06 Mar Mass and Moment of Inertia
In a static [https://www.fea-solutions.co.uk/static-vs-dynamic-analysis/] FEA analysis, the mass [https://www.fea-solutions.co.uk/mass-and-weight/ ] of each part is determined using its density and volume, with the FEA software calculating its mass and centre of gravity (CoG). The CoG is the point where the mass effectively acts.
In a dynamic [https://www.fea-solutions.co.uk/types-of-dynamic-analyses/] FE analysis, the software also calculates an objects moment of inertia. The moment of inertia describes how highly a part resists rotational acceleration about a certain axis. Moment of inertia depends not only on the object’s mass, but also on how that mass is distributed relative to the axis of rotation. In mechanics, flywheels have a large moment of inertia due to their high mass and radius. This makes them resistant to speed fluctuations and capable of smoothing the power output of engines and other machinery. Moments of inertia are particularly important when rotational parts are rotated around another axis, as the moment of inertia will affect the resultant gyroscopic loads induced due to the rotation.
In FEA, for parts where stress results aren’t needed but which still represent a significant mass that could affect the outcome of the analysis, point masses can be used. These are defined by a CoG and its location in the assembly, a mass and a moment of inertia along the CoG. It should be noted that two objects could have the same CoG and mass, but different moments of inertia.
Product of inertia (POI) is a measure of an object’s ‘imbalance’ of mass relative a set of perpendicular axis. If an object’s mass is evenly distributed on XY, YZ and ZX planes, there is no product of inertia, and therefore no imbalance. An example of POI is a car’s tire. For a car to be safe, a tire’s weight should be evenly distributed along the tire’s contact with the ground. If the rim is damaged and the tire becomes unbalanced, the driver is likely to notice an uneasy ride and accelerated wear.
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