EP2096318A1

EP2096318A1 - Impeller for multi-blade fan

Info

Publication number: EP2096318A1
Application number: EP07831608A
Authority: EP
Inventors: Masahito Higashida
Original assignee: Daikin Industries Ltd
Current assignee: Daikin Industries Ltd
Priority date: 2006-11-14
Filing date: 2007-11-12
Publication date: 2009-09-02
Anticipated expiration: 2027-11-12
Also published as: EP2096318A4; WO2008059775A1; EP2096318B1; JP2008121611A

Abstract

An impeller of a multi-blade fan, wherein any gasflow resistance increase is minimized when gas is blown from inside the impeller through an outer peripheral side thereof; and it is possible to integrally mold the impeller from a plastic material, allowing the impeller to rotate at high speed. The impeller (1) of a multi-blade fan is an impeller of a multi-blade fan for sucking in gas along a direction of a rotational shaft (3), and blowing out gas toward a direction intersecting the rotational shaft (3); and comprises a substantially circular endplate (11) that rotates about the rotational shaft (3), a plurality of blades (12), and one or more intermediate rings (14). The plurality of blades (12) is annularly disposed around the rotational shaft (3) on one or both sides of the endplate (11), one end of each of the blades being provided to an outer peripheral portion of the endplate (11). The intermediate ring (14) is formed in a position other than an end of the plurality of blades (12) on a side opposite to the endplate (11), the intermediate ring being formed so as not to overlap the plurality of blades (12) as seen from the direction of the rotational shaft, and so as to link together outer peripheral edges of each of the blades (12).

Description

TECHNICAL FIELD

The present invention relates to an impeller of a multi-blade fan, and particularly to an impeller of a multi-blade fan for sucking in gas along a direction of a rotational shaft, and blowing out gas toward a direction intersecting the rotational shaft.

BACKGROUND ART

Multi-blade fans for sucking in gas along a direction of a rotational shaft and blowing out gas toward a direction intersecting the rotational shaft are often used in air purifiers, air conditioners, and other similar devices in order to blow gas. An impeller in such multi-blade fans mainly comprises a substantially circular endplate that rotates about the rotational shaft; a plurality of blades annularly disposed on the endplate around the rotational shaft, one end of each blade being provided to an outer peripheral portion of the main plate; and an end ring for linking together the other ends of the plurality of blades.
In another impeller associated with such a multi-blade fan, an intermediate ring is disposed between the endplate and the end ring in the direction of the rotational shaft (see Patent Document 1).

