GB2418588A

GB2418588A - Lawnmower blade

Info

Publication number: GB2418588A
Application number: GB0421770A
Authority: GB
Inventors: Paul Stockley
Original assignee: Electrolux Outdoor Products Ltd
Current assignee: Husqvarna UK Ltd
Priority date: 2004-09-30
Filing date: 2004-09-30
Publication date: 2006-04-05
Also published as: WO2006035226A1; GB0421770D0

Abstract

A rotary lawnmower blade 1 comprises a mounting portion 2 provided with means for attaching the blade to the rotary output shaft of a motor. At least one blade member 3, 4 extending outwardly from the mounting portion 2, the or each blade member having a cutting portion 3a, 4a at its leading edge and terminating in a respective tip 3c having a respective upturned portion 3b, 4b at the trailing edge thereof. The or each upturned portion 3b, 4b defines a substantially planar upper surface which is contiguous with the adjacent upper surface of the associated blade tip at an angle to the respective leading edge 3a, 4a.

Description

24 1 8588 P51 5385GB Lawnmower Blade This invention relates to a lawnmower

blade, and in particular to a blade for a lawnrnower which includes a deck defining a cutting chamber, the cutter blade being rotatably mounted within the cutting chamber about a substantially vertical axis.

The invention is applicable to both a so-called "wheeled rotary lawnmower" which includes wheels for mounting the deck, and a hover mower which, in use, has its deck supported above ground datum by air pressure within the deck. In either case, the blade is rotatably driven by a motor mounted on the deck. The motor may be an electric motor or a petrol- fuelled internal combustion engine.

The noise generated by either a wheeled rotary lawnmower or a hover mower is substantial. This noise is a combination of the noise of the engine itself, and that of the rotating blade and, for a hover mower, its impeller (fan). Commonly, the most dominant noise source is the blade (and the fan where applicable). Values depend largely upon the blade/fan size, and the speed of rotation.

A standard petrol mower engine generates approximately 89dB(A) when the engine is positioned on a test rig. It does, however, generate approximately 90dB(A) when positioned on the deck of a mower. The noise generated by a typical rotating blade for such a mower is 92dB(A). These values relate to a typical petrol wheeled rotary lawnmower with a blade diameter of less than 50 cm. Consequently, the total noise generated by such a mower is about 94dB(A). This is, of course subject to production variances such as engine speed and vibration levels, and these can affect the result by up to + 2dB(A).

As the blade of a lawomower rotates, a low pressure area is created behind its trailing edges. These low pressure areas cause turbulence which leads to the noise generated.

The absolute levels of blade noise are dependent upon blade diameter, blade geometry and rotational speed. For a given width of cut, the blade diameter is fixed, so the only possibilities for reducing blade noise are to adjust the rotational speed of the blade or its geometry.

A rotary lawnmower blade includes a central portion designed to be mounted on the vertical output shaft of the lawnmower's motor. The end portions of the blade have leading, cutting edges for cutting grass; and angled, upturned trailing edge portions which generate airflow which is used to carry cut grass clippings to a grass-box associated with the lawamower. The upturned trailing edge portions of the blade contribute substantially to the noise generated by the blade, but are essential to a grass-collecting lawnmower, so that noise reduction cannot be achieved by removing these upturned trailing edge portions whilst ensuring efficient grass collection.

The torque characteristic of small petrol engines used on lawnmowers dictate that the rotational speed of the blade is typically 2700 rpm or more. At this rotational speed, the 1 S area of negative pressure created behind the upturned trailing edge portions of the blade are such that the air flowing over the blade changes direction abruptly as it leaves the trailing edges, thereby causing the airflow to separate and rotate. This rotational turbulent airflow creates noise, and contributes greatly to the overall noise of the product. As mentioned above, aerodynamic blade noise is the most dominant noise source for a grass-collecting rotary lawnmower.

A known low noise blade has upturned portions at each of its tips, the upturned portions being constituted by perforated panels or including pierced holes. The holes reduce the pressure drop behind the blade, which, in turn, reduces the degree of rotational turbulent flow. Lower turbulence results in lower aerodynamic noise. In use, however, the holes quickly become blocked with grass debris, and the noise increases as a result.

Holes and perforations are, therefore, only effective when the blade is clean.

Ever more stringent environmental noise limits require manufacturers to reduce the noise generated by lawnmowers. The aim of the invention is, therefore, to provide a rotary lawnmower blade which has reduced aerodynamic noise generation properties whilst maintaining satisfactory airflow generation characteristics to ensure efficient grass collection, effective throughout the product life.

