ROTOR FOR A CENTRIFUGAL FAN, PARTICULARLY FOR THE FORCED CIRCULATION OF AIR IN CONVENTION COOKING OVENS

Abstract

A centrifugal fan rotor includes radial blades extending from a disc support connectable to a hub for connection to the rotor's shaft. The blades are produced by stamping metal sheet, each having a corresponding blade surface and opposite attachment projection to the disc support. The rotor includes at least a first group of the blades in which each blade of the first group is provided with respective attachment projections upstream of the corresponding blade surface with respect to a direction of rotation of the rotor, and at least a second separate group of the plurality of blades in which each blade is provided with respective attachment projections arranged downstream of the corresponding blade surface with respect to rotation direction, such that each blade of the first and second groups are mirror-symmetrical to each other with respect to a radial center plane of symmetry between the blades extending through rotation axis.

Description

FIELD OF THE INVENTION

The present invention relates to a rotor for a centrifugal fan, in particular for generating forced circulations of air inside convection cooking ovens.

BACKGROUND

Fans of the type indicated are widely used in various technical fields, including, purely by way of example, those in which there is required a forced circulation of air or other gaseous media, for specific applications. Those applications include a typical one in which those fans are used to generate forced circulations of air inside convection cooking ovens.

Those fans normally comprise a rotor which is constructed from sheet metal of small thickness, comprising a disc support to which there is centrally fixed a hub which is intended to be connected to a drive shaft. A plurality of blades, which are also constructed from thin sheet metal and are suitably shaped, are applied to the disc support in order to construct the blade arrangement of the fan.

The invention is directed in particular towards rotors having so-called radial blading, in which the radial blades extend and are defined axially between the disc base support which carries the hub and an opposed support ring which is axially spaced apart from the disc base support, and in which the blades are produced by cold-stamping from sheet metal. That technology provides for the blading to be obtained from a substantially rectangular, planar metal sheet in which there are formed, during the stamping, a series of through-cuts over the thickness of the metal sheet, each cut extending between opposite longitudinal sides of the metal sheet, except for opposite smaller end zones, where there remain defined respective attachment projections of each blade in the metal sheet plane. The subsequent folding of each blade by approximately 90° relative to the metal sheet plane defines the series of blades, where the blade surface therefore extends perpendicularly relative to the plane containing the pair of opposing attachment projections of the blade itself, respectively. The assembly of the blading on the rotor involves folding of the metal sheet which carries the blading into a cylindrical profile so that the blading is wound and closed about the disc support and locked along the circular profile thereof by means of a caulking which locks the projections of each blade on the disc support. Similar fixing by way of caulking the axially opposing projections of each blade is provided along the support ring which is axially opposite the disc support.

The blading structure obtained by stamping sheet metal has definite advantages in the production process of the rotor, but it makes it inevitable that, in the opposing axial ends of each blade, the attachment projections act in the delivery region of the rotor, influencing the flow produced by the rotor in terms of fluid dynamics. Taking into consideration the series of blades of the rotor in a planar development (coincident with the one of the metal sheet before the cylindrical folding), for each blade the respective attachment projections are located upstream of the blade itself with respect to one of the directions of rotation of the rotor, while the projections are located downstream of the corresponding blade when the rotor rotates in the opposite direction of rotation. Since, in the applications inside cooking ovens, the fans which are fitted to those rotors are caused to rotate both in one direction and in the opposite direction, during the same cooking cycle, the fan is subjected to a substantial loss of efficiency and yield when it is actuated in a direction of rotation relative to the activation in the opposite direction, that difference being connected with the above-mentioned configuration of the attachment projections of the blading and the overall disruptive action on the flow of air which they induce in one of the directions of rotation of the fan.

SUMMARY

A main object of the invention is to provide a rotor of the above-mentioned type which is configured to overcome the limitations set out above with reference to the cited prior art.

This object and other objects which will be set out below are achieved by the invention by means of a rotor for a centrifugal fan, which rotor is constructed according to the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the invention will be appreciated more clearly from the following detailed description of some preferred embodiments thereof which are illustrated, by way of non-limiting example, with reference to the appended drawings, in which:

FIGS. 1 and 2 are perspective views of a rotor for a centrifugal fan, which rotor is constructed according to the invention,

FIG. 3 is a perspective view, drawn to an enlarged scale, of a detail of the rotor of the preceding Figures,

FIG. 4 is a schematic, partially sectioned side view of the detail of FIG. 3,

FIG. 5 is a schematic partial view of a series of blades of the rotor of the preceding Figures in an embodiment of a processing step thereof,

FIG. 6 is a schematic view corresponding to that of FIG. 5 in another embodiment of the series of blades of the rotor according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to the aforementioned Figures, there is generally designated 1 a rotor of the radial type for a centrifugal fan, which rotor is constructed according to the present invention. That rotor is particularly configured for assembly inside cooking chambers of convection ovens, for generating a forced circulation of air.

