With mobile cranes, there are three basic components that have individual center of gravity points. These are the superstructure, carrier and boom. Each of these components has its own center of gravity. Depending on how each piece is positioned, it can affect the other's center of gravity. Let’s break down each component and see what it does.
The superstructure, also known as the upper structure, is the revolving frame which holds the operator's cab. Usually the upper structure supports the counterweights on rear, then the boom and/or other attachments on the front. With this component, no matter the position of the crane the center of gravity will always remain the same.
The carrier, also known as the car body, is the undercarriage of the crane. Depending on the type, the carrier usually holds the carrier cab, wheels/tracks, outriggers, etc. It is manufactured for transporting the superstructure. Unlike the superstructure where the center of gravity will always remain the same, the center of gravity will vary on the carrier. This is reliant on the location of the direction of the boom. For example, when the boom is over the rear axles, the center of gravity will be forward of the rear axles. If the boom is perpendicular to the carrier, the center of gravity will be center of the carrier’s width.
The last component to keep in mind is the boom. Just like the other components it as well has a center of gravity to keep in mind. The center of gravity can be affected depending if you are increasing or decreasing the length of the boom. Unless one is using a lattice boom. In this case, the center of gravity will remain the same until a jib or other ancillary items is added.
The boom is a common suspect to tipping accidents. As the boom raises or lowers through an arc, the boom's center of gravity moves toward or away from the tipping axis. When the movement of a hydraulic boom is combined with the movement through the arc, a tipping accident is likely.
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Source: Part 3: Mobile Crane Stability – Taking the Crane Apart
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