The present invention provides a rotary lawnmower blade comprising a mounting portion provided with means for attaching the blade to the rotary output shaft of a motor, and at least one blade member extending outwardly from the mounting portion, the or each blade member having a cutting portion at its leading edge and terminating in a respective tip having a respective upturned portion at the trailing edge thereof, the or each upturned portion defining a substantially planar upper surface which is contiguous with the adjacent upper surface of the associated blade tip at an angle to the respective leading edge, wherein the or each upturned portion lies at an angle of at least 30 to the respective leading edge.

The invention also provides a rotary lawnmower blade comprising a mounting portion provided with means for attaching the blade to the rotary output shaft of a motor, and at least one blade member extending outwardly from the mounting portion, the or each blade member having a cutting portion at its leading edge and terminating in a respective tip having a respective upturned portion at the trailing edge thereof, the or each upturned portion defining a substantially planar upper surface which is contiguous with the adjacent upper surface of the associated blade tip at an angle to the respective leading edge, wherein the or each upturned portion lies at an angle of at least 30 to the respective leading edge, and wherein the upper surface of the or each upturned portion is an imperforate surface.

The invention further provides a rotary lawnmower blade comprising a mounting portion provided with means for attaching the blade to the rotary output shaft of a motor, and at least one blade member extending outwardly from the mounting portion, the or each blade member having a cutting portion at its leading edge and terminating in a respective tip having a respective upturned portion at the trailing edge thereof, the or each upturned portion defining a substantially planar upper surface which is contiguous with the adjacent upper surface of the associated blade tip at an angle to the respective leading edge, wherein the or each upturned portion lies at an angle of at least 30 to the respective leading edge, and wherein the upper surface of the or each upturned portion lies at an angle of at most 40 to the plane of the adjacent upper surface of the associated blade tip.

Advantageously, the or each upturned portion lies at an angle between 30 and 60 to the respective leading edge. Preferably, said angle is between 39 and 45 , and more preferably said angle is substantially 42 .

Conveniently, the or each upturned portion lies at an angle of between 15 and 40 to the plane of the adjacent upper surface of the associated blade tip. Preferably, said angle is between 25 and 29 , and more preferably said angle is substantially 27 .

In a preferred embodiment, the or each upturned portion meets the adjacent upper blade tip surface at a respective bend line. In this case, the or each upturned portion may project rearwardly from the associated bend line by a distance lying between 30 mm and 60 mm, said distance being measured along the plane of the or each upturned portion and in a direction normal to the respective bend line. Preferably, said distance is between 57 mm and 63 mm, and more preferably said distance is substantially 60 mm.

The invention also provides a rotary lawnmower blade comprising a mounting portion provided with means for attaching the blade to the rotary output shaft of a motor, and at least one blade member extending outwardly from the mounting portion, the or each blade member having a cutting portion at its leading edge and terminating in a respective tip having a respective upturned portion at the trailing edge thereof, the or each upturned portion defining a substantially planar upper surface which is contiguous with the adjacent upper surface of the associated blade tip at an angle to the respective leading edge, wherein the or each upturned portion lies at an angle of between 30 and 60 to the respective leading edge, wherein the or each upturned portion lies at an angle of between 15 and 40 to the plane of the adjacent upper surface of the associated blade tip, and wherein the or each upturned portion meets the adjacent upper blade tip surface at a respective bend line, and projects rearwardly from that bend line by a distance lying between 30 mm and 60 mm, said distance being measured along the plane of the or each upturned portion and in a direction normal to the respective bend line.

Advantageously, there are at least two blade members, each of which extends away from the mounting portion, and preferably there are two blade members which extend in opposite directions away from the mounting portion.

In a preferred embodiment, the or each upturned portion has a substantially triangular configuration. In this case, the base of the or each upturned portion may be contiguous with the adjacent upper surface of the associated blade tip.

Advantageously, the blade is made of steel by a stamping and forming operation.

In a preferred embodiment, the or each upturned portion is inwardly offset from the free end of the associated blade member by a distance of between 13 mm and 17 mm.

Preferably, said offset distance is substantially 15 mm.

The invention will now be described in greater detail, by way of example, with reference to the drawings, in which: Figure 1 is a perspective view of a rotary lawnmower blade constructed in accordance with the invention; Figure 2 is a plan view of the blade of Figure 1; Figure 3 is a side view of the blade of Figure 1; Figure 4 is an end view of the blade of Figure 1; Figure 5 shows schematically the tip of the blade of Figure 1 and the tip of a conventional blade; and Figures 6 and 7 illustrate the flow of air over the blade tips of Figure 6.

