Manuel GSPAN

Numerical and experimental modal analysis of a lawnmower body

Nowadays, lawn care by private persons and professional applicants is supported by lawnmowers of various sophistication. Strong competition and varied applications force manufacturers of lawnmowers to continuously enhance their products. Computer-assisted analysis and simulation support the efficiency of product development, or even make it possible. In this master thesis, the basis for a computer-assisted simulation model in terms of a multi-body system of the professional lawn mower MB756, produced by the company VIKING, is laid. Modal analysis, both analytically, numerically and experimentally, provides the natural frequencies and mode shapes of the considered lawnmower. Due to the elaborate geometry of the lawnmower housing, numerical modal analysis is based in a finite element model, solved efficiently with powerful commercial software packages running on fast computers. Numerical modal analysis provides useful information about natural frequencies and mode shapes of the considered object during the development and construction phase. Experimental modal analysis serves essentially the system identification, as well the verification of numerical respectively analytical results. With todays knowledge on measurement systems, accurate test benches can be built very quickly. In the present study, test setup for the investigated lawnmower and applied methodology were validated and verified using a simple beam model. Based on the outcomes it can be concluded that numerical simulation and measurement technology are suitable to identify natural frequencies of this test object with a maximum relative error of three percent. The analytical results of the test beam serve as basis for a convergence analysis investigating various finite mesh configurations. The outcomes of numerical and experimental modal analysis of the lawn mower body, including the four-wheel suspensions, almost coincide with a maximum relative error of eight percent. The development of a multi-body model for the casing considering also wheel suspension and knife assembly could not be implemented in the first step. The knife assembly includes engine, clutch, knife and knife mount. Rigid coupling of the knife assembly as a mass point with rigid massless beams to the casing is shown to be too stiff, and thus, with such a numerical model the experimentally determined eigenmodes could not be confirmed.



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