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Submitted on 1 Jan 1990
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MOWER ATTACHMENT NOISE
G. Mosdzianowski
To cite this version:
G. Mosdzianowski. MOWER ATTACHMENT NOISE. Journal de Physique Colloques, 1990, 51 (C2),
pp.C2-197-C2-200. �10.1051/jphyscol:1990247�. �jpa-00230668�
ler Congres Franqals d'Acoustique 1990
MOWEH ATTACHMENT NOISE
G. MOSDZIANOWSKI
FIGE GmbH Forschungslnstitut DerSusche und Erschiitterungen, Technologiepark,Kaiserstrasse 100, D-5120 Herzogenrath, F.R.G.
Abstract: The noise produced by grass mowing attachments fitted to small tractors and combining a number of rotary blades with a total cutting width of up to 120 cm was investigated. The paper develops a noise reduction concept based on an analysis of sound sources.
1 - INTRODUCTION
The EC Guideline for lawn mowers /!/, which establishes a harmonized measuring procedure and a maximum permitted sound power level dependent on cutting width, basically applies to all powered mowing devices irrespective of the form of cutting mechanism employed. Within the European Community, it is still not clear to what extent mower attachments fitted to small tractors are covered by the envisaged exemption for "non-independent devices". Particularly in the case of multi-blade rotary mower attachments with cutting widths of up to 120 cm, leading manufacturers do not expect to be able to comply with the exacting EC limit of 100 dB(A) in the foreseeable future. Against this background, the Federal Environment Agency (Urn- weltbundesamt, UBA) in Berlin commissioned FIGE to investigate state-of-the-art noise reduction technol- ogy for commercial devices of this type, to analyze the significant sound sub-sources and, finally, to develop a concept for a low-noise prototype, using machines provided by mower manufacturers. A final report on this project will be presented to the UBA in the spring of 1990.
2 - METHODS
Investigations were based on repeated noise measurements with specific modifications of acoustically significant machine components. Instead of the usual tractors, a low-noise electric test bench motor with continuously variable speed was used to power the mower attachments.
Noise measurements were fundamentally carried out as specified in the EC Guideline, using the enveloping surface method on the prescribed "artificial ground". In conformity with the Guideline, both the initial situ ation and the results of noise reduction measures were documented via 6 measuring points in the far field on a measuring hemisphere of 4 m radius, and the A-weighted sound power level determined. To simplify test conditions, however, the comprehensive intermediate tests on the various detail modifications, where only the differences in emissions were of interest, were carried out in a closed test room with 6 measuring points in the near field on a measuring cube with 0.5 m contour distance.
Since the practical effectiveness of the attachments was not to be put at risk, it was necessary to maintain the minimum circumferential velocity of the rotating blades needed for unexceptionable grass cutting and an air flow from the wing blades to the outlet of the grass duct sufficient to ensure reliable removal of the cut grass (cf. Fig. 1). Effective grass removal depends not only on the strength of the air or grass flow, but on its distribution over the cross-sectional area of the grass duct 12,1. Each noise measurement was therefore accompanied by flow measurements in which the air flowvelocity in the grass duct exit plane was continuously determined via a grid of 26 uniformly distributed measuring points. A specially-developed hot-wire ane- mometer with an ultra-compact sensor, linked to a computer, was used for this purpose. The reference for all the mower attachment variants investigated in the tests was the flow profile of the original commercially- proven model.
Résumé: Une analyse expérimentale a été effectuée sur des faucheuses à plusieurs lames utilisées avec des microtracteurs comme tondeuses à gazon, dont la largeur de coupe allait jusqu'à 120 cm. L'étude des différents mécanismes de génération de bruit a permis de développer un concept de réduction du bruit dont les résultats seront présentés.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990247
COLLOQUE DE PHYSIQUE
3
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STATE OF THE ARTA t ~ t a l of 5 different commercial devices with a maximum cutting width of 120 cm were investigated;
technical data and measured emission values for these devices appear in Table 1. It will be evident that all emission levels significantly exceed the 100 dB limit for these devices. Particularly notable were Types la and lb, with severe vibrations in their open cover plates. The following section contains a more detailed discussion of the causes for the high emissions and possible means of reducing them.
4
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SOUND SUB-SOURCESFundamentally, the contributors to mower attachment noise may be sub-divided into:
the transmission,
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the housing andthe cutting space with its rotating wing blades.
Fig. 1 shows a bottom view of a typical mower attachment. The cutting space contains 3 wing blades, rotating in the same direction, and a gradually widening grass removal duct (in the lower half of the illustration); the transmission mounted on the top of the housing is not visible in the figure.
