherein the shape of each rotor blade is such that all
parts of the same rotor blade at the same radial distance from the
axis of rotation are parts of a common respective screw-threaded
spiral around the axis of rotation and all the screw-threaded
spirals for all the rotor blades have the same screw pitch, defined
as the distance travelled in the direction along the axis of
rotation by the spiral in one complete revolution of the spiral
about the axis of rotation. Mathematically, the shape of each rotor
blade can be specified as meeting the requirement that d.theta./dx
is constant and the same for all values of r and the same for all
blades, where r is the radial distance of a part of the blade from
the axis of rotation, x is the distance of the same part of the
blade in the direction along the axis of rotation from an arbitrary
reference axial position, and .theta. is the angle of the same part
of the blade about the axis of rotation from an arbitrary reference
radial direction.
[0009] This aspect of the present invention has particular
application to, but is not limited to, rotors wherein a
blade-bearing portion of the blade-bearing surface of the hub is
concave or flared, that is to say the radial distance of the
surface from the axis of rotation increases with change of position
along the axis of rotation in a particular direction, and the rate
of change of the radial distance also increases with change of
position along the axis of rotation in the same direction.
[0010] Provided that the rotor does not include further
obstructions, in addition to the overlap of the rotor blades with
one another and the increase in hub radius, the screw thread shape
of the rotor blades enables the rotor to be withdrawn from a single
mold piece which defines both faces of each rotor blade, by an
unscrewing rotation of the rotor relative to the mold piece,
without any requirement for the rotor to flex during withdrawal
from the mold. This enables simplification of the design for a
reusable mold, so that in many cases a rotor with axially
overlapping blades can be manufactured using a two-piece mold. This
in turn makes injection molding an economically viable
manufacturing technique in situations where previously it had been
too expensive owing to the large number of mold parts required. The
availability of injection molding at reasonable cost adds to the
range of materials that can be used as compared with the metals
usable in a casting process. For example, injection molding enables
lightweight resin, plastics, glass or ceramic materials to be used.
A reduction in the mass of the rotor may be obtained by use of such
materials in place of the metal (normally aluminum) used when the
rotor is manufactured by metal casting into an expendable mold
formed with a flexible pattern, provided that the necessary
temperature and load-bearing requirements etc can be met. This is
valuable in various contexts, such as where the rotor is used as a
compressor impeller in a turbocharger or supercharger for an
engine, where the mass of the impeller affects the amount of energy
and time taken for it to be accelerated to the correct rotational
velocity when a demand for air compression is made.
[0011] The use of screw-thread spiral rotor blades also allows
benefits in other molding and casting processes. For example, in
the case of sand molding a pattern having the same shape as the
rotor could be used to make a single sand mold piece defining the
shapes of all the rotor blades, and the pattern could then be
removed from the sand mold by unscrewing in a similar way to the
way in which the finished rotor is removed from the mold in the
injection molding example discussed above. Additionally, even if it
is desired to continue to use separate sand mold pieces for each
inter-blade space, the screw thread blade shape means that the
final inter-blade mold block can be inserted between the
previously-positioned mold blocks by a screw spiral motion, thereby
avoiding the need for multiple sand mold blocks to be made and
individually positioned to fill the final inter-rotor space. In
either case, there is a significant simplification when compared
with existing sand molding techniques.
[0012] Accordingly, in another aspect the present invention
provides a method of molding or casting a rotor having a hub and a
plurality of blades in a single piece, in which adjacent rotor
blades overlap axially so that a part of one rotor blade at a first
axial position is at the same angle from the axis of rotation as a
part of another rotor blade at a different axial position along the
axis of rotation, the method comprising (i) filling a mold that
defines a rotor having axially overlapping blades, each part of a
blade at the same radial distance from the axis of rotation
following a screw thread spiral shape with a common screw pitch for
all parts of all blades, with a flowable material, allowing or
causing the material to form a solid rotor, and removing the solid
rotor from the mold by screw-motion rotation or by breaking of the
mold. The flowable material will normally be a liquid that
solidifies to form the rotor.
[0013] In one embodiment, the process involves injection molding
with a plastics, resin, glass or ceramic material, which preferably
contains a reinforcing component which may or may not be fibrous.
For example, the rotor material may be a fiber reinforced plastic.
In another embodiment, the mold is a sand mold which is destroyed
to release the finished rotor. In this case, the rotor may be cast
from metal.