< Patent Document 1>

Japanese Laid-open Patent Publication No. 2000-320490

DISCLOSURE OF THE INVENTION

The above-described impellers of the multi-blade fan are desirably caused to rotate at high speed in order for the impeller to be made smaller, to have gas-blowing performance improvement, and to exhibit other advantageous effects.
However, when the former of the above-described impellers of the multi-blade fan is caused to rotate at high speed, the centrifugal force acting on the impeller increases, the concentricity of the impeller is compromised, and the degree of imbalance increases; therefore, a risk is presented that the impeller will be significantly deformed or damaged.
By contrast, since the latter of the above-described impellers of the multi-blade fan is provided with an intermediate ring, the impeller undergoes less deformation from centrifugal force than the first impeller, and is able to rotate at high speeds. However, the intermediate ring provided in this impeller is positioned through the inner peripheral edges of the blades from the outer peripheral edges of the blades; therefore, a problem of gasflow resistance increase occurs when gas is blown from inside the impeller through an outer peripheral side thereof. Moreover, in a case where the impeller is manufactured as a plastic product, the fact that the intermediate ring is provided through the inner peripheral edges of the blades from the outer peripheral edges of the blades complicates molding the impeller as an integrated plastic product. As with an impeller of a crossflow fan, it may be necessary to perform resin molding with the impeller divided at the location of the intermediate ring into a part nearer the end ring in the direction of the rotational shaft and a part nearer the endplate in the direction of the rotational shaft, and join the plurality of parts together by ultrasonic welding or the like to manufacture the impeller. A problem is accordingly presented in that the impeller becomes time-consuming to manufacture.
An object of the present invention is to provide an impeller of a multi-blade fan, wherein gasflow resistance increase is minimized when gas is blown from inside the impeller toward an outer peripheral side thereof, and wherein the impeller can be integrally molded from a plastic material, allowing the impeller to rotate at high speed.
An impeller of a multi-blade fan according to a first aspect of the present invention is an impeller of a multi-blade fan for sucking in gas along a direction of a rotational shaft, and blowing out gas toward a direction intersecting the rotational shaft; and comprises a substantially circular endplate that rotates about the rotational shaft, a plurality of blades, and one or more intermediate rings. The plurality of blades is annularly disposed around the rotational shaft on one or both sides of the endplate, and one end of each of the blades is provided to an outer peripheral portion of the endplate. The intermediate ring is formed in a position other than an end of the plurality of blades on a side opposite to the endplate, so as not to overlap the plurality of blades as seen from the direction of the rotational shaft, and so as to link together outer peripheral edges of each of the blades.
In the impeller of a multi-blade fan according to the first aspect, the intermediate ring, which is provided in a position other than the end of the plurality of blades on the side opposite to the endplate, is formed so as not to overlap the plurality of blades as seen from the direction of the rotational shaft, and so as to link together the outer peripheral edges of each of the blades. As a result, any gasflow resistance increase when gas is blown from inside the impeller through an outer peripheral side thereof can be minimized, the entirety of the impeller including the intermediate ring can be molded into an integrated plastic product, and high-speed rotation can be achieved.
An impeller of a multi-blade fan according to a second aspect of the present invention is the impeller of a multi-blade fan according to the first aspect of the present invention,
wherein when a length between an end of the blade on the endplate and the end of the blade on the side opposite to the endplate is a blade length, at least a portion of the intermediate ring is formed so as to be provided in an area between a position corresponding to a length of 1/4 times of the blade length from the end of the blade on the endplate toward the end of the blade on the side opposite to the endplate and a position corresponding to a length of 3/4 times of the blade length from the end of the blade on the endplate toward the end of the blade on the side opposite to the endplate.
In the impeller of a multi-blade fan according to the second aspect, deformation of the impeller can be effectively minimized because at least a portion of the intermediate ring is formed so as to be provided in the area between the position corresponding to the length of 1/4 times of the blade length from the end of the blade on the endplate toward the end of the blade on the side opposite to the endplate and the position corresponding to the length of 3/4 times of the blade length from the end of the blade on the endplate toward the end of the blade on the side opposite to the endplate..
An impeller of a multi-blade fan according to a third aspect of the present invention is the impeller of a multi-blade fan according to the second aspect of the present invention, wherein two intermediate rings are formed between the end of the blade on the endplate and the end on the side opposite to the endplate, and a pitch between the intermediate rings is set in a range between one- to three-fourth of the blade length.
In the impeller of a multi-blade fan according to the third aspect, deformation of the impeller can be effectively minimized, because when two intermediate rings are provided between the end of the blade on the endplate and the end on the side opposite to the endplate, the pitch between the intermediate rings is set in a range between one- to three-fourth of the blade length.
An impeller of a multi-blade fan according to a fourth aspect of the present invention is the impeller of a multi-blade fan according to any of the first to third aspects of the present invention, wherein a cross-section of the intermediate ring is substantially rectangular in shape, a length of the intermediate ring in the direction of the rotational shaft being one-half or more of a length of protrusion from the outer peripheral edge of the blade.
In the impeller of a multi-blade fan according to the fourth aspect, deformation of the impeller can be effectively minimized because the cross-section of the intermediate ring is substantially rectangular in shape, the length of the intermediate ring in the direction of the rotational shaft being one-half or more of the length of protrusion from the outer peripheral edge of the blade; and the intermediate ring is imparted with sufficient thickness in the direction of the rotational shaft.
An impeller of a multi-blade fan according to a fifth aspect of the present invention is the impeller of a multi-blade fan according to any of the first to third aspects of the present invention, wherein a cross-section of the intermediate ring is substantially "L"-shaped.
In the impeller of a multi-blade fan according to the fifth aspect, deformation of the impeller can be effectively minimized because the cross-section of the intermediate ring is substantially "L"-shaped, and the intermediate ring is imparted with sufficient thickness in the direction of the rotational shaft.
An impeller of a multi-blade fan according to a sixth aspect of the intermediate ring is the impeller of a multi-blade fan according to any of the first to third aspects of the intermediate ring, wherein a cross-section of the intermediate ring is substantially trapezoidal in shape.
In the impeller of a multi-blade fan according to the sixth aspect, deformation of the impeller can be effectively minimized because the cross-section of the intermediate ring is substantially trapezoidal in shape, and the intermediate ring is imparted with sufficient thickness in the direction of the rotational shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a radial cross-sectional view of an impeller of a multi-blade fan according to one embodiment of the present invention.
FIG. 2 is a radial cross-sectional view of the impeller of a multi-blade fan, and a view showing the shapes of dies for parts corresponding to this cross-sectional view.
FIG 3 is a plan view of the impeller of a multi-blade fan, and a view showing the shapes of dies for parts corresponding to this plan view.
FIG 4 is a radial cross-sectional view of an impeller of a multi-blade fan according to Modification example 1.
FIG. 5 is a radial cross-sectional view of an impeller of a multi-blade fan according to Modification example 2, showing the immediate vicinity of the intermediate ring.
FIG. 6 is a radial cross-sectional view of an impeller of a multi-blade fan according to Modification example 2, showing the immediate vicinity of the intermediate ring.
FIG. 7 is a radial cross-sectional view of an impeller of a multi-blade fan according to another embodiment.