The rotor comprises a plurality of blades 2 which have a direct radial extent and which extend from a disc support 3 which is capable of being fixed to a central hub 4 for securing the drive shaft (not illustrated) for rotation of the fan, with an axis of rotation designated X in the Figures.

The blading of the rotor 1 is obtained by a cold-stamping processing operation of a metal sheet having a small thickness, as described in detail below. The rotor including the blading is preferably produced completely from metal material.

With particular reference to FIG. 5, a planar metal sheet 5 having a rectangular shape (with long sides 5a and short sides 5b) is subjected to a stamping step in which there are formed a series of through-cuts in the thickness of the metal sheet, which cuts are designated 6 and have regular pitch.

Each cut profile 6 extends between opposing longitudinal sides of the metal sheet, except for opposing reduced end zones, so as to remain at a predetermined distance from the edges of the sides 5a. Each profile 6 has a main central portion 6a which has a rectilinear extent and which is connected to the opposing ends by curved portions 6b, respectively.

There remains defined between each pair of contiguous cut profiles 6 a respective blade 2 which comprises a main blade member 2a which defines the blade surface and a pair of axially opposed projections 2b, by means of which the support blade is fixed.

By means of a subsequent folding operation, the main member 2a of each blade is folded through approximately 90° relative to the metal sheet plane so that the pair of corresponding attachment projections 2b of each blade are substantially perpendicular to the blade member 2a following the folding operation.

Following a subsequent folding operation of the stamped metal sheet, in accordance with a cylindrical profile extent (corresponding to the circumferential profile of the support 3), the metal sheet is wound about the disc support in order to be locked along the circumferential edge thereof by means of a caulking which locks the ends of the projections 2b, which are directed along the same side 5a of a metal sheet, at the circumferential edge of the disc support.

There is designated 7 a support ring of the blades, which ring is coaxial relative to the disc support 3 and which is axially spaced apart therefrom. Along the ring 7, there are locked, advantageously by means of a similar caulking operation, the projections 2b of the series of blades which are arranged along the side 5a of the metal sheet opposite the side fixed to the disc 3.

According to a main feature of the invention, the rotor comprises at least a first and a second group of blades which together define the whole plurality of blades 2, in which each blade of the first group is provided with the respective attachment projections 2b which are arranged upstream of the corresponding surface of a blade 2a with respect to a direction V of rotation of the rotor (FIG. 4) and in which each blade of the second group is provided with respective attachment projections 2b which are arranged downstream of the corresponding surface of a blade 2a, with respect to the direction of rotation, so that each blade 2 of the first group is mirror-symmetrical to each blade 2 of the second group with respect to a radial centre plane M of symmetry between the blades, extending through the axis of rotation X of the rotor (FIG. 4).

FIG. 4 illustrates the above-mentioned configuration with reference to a single pair of blades 2 belonging to the respective first and second groups of blades.

According to a first embodiment of the invention, the first and the second groups of blades together form the whole blading of the rotor and the first group comprises at least 25% of the total number of blades of the rotor. According to that configuration, there has been produced an advantageous effect on the recovery of efficiency of the rotor, in one of the directions of rotation of the fan, with respect to a conventional configuration in which all the blades of the blading are orientated in the same manner.

The flow produced by the rotor in the delivery region is influenced by the presence of the attachment projections 2b of each blade. In a conventional configuration of the rotor, with blades all having the attachment projections orientated in the same direction, with respect to a level of efficiency of the fan which can be obtained in a direction of rotation of the fan, the efficiency which can be obtained in the opposite direction of rotation is subjected to a substantial reduction, precisely as a result of the disruptive action brought about by the presence of the attachment projections.

In accordance with the above-mentioned embodiment, the provision of the construction in the rotor of at least ¼ of the total number of blades with the opposite orientation of the attachment projections to that of the remaining portion of blades of the overall series has shown a recovery of efficiency of the rotor, in the direction of rotation with which a rotor of the conventional type is instead subjected to a significant loss of efficiency.