[0014] In the case of a rotor having a flared hub, to be used in a
radial flow device, it is normal for the parts of the rotor blades
at one axial end of the rotor, where the hub is narrow and the
fluid flow is generally axial, to twist around the hub as the blade
extends axially along the hub. This is so that the plane of the
blade is approximately parallel to the direction of relative
movement between the fluid and the rotor taking into account both
the rotation of the rotor and the axial flow speed of the fluid.
The exact angle depends on factors such as the intended rotational
velocity of the rotor in use and the intended volumetric fluid flow
through the device. This twist of the rotor blades around the hub
at the axial flow end defines the screw pitch of the screw-threaded
blades, which must be constant over the whole of all of the blades.
At the other axial end of the rotor, where fluid flow is
substantially radial and the hub surface (having a much greater
radius from the axis of rotation than at the other end of the
rotor) has a greater radial component, this same screw pitch means
that as the blade extends away from the hub surface, in a direction
which includes a substantial axial component owing to the
substantial radial component of the hub surface, the blade surface
must be swept circumferentially around the rotor to some extent,
creating a rake angle between the blade and the hub surface.
Consequently, a screw pitch defined by the appropriate aerodynamic
design for the blades at the axial flow end of the rotor typically
results in a blade rake angle at the radial flow end of the rotor
which is much more extreme than in conventional rotor designs.
Somewhat surprisingly, it has been found in practice that it is
nevertheless possible to design an aerodynamically effective blade
shape for the impeller of a radial flow compressor with a
screw-thread blade shape, notwithstanding the much sharper rake
angle than conventional designs.
[0015] Additionally, it is known in rotors having a flared hub, to
be used in a radial flow device, for the rotor blades at the radial
flow end of the rotor, where the hub is wide, to twist around the
axis of rotation as the blade extends radially along the hub. This
is in order for the plane of the blade at this end of the rotor to
be generally parallel to the direction of relative movement between
the fluid and the rotor taking into account that rotation of the
rotor, the radial flow of the fluid and the circumferential flow of
the fluid. Consequently, with reference to the mathematical
terminology used above, it is possible for .theta. to vary with
variation in r, even at a constant x. The requirement of a
screw-thread shape does not mean that d.theta./dr should be
constant for the same value of x, and the value of .theta. can be
varied with changes in the radial distance r as required for the
fluid dynamics of intended fluid flow through the rotor, provided
that d.theta./dx is kept constant and the same at all places on all
blades. This possibility of varying .theta. with variation in r
applies to all hub shapes.
[0016] An aspect of the present invention provides a rotor, for
driving a fluid by rotation of the rotor about an axis of rotation
or for being driven by a fluid so as to rotate about an axis of
rotation, or a pattern for making a mold part for making a said
rotor, the rotor or pattern comprising (i) a hub having a
blade-bearing surface and (ii) a plurality of rotor blades
extending from the blade-bearing surface of the hub,
[0017] the blade bearing surface of the hub having a first surface
portion at a first radial distance from said axis of rotation and a
second surface portion at a second radial distance, greater than
said first distance, from said axis of rotation, the second surface
portion being spaced from the axis of rotation in the same angular
direction from said axis of rotation as the first surface portion
and being spaced from the first surface portion in a direction
parallel to the axis of rotation, both the first and second surface
portions bearing at least a part of a rotor blade,
[0018] at least some of the rotor blades having a respective
portion that is spaced from a respective portion of another rotor
blade in a direction parallel to the axis of rotation but is at the
same radial distance from the axis of rotation and is spaced from
the axis of rotation in the same angular direction,
[0019] wherein the value of d.theta./dx is constant and the same
for the whole of all blades of the rotor or pattern (where x
represents distance in a direction parallel to the axis of
rotation, and .theta. represents angular direction perpendicular to
the axis of rotation taking the axis of rotation as the origin)
except for differences in the slopes of the blade surfaces arising
solely from variation in the thickness of the rotor blades.
[0020] The blade-bearing surface of the hub may have a third
surface portion at a third radial distance from the axis of
rotation, the third surface portion being spaced from the axis of
rotation in the same angular direction from said axis of rotation
as the first and second surface portions and being midway between
the first and second surface portion in a direction parallel to the
axis of rotation, wherein the third radial distance is less than
the average of the first and second radial distances.
[0021] The blade-bearing surface of the hub may be substantially
parallel to the axis of rotation at the first surface portion and
substantially perpendicular to the axis of rotation at the second
surface portion.
[0022] In the rotor or pattern, the respective portions of at least
one pair of rotor blades may extend circumferentially with respect
to the axis of rotation over an angle of at least 5.degree.
subtended at the axis of rotation. The angle may be at least
10.degree., or even at least 15.degree..