EXPLANATION OF THE REFERENCE NUMERALS

1, 101: Impeller of multi-blade fan
11: Endplate
12: Blade
14, 15: Intermediate ring

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of an impeller of a multi-blade fan according to the present invention are described below with reference to the accompanying drawings.

(1) Structure of impeller of multi-blade fan

FIG. 1 shows an impeller 1 of a multi-blade fan according to one embodiment of the present invention. FIG. 1 is a radial cross-sectional view of the impeller 1 of a multi-blade fan.
The impeller 1 is an impeller of a double-suction fan for sucking in gas from both side along a direction of a rotational shaft, and blowing out gas in a direction intersecting the rotational shaft; the impeller 1 being rotatably driven by a motor 2. Line O-O in the drawing is the axis of rotation of the impeller 1 and the motor 2. The impeller 1 is a plastic product integrally molded using dies, as will be discussed hereinbelow; and mainly comprises an endplate 11, a plurality of blades 12, an end ring 13, and one or more intermediate rings 14.
The endplate 11 is a substantially circular part that rotates about a rotational shaft 3 of the motor 2. A central hole 11a to which the rotational shaft 3 is linked is formed in the endplate 11.
The plurality of blades 12 is annularly disposed around the rotational shaft 3 on both surfaces on the endplate 11. One end of each of the blades is disposed on an outer peripheral portion of the endplate 11, and the blades extend untwisted to both side in the direction of the rotational shaft.
The end ring 13 is positioned on the other end of the blade (i.e., the end on the side opposite to the endplate 11), and is an annular part that links together the outer peripheral edges of the ends of each of the blades 12 on the side opposite to the endplate 11. In the present embodiment, end rings 13 are disposed on both sides of the endplate 11 in the direction of the rotational shaft. The end ring 13 is formed so as not to overlap the plurality of blades 12 as seen from the direction of the rotational shaft.
An intermediate ring 14 is an annular part formed in a position other than the ends of the plurality of blades 12 on the side opposite to the endplate 11, so as not to overlap the plurality of blades 12 as seen from the direction of the rotational shaft, and so as to link together the outer peripheral edges of the blades 12. In the present embodiment, intermediate rings 14 are disposed on both sides of the endplate 11 in the direction of the rotational shaft. When a length between an end of the blades 12 on the endplate 11 and the end of the blades 12 on the side opposite to the endplate 11 is a blade length L1, at least a portion of each of the intermediate rings 14 is formed so as to be provided in the area between the position corresponding to the length of 1/4 times of the blade length L1 from the end of the blades 12 on the endplate 11 toward the end of the blades 12 on the side opposite to the endplate 11 and the position corresponding to the length of 3/4 times of the blade length L1 from the end of the blades 12 on the endplate 11 toward the end of the blades 12 on the side opposite to the endplate 11. The cross-section of each intermediate ring 14 is substantially rectangular in shape, a length L2 of the intermediate ring 14 in the direction of the rotational shaft being one-half or more of a length W of protrusion from the outer peripheral edge of the blades 12. In the present embodiment, each intermediate ring 14 is formed so as to be provided at the position corresponding to the length of substantially half of the blade length L1 from the end of the blades 12 on the endplate 11 toward the end of the blades 12 on the side opposite to the endplate 11 The intermediate ring 14 is substantially rectangular (i.e., substantially square) in cross-section, the length L2 in the direction of the rotational shaft being substantially the same as the protrusion length W.

