U.S. patent application number 14/370894 was filed with the patent office on 2015-02-12 for turbocharger having a connector for connecting an impeller to a shaft.
This patent application is currently assigned to NAPIER TURBOCHARGERS LIMITED. The applicant listed for this patent is Ian Patrick Clare Brown, Jamie Clare, Robert Neil George, Francis Joseph Geoffrey Heyes, Paul Leslie Jacklin, Peter Kay, Trevor Knighton, Christopher John Monaghan, Matthew Elijah Moore, Thomas Jarlath Murray, Kevin John Musson, Geoff Kinpoy Ngao, Osarobo Famous Okhuahesogie, Ian Pinkney, Stuart Michael Potter, Paul Eifion Roach, David Leslie Smith, Alan Martin Taylor, Neil Ryan Thomas, Stephen Wilson. Invention is credited to Ian Patrick Clare Brown, Jamie Clare, Robert Neil George, Francis Joseph Geoffrey Heyes, Paul Leslie Jacklin, Peter Kay, Trevor Knighton, Christopher John Monaghan, Matthew Elijah Moore, Thomas Jarlath Murray, Kevin John Musson, Geoff Kinpoy Ngao, Osarobo Famous Okhuahesogie, Ian Pinkney, Stuart Michael Potter, Paul Eifion Roach, David Leslie Smith, Alan Martin Taylor, Neil Ryan Thomas, Stephen Wilson.
Application Number | 20150044047 14/370894 |
Document ID | / |
Family ID | 45788758 |
Filed Date | 2015-02-12 |
United States Patent
Application |
20150044047 |
Kind Code |
A1 |
Pinkney; Ian ; et
al. |
February 12, 2015 |
TURBOCHARGER HAVING A CONNECTOR FOR CONNECTING AN IMPELLER TO A
SHAFT
Abstract
A connector for connecting an impeller to a shaft is provided.
The impeller has a shaft-side hub extension with a central recess.
The impeller is formed of a material having a greater coefficient
of thermal expansion than the material of the shaft. The connector
is inserted into the recess to frictionally connect an outwardly
facing surface of the connector with a radially inner surface of
the hub extension. The connector has a threaded portion carrying a
thread which screws onto a corresponding threaded portion of the
shaft, such that the connector provides a rotationally fixed
connection between the impeller and the shaft. The connector is
formed of a material having a coefficient of thermal expansion
which is greater than the coefficient of thermal expansion of the
material of the shaft.
Inventors: |
Pinkney; Ian; (Lincoln,
GB) ; Okhuahesogie; Osarobo Famous; (Lincoln, GB)
; Roach; Paul Eifion; (Lincoln, GB) ; Thomas; Neil
Ryan; (Lincoln, GB) ; Brown; Ian Patrick Clare;
(Nottingham, GB) ; Kay; Peter; (Lincoln, GB)
; Wilson; Stephen; (Lincoln, GB) ; Smith; David
Leslie; (Welton Le Marsh, GB) ; George; Robert
Neil; (Lincoln, GB) ; Jacklin; Paul Leslie;
(Lincoln, GB) ; Ngao; Geoff Kinpoy; (Lincoln,
GB) ; Musson; Kevin John; (Lincoln, GB) ;
Moore; Matthew Elijah; (Lincoln, GB) ; Clare;
Jamie; (Grantham, GB) ; Murray; Thomas Jarlath;
(Lincoln, GB) ; Potter; Stuart Michael;
(Burton-upon-Stather, GB) ; Monaghan; Christopher
John; (Lincoln, GB) ; Taylor; Alan Martin;
(Lincoln, GB) ; Heyes; Francis Joseph Geoffrey;
(Lincoln, GB) ; Knighton; Trevor; (Doncaster,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Pinkney; Ian
Okhuahesogie; Osarobo Famous
Roach; Paul Eifion
Thomas; Neil Ryan
Brown; Ian Patrick Clare
Kay; Peter
Wilson; Stephen
Smith; David Leslie
George; Robert Neil
Jacklin; Paul Leslie
Ngao; Geoff Kinpoy
Musson; Kevin John
Moore; Matthew Elijah
Clare; Jamie
Murray; Thomas Jarlath
Potter; Stuart Michael
Monaghan; Christopher John
Taylor; Alan Martin
Heyes; Francis Joseph Geoffrey
Knighton; Trevor |
Lincoln
Lincoln
Lincoln
Lincoln
Nottingham
Lincoln
Lincoln
Welton Le Marsh
Lincoln
Lincoln
Lincoln
Lincoln
Lincoln
Grantham
Lincoln
Burton-upon-Stather
Lincoln
Lincoln
Lincoln
Doncaster |
|
GB
GB
GB
GB
GB
GB
GB
GB
GB
GB
GB
GB
GB
GB
GB
GB
GB
GB
GB
GB |
|
|
Assignee: |
NAPIER TURBOCHARGERS
LIMITED
Lincoln, Lincolnshire
GB
|
Family ID: |
45788758 |
Appl. No.: |
14/370894 |
Filed: |
December 10, 2012 |
PCT Filed: |
December 10, 2012 |
PCT NO: |
PCT/GB2012/053082 |
371 Date: |
July 7, 2014 |
Current U.S.