[0023] Another aspect of the present invention provides a rotor, or
a pattern for making a mold part for making a rotor, having a hub
and a plurality of rotor blades extending from the hub,
wherein,
[0024] for each rotor blade, all portions of the rotor blade at the
same radial distance from the axis of rotation of the rotor form
part of the same screw thread spiral, and the screw pitch of the
screw thread spiral is the same at all radial distances of the
blade and is the same for all of the rotor blades,
[0025] at least some of the rotor blades overlap axially at least
partially in the sense that a part of one rotor blade at the same
polar co-ordinates from the axis of rotation as a part of another
rotor blade but is spaced axially therefrom, and
[0026] at least a part of the hub has a diameter which varies with
distance along the axis of rotation.
[0027] Another aspect of the present invention provides apparatus
for driving a gas, the apparatus comprising an impeller that is a
rotor according to any of the aforementioned aspects of the
invention.
[0028] Another aspect of the present invention provides a
turbocharger or supercharger for an internal combustion engine,
comprising a rotor according to any of the aforementioned aspects
of the invention.
[0029] Another aspect of the present invention provides a mold part
for use in making a rotor, the mold part having a recess for
receiving molding material in use, said recess having recess
portions that define both faces of each blade of the rotor, all
parts of each said recess portion having a common value of
d.theta./dx, where x represents distance in a direction parallel to
the axis of rotation, and .theta. represents angular direction
perpendicular to the axis of rotation taking the axis of rotation
as the origin, except for differences in the slopes of the surfaces
of the recess portions arising solely from variation in the width
of the recess portions in the direction of the thickness of the
rotor blades, and at least some of said recess portions overlapping
another said recess portion in the axial direction of the
rotor.
[0030] Part of the surface of said recess of the mold part may
define a blade-bearing surface of a hub of the rotor, such that
said part of the surface of said recess has respective portions
axially spaced from each other, with reference to the axis of
rotation of the rotor, which portions are at different radial
distances from said axis of rotation.
[0031] The part of the recess surface defining a blade-bearing
surface may be convex so that the part of the recess surface is
less parallel to the axis of rotation of the rotor where it is
further from said axis of rotation.
[0032] Another aspect of the present invention provides a method of
making a rotor comprising filling the recess of a mold part as
aforementioned with a flowable material, causing or allowing the
flowable material to solidify, and removing the solidified material
from the mold part by moving it relative to the mold part with a
screw motion.
[0033] The step of filling a mold part with a flowable material may
comprise injecting the flowable material under pressure. The
flowable material may be a settable plastics resin. The plastics
resin may contain re-inforcing material.
[0034] Another aspect of the present invention provides a method of
making the aforementioned mold part comprising immersing or
embedding a rotor pattern in a mold-forming material, allowing or
causing the mold-forming material to solidify, and withdrawing the
pattern from the solidified material by movement with a relative
screw motion to leave the recess in the solidified material. The
mold-forming material may be sand or mostly sand.
[0035] Another aspect of the present invention provides a method of
making a rotor comprising making a mold part by the aforementioned
method, filling the recess left by the pattern with a molding
material, allowing or causing the molding material to solidify, and
releasing the solidified molding material by destroying the mold
part.
[0036] The molding material may be molten metal, and the step of
allowing or causing the molding material to solidify may comprise
allowing or causing the molten metal to cool.
[0037] Another aspect of the present invention provides an
inter-blade mold part for a mold for use in making a rotor by
molding or casting, the inter-blade mold part having a first
surface that defines a surface on one side of a blade of a rotor
and a second surface that defines a surface on the other side of
another blade of the rotor,
[0038] wherein for both of the first and second surfaces of the
inter-blade part, all parts of the surface that define a blade
surface of a rotor slope at a common screw pitch, with reference to
an axis of rotation of the rotor, except for differences in the
slopes of the first and second surfaces arising solely from
variation in the thickness of the blades. The inter-blade mold part
may be sand or mostly sand.
[0039] Another aspect of the present invention provides a sand
mold-forming pattern for defining an inter-blade sand mold part as
aforementioned.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] Embodiments of the present invention, given by way of
non-limiting example, will be discussed with reference to the
accompanying drawings.
[0041] FIG. 1 is a schematic front view of a rotor embodying the
present invention.
[0042] FIG. 2 is a schematic side view of the rotor of FIG. 1.
[0043] FIG. 3 is a schematic front view of the hub of the rotor of
FIG. 1.
[0044] FIG. 4 is a schematic side view of the hub of the rotor of
FIG. 1.
[0045] FIG. 5 is a schematic front view of a known rotor provided
for comparison.