In accordance with a second embodiment of the invention, N indicating the total number of blades of the rotor, the first and second groups of blades each comprise a number of blades N′ having the following values:

• • N′=N/2 if N is an even number, or
  • N′=(N/2)+/−0.5 if N is an odd number.

In this case, there is a balanced distribution of the blades and the respective attachment projections on the rotor, which allows the same level of efficiency of the fan to be achieved in both directions of rotation. That configuration represents the preferred selection for an effective balancing of the fan.

According to a preselected configuration, each of the first and second groups is formed by mutually adjacent blades in the series of blades so as to involve a semi-cylindrical portion of the total cylindrical extent of the rotor. FIG. 6 schematically illustrates a pair of metal sheets 5 which are in mutual alignment and positioned with a symmetrical arrangement of the cuts 6 which are formed in each metal sheet, whose overall extent allows the complete cylindrical winding about the support 3 in order to construct the rotor. In other words, each metal sheet 5 corresponds to each of the two semi-cylindrical extents which form the rotor and is intended to act on a portion of the semi-circumference of the circular profile of the disc support to which the series of blades is fixed.

It should be noted that such a configuration may be obtained by providing pairs of metal sheets 5 (with respective cuts 6) which are equal to each other but which have opposing relative support, as clearly illustrated in FIG. 6.

In another variant, there may be provision for the series of blades to be formed by a sequence of blades in which a blade of the first group alternates with a blade of the second group, the entire blading of the rotor being formed on a single metal sheet which is stamped and folded into a cylindrical shape for locking on the disc support.

It will be understood that other configurations are possible in which there are provided series of blades of the first and second groups which are different from those described above, for example, by providing alternating series of sub-groups of blades with the same orientation.

The invention thereby achieves the objectives set out and results in the advantages set out with respect to the known solutions.

Claims

1. A rotor for a centrifugal fan comprising a plurality of radial blades extending from a disc support which is configured to be connected to a hub for connection to a rotary shaft of the rotor, said blades being of the type produced by stamping metal sheet, in which a planar sheet of substantially rectangular shape is provided, during stamping, with a sequence of cuts over a thickness of the metal sheet, each cut extending between opposite longitudinal sides of the metal sheet, with the exception of opposite smaller end zones where respective attachment projections of each blade with respect to the metal sheet plane continue to be defined, and in which successive folding of each blade by approximately 90° with respect to the metal sheet plane forms a series of blades, and in which an assembly of said series of blades on the rotor involves folding of the metal sheet carrying the blading into a cylindrical profile, such that the metal sheet is wound about the disc support in order to be locked along a circular profile of the disc support by means of caulking which locks the attachment projections of the blades, extending from the same side of the series, on the disc support, a support ring being provided, axially opposite the disc support, for fixing by caulking of the attachment projections of the axially opposite blades extending from the opposite side of the series, wherein the rotor further comprises at least a first group of said plurality of blades in which each blade of said first group is provided with respective attachment projections arranged upstream of the corresponding blade surface with respect to a direction of rotation of the rotor, and at least a second separate group of said plurality of blades in which each blade is provided with respective attachment projections arranged downstream of the corresponding blade surface with respect to said direction of rotation, such that each blade of the first group is mirror-symmetrical to each blade of the second group with respect to a radial center plane of symmetry between the blades extending through the axis of rotation of the rotor.

2. A rotor according to claim 1, wherein the first and the second group of blades together form the whole blading of the rotor and the first group comprises 25% of the overall number of blades of the rotor.

3. A rotor according to claim 2, wherein, where N indicates the overall number of blades of the rotor, the first and second group of blades each comprise a number of blades N′ having the following values:

N′=N/2 if N is an even number, or

N′=(N/2)+/−0.5 if N is an odd number.

4. A rotor according to claim 3, wherein each of the first and second groups is formed by blades adjacent to one another in the series of blades so as to involve a semi-cylindrical portion of the overall cylindrical extent of the rotor.

5. A rotor according to claim 4, wherein each of the first and second groups of blades is formed from a respective and separate metal sheet for stamping of the blades of the corresponding group, each metal sheet being configured to act on a semi-circumferential section of the circular profile of the disc support to which the series of blades is fixed.

6. A rotor according to claim 3, wherein the series of blades is formed by a sequence of blades in which a blade of the first group alternates with a blade of the second group, all the blading of the rotor being formed on a single metal sheet stamped and folded into a cylindrical shape for locking on the disc support.

7. A rotor according to claim 1, configured as a rotor with radial blading.