Class: |
416/95 |
Current CPC
Class: |
F05D 2300/50212
20130101; F04D 29/023 20130101; F04D 29/266 20130101; F04D 29/601
20130101; F16D 1/0835 20130101; F05D 2300/173 20130101; F04D
29/5853 20130101; F05D 2220/40 20130101; F01D 5/025 20130101; F04D
29/284 20130101; F05D 2300/171 20130101; F04D 25/024 20130101 |
Class at
Publication: |
416/95 |
International
Class: |
F04D 29/58 20060101
F04D029/58; F04D 29/60 20060101 F04D029/60 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 10, 2012 |
GB |
1200403.2 |
Claims
1. A connected impeller and shaft, the impeller having a shaft-side
hub extension with a central blind hole recess, the impeller being
fitted with a connector which connects the impeller to the shaft,
and the impeller being formed of a material having a greater
coefficient of thermal expansion than the material of the shaft,
wherein: the connector is inserted into the recess to frictionally
connect an outwardly facing surface of the connector with a
radially inner surface of the hub extension, the frictional
connection between the outwardly facing surface of the connector
and the radially inner surface of the hub extension transmitting,
in use, substantially all of the torque between the shaft and the
impeller; and the connector has a threaded portion carrying a
thread which screws onto a corresponding threaded portion of the
shaft, such that the connector provides a rotationally fixed
connection between the impeller and the shaft; charaterised in
that: the connector is formed of a material having a coefficient of
thermal expansion which is greater than the coefficient of thermal
expansion of the material of the shaft, the value of
(.alpha..sub.c-.alpha..sub.s)/(.alpha..sub.i-.alpha..sub.s) being
greater than 0.2 and less than 0.9, where .alpha..sub.c is the
coefficient of thermal expansion of the connection, .alpha..sub.i
is the coefficient of thermal expansion of the impeller,
.alpha..sub.s is the coefficient of thermal expansion of the shaft
.
2. (canceled)
3. (canceled)
4. (Canceled)
5. A connected impeller and shaft according to claim 1, wherein the
connector is formed of a material having a greater strength than
the material of the impeller.
6. (canceled)
7. A connected impeller and shaft according to claim 1, wherein the
threaded portion of the connector is within the central recess.
8. A connected impeller and shaft according to claim 1, wherein the
connector and/or the impeller has one or more centring portions
having respective engagement surfaces which engage with one or more
corresponding centring portions of the shaft, the threaded portion
of the connector and the centring portions of the connector and/or
the impeller being distributed along the impeller axis.
9. A connected impeller and shaft according to claim 1, wherein the
impeller has a casing and the connector and/or the hub extension
forms a seal with a section of the casing.
10. A connected impeller and shaft according to claim 1, wherein
the connector is formed with or carries a circumferential oil
thrower formation at its radially outer surface.
11. (canceled)
12. (canceled)
13. A turbocharger having the connected impeller and shaft of claim
1.
Description
CROSS-REFERENCE TO RELATED U.S. APPLICATIONS
[0001] Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT Not applicable.