[0046] FIG. 6 is a schematic side view of the rotor of FIG. 5.
[0047] FIG. 7 is a schematic front view of the hub of the rotor of
FIG. 5.
[0048] FIG. 8 is a schematic side view of the hub of the rotor of
FIG. 5.
[0049] FIG. 9 is a schematic plan view of a mold base part of a
mold for making the rotor of FIG. 1.
[0050] FIG. 10 is a schematic side view of the mold base part of
FIG. 9.
[0051] FIG. 11 is a schematic plan view of a mold lid part for use
with the mold base part of FIG. 9.
[0052] FIG. 12 is a schematic side view of the mold lid part of
FIG. 11
[0053] FIG. 13 is a schematic end view of an inter-blade mold part,
together with views of the box parts that can be assembled into a
pattern box for making it.
[0054] FIG. 14 is a schematic end view of an axial flow rotor that
can be made by a manufacturing process embodying the present
invention.
[0055] FIG. 15 is a schematic side view of the rotor of FIG.
14.
[0056] FIG. 16 is a schematic end view of a further axial flow
rotor that can be made by a manufacturing process embodying the
present invention.
[0057] FIG. 17 is a schematic side view of the rotor of FIG.
16.
[0058] FIG. 18 is a schematic top view of a turbocharger or other
supercharger using the rotor of FIG. 1 as an impeller.
[0059] FIG. 19 is a schematic side view of the turbocharger or
other supercharger of FIG. 18, with the shroud partially cut
away.
DETAILED DESCRIPTION OF THE DRAWINGS
[0060] The disclosed embodiments are provided by way of example,
and the invention is not limited thereto.
[0061] A first embodiment of the present invention is shown in
FIGS. 1 and 2. FIG. 1 is a front view of a rotor suitable for use
as an impeller in a radial flow (centrifugal flow) air compressor
or pump, such as a supercharger or turbocharger for an internal
combustion engine. FIG. 2 is a side view of the rotor of FIG. 1.
FIGS. 3 and 4 are corresponding front and side views showing the
hub of the rotor of FIGS. 1 and 2, without the rotor blades. By way
of comparison, FIGS. 5 to 8 are views, corresponding respectively
to FIGS. 1 to 4, of a known rotor suitable for the same use.
[0062] FIGS. 18 and 19 show the rotor of FIGS. 1 to 4 mounted as
the impeller in a supercharger or turbocharger. The impeller is
mounted in a shroud 10 (which is cut away in FIG. 19 so that the
impeller is visible), which also defines an air inlet 12 and an air
outlet 14. The impeller is driven by a drive means 16, which is a
turbine (normally driven by exhaust gases) in a turbocharger and is
some other drive means (such as an electric motor, or a coupling to
take drive from the engine being supercharged) in other types of
supercharger.
[0063] As can be seen from the drawings, both the rotor of the
embodiment and the comparison rotor have a hub 1 having a concave
flared blade-bearing hub surface 3, in which the radial distance of
the surface 3 from the axis of rotation 4 of the hub increases with
change in position in a direction along the axis of rotation, and
the rate of increase of the radial distance of the surface 3 also
increases with change in axial distance in the same direction, so
that the direction normal to the blade-bearing surface 3 is closer
to being parallel with the axial direction at points on the
blade-bearing surface 3 which have a greater radial distance from
the axis of rotation 4. Each of the rotors has sixteen blades 5, 7.
Eight rotor blades 5 extend over the full axial length of the
blade-bearing surface 3, and eight blades 7 extend over only part
of the length of the blade-bearing surface 3, where the surface is
widest. A hole 8 through the rotor, centered on the axis of
rotation, allows it to be mounted on a shaft for rotation. Lines 9
in FIGS. 3, 4, 7 and 8 show the positions where respective blades
5, 7 contact the blade-bearing surface 3. These lines are sometimes
known as the hub-lines of the blades or as the root-lines of the
blades.
[0064] The individual blades 5, 7 of the rotors each wrap around
the hub 1 to some extent, so that the angle .theta. of the blade
from the axis of rotation 4 varies along the length of the blade.