Transmissions conveying power from the input shaft to the cutting tools do not constitute a fundamental noise problem on mower attachments. Systems with input bevel-gear drives, rigid shafts and additional bevel-gear drives to the rotating blades (e.g. Types l a and lb) are undoubtedly unfavourable from the noise standpoint. Conversely, low-noise conditions can be achieved with a V-belt drive to the blades, as in Type 3.
The housing of a mower attachment also functions as an acoustic screen between the cutting space and the external space above it. It should therefore completely enclose the cutting space, except for the indispensable mowing gap between the housing and the ground, and should be made of relatively solid material to achieve high airborne sound insulation. Loose, vibrating sub-components should be avoided.
The cutting space and the rotating blades may be regarded as a single complex decisively influencing total sound radiation. Investigations were therefore focussed on a device with relatively quiet transmission and a relatively solid housing (Type 3). Specific modifications were made to this version, singly and in combination, in terms of blade shape, cutting space geometry and working speed.
5
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NOISE REDUCTION CONCEPTThe wing blades were an initial target for noise-influencing modifications. Different shape variants (length, width, cross-section,wing shape) were investigated. As expected, a reduction of the wing flow surfaces proved particularly effective. In a first successful variant, the only change made in relation to the original blade geometry was to reduce wing height (by approximately 40%). In a second variant, based on the literature /u, the wings were folded at an angle of 45" to the longitudinal axis of the blade and inclined at an angle of 45" to the cutting plane. A third variant adopted the above shape with a wing folded at 45" to the blade axis, but the flow surface was reduced still further by selecting an angle of inclination of only 35", to achieve still lower noise emissions.
These noise-reducing modifications to the wing blades could, however, represent only part of the solution.
Quieter blades with altered flow behaviour cannot be allowed to affect the necessary air or cut-grass flows and thus detract excessively from the mower attachment's effectiveness.
Decisive improvements were therefore also made in flow conditions throughout the cutting space. Adjacent blades moving in opposite directions naturally produce eddy flows in the boundary zone, causing additional noise emissions (cf. Fig. 1). The two component flows partially compensate each other, restricting the resulting total flow. Extra guide plates between the blade paths, with the smoothest possible transitions to the grass duct, were used to achieve significant improvements in the airflow.
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RESULTSThe various modifications made in order to reduce noise frequently also reduce the air or cut grass flow.
From the complete range of variants investigated, only those attaining a considerable reduction in level without excessive flow reductions are of interest. A combination of improved blade geometry, additional guide plates and reduced working speed has proved to be the optimum solution. The improved blades (wings folded at 45" and inclined at 45") are characterized by relatively small flow surfaces, but the guide plates in the cutting space result in more effective airflows, finally permitting an approximately 20 % reduction in speed and an overall improvement in emissions of more than 6 dB. Fig. 2 contrasts this result (right) with the initial situation (left). In each case, the flow profile at the outlet is shown, with flow speed isozones at intervals of 2 m/s. It will be apparent that the improved variant has a more uniform profile, with virtually no change in the total flow (676 Vs as against 663 Us), and an emission level of 97 dB, some 6 dB below the original figure and thus 3 dB below the limit.
Fig. 3 collates all results: individual modifications (reduced speed, additional guide plates, improved wing blades) either achieve improved emissions at the cost of less favourable flow values (A, C) or result neither in significantly improved levels nor in worsened flows (B). Combinations of any two of these measures fail to reduce levels adequately (A
+
B) or achieve considerable reductions in level but substantially worsen flows (A+
C, B+
C). Only a combination of all three types of modification leads to the desired effect.The low-noise prototype has proved successful in initial mowing tests. Further practical tests will, however, be necessary in order to obtain an optimum balance between low-noise and mowing efficiency. It is also essential that a sufficiently low-noise tractor be used to power the mower attachment (emission level below 97 dB), in order to ensure that the total emission for the combined unit is minimized and, particularly, that the 100 dB limit is not exceeded.
111 84/538/EEC, OJ No. L300,19.11.1984, p. 171 and 87/252/EEC, OJ No. L 117,05.05.1987, p. 22.
I21 Scheufler, B., doctoral thesis, Institut fiir Landmaschinen, Technische Universitat Braunschweig, BraunschweiglFRG (1984), 171 pages.
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Table 1: Technical data and sound power levels of commercial mower attachment
Fig. 1: Typical mower attachment, bottom view
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= 663 l/s LWA = 103,4 dB $ = 676 11s LWA = 97,l dB initial situation optimum solution
Fig. 2: Variations of air speed in the exit plane of the grass duct
Fiq. 3: Influence of various modifi- cations, singly and in com- bination, on volume flow V and on sound power level LWA
A speed reduction B deflectors
C improved blade wings
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A B C A*B A* B e A*B<
