Wednesday, July 23, 2014

Load Composition Part 4

Balance
Stability Triangle formed by connecting the three support points of a powered industrial truck's suspension system.
Figure 8. Stability Triangle formed by connecting the three support points of a powered industrial truck's suspension system.
Understanding why forklifts tipover and why loads become unstable and fall is very important to preventing accidents. Important factors that affect a forklift's balance are:
  • Center of Gravity

  • Shifting Center of Gravity
Center of Gravity

Potential Hazards:

While operating a forklift, be aware of the following:
  • Tipover

  • Falling load
Use special care when carrying a wide load. Turn slowly to prevent the load from shifting.
Figure 9. Use special care when carrying a wide load. Turn slowly to prevent the load from shifting.
Requirements and Recommended Practices:
  • Distribute the weight evenly when carrying irregular sized loads (Figure 9). [29 CFR 1910.178 App A]

  • Keep the center of gravity of the load as near as possible to the center going horizontally across the forks.

  • Keep the center of gravity of the load as near to the front wheels as possible.
     
Shifting Center of Gravity

The Center of Gravity (CG) is in the center of a symmetrical load but is off center in an irregular load. In the third example, the CG is outside the boundaries of the object.
Figure 10. The Center of Gravity (CG) is in the center of a symmetrical load but is off center in an irregular load. In the third example, the CG is outside the boundaries of the object.

A 4,000 pound truck is balanced by a 4,000 pound load.
Figure 11. A 4,000 pound truck is balanced by a 4,000 pound load.

Notice the center of gravity of the load and truck system shift forward toward the front wheels as the load is engaged.
Figure 12. Notice the center of gravity of the load and truck system shift forward toward the front wheels as the load is engaged.

 
All objects have a specific center of gravity. Gravity is a force that always pulls objects toward the earth's core. Center of gravity means the point on an object at which all of the object’s weight is concentrated and all of the parts balance each other. For symmetrical loads, the center of gravity is at the middle of the load in terms of the load’s length, width and height. (Figure 10). Since the capacity of the forklift is based on the assumption of a cube having the center of gravity in the middle, the shape and position of the actual load are key factors when determining whether a load can be carried safely.

When a load is placed on a forklift, the key concept is the combined center of gravity of the forklift and the load. For example, a typical unloaded forklift weighing 4000 pounds may have its center of gravity about 10 inches (25.4 cm) above and two feet (0.6 m) behind the front axle, about half way up the truck body. The heavy counterweight located toward the rear of the forklift places the center of gravity toward the rear, which keeps the forklift from tipping forward. In the meantime, a 4,000 pound load consisting of a cube with even weight distribution has a CG in its center. When the load is placed on the forklift, the combined center of gravity of the forklift and the load will move forward, but the forklift will not tipover so long as the weight of the load is centered and does not exceed the capacity stated on the data plate. But if the load is too heavy, or if it is placed at the end of the forks so that the load center distance is increased, the excessive load moment will cause the forklift to tip forward. Remember, when the forklift engages a load, the combined center of gravity of both the load and the truck system shift forward from the center of gravity of the unloaded forklift. (Figure 11 and 12)


Potential Hazards:

Be aware of tipover or falling loads while:
  • Operating a forklift, as the center of gravity shifts.
     
  • Engaging or depositing a load.
Requirements and Recommended Practices:
  • Handle loads within the capacity of the truck as stated on the data plate. [29 CFR 1910.178(o)(2)]

  • Do not operate a forklift if the back wheels begin to lift off the ground. This is an indication that the forklift is overloaded. The center of gravity has shifted too far forward over the axle of the front wheels and the forklift may teeter on the wheels.

  • Handle only stable or safely arranged loads. Exercise caution when handling off-center loads that cannot be centered. [29 CFR 1910.178(o)(1)]
  • Tilt the mast forward cautiously when positioning the load onto the stack. [29 CFR 1910.178 App A]

  • Never travel with the load elevated. Elevating the load increases the load center distance by shifting the center of gravity upward and forward, making the forklift and the load less stable (Figure 5).

  • Adjust long or high (including multiple-tiered) loads which may affect capacity. [29 CFR 1910.178(o)(3)]

  • Keep the center of gravity of the load as low to the ground and as close to the front wheels as possible:

    • Carry the load at the lowest position possible, 4 to 6 inches from the ground.

    • Tilt the mast back and position the heaviest part of the load against the carriage.

    • Travel with the mast tilted back to stabilize the load.

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