ME210 - Engineering Design Graphics



	Mass Properties Analysis

"Mass Properties" are physical attributes of a mechanical part that relate to how the component will behave in an environment. These properties include familiar attributes such as the weight and volume of the object. They also include more advanced physical information such as its center of mass and moments of inertia. Below is Table 1 that describes the type of Mass Properties that one can generate from a solid computer model.

Table 1 - SOLID MODEL MASS PROPERTIES


1.	SURFACE AREA: The surface area is the total area of the boundary surfaces defining the solid model.

2.	BOUNDING BOX: The bounding box is a rectangular box that encloses the solid model and fits exactly the model's upper and lower X, Y, and Z extents.

3.	VOLUME: The volume of a body is the total volume of space enclosed by its boundary surfaces.

4.	CENTROID: Centroid of a volume is the origin of coordinate axes for which first moments of the volume are zero. It is considered center of a volume. For a homogeneous body in a parallel gravity field, mass center and center of gravity coincide with the centroid.

5.	MASS: The mass of a body is the measure of its property to resist change in its steady motion. The mass depends on the volume of the body and the density of the material of which the body is made.

6.	MOMENTS OF INERTIA: A moment of inertia is the second moment of mass of a body relative to an axis, usually X, Y, or Z. It is a measure of body's property to resist change in its steady rotation about that axis. It depends on body's mass and its distribution around the axis of interest.

7.	PRODUCTS OF INERTIA: Products of inertia are expressions similar to moments of inertia. They describe the lack of symmetricity in mass distribution relative to two axes of interest.

8.	PRINCIPAL MOMENTS OF INERTIA: Principal moments of inertia are extreme (maximal, minimal) moments of inertia for a body. They are associated with principal axes of inertia which have origin at the centroid, and the direction of each usually given by the three unit-vector components. For these axes, the products of inertia are zero.

9.	RADII OF GYRATION: The radius of gyration is the distance from the axis of interest where all the mass can be concentrated while still yielding the same.

Below in Figure 1 is a solid model of the Rocker Arm mechanical component. Following the picture in Table 2 is a computer printout of its Mass Properties.

Figure 1 - A Solid Model of the Rocker Arm.

Table 2 - Computer Printout of the Mass Properties Report for the Rocker Arm.

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SOLIDS

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Mass:

2.4286

Volume:

2.4286

Bounding box:

-0.8750 -- 0.8750

-0.8750 -- 2.2500

0.0000 -- 1.0000

Centroid:

0.0000

0.4919

0.4567

Moments of inertia:

2.9722

1.1745

2.7730

Products of inertia:

XY:

0.0000

YS:

0.4457

SX:

0.0000

Radii of gyration:

1.1063

0.6954

1.0685

Principal moments and X-Y-Z directions about centroid:

1.8779 along [1.0000 0.0000 0.0000]

0.6614 along [0.0000 0.9979 -0.0654]

2.1918 along [0.0000 0.0654 0.9979]

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		© 2000 ME College of Engineering at UT Austin