GB2411567A

GB2411567A - Lawnmower blade

Info

Publication number: GB2411567A
Application number: GB0404901A
Authority: GB
Inventors: Paul Stockley
Original assignee: Electrolux Outdoor Products Ltd
Current assignee: Husqvarna UK Ltd
Priority date: 2004-03-04
Filing date: 2004-03-04
Publication date: 2005-09-07
Also published as: GB0404901D0

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, and at least one blade member 3, 4 extending outwardly from the mounting portion. The or each blade member 3, 4 terminates in a respective tip having a leading, cuffing edge portion 3a, 4a, an upturned intermediate portion 3b, 4b and a trailing edge portion 3c, 4c. The profile of the or each tip portion is such that its upturned intermediate portion 3b, 4b merges with its trailing edge portion 3c, 4c at a point of inflexion.

Description

1 241 1567 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 only 89dB(A) when the engine is positioned on a test rig. It does, however, generate 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 lawnmower 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 lawnmower. 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 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.

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 lawomower blade which has reduced aerodynamic noise generation properties whilst maintaining satisfactory airflow generation characteristics to ensure efficient grass collection.

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 terminating in a respective tip having a first, upturned portion and a second, trailing edge portion, the profile of the or each tip being such that the general direction of its first portion makes a greater angle with the plane of rotation of the blade than the general direction of the second portion.

The term "general direction" should be taken to mean the direction between the start and the end of the relevant blade tip portion.

Preferably, there are at least two blade members, each of which extends away from the mounting portion.

In a preferred embodiment, there are two blade members which extend in opposite directions away from the mounting portion. In this case, each blade member may be laterally offset with respect to the central longitudinal axis of the blade.

In another preferred embodiment, the blade is constituted by a disc, the central portion of the disc constituting the mounting portion, and the or each blade member forming part of the disc, the or each tip being constituted by a respective partially cut-out, downwardly-inclined portion formed at a respective outer circumferential portion of the disc.

Advantageously, the or each first portion is upwardly concave. Alternatively, the or each first portion is planar.

Preferably, the or each second portion has first and second sections, the or each first section having an upwardly convex surface contiguous with the adjacent first portion, and the or each second section having an upper surface which forms a continuation of the respective upwardly convex surface of the adjacent first section. Conveniently, each said upper surface is a planar upper surface.

Alternatively, the or each second portion has a planar upper surface that is contiguous with the adjacent first portion.

In a preferred embodiment, a cutting edge is formed at the leading edge of the first portion. Alternatively, a third portion is provided at the leading edge of the first portion, a cutting edge being formed at the leading edge of the third portion.

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

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 first form of rotary lawnmower blade l O constructed in accordance with the invention; Figure 2 is a representation of the tip of the blade of Figure I compared with the tip of a conventional rotary lawnmower blade; Figures 3 to 6 show alternative blade tip configurations; and Figure 7 shows a perspective view (shown upside-down) of a second form of rotary lawnmower blade constructed in accordance with the invention.

Referring to the drawings, Figure 1 shows a rotary lawnmower blade 1 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 members being laterally offset with respect to the cenka1 longitudinal axis of the blade 1. The blade 1 is made of steel by stamping from a steel blank and then shaping using a forming tool. The shape of the blade 1 is chosen to minimise wastage of steel in the stamping and forming operations.

Each blade member 3, 4 has a tip formed with a sharpened leading edge 3a, 4a, an upwardly concave upturned portion 3b, 4b and a trailing edge portion 3c, 4c which projects to the rear with respect to the direction of rotation (indicated by the arrow R) of the blade 1.

Each trailing edge portion 3c, 4c includes an upwardly convex portion 5 which is contiguous with the respective concave portion 3b, 4b, and a flat portion 6 which is contiguous with the convex portion 5. Each of the flat portions 6 is inclined at a shallow angle to the horizontal, and constitutes a smooth continuation of the surface of the associated convex portion 5, the profile of each blade tip being such that its concave and convex portions meet at a point of inflexion.

