GB2367596A

GB2367596A - Fan rotor construction

Info

Publication number: GB2367596A
Application number: GB0024566A
Authority: GB
Inventors: Henry Riddoch
Original assignee: NMB; Minebea Co Ltd
Current assignee: NMB; Minebea Co Ltd
Priority date: 2000-10-06
Filing date: 2000-10-06
Publication date: 2002-04-10
Also published as: AU2001286108A1; GB0024566D0; WO2002029253A1

Abstract

A fan sub-assembly comprises a hub 1 and a number of blade assemblies 7. The hub 1 has a number of blade attachment sites which may comprise dovetail slots 6 formed by grooves 5 in elongate strips 4. Each blade assembly 7 includes an attachment plate 8 which may have protruding tongues 10 forming a dovetail plate, and may be attached to the hub 1 by sliding the attachment plate 8 into a slot 6 until an edge of the plate 8 abuts a lip 2a of the hub 1. The attachment plates 8 may have inner surfaces 9 which are contoured to match the outer surface 3 of the hub 1, and each plate 8 may be interference fit into a slot 6. Blades 12 of the blade assemblies 7 may overlap when the fan is assembled ( see figure 4).

Description

"A FAN SUB-ASSEMBLY" THIS INVENTION relates to fans, and in particular to small, axial cooling fan sub-assemblies, such fans having diameters of approximately 100 to 150mm.

A conventional method of manufacturing cooling fans employs a two-part open and shut mould tool to manufacture a hub of the fan and the blades thereof as a unitary item-the so-called fan sub-assembly. Once the sub-assembly has been manufactured, it may be combined with a motor and a housing, which substantially surrounds the fan sub-assembly to aid mounting and prevent damage, to form a complete fan. In use, the two parts of the mould, when placed together, define a space of the same shape as the fan sub-assembly to be manufactured. The space between the two parts of the mould is filled with a suitable molten material which is allowed to cool and harden. Once cooling is complete, one part of the mould is lifted away from the other, and the fan subassembly formed thereby may be removed.

Although this form of open and shut two-part moulding tool is efficient and widely-used, fan sub-assemblies formed thereby suffer from drawbacks, as will be explained below.

Due to the fact that, once the cooling of a moulded article is completed, the two parts of the mould tool must be linearly removable from one another, there are many shapes that it is not possible to mould by this method. For instance, once the cooling step is complete, all parts of each half of the mould tool must be presented with a clear, unobstructed path, along which to be moved away from the moulded article. This requirement clearly limits the range of shapes that can be formed by the two-part moulding method.

For this reason, a fan sub-assembly manufactured by the above-described twopart moulding method must necessarily include a space, when viewed along the line of removal of the two halves of the mould, between each of the blades thereof. Conventionally, such fan sub-assemblies are formed with a substantially cylindrical hub with a plurality of blades extending radially outwardly therefrom, and are manufactured such that the two halves of the mould tool used to form a fan sub-assembly are separated from one another in a direction parallel to the axis of rotation of the fan i. e. parallel with the axis of symmetry of the cylindrical hub. If the blades overlap one another, then the two halves of the mould tool used to manufacture the fan sub-assembly cannot be removed from the fan sub-assembly once moulding thereof is complete.

The fact that such gaps must exist between the blades of a conventional moulded fan sub-assemblies presents aerodynamic drawbacks and limits the performance and efficiency of the resultant fan. Firstly, the chord length of each blade (i. e. the linear distance between the leading and trailing edges thereof) is limited, due to the fact that the radial space that each blade may occupy around the hub is limited to less than 21tin radians, where n is the number of blades. In addition, due to the fact that relatively little space on the outer surface of the hub is afforded to each blade, the angle of attack of each blade (i. e. the angle between the plane of the blade and the direction of linear movement of the blade) must be relatively large, as there is insufficient space on the rim of the hub for each blade to have a shallow angle of attack. The short chord length and steep angle of attack of the blades of conventional moulded fan sub-assemblies mean that, at low rotational speeds and high pressures, the fan is prone to stalling. Stalling occurs when the flow of air over each blade has a strong adverse pressure gradient (i. e. the pressure is increasing rapidly in the direction of air flow). If the pressure gradient is sufficiently strong, the velocity of air near the surface of the blade will fall below zero, and subsequently be reversed in direction. When this occurs, a large region of "dead air"surrounds the blade, leading to a drastic fall in the efficiency and effectiveness thereof. A blade with a short chord length and steep angle of attack is, as described above, more prone to stalling than one with a longer chord length and shallower angle of attack.