(2) Molding impeller of multi-blade fan

Molding the impeller 1 of a multi-blade fan according to the present embodiment will now be described with reference to FIGS. 2 and 3. FIG. 2 shows a radial crows-sectional view of the impeller 1 of a multi-blade fan, as well as the shape of dies 4, 5, 6, 7 for the parts corresponding to this cross-sectional view. FIG. 3 shows a plan view of the impeller 1 of a multi-blade fan, as well as the shape of dies 4, 6, 7 for the parts corresponding to this plan view.
The impeller 1 is integrally molded from plastic material and formed using two pairs of dies 4, 5 and 6, 7.
When the dies 4, 5 are fitted together in the direction of the rotational shaft, an endplate-forming part 4a of the die 4 and an endplate-forming part 5a of the die 5 form the endplate 11 including the central hole 11 a (exclusive of the outer peripheral edges in the radial direction); blade-forming parts 4b, 5b of the dies 4, 5 form the blade 12 (exclusive of the outer peripheral edge in the radial direction); and outer peripheral edge-forming parts 4c, 5c (part 5c is not shown in the drawing) of the dies 4, 5 form an inner peripheral edge of the end ring 13 in the radial direction and an inner peripheral edge of the intermediate ring 14 in the radial direction.
When the dies 6, 7 are fitted together in a direction perpendicular to the rotational axis line O-O (i.e., the radial direction), outer peripheral edge-forming parts 6a, 7a form the outer peripheral edge of the endplate 11 in the radial direction, and the outer peripheral edge of the blade 12 in the radial direction. End ring-forming parts 6b, 7b form the end ring 13 (exclusive of the inner peripheral edge of the end ring 13 in the radial direction); and intermediate ring-forming parts 6c, 7c form the intermediate ring 14 (exclusive of the inner peripheral edge of the intermediate ring 14 in the radial direction).
Removing the dies 4, 5 in the direction of the rotational shaft and removing the dies 6, 7 in the radial direction thus allows the impeller 1 of a multi-blade fan according to the present embodiment to be molded as an integrated plastic product.

(3) Characteristics of the impeller of a multi-blade fan according to the present embodiment

The impeller 1 of a multi-blade fan according to the present embodiment has the following characteristics.

(A)

In the impeller 1 of a multi-blade fan according to the present embodiment, the intermediate ring 14, which is provided in the position other than the ends of the plurality of blades 12 on the side opposite to the endplate 11, is formed so as not to overlap the plurality of blades 12 as seen from the direction of the rotational shaft, and so as to link together the outer peripheral edges of each of the blades 12. As a result, when the gasflow F (see FIG. 1) that flows into the impeller 1 from the side opposite to the endplate 11 in the direction of the rotational shaft of the impeller 1 is blown from inside the impeller 1 toward the outer peripheral side thereof, any gasflow resistance increase can be reduced by the intermediate ring 14 more compared to a conventional intermediate ring being used. In addition, the entirety of the impeller 1 including the intermediate ring 14 can be molded into an integrated plastic product.
Any gasflow resistance increase when gas is blown from inside the impeller 1 toward the outer peripheral side thereof can thus be minimized in the impeller 1 of a multi-blade fan according to the present embodiment, the entirety of the impeller 1 including the intermediate ring 14 can be molded into an integrated plastic product, and high-speed rotation can be achieved.

(B)

In the impeller 1 of a multi-blade fan according to the present embodiment, deformation of the impeller 1 can be effectively minimized because at least a portion of the intermediate rings 14 is formed so as to be provided in the area between the position corresponding to the length of 1/4 times of the blade length L1 from the end of the blade 12 on the endplate 11 toward the end of the blade 12 on the side opposite to the endplate 11 and the position corresponding to the length of 3/4 times of the blade length L1 from the end of the blade 12 on the endplate 11 toward the end of the blade 12 on the side opposite to the endplate 11.