REFERENCE TO AN APPENDIX SUBMITTED ON COMPACT DISC
[0003] Not applicable.
BACKGROUND OF THE INVENTION
[0004] 1. Field of the Invention
[0005] The present invention relates to a connector for connecting
an impeller to a shaft, and in particular, but not exclusively, for
connecting an impeller of a turbocharger to a turbocharger
shaft.
[0006] 2. Description of Related Art Including Information
Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
[0007] Turbocharger impellers are typically made of aluminium
alloys to provide low rotational inertia with reasonable strength
at a commercially-acceptable cost. Attachment of the impeller to
the steel turbocharger shaft is achieved in various ways. For
example, because of the relative weakness of aluminium and the
small diameter of the shaft, one option is to provide the impeller
with a steel insert containing a screw-threaded socket which can be
threaded on to the shaft. This arrangement can take a higher torque
than a connection in which the shaft is directly threaded into the
aluminium body (the torque is proportional to the power transmitted
across the joint, and so the impeller can be used at a higher
pressure ratio than one in which there is a direct threaded
connection).
[0008] Typically, such an insert is fitted into the impeller by
shrink fitting; the aluminium body of the impeller is heated to
expand the bore which is to receive the steel insert, while the
insert is cooled, for example using liquid nitrogen, before being
inserted into the bore. The resultant interference connection is
restricted by the temperature to which the aluminium can be heated
before its material properties are affected, and by the temperature
to which the steel can be cooled.
[0009] While the arrangement described can perform satisfactorily,
a problem can arise during cycling of the turbocharger from rest to
full load. As the turbocharger starts to spin, the joint is
affected by centrifugal forces, whereby the aluminium grows
outwards away from the steel insert. This reduces the interference
force between the insert and the impeller, and due to design
constraints it has been found that this reduction tends to be
greater at one end of the insert than at the other. Consequently,
the insert is gripped more firmly at one of its ends than at the
other. The turbocharger then starts to heat up, and because of the
different thermal coefficients of expansion of the aluminium alloy
and the steel, the aluminium grows axially more than the steel,
causing the two metals to slide over each other, except at the
location where the impeller still grips the insert firmly. On
shutdown, the centrifugal stresses are removed, but the thermal
stresses remain for some minutes as the turbocharger cools. In this
process, the point of grip of the impeller on the insert changes
from one end to the other, and as the turbocharger cools, the
insert "walks" along the impeller.
[0010] In certain very cyclic conditions (for example fast ferry
applications in high ambient temperatures), it has been observed
that the insert can move so far along the impeller that
turbocharger failure can occur. Although the effect can be
mitigated to some degree by increasing the original interference
between the components, for the reasons mentioned above this
solution is limited, and it is therefore desirable to achieve a
design which ensures that the point of grip remains at the same
location during the operating cycles, rather than shifting from one
end of the insert to the other.
[0011] Accordingly, EP1394387 proposes an outer steel constraining
ring which reinforces the frictional contact between aluminium
impeller and the insert. Since the ring does not expand as much as
the impeller body as the turbocharger heats up, the point of grip
between the impeller and the insert remains within the axial extent
of the ring during the whole operating cycle of the turbocharger,
thereby preventing the tendency of the impeller to "walk" along the
insert. As a consequence, the operating life of the turbocharger
can be considerably extended in comparison with the conventional
turbocharger without the constraining ring.
[0012] However, the assembly of such a joint is relatively complex.
First the insert and impeller bore are manufactured to tight
tolerances. Then typically the insert is cooled and the impeller
heated, and the insert is placed within the impeller bore at a hub
extension of the impeller. As the insert warms up and the impeller
cools, a shrink fit joint is formed, but because of the
non-axisymmetrical shape of the impeller, some distortion occurs
within the impeller. Generally, the outer surface of the impeller
hub extension must therefore be reground to be axisymmetric so that
it will be suitable for the outer joint with the constraining ring.
A further ring may then be shrunk onto a flange portion of the
insert to prevent the constraining ring from coming off the
impeller.
BRIEF SUMMARY OF THE INVENTION
[0013] It would be desirable to provide a connection between an
impeller and a shaft which is simpler to install, but one that can
transmit high torques and can prevent or reduce any tendency of the
impeller to "walk".
