Wednesday, July 30, 2014

Load Composition Part 5

Stability
Longitudinal stability is lost when the center of gravity moves too far forward.
Figure 13. Longitudinal stability is lost when the center of gravity moves too far forward.

Maintaining stability of a powered industrial truck is easier once you understand a few basic principles. There are many aspects of a vehicle's stability:
  • The Stability Triangle
  • Longitudinal Stability
  • Lateral Stability
  • Dynamic Stability
NOTE: This discussion focuses on sit-down counterbalanced forklifts. "Counterbalanced" means that the truck is weighted in the back with counterweight so that it will not tipover when the load is placed on the front. The counterweight is located between or behind the rear wheels and provides a weight in the back to "counterbalance" the weight of the load in the front. For a brief summary of other types of forklifts, see Types and Fundamentals.
 
The Stability Triangle

When the vehicle is loaded, the combined center of gravity (CG) shifts toward line B-C. Theoretically the maximum load will result in the CG at the line B-C. In actual practice, the combined CG should never be at line B-C.
Figure 14. When the vehicle is loaded, the combined center of gravity (CG) shifts toward line B-C. Theoretically the maximum load will result in the CG at the line B-C. In actual practice, the combined CG should never be at line B-C.

The forklift will not tip over as long as the Combined Center of Gravity of the truck and load system remains within the Stability Triangle.
Figure 15. The forklift will not tipover as long as the Combined Center of Gravity of the truck and load system remains within the Stability Triangle.

If the CG shifts outside the boundaries of the stability triangle, the truck will tipover.
Figure 16. If the CG shifts outside the boundaries of the stability triangle, the truck will tipover.
Almost all counterbalanced powered industrial trucks have a three-point suspension system, that is, the vehicle is supported at three points. This is true even if the vehicle has four wheels. The truck's steer axle is attached to the truck by a pivot pin in the axle's center. When this point is connected to the front wheels with imaginary lines, this three-point support forms a triangle called the stability triangle (Triangle ABC where Point A is the pivot point in the rear axle and Points B and C are the front wheels). (Figure 14) [A-4.1, 29 CFR 1910.178 Appendix A] So long as the center of gravity remains within this stability triangle, the truck is stable and will not tip over.

When the forklift is not loaded, the location of the forklift's center of gravity is the only factor to be considered in determining its stability. In Figure 14, the center of gravity is between the axle of the steer wheels at A and the drive wheels at B-C and it is marked with the arrow as the Vehicle Center of Gravity (Unloaded).
In Figure 15, the combined center of gravity of the forklift and its maximum load shifts forward toward the load so that it is now located on the line representing the front axle, at the very edge of the stability triangle. While the loaded forklift is still theoretically stable, in practice the combined center of gravity should never reach this line because sudden stops, starts, and turns could shift the center of gravity further out and destabilize the forklift. 

As seen in Figures 16 and 17, a shift of the center of gravity occurs as the forklift is loaded. The forklift is more stable when it is properly loaded than when it is unloaded. However, improper loading, such as loading the forklift beyond its capacity, or loading an oversize or wide load without adjusting the weight, will cause the forklift to tipover, either laterally on its side or longitudinally forward. The direction of the tipover will depend on where the combined center of gravity shifts outside the stability triangle.

Additional Information:

The Powered Industrial Truck Standard has a non-mandatory Appendix which more fully describes the forces involved and includes figures and definitions:
[29 CFR 1910.178 Appendix A]
  • Lateral stability is a truck's resistance to overturning sideways.
  • Dynamic stability refers to the idea that an unloaded forklift's center of gravity and a loaded forklift's combined center of gravity can shift outside of the stability triangle as a result of certain movements, such as sudden stops and starts, turns, or operating on grades.
  • Line of action is an imaginary vertical line through an object's center of gravity.
  • Load center is the horizontal distance from the fork's or other attachment's vertical face to the line of action through the load's center of gravity.
  • Moment is the product of the object's weight times the distance from a fixed point (usually the fulcrum). In the case of a powered industrial truck, the distance is measured from the point at which the truck will tipover to the object's line of action. The distance is always measured perpendicular to the line of action. [29 CFR 1910.178 Appendix A]
The combined center of gravity of the truck and load system shifts forward outside the stability triangle, as the load's moment is greater than the vehicle's moment, and the forklift tips forward, pivoting on the front axle or fulcrum.
Figure 17. The combined center of gravity of the truck and load system shifts forward outside the stability triangle, as the load's moment is greater than the vehicle's moment, and the forklift tips forward, pivoting on the front axle or fulcrum.

The forklift teeters and finds its balance point. As the load is added to the seesaw, the moment is increased on the right side. The loaded forklift reverses and finds a new balance point at its combined center of gravity.
Figure 18. The forklift teeters and finds its balance point. As the load is added to the seesaw, the moment is increased on the right side. The loaded forklift reverses and finds a new balance point at its combined center of gravity.
View Animation


Use extra caution when carrying loads that approach a vehicle's maximum design limits.
Figure 19. Use extra caution when carrying loads that approach a vehicle's maximum design limits.
Requirements and Recommended Practices:
  • Maintain stability. Keep the combined center of gravity within the stability triangle.
  • Do not accelerate rapidly or brake suddenly. Sudden changes in direction may also shift the combined center of gravity outside the vehicle's stability triangle and destabilize it.
  • Do not turn rapidly. The combined center of gravity may shift outside the stability triangle and may cause the vehicle to tipover to the left or right.
  • Never turn on a grade or ramp. Even a 10 percent grade may shift the combined center of gravity outside the stability triangle and cause the vehicle to roll over laterally.
  • Cross an obstacle (railroad tracks, beam, pot hole) at a 45 degree angle, so both wheels do not elevate simultaneously.
  • Maintain control of your vehicle at all times. Adjust your speed to match the conditions. Be aware and anticipate dangerous motions and avoid them.
  • Consider the dynamic forces that result when the vehicle and load are put into motion. The weight's transfer and the resultant shift in the center of gravity due to the dynamic forces created when the machine is moving, braking, cornering, lifting, tilting, and lowering loads, etc., are important stability considerations. [A-7.1. 29 CFR 1910.178 Appendix A]
  • When determining whether a load can be safely handled, the operator should exercise extra caution when handling loads that are close to the truck's stated capacity.

    If an operator must handle a maximum load:
    • The load should be carried at the lowest position possible,
    • The truck should be accelerated slowly and evenly, and
    • The forks should be tilted forward cautiously.
  • However, no one rule can cover all eventualities. [A-7.2. 29 CFR 1910.178 Appendix A]
     

1 comment:

  1. Each different kind of forklift has a unique center of gravity, depending on the make, model, and type of machine. If the driver isn't educated in the dangers and risks, There could be deadly consequences. I am certainly glad I stumbled upon this post so that I will be better educated behind the wheel.
    Forklift Parts Calgary

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