Referring to the drawings, Figure I shows a rotary lawnmower blade I having a central portion 2 which is suitably apertured for detachably mounting the blade to the output shaft (not shown) of the motor of the lawnmower. Blade members 3 and 4 extend in opposite directions from the central portion 2. The blade 1 is made of steel by stamping from a steel blank and then shaping using a forming tool.

Each blade member 3, 4 has a tip formed with a sharpened leading edge 3a, 4a, and a trailing edge portion 3b, 4b which projects to the rear with respect to the direction of rotation (indicated by the arrow R) of the blade I. Each trailing edge portion (wing) 3b, 4b is generally triangular, and is set back slightly from the adjacent distal end 3c, 4c of the associated leading edge 3a, 4a. Each wing 3b, 4b is angled upwardly at about 27 with respect to its blade member 3, 4, the bend line 3d, 4d of that wing being about 42 to the respective leading edge 3a, 4a. The wings 3b, 4b are relatively long, extending about 60 mm behind their bend lines 3d, 4d in directions normal to the bend lines. This ensures that the blade I generates the same air flow as blades of a conventional design, whilst its geometry generates an efficient, low- turbulence airflow, and hence a low level of aerodynamic noise. The blade 1 does, therefore, produce a sufficiently large airflow to satisfy the requirements for collection of grass clippings but by the blade.

As the wings 3b, 4b are angled about the bend lines 3d, 4d with respect to the leading edges 3a, 4a, the frontal area of each wing is smaller than would be the case if the wings were upturned about bend lines parallel to the cutting edge (as is the case with known blades). This reduction in wing frontal area results in a reduced disturbing of air as the blade I is rotated. Although the wings 3b, 4b are angled up from the respective back edges of the blade 1, as is the case with many blade designs, air spills off the tips of the blade due to centrifugal force. As it spills, the air stream separates and rotates creating turbulence. With the blade 1, however, as the wings are angled across the ends of the blade, the air flowing off the wing tips is entrained upwards into the collection airflow.

This creates a less turbulent flow, and hence generates less noise, for the same airflow volume.

The reasons for this noise reduction will now be described with reference to Figures 5 to 7. Thus, as shown in Figure 5, which shows schematically the tip of a blade member 3 of the blade I of the present invention and the tip of a conventional blade, these tips being designated 3' and 3" respectively. Figures 6 and 7 show the two tips 3' and 3", and illustrate the flow of air over those tips when the blades are in use. As shown in Figure 7, the conventional blade 3 has an upturned portion 3"b which has a substantially larger area than the upturned portion (wing) 3b of the blade 1, and the upturned portion 3"b is bent about a bend line 3"d which is substantially parallel to the leading edge 3"a of that blade. As indicated in Figure 5, the area of the wing 3"b is 930 mm2, as compared with the area 645 mm2 of the wing 3b. It will be apparent, therefore, that the wing 3b of the blade I has a considerably smaller area which can disturb air as the blade I is rotated as is the case with a conventional blade.

The design of the blade I is such as to produce an equivalent air flow to that of the conventional blade, despite having a smaller disturbing area. In this connection, the disturbing area is defined as the surface area of the blade wing normal to the direction of motion. As the disturbing area of the blade I is about 30% less than that of a conventional blade, the blade of the present invention generates a more efficient, less turbulent airflow.

Where a lawnmower blade is driven by an electric induction motor at 3000 rpm, 355 watts of power are needed to drive the motor with no blade fitted. If a completely flat blade is fitted, the power needed is 400 watts, a flat blade being the most efficient form of blade from a power point of view. The blade I of the present invention can be driven at 3000 rpm using only 440 watts, whereas the conventional blade requires 475 watts. Thus, not only is the blade I quieter than the conventional blade, but it is more efficient due to the reduced aerodynamic drag.

Figures 6 and 7 show the resultant air flow produced by the respective blades around the associated cutting chambers. The resultant air flow travels around the chamber in a circular manner a velocity Vairflow, Vairflow being about l/lOth the velocity of the blade itself (Vbladc). As the disturbing area Be behind the wing 3b of the blade I is substantially smaller than the disturbing area We of a conventional blade, the blade 1 creates considerably less turbulence, and so is quieter. The angle with which the wing 3b cuts through the airflow Vairflow also helps to reduce turbulence and hence noise.

This reduced turbulence is analogous to the reduction in turbulence that results from positioning an angled deflector at the front of a lorry when compared with a lorry having no such deflector which presents a blunt surface substantially at right-angles to the airflow when the lorry is moving.

