Principles of Lifting Loads
Fulcrum Principle
The fulcrum is the forklift's axis of rotation when it tips over. Whether a forklift is stable depends on the its load moment at one end of the system being greater than, equal to, or less than the forklifts moment at the system's other end.
This principle can be seen in the way a see-saw works: that is, if the product of the load and distance from the fulcrum (moment) is equal to the moment at the device's other end, the device is balanced and it will not move. However, if there is a greater moment at one end of the device, the device will try to move downward at the end with the greater moment.
During a forward tip, the forklift rotates around the line through the points where the front wheels contact the ground. The forklift's weight and counterweight help balance the load carried on the forks.
Center of Gravity (CG)
The center of gravity (CG) is the point at which an object's weight can be considered concentrated. An unloaded forklift has its own CG, and a load has its own CG. When the forklift picks up a load, the forklift and load form a combined system with a new combined center of gravity.
The position of the combined CG changes with the weight and position of the load. As a load is moved forward or raised, the combined CG also shifts. The location of the combined CG is an important factor in determining forklift stability.
Stability Triangle
Almost all counterbalanced powered industrial trucks have a three-point suspension system, that is, the vehicle is supported at three points.
Stability Triangle
This is true even if the vehicle has four wheels. The forklift's steer axle is attached to the truck by a pivot pin in the axle's center. When the points are connected with imaginary lines, this three-point support forms a triangle called the stability triangle.
The two front wheels form two points of the stability triangle. The third point is the pivot point at the center of the rear steer axle.
Line of Action: For the stability triangle, the line of action is an imaginary vertical line extending through the combined center of gravity (CG) to the ground. When the vehicle's line of action falls within the stability triangle, the vehicle is stable and will not tip over. However, when the vehicle's line of action or the vehicle/load combination falls outside the stability triangle, the vehicle is unstable and may tip over.
For example, a heavier load or a load positioned farther forward shifts the combined CG toward the front of the stability triangle. Raising or tilting a load can also change the combined CG. Turning, accelerating, braking, traveling on a slope, or operating on uneven surfaces will affect the line of action and forklift stability.
Stability Pyramid:
The stability triangle represents the forklift's support area at ground level, but forklift stability is three-dimensional. One way to visualize this is to imagine a stability pyramid extending upward from the stability triangle.
Stability Pyramid
As the pyramid extends upward, its boundaries become narrower. This illustrates an important stability principle: as the combined CG rises, there is less room for its line of action to move before reaching a stability boundary.
One way to understand how a forklift's stability pyramid affects stability is to visualize a child riding a tricycle, which has a similar stability structure. If the child turns the tricycle too sharply, the combined center of gravity (CG) of the child and tricycle may move outside the stability boundaries, causing the tricycle to tip over.
Line of Action: For the stability pyramid, the vertical line of action must remain within the boundaries of the stability pyramid which narrows as the elevation of the combined CG rises.
This is why operators should travel with the load lowered to the appropriate traveling position rather than elevated. An elevated load raises the combined CG and reduces the margin for movement before a stability pyramid boundary is reached. Turning, braking, accelerating, traveling on slopes, or traveling over uneven surfaces can further affect stability. If the line of action moves outside the applicable stability boundary, the forklift can tip over.
Knowledge Check Choose the best answer for the question.
2-3. Which term states that a forklift has two weights (load weight and forklift counterweight), which are balanced in a way similar to a playground seesaw?
You forgot to answer the question!