It is an object of the present invention to seek to provide a small, axial cooling fan sub-assembly that alleviates some or all of the above drawbacks.

Accordingly, one aspect of the present invention provides an axial cooling fan sub-assembly, comprising: a hub having a main body and a plurality of blade attachment sites, each blade attachment site being located on an outer surface of the main body and being shaped to receive and retain a blade; and a plurality of blades, each blade having an attachment portion shaped to be received in a blade attachment site of the hub.

A further aspect of the present invention provides a method of constructing a small, axial cooling fan sub-assembly, comprising the steps of : providing a hub having a main body and a plurality of blade attachment sites, each blade attachment site being located on an outer surface of the main body and being shaped to receive and retain a blade; providing at least one blade comprising an attachment portion and a blade portion ; and attaching the at least one blade to the hub.

In order that the present invention may be more readily understood, embodiments thereof will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a perspective view of a hub embodying the present invention; Figure 2 is a perspective view of a blade embodying the present invention; Figure 3 is a perspective view of a fan sub-assembly embodying the present invention; Figure 4 is a plan view of a fan sub-assembly embodying the present invention; Figure 5 is a plan view of a fan manufactured with a two part mould; and Figure 6 is a schematic side view comparing the blade profiles of the fan of Figure 5 and the fan of Figure 3.

Turning firstly to Figure 1, a hub 1 for use with a fan sub-assembly embodying the present invention is substantially cylindrical in shape. One of the ends of the cylindrical hub 1 is closed, forming a circular upper surface 2 of the hub 1.

The other end of the hub 1 is open, the hub 1 being substantially hollow. The upper surface 2 of the hub 1 has a slightly greater radius than the curved outer surface 3 and therefore protrudes slightly over the edge thereof to form a lip 2a.

The interior of the hub 1 is dimensioned to receive the rotor cup of an externalrotor brushless DC motor (not shown). Elongate strips 4 are provided at equally-spaced intervals around the curved outer surface 3 of the hub 1, the elongate strips 4 extending from the lip 2a, parallel with the central axis of the cylindrical hub 1 across the outer curved surface 3 to the open end of the hub 1. The elongate strips 4 are undercut on opposite sides of their length, each undercut forming one half of a dovetail groove 5. The grooves 5 are typically in the region of 1mm deep. Thus, each elongate strip 4 has a"T"shaped cross section when viewed along the central axis of the cylindrical hub 1. The combination of a pair of dovetail grooves 5 from adjacent strips 4, which grooves 5 face one another across the outer curved surface 3, forms a dovetail slot between the adjacent strips 4.

In a preferred embodiment of the present invention, there are seven such elongate strips 4 equally spaced around the outer surface 3 of the hub 1 creating seven dovetail slots 6. It will be clear to a skilled person that more or fewer dovetail slots 6 may be provided.

Figure 2 is a perspective view of a blade assembly 7 for use with a fan embodying the present invention. The blade assembly 7 comprises an attachment plate 8 and a blade 12. The attachment plate 8 is substantially rectangular and has a curved profile. The curvature of an inner surface 9 of the attachment plate 8 conforms to that of the outer curved surface 3 of the hub 1.

Each of two opposing edges of the attachment plate 8 has a protruding tongue 10, which is of reduced thickness with respect to the remainder of the attachment plate 8. Each rib 10 is flush with the inner surface 9 of the attachment plate 8 to create a dovetail plate.

The blade 12 is formed integrally with the attachment plate 8 and attached at the blade root 13 to an outer surface 11 thereof. The blade 12 is in the form of an aerofoil, and lies substantially in a plane perpendicular to that of the attachment plate 8. The leading edge of the blade 12 is located in one comer of the attachment plate 8, the blade 12 subtending an angle of approximately 15 to the edges of the attachment plate 8 that do not have ribs 10 provided thereon.

This angle represents the angle of attack of the blade 12. The blades 12 are preferably twisted slightly so that the angle of attack of the blades 12 reduces from the root 14 of the blade to its tip 15.