(C)

In the impeller 1 of a multi-blade fan according to the present embodiment, deformation of the impeller 1 can be effectively minimized because the cross-section of the intermediate ring 14 is substantially rectangular in shape, the length L2 in the direction of the rotational shaft being one-half or more of a length W of protrusion from the outer peripheral edge of the blade 12; and the intermediate ring 14 is imparted with sufficient thickness in the direction of the rotational shaft.

(4) Modification example 1

In the abovedescribed embodiment, one intermediate ring 14 is formed between the end of the blade 12 on the endplate 11 and the end of the blade 12 on the side opposite the endplate 11; however, a plurality of intermediate rings may be formed, and these may be configured so as to reduce the degree to which the impeller is deformed by centrifugal force.
For example, as shown in FIG. 4, two intermediate rings 15 can be formed between the end of the blade 12 on the endplate 11 and the end of the blade 12 on the side opposite to the endplate 11. In this case also, at least a portion of each of the intermediate rings 15 is desirably formed so as to be provided in the area between the position corresponding to the length of 1/4 times of the blade length L1 from the end of the blades 12 on the endplate 11 toward the end of the blades 12 on the side opposite to the endplate 11 and the position corresponding to the length of 3/4 times of the blade length L1 from the end of the blades 12 on the endplate 11 toward the end of the blades 12 on the side opposite to the endplate 11, making it possible for deformation of the impeller 1 to be effectively minimized in the same manner as in the abovedescribed embodiment. A pitch P between the two intermediate rings 15 formed between the end of the blade 12 on the endplate 11 and the end of the blade 12 on the side opposite to the endplate 11 is set in a range between one- to three-fourths of the blade length L1, whereby deformation of the impeller 1 can be effectively minimized.
The impeller 1 of a multi-blade fan according to the present modification example may also be provided with intermediate ring-forming parts on the dies that are removed toward the radial direction (dies 6, 7 shown in FIGS. 2 and 3) in accordance with the location where the intermediate ring 15 is formed. As a result, the entirety of the impeller 1 can be molded into an integrated plastic product, as with the abovedescribed embodiment.

(5) Modification example 2

In the abovedescribed embodiment and Modification example 1, the intermediate rings 14, 15 are substantially rectangular in cross-section; however, it is possible to use an intermediate ring having any of a variety of cross-sectional shapes as long as the cross-sectional shape is not difficult to remove the die (dies 6, 7 shown in FIGS. 2 and 3) toward the radial direction when the impeller is molded into an integrated plastic product.
For example, as shown in FIG. 5, the intermediate rings 14, 15 may be substantially "L"-shaped in cross-section so as to be imparted with adequate thickness in the direction of the rotational shaft. In this case, the intermediate rings 14, 15 have parts 14a, 15a extending in the direction of the rotational shaft, and parts 14b, 15b protruding toward the outer periphery in the radial direction to a greater degree than the parts 14a, 15a. However, the parts 14b, 15b are desirably formed so as to be disposed on the side nearer the endplate 11 in order that the gasflow F can flow through the impeller 1 with minimal obstruction.
As shown in FIG. 6, the intermediate rings 14, 15 may also be substantially trapezoidal in cross-section, so as to be imparted with adequate thickness in the direction of the rotational shaft. In this case, the intermediate rings 14, 15 are formed so that the thickness in the direction of the rotational shaft decreases outward from the outer periphery of the blade 12 along the radial direction, as shown in FIG. 6. Moreover, the intermediate rings 14, 15 are desirably formed so that the surface farther from the endplate 11 inclines toward the end ring 13 outward from the outer periphery of the blade 12 along the radial direction, so that the gasflow F can flow through the impeller 1 with minimal obstruction.

(6) Other embodiments

An embodiment and modification examples of the present invention have been described above with reference to the accompanying drawings; however, the specific structure is not limited to this embodiment and modification examples, and variations may be possible as long as there is no departure from the scope of the present invention.