As each trailing edge portion 3c, 4c extends from the upper edge of the contiguous upturned portion 3b, 4b at a shallow angle, the departure angle of the airflow from the actual trailing edges of the blade members 3, 4 is substantially less than that for conventional rotary lawnmower blades. This is illustrated in Figure 2 which shows the profile A of the conventional blade and the profile B of the blade of Figure 1 at the tips thereof. Figure 2 also shows the respective airflow patterns C and D associated with the blade profiles A and B. As will be apparent, the airflow C over the conventional blade is subjected to a substantial change in the angle of its flow as it leaves the trailing edge, and the change in the angle of airflow D for the blade of Figure 1 is considerably less. Consequently, the airflow D is much smoother than the airflow C, so that the rotary blade of Figure I produces substantially less turbulence at the trailing edges of its blade tips, leading to a substantial (1.3dB) noise reduction. Moreover, the blade of Figure I produces the same overall airflow as that of the conventional blade, and so produces a sufficiently large airflow to satisfy the requirements for collection of grass clippings cut by the blade.

It will be apparent that modifications could be made to the blade described above. For example, it is not essential that the portions 6 at the extreme trailing edge portions of the blade tips are flat, provided those portions constitute smooth continuations of the surfaces of the associated convex portions 5.

Figures 3 to 6 show the profiles of alternative blade tip configurations. Thus, Figure 3 shows a blade tip having a substantially planar upturned portion 13b and a substantially planar trailing edge portion 13c. Figure 4 shows a blade tip configuration having an upwardly-concave upturned portion 23b and a trailing edge portion 23c having first and second upwardly-convex portions 24 and 25, these two convex portions having different curvatures. Figure 5 shows a blade tip configuration having planar upturned and trailing edge portions 33b and 33c respectively, the leading edge of the upturned portion 33b being formed as a sharpened leading edge 33a. Figure 6 shows a blade tip configuration similar to that of Figure 5, having an upwardly-concave upturned portion 43b and a planar trailing edge portion 43c. Here again, the leading edge of the upturned portion 43b is formed with a cutting edge 43a.

A further reduction in the magnitude of negative pressure behind the upturned portion 3b, 4b of the blade I could be achieved by providing a hole or a series of holes in those portions, to allow some air to bleed through.

Figure 7 shows a modified lawnmower blade 51, the blade being formed from a disc la made of steel. The disc is formed with downwardly-inclined, partially cut-out tabs 53 and 54 at diametrically-opposed positions at the circumferential edge of the disc.

The central portion 52 of the disc 5 la is suitably apertured for detachably mounting the blade 51 to the output shaft (not shown) of the motor of the lawnmower. The tabs 53 and 54 are equivalent to the tips of the blade members of the embodiments described earlier, and these tabs can take any of the forms shown in any one of Figures 1 and 3 to 6.

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 terminating in a respective tip having a first, upturned portion and a second, trailing edge portion, the profile of the or each tip being such that the general direction of its first portion makes a greater angle with the plane of rotation of the blade than the general direction of the second portion.
2. A blade as claimed in claim 1, wherein there are at least two blade members, each of which extends away from the mounting portion.
3. A blade as claimed in claim 2, wherein there are two blade members which extend in opposite directions away from the mounting portion
4. A blade as claimed in claim 3, wherein each blade member is laterally offset with respect to the central longitudinal axis of the blade.
5. A blade as claimed in claim 1, wherein the blade is constituted by a disc, the central portion of the disc constituting the mounting portion, and the or each blade member forming part of the disc, the or each tip being constituted by a respective partially cut-out, downwardly-inclined portion formed at a respective outer circumferential portion of the disc.
6. A blade as claimed in any one of claims I to 5, wherein the or each first portion is upwardly concave.
7. A blade as claimed in any one of claims 1 to 5, wherein the or each first portion is planar.
8. A blade as claimed in claim 6 or claim 7, wherein the or each second portion has first and second sections, the or each first section having an upwardly convex surface contiguous with the adjacent first portion, and the or each second section having an upper surface which forms a continuation of the respective upwardly-convex surface of the adjacent first section.
9. A blade as claimed in claim 8, wherein each said upper surface is a planar upper surface.
10. A blade as claimed in claim 6 or claim 7, wherein the or each second portion has a planar upper surface that is contiguous with the adjacent first portion.
11. A blade as claimed in any one of claims 1 to 10, wherein a cutting edge is formed at the leading edge of the first portion.
12. A blade as claimed in any one of claims I to 10, wherein a third portion is provided at the leading edge of the first portion, a cutting edge being formed at the leading edge of the third portion
13. A blade as claimed in any one of claims I to 12, wherein the blade is made of steel by a stamping and forming operation.