A substantial portion of the blade 12, in the present example just under one half of the chord length of the blade 12, projects past the edge of the attachment plate 8 so that the trailing edge of the blade 12 is free from the attachment plate 8. The part of the root 13 which is not attached to the attachment plate 8 continues to follow the same curvature of the attachment plate 8.

Figure 3 shows an assembled fan 16 comprising the hub I and seven blade assemblies 7. The width of the attachment plate 8 of each blade assembly 7 is the same as the width of the dovetail slot 6 between any two adjacent elongate strips 4 on the curved outer surface 3 of the hub 1. In order to attach each of the blade assemblies 7 to the hub 1, the attachment plate 8 of each blade assembly 7 is slid into a free dovetail slot 6 between two of the elongate strips 4 provided on the outer surface 3 of the hub I. The elongate ribs 10 are received in a slide fit in the dovetail grooves 5 formed between the elongate strips 4 and the curved surface 3 of the hub 1. The attachment plates 8 of the blade assemblies 7 are slid into this position in a direction towards the upper surface 2 of the hub 1, until an edge of each attachment plate 8 abuts against the lip 2a of the upper surface 2. The attachment plate 8 can be glued,ultrasonically welded, or fixed by any suitable method into the dovetail slot 6 or the fit between the attachment plate 8 and the slot 6 can be constructed to provide an interference fit between the plate 8 and the slot 6.

Referring to Figures 4 and 5, it is important to note that, in the assembled fan embodying the invention shown in Figure 4, the blades 12 overlap one another in that the trailing edge (or a portion thereof) of one blade 12 lies underneath the leading edge of an adjacent blade 12 when the fan 16 is viewed along the central axis of the hub 1. This overlapping arrangement is impossible for a conventional fan (as shown in Figure 5) manufactured using a two-part mould.

Because the blades 12 can overlap one another in fans embodying the present invention, the angle of attack of the blades 12 can be reduced and the blade density for the fan increased.

Figure 6 shows a comparison of the profiles of a blade 12 of a fan manufactured using a two part mould as discussed above, and a blade 12 of a fan 16 embodying the present invention. Due to the fact that the blades 12 of the fan 16 embodying the present invention may overlap each other in the direction parallel to the axis of rotation of the fan 16, the chord length C2 of each blade 12 is relatively long. In contrast, the chord length Ci of the blade of the fan manufactured using a two part mould must be relatively short as overlapping is prohibited. In addition, the angle of attack a2 of the blade 12 of the fan 16 embodying the present invention with respect to the direction of movement of the blade 12 (indicated on Figure 6 by reference numeral 17) is shallower than the angle of attack ai of the blade of the fan manufactured using a two part mould. As described above, the increased chord length and decreased angle of attack of the blades 12 of the fan 16 embodying the present invention makes the fan 16 less likely to stall when the fan 13 is rotating at low speeds and under high pressure.

It will be apparent to a person skilled in the art that the present invention provides a fan, the components of which, whilst exploiting the advantages of two-part mould manufacturing methods, do not suffer from the drawbacks associated with conventional fans manufactured by this method. In addition, if individual blades 12 of the fan 16 become broken or damaged, they may be individually replaced without the necessity of replacing the entire fan 16.

In the present specification"comprises"means"includes or consists of'and "comprising"means"including or consisting of.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

CLAIMS 1. An axial cooling fan sub-assembly, comprising: a hub having a main body and a plurality of blade attachment sites, each blade attachment site being located on an outer surface of the main body and being shaped to receive and retain a blade; and a plurality of blades, each blade having an attachment portion shaped to be received in a blade attachment site of the hub.
2. A sub-assembly according to any preceding claim, wherein at least one of the blade attachment sites comprises at least one elongate strip, parallel to the outer surface of the hub and spaced therefrom, and having a groove defined therein.
3. A sub-assembly according to Claim 4, wherein the at least one blade attachment site comprises two elongate strips, the grooves defined thereby forming a dovetail slot.
4. A sub-assembly according to any preceding claim, wherein the attachment portion is substantially planar, and substantially follows the contours of a portion of the outer surface of the main body of the hub.
5. A sub-assembly according to any one of Claims 6 to 8, wherein the attachment portion has at least one protruding lip.
6. A sub-assembly according to Claim 5, wherein the attachment portion has two protruding lips, the attachment portion being formed as a dovetail plate.
7. A sub-assembly according to any preceding claim, wherein the blade portion lies substantially in a plane that is substantially perpendicular to the attachment portion.
8. A sub-assembly according to any preceding claim, wherein each attachment portion is receivable in a slide fit in one of the blade attachment sites.
9. A sub-assembly according to Claim 8, wherein each attachment portion is receivable in an interference fit in one of the blade attachment sites.
10. A sub-assembly according to any preceding claim wherein, when the plurality of blades are received in the blade attachment sites and the fan is viewed along an axis of rotation thereof, each blade overlaps with at least one adjacent blade.
11. A sub-assembly substantially as hereinbefore described, with reference to accompanying Figures 1 to 4.

