The center of gravity (CG) is the primary factor governing the safety and stability of a forklift. This heavy machinery is inherently unstable by design, but it becomes stable under specific operating conditions. Understanding the location and movement of the combined center of gravity is necessary for preventing dangerous tipping accidents. Safe forklift operation rests on maintaining balance by managing this invisible, shifting point of equilibrium.
Defining Center of Gravity
The overall balance of a loaded forklift is determined by the interaction of two distinct centers of gravity. The first is the forklift’s own center of gravity, often called the Fixed CG. This point is determined by the truck’s design and the distribution of its permanent components, including the engine, mast, and the heavy counterweight. For an unloaded forklift, this point is generally constant and typically rests toward the rear axle.
The second component is the center of gravity of the load, known as the Variable CG. This point changes with every lift because it depends on the size, shape, and weight distribution of the material being carried. For a symmetrical load, the Variable CG is at the geometric center. However, for irregular items, operators must estimate where the weight is concentrated, and the placement of the load on the forks also makes this point highly variable.
What is the Combined Center of Gravity?
The Combined Center of Gravity (CCG) is the theoretical point where the weight of the entire system—the forklift and its load—is perfectly balanced. This point represents the total mass of the machine and the material being moved. The CCG is dynamic and moves constantly during operation, unlike the forklift’s Fixed CG.
As soon as a load is picked up, the forklift’s original center of gravity shifts forward and upward, merging with the load’s Variable CG to create the CCG. The front axle acts as the pivot point, similar to a seesaw. The CCG is the point where the weight of the counterbalanced machine and the weight of the load perfectly counteract each other.
Understanding the Stability Triangle
The physical space the Combined Center of Gravity must remain within for safe operation is called the stability triangle. This imaginary area is formed by connecting three points of support: the center of the rear axle and the two front load wheels. The forklift remains stable only as long as the vertical line extending down from the CCG stays inside the boundaries of this triangle.
The stability triangle defines the limits of the forklift’s longitudinal and lateral stability. If the CCG moves outside the front boundary, usually due to an overloaded or high-lifted load, the forklift will tip forward. If the CCG shifts outside the side boundaries, often during a turn, the machine will tip sideways. As the load is raised, the triangle effectively becomes a pyramid in three dimensions, requiring the CCG to stay within a smaller space for stability.
Factors That Shift the Combined Center of Gravity
Several operational factors directly influence the position of the Combined Center of Gravity, increasing the risk of a tip-over accident. Lifting the load higher shifts the CCG vertically upward, making the forklift less stable. Tilting the mast forward also shifts the CCG forward, pushing it closer to the stability triangle’s front edge.
Driving dynamics also affect the CCG, even with a properly loaded machine. Centrifugal force generated during acceleration, sudden braking, or sharp turning causes the CCG to shift rapidly, especially in the lateral direction. Operating the forklift on an uneven surface, such as a slope or a pothole, can also temporarily shift the CCG outside the stability boundaries.
Practical Implications for Safe Forklift Operation
Understanding the Combined Center of Gravity is the theoretical basis for nearly all safety rules for operators. The primary rule derived from CCG principles is to travel with the forks positioned as low as possible, ideally four to six inches off the ground, to keep the CCG low and maximize stability. Operators must also consult the forklift’s data plate, which lists the maximum load capacity based on a specific load center distance.
This capacity rating is calculated to ensure the CCG remains within the stability triangle under controlled conditions. Moving with smooth, controlled movements, rather than sudden stops or sharp turns, prevents the CCG from shifting outside the stability boundaries. Loads should always be centered and pressed firmly against the carriage to keep the load’s center of gravity as close to the machine as possible.