(A)

There has been provided in the abovedescribed embodiment and modification examples a description of the present invention applied to an impeller of a double-suction multi-blade fan; however, the present invention may also be applied to an impeller of a single-suction multi-blade fan.
For example, FIG 7 shows an impeller 101 of a multi-blade fan rotatably driven by a motor 2 and comprising an endplate 11, a plurality of blades 12, and an end ring 13, wherein an intermediate ring 14 similar to that in the abovedescribed embodiment is formed between the endplate 11 and the end ring 13 in the direction of the rotational shaft. As a result, it is possible to establish a structure in which one intermediate ring 14 is formed between the end of the blade 12 on the endplate 11 and the end of the blade 12 on the side opposite to the endplate 11, as with the abovedescribed embodiment.
In the impeller 101 of the present embodiment, it is possible to establish not only a structure in which one intermediate ring 14 is formed, but a structure in which there are two intermediate rings 15 formed between the end of the blade 12 on the endplate 11 and the end of the blade 12 on the side opposite to the endplate 11, as with the above-mentioned Modification example 1; or a structure in which an "L"-shaped or trapezoidal intermediate ring 14, 15 is formed between the end of the blade 12 on the endplate 11 and the end of the blade 12 on the side opposite to the endplate 11, as with the above-mentioned Modification example 2.
Since one end of each of a plurality of the blades 12 is provided to only one surface of the endplate 11, the specific shape of the dies will be different from that in the abovedescribed embodiment and modification examples. However, as with the abovedescribed embodiment and modification examples, in the impeller 101 of a multi-blade fan according to the present embodiment, the entirety of the impeller 101 including the intermediate ring 14 can also be molded into an integrated plastic product using dies that are removed in the direction of the rotational shaft and dies that are removed toward the radial direction.

(B)

In the abovedescribed embodiment and modification examples, the intermediate ring 14, 15 are provided to a structure in which one end of each of a plurality of the blades 12 is provided to an endplate 11, and the other ends of each of the blades 12 are linked together by an end ring 13. However, this structure is not provided by way of limitation; the intermediate rings 14, 15 may also be provided to a structure in which the other end of each of the blades 12 is not linked together by the end ring 13.

INDUSTRIAL APPLICABILITY

Using the present invention as an impeller of a multi-blade fan allows any gasflow resistance increase to be minimized when gas is blown from inside the impeller through an outer peripheral side thereof. Moreover, having the impeller capable of being integrally molded from a plastic material allows it to rotate at high speed.

Claims

An impeller (1, 101) of a multi-blade fan is an impeller of a multi-blade fan for sucking in gas along a direction of a rotational shaft, and blowing out gas in a direction intersecting the rotational shaft,
the impeller of a multi-blade fan comprising:
a substantially circular endplate (11) that rotates about the rotational shaft;

a plurality of blades (12) annularly disposed around the rotational shaft on one or both sides of the endplate, one end of each of the blades being provided to an outer peripheral portion of the endplate; and

one or more intermediate rings (14, 15) formed in a position other than an end of the plurality of blades on a side opposite to the endplate, the intermediate ring being formed so as not to overlap the plurality of blades as seen from the direction of the rotational shaft, and so as to link together outer peripheral edges of each of the blades.
The impeller (1, 101) of a multi-blade fan according to claim 1, wherein when a length between an end of the blade (12) on the endplate and the end of the blade on the side opposite to the endplate is a blade length (L1), at least a portion of the intermediate ring (14, 15) is formed so as to be provided in an area between a position corresponding to a length of 1/4 times of the blade length from the end of the blade on the endplate toward the end of the blade on the side opposite to the endplate and a position corresponding to a length of 3/4 times of the blade length from the end of the blade on the endplate toward the end of the blade on the side opposite to the endplate.
The impeller (1, 101) of a multi-blade fan according to claim 2, wherein two intermediate rings (14, 15) are formed between the end of the blade (12) on the endplate and the end on the side opposite to the endplate; and
a pitch (P) between the intermediate rings is set in a range between one- to three-fourth of the blade length (L1).
The impeller (1, 101) of a multi-blade fan according to any of claims 1 to 3, wherein a cross-section of the intermediate ring (14, 15) is substantially rectangular in shape, a length (L2) of the intermediate ring in the direction of the rotational shaft being one-half or more of a length (W) of protrusion from the outer peripheral edge of the blade (12).
The impeller (1, 101) of a multi-blade fan according to any of claims 1 to 3, wherein a cross-section of the intermediate ring (14, 15) is substantially "L"-shaped.
The impeller (1, 101) of a multi-blade fan according to any of claims 1 to 3, wherein a cross-section of the intermediate ring (14, 15) is substantially trapezoidal in shape.