GB2090338A

GB2090338A - Centrifugal fan

Info

Publication number: GB2090338A
Application number: GB8041573A
Authority: GB
Original assignee: Birmid Qualcast Home & Garden
Current assignee: Birmid Qualcast Home & Garden
Priority date: 1980-12-31
Filing date: 1980-12-31
Publication date: 1982-07-07
Also published as: GB2090338B

Abstract

The fan is provided with an annular internal passage (17) between a central hub (13) and a generally frusto-conical wall (10). First and second sets of upstanding blades (11, 16) are angularly spaced around the hub and connected to respective opposite surfaces of the wall (10). Each set of blades respectively impels air along an independent air flow path having an axial air intake, the air impelled by the blades (11) being discharged radially, whilst that from blades (16) is discharged both radially and axially through the internal passage (17). When fitted to an air cushion lawn mower, blades (11) provide the lift air whilst blades (16) draw in cooling air over the motor driving the fan. <IMAGE>

Description

SPECIFICATION Centrifugal fans An air cushion mower normally includes a centrifugal fan mounted for rotation about a substan tiallyvertical axis beneath a certral opening in a deck forming the roof of a plenum chamber. Air drawn axially through the central opening in the deck is radially discharged by the fan into the plenum chamber to form the air cushion.

The fan motor is normally supported on the deck and enclosed within a protective hood. To avoid overheating it is common practice to provide a cooling flow of air overthe motor, either by means of an additional fan or by using the main lift fan. In either case, the two streams of air (the main lift air and the motor cooling air) are combined or recombined with one another either in the fan itself or just above the fan in the region of the central opening in the deck.

The motor cooling air is preferably filtered at its inlet, but the main lift air is not. There is, therefore, a risk of the fan becoming temporarily clogged with grass cuttings carried in by the main lift air, and this can then block, or severely reduce, the flow of motor cooling air with the result that the motor may be damaged by overheating.

In accordance with the present invention there is provided a centrifugal fan which includes means for radially discharging a flow of air from a first axial air intake, and at least one generally axially extending passage for the flow of air from a second axial air intake. In this mannerthe air flows from the two intakes are kept separate from one another until the respective flows have left the fan. The two intakes are preferably concentric, with one lying within the other.

When such a fan is fitted to the output shaft of a motor driving an air cushion mower, the main lift air can be kept apart from the motor cooling air until the respective airflows have been discharged by the fan into the plenum chamber. Any blocking ofthe unfiltered main lift air does not therefore affect the flow of motor cooling air.

The axially extending passage preferably receives the motor cooling air and has its outlet into the low pressure area immediately beneath the fan. This has the added advantage of reducing the undesirable suction effect which this low pressure area normally causes.

The axially extending passage is preferably traversed by at least one vane so that the fan performs the dual function of providing the lift air and drawing in the motor cooling air without the need for an additional fan.

In a preferred embodiment of the invention the centrifugal fan consists of an open-topped inverted dish or bowl surrounding a central support formed with a bore for receiving the output shaft of the fan motor, the central support member being connected to the underneath surface of the inverted dish or bowl by an inner set of vanes aligned with the second air intake, and the upper surface of the dish or bowl including an outer set of vanes aligned with the first air intake.

By way of example only, a centrifugal fan embodying the invention will now be described with reference to the accompanying drawings, in which Fig. 1 is a top plan view of the centrifugal fan; Fig. 2 is a sectional view through the fan; Fig. 3 is an underneath plan view of the fan; Fig. 4 is a top perspective view of the fan; and, Fig. 5 is a partly sectioned view illustrating the fan fitted to an air cushion mower.

Referring to these drawings, the fan consists of an inverted bowl 10 having a top surface with an outer set of upstanding vanes 11 curving inwardly from the circumference.

The bowl is open at the top and surrounds a hollow frustum 13 having a central bore 14 for receiving the output shaft of a fan motor.

The frustum 13 is connected to the underneath surace ofthe bowl 10 by means of an inner set of skewed vanes 16 so that a number of axially extending tunnels or passages 17 are formed beween the adjacent vanes 16.