[0014] Accordingly, in a first aspect the present invention
provides a connector for connecting an impeller to a shaft, in
particular for connecting an impeller of a turbocharger to a
turbocharger shaft, the impeller having a shaft-side hub extension
with a central recess, and the impeller being formed of a
respective material having a greater coefficient of thermal
expansion than the material of the shaft, wherein: [0015] the
connector is inserted into the recess to frictionally connect an
outwardly facing surface of the connector with a radially inner
surface of the hub extension; [0016] the connector has a threaded
portion carrying a thread which screws onto a corresponding
threaded portion of the shaft, such that the connector provides a
rotationally fixed connection between the impeller and the shaft;
and [0017] the connector is formed of a material having a
coefficient of thermal expansion which is greater than the
coefficient of thermal expansion of the material of the shaft.
[0018] By forming the connector from a material having such a
coefficient of thermal expansion, the differential thermal forces
which encourage the impeller to "walk" can be reduced, thereby
reducing any tendency of the impeller to "walk" while maintaining
the torque capacity of the joint. In addition, regrinding of the
hub extension can also be avoided after fitting of the connector,
as it is usually unnecessary to fit a constraining ring of the type
described in EP1394387 to the hub extension.
[0019] A second aspect of the invention provides an impeller having
a shaft-side hub extension with a central recess and fitted with a
connector according to the first aspect, the connector being
frictionally connected at an outwardly facing surface with a
radially inner surface of the hub extension.
[0020] A third aspect of the invention provides the impeller fitted
with a connector of the second aspect, which impeller is connected
to a shaft having a corresponding threaded section, the thread of
the threaded portion of the connector screwing onto the
corresponding threaded portion of the shaft.
[0021] A fourth aspect of the invention provides a turbocharger
having the connected impeller and shaft of the third aspect.
[0022] Optional features of the invention will now be set out.
These are applicable singly or in any combination with any aspect
of the invention.
[0023] The central recess may be a blind hole (i.e. with an end
surface). Thus, the impeller may not have a through-hole extending
from one side to another of the impeller.
[0024] The outwardly facing surface of the connector may be
approximately cylindrically shaped. The radially inner surface of
the shaft-side hub extension of the impeller which frictionally
connects with the outwardly facing surface may be correspondingly
approximately cylindrical.
[0025] The frictional connection between the connector and the hub
extension can be achieved by e.g. press fitting or shrink fitting.
In particular, as the connector material has a higher coefficient
of thermal expansion than that of a conventional connector, shrink
fitting can be used to produce a tighter interference with the
impeller, while maintaining the temperatures to which the connector
is cooled and the impeller is heated during fitting.
[0026] The connector (which provides the outwardly facing surface
and the threaded portion) can be formed as a unitary body.
[0027] To provide the rotationally fixed connection, the threads
can be positive-locking, e.g. tapered. However, another option is
for the connector to have an abutment surface (e.g. provided by a
flange portion) which engages a corresponding abutment surface
(e.g. provided by a shoulder) of the shaft when the thread portions
are screwed together, thereby tightening the threads to provide the
rotationally fixed connection.
[0028] The threads carried by the threaded portion of the connector
may be protected by a helicoil formation fitted to the connector.
As the material of the connector may be less strong than the
material of the shaft, the helicoil formation can thereby prevent
damage to the threads of the connector.
[0029] The threaded portion of the connector can be within the
central recess. In this way, an axially compact arrangement can be
achieved.
[0030] Preferably, the frictional connection between the outwardly
facing surface of the connector and the radially inner surface of
the hub extension transmits, in use, substantially all of the
torque between the shaft and the impeller.
