An experimental assessment of lawnmower blade loading

Abstract

An experimental study of the loading to which a lawnmower blade is subjected during normal operation has been performed. The blade of a consumer-grade walk-behind electric lawnmower was instrumented with strain gauge bridges and strain data were collected using a slip ring assembly and personal-computer-based data acquisition system with integral amplifier. Blade strains were monitored over a range of grass-cutting conditions. It was found that blade strain was dependent principally upon the rotational speed, with no perceptible effect due to cutting conditions, other than the indirect effect due to blade speed changes. The measured strains also compared well with calculated strains, particularly at low speeds. At higher speeds, there were small differences between measured and calculated strains and these differences are attributed to the effects of blade deflection. The results of this study suggest that blade stresses can be accurately calculated using models which include only the effects of rotation. Relative to other possible types of loading (i.e. aerodynamic and grass impacts), rotational effects are easily modelled. At higher speeds, models should include the effects of large-displacement non-linearities to account for the effects of blade deflection. These findings will enable engineers and designers to perform analyses that will minimize the repetitive and often extended empirical testing in arriving at a final mower blade design. Where empirical testing is required, these results indicate that such testing should be based on blade rotation rather than on static fixtures in which cyclic deflection of the blade is applied.