An additional benefit comes from the wings 3b, 4b being positioned slightly inboard (substantially 15 mm) from the tips of the blade 1. As a result, the rotational speed of the tips of the wings 3b, 4b is around 7% lower, which has an additional noise benefit.

It will be apparent that modifications could be made to the blade described above. In particular, although each of the wings 3b, 4b defines a substantially planar upper surface, the upper wing surfaces could be slightly concave or slightly convex, and the term "substantially planar upper surface" as used throughout this specification should be construed accordingly.

Claims

Claims 1. A rotary lawnmower blade comprising a mounting portion provided

with means for attaching the blade to the rotary output shaft of a motor, and at least one blade member extending outwardly from the mounting portion, the or each blade member having a cutting portion at its leading edge and terminating in a respective tip having a respective upturned portion at the trailing edge thereof, the or each upturned portion defining a substantially planar upper surface which is contiguous with the adjacent upper surface of the associated blade tip at an angle to the respective leading edge, wherein the or each upturned portion lies at an angle of at least 30 to the respective leading edge, and wherein the upper surface of the or each upturned portion is an imperforate surface.
2. A rotary lawnmower blade comprising a mounting portion provided with means for attaching the blade to the rotary output shaft of a motor, and at least one blade member extending outwardly from the mounting portion, the or each blade member having a cutting portion at its leading edge and terminating in a respective tip having a respective upturned portion at the trailing edge thereof, the or each upturned portion defining a substantially planar upper surface which is contiguous with the adjacent upper surface of the associated blade tip at an angle to the respective leading edge, wherein the or each upturned portion lies at an angle of at least 30 to the respective leading edge, and wherein the upper surface of the or each upturned portion lies at an angle of at most 40 to the plane of the adjacent upper surface of the associated blade tip.
3. A blade as claimed in claim 1 or claim 2, wherein the or each upturned portion lies at an angle between 30 and 60" to the respective leading edge.
4. A blade as claimed in claim 3, wherein said angle is between 39 and 45 .
5. A blade as claimed in claim 4, wherein said angle is substantially 42 .
6. A blade as claimed in any one of claims 1 to 5, wherein the or each upturned portion lies at an angle of between 15 and 40 to the plane of the adjacent upper surface of the associated blade tip.
7. A blade as claimed in claim 6, wherein said angle is between 25 and 29 .
8. A blade as claimed in claim 7, wherein said angle is substantially 27 .
9. A blade as claimed in any on of claims I to 8, wherein the or each upturned portion meets the adjacent upper blade tip surface at a respective bend line.
10. A blade as claimed in claim 9, wherein the or each upturned portion projects rearwardly from the associated bend line by a distance lying between 30 mm and 60 mm, said distance being measured along the plane of the or each upturned portion and in a direction normal to the respective bend line.
A blade as claimed in claim 10, where said distance is between 57 mm and 63 mm.
12. A blade as claimed in claim 11, wherein said distance is substantially 60 mm.
13. A rotary lawnmower blade comprising a mounting portion provided with means for attaching the blade to the rotary output shaft of a motor, and at least one blade member extending outwardly from the mounting portion, the or each blade member having a cutting portion at its leading edge and terminating in a respective tip having a respective upturned portion at the trailing edge thereof, the or each upturned portion defining a substantially planar upper surface which is contiguous with the adjacent upper surface of the associated blade tip at an angle to the respective leading edge, wherein the or each upturned portion lies at an angle of between 30 and 60 to the respective leading edge, wherein the or each upturned portion lies at an angle of between 15 and 40" to the plane of the adjacent upper surface of the associated blade tip, and wherein the or each upturned portion meets the adjacent upper blade tip surface at a respective bend line, and projects rearwardly from that bend line by a distance lying between 30 mm and 60 mm, said distance being measured along the plane of the or each upturned portion and in a direction normal to the respective bend line.
14. A blade as claimed in any one of claims 1 to 13, wherein there are at least two blade members, each of which extends away from the mounting portion.
15. A blade as claimed in claim 14, wherein there are two blade members which extend in opposite directions away from the mounting portion.
16. A blade as claimed in any one of claims I to I S. wherein the or each upturned portion has a substantially triangular configuration.
17. A blade as claimed in claim 16, wherein the base of the or each triangular upturned portion is contiguous with the adjacent upper surface of the associated blade tip.
18. A blade as claimed in any one of claims 1 to 17, wherein the blade is made of steel by a stamping and forming operation.
19. A blade as claimed in any one of claims I to 18, wherein the or each upturned portion is inwardly offset from the free end of the associated blade member by a distance of between 13 mm and 17 mm.
20. A blade as claimed in claim 19, wherein said offset distance is substantially 15 mm.