[0031] The connector may be formed of a material having a greater
strength than the material of the impeller. The connector may be
formed of a material having a lower coefficient of thermal
expansion than the material of the impeller. For example, the shaft
can be formed of steel (e.g. a high strength steel), which
typically has a coefficient of thermal expansion of about
11.times.10.sup.-6/K , and the impeller can be formed of aluminium
alloy, which typically has a coefficient of thermal expansion of
about 22.7.times.10.sup.-6/K. Preferably the connector is formed of
a material that is resistant to galling with the shaft. The
connector can be formed, for example, of magnesium alloy, bronze,
brass or stainless steel. Generally, a value for the coefficient of
thermal expansion of the connector that is equal to or close to
that of the impeller is preferred for reducing the differential
thermal forces which encourage the impeller to "walk". Therefore,
preferably the value of
(.alpha..sub.c-.alpha..sub.s)/(.alpha..sub.i-.alpha..sub.s) is
greater than 0.2, and more preferably greater than 0.3 or 0.4,
where, .alpha..sub.c is the coefficient of thermal expansion of the
connector, .alpha..sub.i is the coefficient of thermal expansion of
the impeller, and .alpha..sub.s is the coefficient of thermal
expansion of the shaft. However, a risk of a coefficient of thermal
expansion of the connector which is much greater than that of the
shaft is that the resultant stretching of the shaft at high
temperatures could lead to shaft breakage. Therefore, at least for
typical materials for the impeller and shaft (such as respectively
aluminium alloy and steel), preferably the value of
(.alpha..sub.c-.alpha..sub.s)/(.alpha..sub.i-.alpha..sub.s) is less
than 0.9, and more preferably less than 0.8 or 0.7. However, this
does not exclude that the value of
(.alpha..sub.c-.alpha..sub.s)/(.alpha..sub.i-.alpha..sub.s) can be
equal to or greater than 1. In particular, if the value of
(.alpha..sub.i-.alpha..sub.s) is reduced, then higher values of
(.alpha..sub.c-.alpha..sub.s)/(.alpha..sub.i-.alpha..sub.s) can be
adopted without risk of shaft breakage. Thus one option is to form
the impeller of a material having a relatively low coefficient of
thermal expansion, such as silicon carbide reinforced aluminium
alloy which, depending on the volume of silicon carbide, typically
has a coefficient of thermal expansion in the range of from 14 to
17.times.10.sup.-6/K. In such cases, a relatively high coefficient
of thermal expansion for the connector not only can reduce any
tendency of the impeller to "walk", but also can assist with the
production of a shrink fitted frictional connection between the
connector and the hub extension.
[0032] The connector and/or the impeller may have one or more
centring portions having respective engagement surfaces which
engage with one or more corresponding centring portions of the
shaft, the threaded portion of the connector and the centring
portions of the connector and/or the impeller being distributed
along the impeller axis. The thread surface of the connector and
the engagement surfaces of the connector and/or the impeller can
face radially inwardly, and the respective diameters on the shaft
of the thread and the engagement surfaces can then decrease towards
the impeller.
[0033] Generally the impeller has a casing, and the connector
and/or the hub extension can then form a seal with a section of the
casing. For example, the seal can include a sealing ring, which may
be carried by the casing section and which may be received by a
corresponding circumferential recess formed on an outer surface of
the connector and/or the hub extension. The sealing ring may have
one or more annular grooves on its radially inner face, and the
recess may have corresponding circumferential ribs which are
received in the grooves. Another option is for the seal to include
a labyrinth seal, with formations on facing surfaces of the casing
section and the connector and/or the hub extension forming the
labyrinth.
[0034] The connector may be formed with or may carry a
circumferential oil thrower formation at its radially outer
surface.
[0035] Further optional features of the invention are set out
below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] Embodiments of the invention will now be described by way of
example with reference to the accompanying drawings in which:
[0037] FIG. 1 is a sectional elevation through a turbocharger
impeller joined to a shaft by a connector in accordance with an
embodiment of the invention;
[0038] FIG. 2 is a close-up schematic view of a seal between a
section of a casing of the impeller of FIG. 1 and a hub extension
of the impeller;
[0039] FIG. 3 is a close-up schematic view of a seal between a
section of a casing of an impeller and a sleeve portion of a
further embodiment of the connector; and
[0040] FIG. 4 shows schematically a sectional elevation of a
further embodiment of the connector.
