Now we’ll calculate the area moment of inertia for the rectangular section centered on this line CD. We would then assume that one of the lines will pass through the rectangular section’s base. We’ll start with one rectangular section ABCD, as shown in the figure below. I y = hb 3/ 12 Calculating Moment of Inertia of Rectangle Section We’ll get the following equation as a result: (3) A Centroidal Axis Perpendicular to Its Baseīy alternating the dimensions b and h from the first equation given above, we can determine the moment of inertia of a rectangle by taking the centroidal axis perpendicular to its base. D is said to be the perpendicular distance between the x and x’ axes.I x = moment of inertia in arbitrary axis.If we recognize the moment of inertia of the non-centroidal axis with respect to a centroidal axis parallel to the first, we can find it here. The parallel axis theorem could be used to calculate the area moment of inertia of any shape present in any parallel axis. It is seamlessly determined by applying the Parallel Axis Theorem because the rectangle centroid is located at a distance equal to h/2 from the base. The moment of inertia of a rectangle has been expressed as follows when an axis passes through the base: (2) Moment of Inertia of Rectangle An Axis Passing Through Its Base The rectangle width (a dimension parallel to the axis) has been denoted by b, and the height is denoted by h (dimension perpendicular to the axis). In the particular instance where the axis passes through the centroid, the moment of inertia of the rectangle is given as follows: When sizing linear systems, the most important use for mass moment of inertia is probably in motor selection, where the ratio between the load inertia and the motor inertia is a critical performance factor.(1) Moment of Inertia of Rectangle An Axis Passing Through Its Centroid The mass moment of inertia equation for a point mass is simply:įor a rigid body, the mass moment of inertia is calculated by integrating the mass moment of each element of the body’s mass: Mass moment of inertia, like planar moment, is typically denoted “I,” but unlike planar moment, the units for mass moment of inertia are mass-distance squared (slug-ft 2, kgm 2). It has the same relationship to angular acceleration that mass has to linear acceleration. Mass moment of inertia (also referred to as second moment of mass, angular mass, or rotational inertia) specifies the torque needed to produce a desired angular acceleration about a rotational axis and depends on the distribution of the object’s mass (i.e. I = planar moment of inertia Mass moment of inertia Cantilever beam with a concentrated load at the free end Unsupported shafts are also analyzed using beam deflection calculations. In linear systems, beam deflection models are used to determine the deflection of cantilevered axes in multi-axis systems. The planar moment of inertia of a beam cross-section is an important factor in beam deflection calculations, and it is also used to calculate the stress caused by a moment on the beam. The equation for polar moment of inertia is essentially the same as that of planar moment of inertia, but the distance used is distance to an axis parallel to the area’s cross-section. Second moment of area can be either planar or polar. Polar moment of inertia describes an object’s resistance to torque, or torsion, and is used only for cylindrical objects. Planar moment of inertia is expressed as length to the fourth power (ft 4, m 4). If it’s unclear which type of moment is specified, just look at the units of the term. Terminology varies, and sometimes overlaps, for planar moment and mass moment of inertia. Planar moment of inertia (also referred to as second moment of area, or area moment of inertia) defines how an area’s points are distributed with regard to a reference axis (typically the central axis) and, therefore, its resistance to bending. But it’s critical to know which type of inertia-planar moment of inertia or mass moment of inertia-is given and how it affects the performance of the system. Moment of inertia is an important parameter when sizing and selecting a linear system.
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