With reference to Fig. 5, the fan is shown attached to the output shaft 21 of a motor 20 secured to deck 22 of an air cushion mower. The axially extending passages 17 in the fan are aligned with openings 23 in the motor end frame 24 so that motor cooling air (which passes both through the motor stack and between the stack and a baffle 26 surrounding the motor) is drawn by the inner set of vanes 16 through the openings 23 and the passages 17 as shown by the arrowed flow paths. This air is discharged into what is normally a low pressure zone of the plenum chamber 25 immediately beneath the fan, but the effect of the discharged air is to increase the pressure in this zone and reduce the undesirable suction effect. The outer set of blades 11 on the top surface of the fan sweep around the edge of a central opening 27 in the deck 22 and draw in the main lift air in a conventional manner.

It can be seen that the motor cooling air and the main lift air are kept apart from one another until they have been discharged by the fan into the plenum chamber. This not only ensures a free flow path for the motor cooling air irrespective of possible clogging in the main lift air path, but it allows greater freedom of design to achieve optimum efficiency.

For example, the cooling air flow may comprise between 20% and 30% of the total airflow and since this is discharged directly into the plenum chamber, the gross throughput which has to be delivered by the main lift air vanes 11 can be correspondingly reduced. This may increase the efficiency of the fan and/or allow the use of smaller fans.

The shaft 21 also carries a rotary cutting element which, in this example, consists of a spool of flexible line having a free end 28 which, in use, is flung outwards by centrifugal forces to perform a flail-type cutting action.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Centrifugal fans An air cushion mower normally includes a centrifugal fan mounted for rotation about a substan tiallyvertical axis beneath a certral opening in a deck forming the roof of a plenum chamber. Air drawn axially through the central opening in the deck is radially discharged by the fan into the plenum chamber to form the air cushion. The fan motor is normally supported on the deck and enclosed within a protective hood. To avoid overheating it is common practice to provide a cooling flow of air overthe motor, either by means of an additional fan or by using the main lift fan. In either case, the two streams of air (the main lift air and the motor cooling air) are combined or recombined with one another either in the fan itself or just above the fan in the region of the central opening in the deck. The motor cooling air is preferably filtered at its inlet, but the main lift air is not. There is, therefore, a risk of the fan becoming temporarily clogged with grass cuttings carried in by the main lift air, and this can then block, or severely reduce, the flow of motor cooling air with the result that the motor may be damaged by overheating. In accordance with the present invention there is provided a centrifugal fan which includes means for radially discharging a flow of air from a first axial air intake, and at least one generally axially extending passage for the flow of air from a second axial air intake. In this mannerthe air flows from the two intakes are kept separate from one another until the respective flows have left the fan. The two intakes are preferably concentric, with one lying within the other. When such a fan is fitted to the output shaft of a motor driving an air cushion mower, the main lift air can be kept apart from the motor cooling air until the respective airflows have been discharged by the fan into the plenum chamber. Any blocking ofthe unfiltered main lift air does not therefore affect the flow of motor cooling air. The axially extending passage preferably receives the motor cooling air and has its outlet into the low pressure area immediately beneath the fan. This has the added advantage of reducing the undesirable suction effect which this low pressure area normally causes. The axially extending passage is preferably traversed by at least one vane so that the fan performs the dual function of providing the lift air and drawing in the motor cooling air without the need for an additional fan. In a preferred embodiment of the invention the centrifugal fan consists of an open-topped inverted dish or bowl surrounding a central support formed with a bore for receiving the output shaft of the fan motor, the central support member being connected to the underneath surface of the inverted dish or bowl by an inner set of vanes aligned with the second air intake, and the upper surface of the dish or bowl including an outer set of vanes aligned with the first air intake. By way of example only, a centrifugal fan embodying the invention will now be described with reference to the accompanying drawings, in which Fig. 1 is a top plan view of the centrifugal fan; Fig. 2 is a sectional view through the fan; Fig. 3 is an underneath plan view of the fan; Fig. 4 is a top perspective view of the fan; and, Fig. 5 is a partly sectioned view illustrating the fan fitted to an air cushion mower. Referring to these drawings, the fan consists of an inverted bowl 10 having a top surface with an outer set of upstanding vanes 11 curving inwardly from the circumference. The bowl is open at the top and surrounds a hollow frustum 13 having a central bore 14 for receiving the output shaft of a fan motor. The frustum 13 is connected to the underneath surace ofthe bowl 10 by means of an inner set of skewed vanes 16 so that a number of axially extending tunnels or passages 17 are formed beween the adjacent vanes 16. With reference to Fig. 5, the fan is shown attached to the output shaft 21 of a motor 20 secured to deck 22 of an air cushion mower. The axially extending passages 17 in the fan are aligned with openings 23 in the motor end frame 24 so that motor cooling air (which passes both through the motor stack and between the stack and a baffle 26 surrounding the motor) is drawn by the inner set of vanes 16 through the openings 23 and the passages 17 as shown by the arrowed flow paths. This air is discharged into what is normally a low pressure zone of the plenum chamber 25 immediately beneath the fan, but the effect of the discharged air is to increase the pressure in this zone and reduce the undesirable suction effect. The outer set of blades 11 on the top surface of the fan sweep around the edge of a central opening 27 in the deck 22 and draw in the main lift air in a conventional manner. It can be seen that the motor cooling air and the main lift air are kept apart from one another until they have been discharged by the fan into the plenum chamber. This not only ensures a free flow path for the motor cooling air irrespective of possible clogging in the main lift air path, but it allows greater freedom of design to achieve optimum efficiency. For example, the cooling air flow may comprise between 20% and 30% of the total airflow and since this is discharged directly into the plenum chamber, the gross throughput which has to be delivered by the main lift air vanes 11 can be correspondingly reduced. This may increase the efficiency of the fan and/or allow the use of smaller fans. The shaft 21 also carries a rotary cutting element which, in this example, consists of a spool of flexible line having a free end 28 which, in use, is flung outwards by centrifugal forces to perform a flail-type cutting action. CLAIMS