DETAILED DESCRIPTION OF THE INVENTION
[0041] Referring first to FIG. 1, an aluminium alloy impeller 1 is
fitted on to a steel turbocharger shaft 2 by means of a connector
3. The alloy of which the impeller is made (known in the U.S.A. by
the designation "2618A") has a relatively high strength for use up
to a temperature of about 200.degree. C., having a composition of
aluminium with about 2.5 wt. % copper and smaller amounts of
magnesium, iron and nickel.
[0042] The alloy of the impeller 1 has a coefficient of thermal
expansion of about 22.7.times.10.sup.-6/K, and the steel of the
shaft 2 has a coefficient of thermal expansion of about
11.times.10.sup.-6/K. The material of the connector 3 preferably
has a coefficient of thermal expansion such that the value of
(.alpha..sub.c-.alpha..sub.s)/(.alpha..sub.i-.alpha..sub.s) is
greater than 0.2, and more preferably greater than 0.3 or 0.4. For
example, the connector 3 may be made of magnesium alloy
(coefficient of thermal expansion of about 26.times.10.sup.-6/K),
bronze (coefficient of thermal expansion typically of about
18.times.10.sup.-6/K, although as high as 20-21.times.10.sup.-6/K
for manganese-bronze), brass (coefficient of thermal expansion of
about 18.7.times.10.sup.-6/K) or stainless steel (coefficient of
thermal expansion of in the range of 16-17.3.times.10.sup.-6/K).
Such alloys can also be resistant to galling with the steel of the
shaft 2.
[0043] The connector 3 is of cup-like shape and has an outer
surface 14 for connecting to the impeller 1, a threaded portion 12
with a threaded bore 11 forming the base of the cup, and a flange
portion 8 around the mouth of the cup.
[0044] The shaft 2 is formed at its end with a first shoulder 4
surrounding a cylindrical centring portion 5, and a screw-threaded
portion 7 of further reduced diameter extending from the end of the
centring portion. The connector 3 is inserted into a blind central
recess formed in the hub extension H, with the outer surface 14 of
the connector 3 frictionally connected to the radially inner
surface of the hub extension H. The flange portion 8 of the
connector 3 engages against a shaft-side end face 9 of the hub
extension H to determine the relative axial positions of the
connector 3 and the hub extension H. The flange portion 8 is
engaged on its other side by the shoulder 4 on the shaft 2. The
centring portion 5 of the shaft is received in a corresponding
centring portion 10 of the connector in a close, but not tight,
fit. The threaded bore 11 engages on the screw-threaded portion 7
of the shaft. The threaded portion 12 has a small clearance from
the end of the recess.
[0045] The connector 3 is fitted on to the hub extension H by
cooling the connector 3 to cause it to shrink and by heating the
impeller to cause the hub extension H to expand, and then inserting
the connector 3 into the central recess of the hub extension H
until the flange portion 8 contacts the end face 9 of the hub
extension H. On returning from their thermal excursions, the
connector 3 and hub extension H frictionally grip across the outer
surface 14 of the connector 3 and the radially inner surface of the
hub extension H. The outer surface 14 extends over and thereby
frictionally contacts most of the axial length of the hub extension
H.
[0046] The outer diameter of the flange portion 8 is provided with
an oil capture/thrower ring R, which in this embodiment of the
invention is machined into the flange portion 8. Another option,
however, is to form the ring R as a separate component.
[0047] As shown better in FIG. 2, a section 15 of the impeller
casing and the outer surface of the hub extension H are in close
proximity to help provide a rotating oil and pressure seal between
the impeller 1 and the casing. To improve the seal, the hub
extension H has a recess 13 on its outer surface which is bounded
at one end by the flange portion 8 of the first component of the
connector and which receives a sealing ring 16 carried by the
casing section 15. To reduce wear between the sealing ring 16 and
the hub extension H, the casing section 15 has a small abutment
surface 20 on the shaft side (right hand in FIG. 1) of the seal
ring 16 and against which the sealing ring 16 rests. To provide
enhanced sealing, the sealing ring 16 has annular grooves 18 on its
radially inner face, and the recess has corresponding
circumferential ribs 17 which are received in the grooves, as
described in EP A 1130220. Alternatively, however, the sealing ring
can be a plain ring (i.e. without grooves) received in a plain
recess (i.e. without ribs). The sealing ring 16 co-operates with
the casing section 15 and serves to retain lubricating oil to the
shaft side of the assembly and compressed air to the impeller side
of the assembly (left hand in FIG. 1). The compressed air is
contained between the body of the impeller 1, the hub extension H
with its sealing ring 16, and the impeller casing, within which the
impeller assembly is mounted for rotation on overhung bearings (not
shown).