11. A sub-assembly according to any preceding claim, wherein the axial cooling fan sub-assembly is a small axial cooling fan sub-assembly.

12. A method of constructing a small, axial cooling fan sub-assembly, comprising the steps of : providing a hub having a main body and a plurality of blade attachment sites, each blade attachment site being located on an outer surface of the main body and being shaped to receive and retain a blade; providing at least one blade comprising an attachment portion and a blade portion; and attaching the at least one blade to the hub.

13. A method according to Claim 12, further comprising the steps of : providing at least one groove at each blade attachment site; providing at least one elongate lip on the attachment portion of the at least one blade; and placing the at least one elongate lip in the at least one groove in a slide fit.

14. A method according to Claim ! 3, further comprising the steps of : providing a further groove at each blade attachment size; providing a further elongate lip on the attachment portion of the at least one blade; and placing the further elongate lip in the further groove in a slide fit.

15. A method according to any one of Claims 12 to 14, further comprising the step of attaching at least one further blade to the hub.

16. A method according to any of Claims 12 to 15, wherein the hub is manufactured by means of a two-part mould.

17. A method according to any one of Claims 12 to 16, wherein the at least one blade is manufactured by means of a two-part mould.

18. A sub-assembly substantially as hereinbefore described, with reference to accompanying Figures 3 and 4.

19. A method substantially as hereinbefore described, with reference to the accompanying drawings. 20. Any novel feature or combination of features described herein. Amendments to the claims have been filed as follows CLAIMS

1. An axial cooling fan sub-assembly, comprising: a hub having a main body and a plurality of blade attachment sites, each blade attachment site being located on an outer surface of the main body and being shaped to receive and retain a blade ; and a plurality of blades, each blade having an attachment portion shaped to be received in a blade attachment site of the hub and a blade portion and, when the plurality of blades are received in the blade attachment sites and the fan is viewed along an axis of rotation thereof, each blade overlaps an adjacent blade.

2. A sub-assembly according to claim 1, wherein the blade attachment sites comprises two elongate strips, parallel to the outer surface of the hub and spaced therefrom, and each elongate strip having a groove defined thereby forming a dovetail slot.

3. A sub-assembly according to any preceding claim, wherein the attachment portion is substantially planar, and substantially follows the contours of a portion of the outer surface of the main body of the hub.

4. A sub-assembly according to any preceding claim, wherein the attachment portion has two protruding tongues, the attachment portion being formed as a dovetail plate.

5. A sub-assembly according to any preceding claim, wherein the blade portion lies substantially in a plane that is substantially perpendicular to the attachment portion. 6. A sub-assembly according to any preceding claim, wherein each attachment portion is receivable in a slide fit in one of the blade attachment sites.

7. A sub-assembly according to Claim 6, wherein each attachment portion is receivable in an interference fit in one of the blade attachment sites.

8. A sub-assembly according to any preceding claim, wherein the axial cooling fan sub-assembly is a small axial cooling fan sub-assembly, a fan having a diameter of approximately 100 to 150mm.

9. A method of constructing a small, axial cooling fan sub-assembly, a fan having a diameter of approximately 100 to 150mm, comprising the steps of : manufacturing a hub by means of a two-part mould, the hub having a main body and a plurality of blade attachment sites, each blade attachment site being located on an outer surface of the main body and being shaped to receive and retain a blade; manufacturing blades by means of a two-part mould, each blade comprising an attachment portion and a blade portion ; and attaching the blades to the hub, such that when the fan is viewed along an axis of rotation thereof, each blade overlaps an adjacent blade.

10. A method according to Claim 9, further comprising the steps of : providing two grooves at each blade attachment site ; providing two protruding tongues on the attachment portion of each blade ; and placing the protruding tongues in the grooves in a slide fit.