As can be seen in FIGS. 1 and 5, this has the result that each
blade 5, 7 has a part which axially overlaps a part of another
blade 5, 7. In this context, the term "axially overlaps" means that
the respective parts of the two blades are each at the same radial
distance r from the axis of rotation and both at the same angle
.theta. with respect to the axis of rotation, and are separated
merely by a distance x in a direction parallel to the axis of
rotation. This axial overlapping of the blades 5, 7, together with
the widening of the blade-bearing surface 3 of the hub 1 with
change of distance in the axial direction, means that it is
impossible for these rotors to be made by molding in a two-part
mold with the mold parts being separated axially. For example, a
mold part defining the blade rear surface of the front part of one
of the full-length blades 5 cannot be removed forwardly, because
the blade 5 is in front of it, but cannot be removed rearwardly
because of obstruction by part of a part-length blade 7 and because
of obstruction by part of the hub 1, which are both axially in line
with the front part of the full-length blade 5. In the case of the
rotor of FIGS. 5 to 8, any mold which could be removed from the
rotor after manufacture, without destruction of the mold, would
have to be a very complex shape with many separate pieces. In
practice, the rotor of FIGS. 5 to 8 is made by casting it out of a
lightweight metal such as aluminum in a plaster of Paris mold, and
the mold is broken to release the rotor. In order to form the mold,
a flexible pattern made from silicone rubber has to be used, so
that the pattern can be removed from the mold without damaging the
mold.
[0065] The rotor of FIGS. 1 to 4, which embodies the present
invention, has blades 5, 7 each of which follow a screw-thread
shape with a common screw pitch. That is to say, the change
.delta.x in the position of the blade in the direction parallel to
the axis of rotation 4 for a given change .delta..theta. in angular
position around the axis of rotation 4 without variation in the
radial distance r from the axis of rotation 4 is constant and the
same over the whole length of all blades 5, 7. As a result, if the
rotor as a whole is moved rearwardly with a screw-type movement,
both rotating and moving axially at the same time with the same
screw pitch as the rotor blades 5, 7, each rotor blade will move
along a path made up entirely of previous positions of that blade
and the region rearwardly of previous positions of the
blade-bearing surface 3. This means that if the blades 5, 7 and the
blade-bearing surface 3 of the hub 1 are embedded in a single mold
part, which defines both the front and the rear surfaces of each of
the blades 5, 7 and defines the blade-bearing surface 3 of the hub
1, the rotor can be removed from the mold part by an unscrewing
motion without any obstruction between the rotor and the mold part
during removal. This enables a reusable mold to have a small number
of parts, for example it may be a two-part mold. Consequently,
manufacturing the rotor using a reusable mold is much simpler and
cheaper with the rotor of FIGS. 1 to 4, embodying the present
invention, than it is with the rotor of FIGS. 5 to 8.
[0066] FIGS. 9 and 10 show schematically a mold part 11 for molding
the rotor of FIGS. 1 to 4. As shown by the broken lines in FIG. 10,
the mold part 11 defines the entire shape of all of the rotor
blades 5, 7 and defines the shape of the blade-bearing surface 3 of
the hub 1. When the mold part 11 is seen in plan view, as in FIG.
9, only the hub lines 9 of the blades can be seen, and the
remainder of the blade shapes are defined by slots disappearing
into the body of the mold part 11 with the hub lines 9 being the
openings of the slots in the surface of the mold part 11.
Nevertheless, as mentioned above, it is possible to remove the
molded rotor from the mold part 11 without damage either to the
rotor or to the mold part 11 by an unscrewing movement, because
this movement will cause the rotor blades 5, 7 to slide along the
slots and out of the mold part 11. In the molding process, the
shaft hole 8 will be formed by a core and the mold part 11 has a
small recess 13 extending beyond the end of the rotor shape, in
order to receive this core.
[0067] In the mold part 11, the slots which define the shape of the
rotor blades 5, 7 are slightly tapered so that each rotor blade
becomes thinner as it extends away from the blade-bearing surface
3. Consequently, during the unscrewing movement for removing the
molded rotor from the mold part 11, each individual portion of a
rotor blade 5, 7 will move into a portion of the corresponding slot
in the mold part 11 which is slightly wider than the portion of the
blade. This facilitates removal of the rotor from the mold part 11
by reducing the tendency of the blade surface to bind with the
surface of the slot as the blade slides through the slot. This is
equivalent to the known practice of providing a draft angle on a
molded product, to facilitate separation from the mold.
[0068] FIGS. 11 and 12 show schematically another mold part 15
which is arranged to co-operate with the mold part 11 for molding
the rotor of FIGS. 1 to 4. In effect, the mold part 11 of FIGS. 9
and 10 forms a mold base, and the mold part 13 forms a mold lid,
closing the base.