[0048] After the connector 3 is fitted on to the hub extension H,
the screw-threaded portion 7 of the shaft 2 is screwed onto the
threaded portion 12 of the connector 3, the respective centring
portions 5, 10 ensuring the shaft aligns with the axis of the
impeller. The threads are screwed until opposing surfaces of the
flange portion 8 and shoulder 4 come into abutment, which causes
the threads to tighten and provides a rotationally fixed connection
between the impeller 1 and the shaft 2.
[0049] Advantageously, by forming the connector 3 from a material
having an intermediate coefficient of thermal expansion, the
differential thermal forces acting across the frictional connection
between the connector 3 and the impeller 1 can be reduced relative
to a connector formed from the a material having the same
coefficient of thermal expansion as that of the shaft. In this way,
the tendency for the impeller to "walk" can also be reduced, which
allows the impeller to be driven by a higher torque and therefore
increases the maximum pressure ratio of the impeller. In addition,
by containing the threaded connection between the connector 3 and
the shaft 2 in the central recess of the hub extension H, an
axially compact arrangement is achieved. The frictional connection
between the connector 3 and the impeller transmits, in use,
substantially all of the torque between the shaft 2 and the
impeller 1. Further, as there is no need to fit a constraining ring
of the type described in EP1394387 to the hub extension H,
regrinding operations can be avoided during fitting of the
connector 3.
[0050] If there is any tendency for the impeller 1 to "walk",
advantageously this can be monitored by measuring the size of the
gap that would open up between the flange portion 8 and the end
face 9. For this reason, it is preferred that the flange portion 8
and the end face 9 determine the relative axial positions of the
connector 3 and the hub extension H. Alternative pairs of facing
features that could be configured to abut each and thereby
determine the relative axial positions (such as the threaded
portion 12 and the end of the recess) are less amenable to
inspection.
[0051] FIG. 3 is a close-up schematic view of a seal between a
section of a casing of an impeller and the flange portion 8 of a
further embodiment of the connector 3. In this case, instead of a
seal formed by a sealing ring, the hub extension H and flange
portion 8 on one side and the casing section 15 on the other side
have engaging surfaces 19 carrying respective sets of machined
grooves which interlock to form a labyrinth seal.
[0052] FIG. 4 shows schematically a sectional elevation of a
further embodiment of the connector. This embodiment is similar to
the embodiment of FIG. 1 except that the shaft 2 has two centring
portions 5a, 5b, and the connector has two corresponding centring
portions 10a, 10b. The threaded portions 7, 12 of the shaft 2 and
the connector are located axially between the engaging pairs of
centring portions such that, on each of the shaft and the
connector, the respective diameters of the threaded portions and
the centring portions decrease towards the impeller. A further
difference relative to the embodiment of FIG. 1 is that the threads
are tapered, so that merely screwing the threaded portions 7, 12
together results in a rotationally fixed connection between the
impeller 1 and the shaft 2.
[0053] While the invention has been described in conjunction with
the exemplary embodiments described above, many equivalent
modifications and variations will be apparent to those skilled in
the art when given this disclosure. For example, in embodiments
such as that of FIG. 4 in which the shaft has a centring portion 5b
which is at the base of the recess, instead of the connector having
a centring portion 10b, the impeller may have a centring portion at
the base of the recess that engages with the centring portion 5b of
the shaft. In another example, the threads carried by the threaded
portion 12 of the connector 3 may be protected by a helicoil
formation to prevent damage to the threads of the connector 3 from
the stronger material of the shaft 1. Accordingly, the exemplary
embodiments of the invention set forth above are considered to be
illustrative and not limiting. Various changes to the described
embodiments may be made without departing from the spirit and scope
of the invention.
[0054] All references referred to above are hereby incorporated by
reference.
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