6-component propeller blade/shaft balance provides important insight


René Bosman & Do Ligtelijn r.bosman@marin.nl


MARIN designed a podded propulsor model with an internal motor and rotating 6-component force and moment shaft balance.


FEM and 3D CAD software the balance design can be optimised in such a way that it will not fail as a result of high stress concentrations.


After it is designed, the metal balance is accurately machined with lathe, milling and wire EDM manufacturing techniques. The strain gauges are applied on the balance very carefully and with great accuracy. A special flexible coating is applied on the strain gauges and on the lacquer wires to make the balance watertight. This coating is so flexible that it has no influence on the creep and hysteresis properties of the balance.


Setup for measuring forces and moments on one specific blade (1) T


his rotating balance is designed to measure the 3 forces and 3 moments directly on the propeller


shaft, without any interference from seals or bearings. By applying a special bypass the balance can be used to measure forces and moments on one propeller blade.


It has been used during the last decade in Joint Industry Projects and commercial projects to measure ice loads on propeller blades and effects of cavitation and ventilation, amongst other applications.


18 report


Design and manufacturing During the design process of a forces and moments balance for model testing, it is always challenging to fulfil the specifications for measuring hydrodynamic loads. This is es- pecially when considering that these loads have to be corrected for gravity and inertia loads as well. The natural frequencies of the measuring setup should not interfere with the signal frequencies that have to be measured. In a limited space the balance should measure the forces and moments with an acceptable accuracy. With the current


The excitation voltage of the Wheatstone strain gauge bridges and the measuring signals are transferred to the fixed part of the test setup by a 24 channels’ slip ring set. In the near future this will be done by an inductive telemetry system to avoid slip ring maintenance issues.


Calibration and accuracy The calibration of the balance is carried out by our calibration team with specially made tools and standard calibrated dead weights. For this balance a load schedule with 230 calibration points was designed with 18 different load cases, which is necessary to describe the linear and quadratic terms of the calibration matrix.


After the calibration, we analyse the results to check if it is done in a statistically acceptable manner. Hereby it is certified that there is enough information to describe


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