[0069] The mold lid part 15 is formed integrally with the core 17
for forming the shaft hole 8 of the rotor. Additionally, the mold
lid part 15 has another core 19 which fills part of the volume of
the hub 1 of the rotor, in order to reduce the amount of material
used to form the rotor and thereby reduce its mass. The core 19 is
spaced from the core 17, and is also shaped so that when the mold
is closed it is suitably spaced from the surface of the mold base
part 11 that defines the blade-bearing surface 3 of the hub, to
allow the hub to have sufficient thickness of material for
robustness.
[0070] The core 19 has four recesses 21 extending radially inward
from its circumference, which in use will define four corresponding
protrusions on the rear surface of the hub 1 when the rotor is
formed. These recesses 21 create an interaction between the mold
lid part 15 and the molded rotor to allow the lid part 15 to drive
the rotor in rotation in a manner similar to the manner in which a
screwdriver drives a screw. After the rotor has been formed in the
mold, it is initially removed from the mold base part 1 1 by an
unscrewing motion driven by rotating the mold lid part 15 while
withdrawing it from the mold base part 11. The mold lid part 15
also has holes 23 for ejector pins, for separating the molded rotor
from the mold lid part 15 after it has been removed from the mold
base part 11 by unscrewing.
[0071] In principle, either or both of the cores 17, 19 could be
provided as separate pieces. However, it will normally be most
convenient to make both cores integral with the mold lid part 15.
As shown in the drawings, the shaft hole 8 in the rotor is
cylindrical, and accordingly the corresponding mold core 17 is
cylindrical. However, if an alternative cross-section is required,
such as a square-section shaft hole or a splined shaft hole, the
core 17 is given an appropriate shape. Because the ejector pins
separate the molded rotor from the mold lid part 15 by axial
movement rather than by rotational unscrewing, such alternative
cross-sectional shapes for the shaft hole do not prevent removal of
the rotor from the mold.
[0072] It should be noted that FIGS. 9 to 12 are schematic views
showing the main features of the mold parts, and additional
features familiar to those skilled in the art will also be present,
such as holes for injection of the material from which the rotor is
to be made, even though they are not shown.
[0073] As will be understood from the foregoing description, the
rotor shape of FIGS. 1 to 4 is suitable for use in manufacturing a
rotor by injection molding of a lightweight material such as fiber
reinforced plastic, using a two-piece mold. As compared with the
rotor shape of FIGS. 5 to 8, which cannot economically be made by
injection molding and is therefore made by casting metal into a
disposable mold, this allows a reduction in the mass of the rotor
and consequently a reduction in the energy required to accelerate
the rotor. This is advantageous in a variety of contexts. For
example, if the rotor is used as the impeller in a turbocharger,
where the limiting factor in impeller acceleration is normally the
availability of energy to accelerate it at the initial moment of
demand for turbocharger operation, reduction in the energy required
for impeller acceleration may permit a faster response. If the
rotor is used as the impeller of an electrically driven
supercharger, the advantage provided by the reduced inertia of the
impeller can be taken either as a faster acceleration of the
impeller or as a reduction in the electrical current required to
accelerate it. A further potential advantage in the reduced mass of
the rotor is that if the rotor fractures during operation the mass
of the rapidly moving parts of the broken rotor is less. In some
applications it is necessary for safety reasons that the shroud
around the rotor, or some other enclosure, is designed to be strong
enough to contain the rapidly moving parts of a rotor that
fractures during rotation. A reduction in the mass of those broken
parts permits a corresponding reduction in the strength required
for the shroud or other containing part, potentially permitting the
shroud or other part to be made from a thinner material or itself
be made from a lightweight plastic material instead of from metal.
Thus the overall weight of the component containing the rotor can
be reduced.
[0074] The feature that all portions of all blades of a rotor
follow a common screw pitch is useful in other molding and casting
processes, in addition to its benefits for injection molding. In
any molding or casting process in which the mold is to be reused,
rather than destroyed to release the rotor, the ability to remove
the rotor from a mold part by an unscrewing movement will normally
allow a significant reduction in the number of mold parts which are
needed. Additionally, in manufacturing methods in which the mold is
destroyed in order to release the rotor, this shape of rotor blade
will normally allow simplification in the process of making the
mold. For example, if a rotor having the shape of the embodiment of
FIGS. 1 to 4 is made by pouring molten metal into a disposable
plaster of Paris mold, the pattern used to form the mold can be
withdrawn from the mold by an unscrewing movement, without the need
for the pattern to flex. Accordingly, it ceases to be necessary to
use a silicone rubber pattern and a rigid pattern can be used
instead. The plaster of Paris mold made in this way will be
substantially as shown in FIGS. 9 and 10. However, the recess 13
would be replaced by the core 17, which in this case would be
integral with the mold part defining the shape of the blades 5, 7
and the blade-bearing surface 3 of the hub. The ability to use a
solid pattern for forming the plaster of Paris mold provides a
greater choice of materials from which the pattern may be made, and
also avoids any potential manufacturing problems or inaccuracies
that may result from the use of a flexible pattern.
