DAMPING PROPERTIES OF PLUM TREES SHAKEN AT THEIR TRUNKS

Abstract

Investigations of the energy requirements of tree shakers have shown that the measured energy far exceeded theenergy predicted by theoretical calculations. Two main reasons have been established for this: first, a given root and soil massis also part of the vibration system, and second, the existing methods for determining damping coefficients for soils and thetree canopy (drop test, hammer striking) are not suitable, especially if displacements are of great amplitudes. In the currentstudy, a direct energy method was used to measure effective damping coefficients of the root-soil body and of primary limbs.Measurements have shown that the vibrating soil mass has a large damping capacity, and its logarithmic decrement approaches. (i.e., the energy requirement equals the theoretical working capacity of the vibrating system). Measurements werecarried out in a 12-year-old plum orchard. Trunk diameters varied between 130 and 250 mm, the respective canopy radiusvaried between 2.2 and 3.0 m, and each tree had five to seven primary limbs. The total damping loss originated mainly fromthe vibrating soil mass and tree canopy. A calculation method was developed to determine these loss contributions. By varyingthe attachment height on the trunk, different contribution ratios were achieved and the calculated and measured values werecompared.