1. A fan comprising first and second blade means respectively impelling air along two independent airflow paths, each path having an axial air intake.

2. A centrifugal fan comprising first and second blade means respectively impelling air along two independent airflow paths, each path having an axial air intake.

3. A fan according to claim 1 or claim 2 in which the two intakes are concentric.

4. Afan according to claim 3 in which one of the intakes lies within the other.

5. A fan according to any one of the preceding claims in which the air flowing along a first of the paths is discharged at a higher level than the air from the second path when the fan is rotated about a gen erallyvertical axis.

6. A fan according to any one of the preceding claims in which the air flowing along a first ofthe paths is discharged radially and the air flowing along the second path is discharged axially and/or radially through an internal passage.

7. A fan according to claim 6 in which the passage is an annular passage formed between a central hub and a generallyfrusto-conical wall surrounding the hub.

8. A fan according to claim 7 in which the first blade means comprise a plurality of upstanding blades angularly spaced around the hub and connected to the underneath surface of the said wall, and in which the second blade means comprise a further plurality of upstanding blades angularly spaced around the top surface of the wall.

9. A lawn mower of the type which, in use, rides on a cushion of air, the mower including a centrifugal fan for generating the cushion of air beneath a generally dome-shaped deck, and a motorfor driving the fan, the shroud and the fan including first and second blade means respectively impelling air along two independent airflow paths, each path having an axial air intake, and the arrangement being such that the first air intake communicates with the atmosphere to draw in lift air for the said cushion while the second air intake communicates with an internal passage in the shroud to drawn in cooling air overthe motor.

10. A lawn mower according to claim 9 in which the lift air flowing along the first path is discharged radially from the fan, and the cooling air flowing along the second path is discharged into a zone immediately beneath the fan.

11. A centrifugal fan substantially as herein described with reference to the accompanying drawings.