[0075] Sand molding may be the preferred method of making an
integral one-piece rotor if the diameter of the rotor is greater
than about 30 cm. The sand mold for a rotor may be assembled from a
large number of mold parts. For example, there may be a separate
mold part for each inter-blade space together with a core defining
the shaft hole 8. Each inter-blade mold part will define the shape
of one surface of one blade and the facing surface of an adjacent
blade, and also the portion of the blade-bearing surface 3 of the
hub which lies between the two blades. Each inter-blade mold part
will be made using a pattern, into which the sand is packed in
order to form an appropriately shaped sand block. If a rotor having
the shape of the embodiment of FIGS. 1 to 4 is to be made using
sand molding, it becomes possible to make a single pattern for the
rotor that defines the shapes of all the blades 5, 7 and the
blade-bearing surface 3. The pattern will have the shape shown in
FIGS. 1 and 2. This pattern can be used with a sand box to create a
single sand mold piece having the shape shown in FIGS. 9 and 10,
and the pattern can be removed from the sand mold by an unscrewing
motion without damaging the mold. If desired, a separate core mold
piece may be used to define the shaft hole 8. This greatly
simplifies the process of making the sand mold for the rotor, for
two reasons. First, the number of mold parts to be made is greatly
reduced since there is no longer a separate mold part for each
inter-blade space. Second, this method avoids the need for a step
of assembling the mold by moving a large number of separate
inter-blade mold parts into the correct positions with the
necessary degree of accuracy.
[0076] Even if it is still desired to make the sand mold for such a
rotor using separate inter-blade mold parts, an advantage is
obtained at the time of assembling the parts into the finished
mold. FIG. 13 shows an inter-blade mold part 25 for a rotor and the
separated parts 27a to 27f of a pattern box for making the mold
part. To make the inter-blade mold part 25, the pattern box parts
are joined together to form a pattern box that is packed with sand
to form the mold part. When the sand has hardened, the parts 27a to
27f are separated to release the inter-blade mold part 25 thus
formed.
[0077] Because the angle of each blade with respect to the
direction of the axis of rotation twists as the radial distance of
the blade from the axis of rotation changes, and the blade also
sweeps around the axis of rotation as its position in a direction
parallel to the axis of rotation varies, the inter-blade mold part
25 has a complex shape, such that it is impossible for all the
required inter-blade mold parts 25 to be assembled into their
correct positions by moving each into place in turn either by a
radial or an axial movement. In the past, some designs of rotor
blade shape have had the consequence that it is impossible to slide
the final inter-blade mold part into position at all, because of
obstruction of its path by the adjacent mold parts which are
already in position. In such cases, it is necessary to create
several smaller mold parts, each defining a respective part of the
total inter-blade space, and these smaller mold parts each have to
be inserted and positioned in turn to fill the final inter-blade
space as the mold is assembled. This requires additional patterns,
for making the special mold parts, and a more complex assembly
operation as the mold parts are assembled into the final
inter-blade space. However, if all the rotor blades follow a common
screw thread pitch, it must be possible to insert the final
inter-blade mold part 25 into its correct position, after all the
other inter-blade mold parts 25 have been placed in their
positions, by a screw spiral motion so that the blade-defining
surfaces of the mold part 25 follow the paths of the blades they
define. The screw thread shape of the blades means that the
inter-blade mold part 25 will be able to follow this line of
movement into its correct position without being obstructed by any
other inter-blade mold part 25.
[0078] In order to design a radial flow rotor with a concave flared
blade-bearing hub surface 3, similar to the rotor of FIGS. 1 to 4,
the designer will normally start from the shape of the
blade-bearing surface 3 and the hub lines 9. The shape of the
blade-bearing surface may be limited to some extent by external
considerations, such as the total shape and size of the space
allowed for the rotor in the design of a larger installation of
which it will form a part. Within such constraints, the designer
will use their experience to create a design which is likely to be
useful. The designer will particularly need to take into account
the velocity and volumetric flow rate of fluid that the rotor is
intended to handle and consequently the angle of relative motion
between the fluid and the rotor during operation. Normally, the hub
lines 9 will be designed to be generally parallel to the intended
fluid flow in use at each end of the hub 1, and between the two
ends of the hub the hub lines 9 will normally be selected to
provide a substantially constant relative speed of the fluid flow,
relative to the blade-bearing surface 3 of the hub, or else a
smoothly varying speed, in order to avoid localized fluid
acceleration or deceleration which can disrupt the smooth flow of
fluid through the rotor.
[0079] Because the blade-bearing surface 3 of the hub is
substantially parallel to the axis of rotation 4 at the narrow end
of the hub 1, the angle of the hub lines 9 of the blades at this
end of the blade-bearing surface 3 tends to determine the screw
pitch d.theta./dx. Consequently, the blade shapes are determined by
extending the blades away from each point of each hub line
maintaining a constant radial distance r from the axis of rotation
4 and following the screw pitch d.theta./dx, until the blades reach
a predefined virtual surface for the blade tips or shroud lines, or
if the shape of the shroud around the rotor is not predefined,
until the blades define a shroud shape which the designer deems to
be suitable. As mentioned above, it is preferred that the blades
are tapered so as to become thinner as they move away from the hub
line, in order to facilitate removable of the blades 5, 7 from the
mold base part 11. This is achieved by selecting an appropriate
draft angle for the blade surfaces as they extend away from the hub
line 9. The thickness of the hub lines 9 and the magnitude of the
draft angle can be adjusted, if desired, so as to optimize the
blade thickness.
[0080] The resulting rotor design can then be subjected to fluid
dynamic analysis in order to estimate how it will perform in
practice, and to assess whether the blade angles of the rotor
design are correct. If necessary, the rotor design can be varied by
adjusting the hub lines 9, both to change the overall path of the
blades 5, 7 over the blade-bearing surface 3 and to adjust the
screw pitch d.theta./dx. The revised design can then be optimized
and subjected to fluid dynamic analysis, and the design adjustment
process can be repeated as necessary, until a satisfactory
conclusion is reached.
[0081] Because the radius of the blade-bearing surface 3 is much
greater at the radial flow end of the hub 1 than at the axial flow
end of the hub 1, even a slight twist of the hub lines 9 of the
blades around the axis of rotation 4 at the axial flow end of the
hub will define a screw pitch d.theta./dx which causes the radial
flow ends of the blades 5, 7 to be swept sideways for a
considerable circumferential distance for a small change in axial
distance. Consequently, at the circumferentially outermost parts of
the rotor, the blades 5, 7 lean over with a considerable rake
angle, as is clearly visible in FIG. 2. By comparison, the blades
5, 7 of the known rotor of FIGS. 5 to 8 do not lean over at their
radial flow ends where the rotor diameter is largest, as can be
seen in FIG. 6. This rake angle or "blade lean" is the inevitable
consequence of maintaining a constant screw thread pitch
d.theta./dx over all parts of all blades 5, 7, together with the
need to angle the axial flow ends of the blades 5, 7 to accommodate
the direction of relative movement between the blades and the
fluid. Since the rake angle can affect the flow of fluid through
the rotor, it might be thought that this unusual rake angle would
make it difficult to obtain satisfactory fluid dynamic performance
of the rotor. However, aerodynamic analysis and subsequent testing
of a prototype rotor substantially in accordance with FIGS. 1 to 4
showed that satisfactory performance could be obtained.
[0082] The embodiment of FIGS. 1 to 4 has a hub 1 the diameter of
which varies with axial distance along the hub, and the rotor is
suitable for radial fluid flow. FIGS. 14 to 17 show rotors having
cylindrical hubs, which are suitable for axial fluid flow through
the rotor. In these rotors, the shape of the hub 1 does not
obstruct axial separation of the rotor from a mold part. However,
these rotor designs have substantial axial overlap between adjacent
rotor blades, thereby preventing separation of the rotor from a
two-piece mold by axial movement. Nevertheless, as with the
embodiment of FIGS. 1 to 4, the rotors of FIGS. 14 to 17 have rotor
blades 7 which conform to a screw thread shape, so that a rotor or
molding pattern can be separated from a mold part by a screw motion
without destruction of the mold part. Accordingly, the impellers of
FIGS. 14 to 17 may also be manufactured by a molding process in
which the mold is not destroyed, such as injection molding, with a
conveniently small number of mold pieces, for example using a
two-piece mold. Additionally, if such impellers are made using a
molding or casting process in which the mold is destroyed, such as
sand molding or casting into a plaster of Paris mold, the
advantages described above for the methods of making or assembling
the mold can also be obtained with these designs of rotor.
[0083] The examples and embodiments of the present invention that
have been described herein are provided by way of non-limiting
illustration, and those skilled in the art will understand that
many variations are possible that fall within the scope of the
present invention as defined by the following claims.
* * * * *