Wednesday, July 2, 2014

Load Composition Part 3

Maximum Load Moment
Lifting a 5 lb box directly up has the effect of lifting 5 lbs.
Figure 5. Lifting a 5 lb box directly up has the effect of lifting 5 lbs.

 As the distance increases from the shoulder, the moment, or apparent weight increases so that a 5 pound box seems to weigh 12 pounds.
Figure 6. As the distance increases from the shoulder, the moment, or apparent weight increases so that a 5 pound box seems to weigh 12 pounds.
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The way in which weight is distributed changes the amount of weight the lift truck will safely carry. You can experience this for yourself by doing the following activity:

Lift a 5 pound box. As you extend your arms, the center of the box’s weight moves a greater distance from your body, so the box feels heavier and you will tend to fall forward. The same idea of increasing the load center distance applies to a playground see-saw: the farther you sit from the middle, the more you increase the load center distance and the more force you put on that end. The same principle—increasing the load center distance—can cause a forklift to tipover.

When the load center distance increases, it is actually increasing something called the "Load Moment":

Load Moment is the product of the object's weight multiplied by the object’s distance from the fulcrum, which is a fixed point that acts as the pivot point. On a sit-down counterbalanced forklift, the fulcrum or pivot point is the axle of the front wheels. It is this product, or Load Moment, which determines how much overturning force is being applied to the forklift.

Load Moment = Weight X Distance

Because the overturning force depends on both the weight of the load and the load’s distance from the pivot point, a forklift’s capacity is always stated in terms of both: the load’s weight and its load center distance. For example, if a forklift’s capacity as stated on its data plate is “3,000 pounds at a 24 inch load center,” this means that the Load Moment cannot safely exceed 72,000 inch-pounds (24-in. x 3,000 lb = 72,000 inch-pounds.) If the load center distance for the actual load is greater than the standard 24 inches, the only way to keep the Load Moment from exceeding 72,000 inch-pounds is to reduce the load. The easiest way to determine the maximum load when the load center distance is greater than the distance stated on the data plate is to divide the maximum Load Moment by the actual load center distance. For example:

If a load is 60 inches long (30-inch load center) then the maximum that this load can weigh is:   

72,000 inch-pounds / 30 in-load center = 2,400 pounds
Improperly distributed loads may tip the forklift if the maximum load moment is exceeded.
Figure 7. Improperly distributed loads may tip the forklift if the maximum load moment is exceeded.
Potential Hazards:

While carrying a load near the maximum allowable capacity, be aware of the following:
  • Danger of tipover

  • Danger of losing load

  • Danger of being struck by falling load
Requirements and Recommended Practices:
  • Calculate a maximum allowable load moment to determine whether an unusual load, such as one that is longer than 48 inches (i.e., the load center distance would be greater than 24 inches) or that has an offset center of gravity (i.e., uneven weight distribution) can be handled safely.

  • Minimize the load center distance measured from the back of the forks to the center of the load. This allows the forklift to carry more weight.

    As illustrated in Figure 7, a truck that has a 4,500 pound capacity at a 24-inch load center will tipover if a 60-inch load is positioned lengthwise. Positioning the load in this way increases the load center distance to 30 inches and increases the load moment by 27,000 inch-pounds.

    In Figure 7 the forklift safely carries the 4,500 pound load at a load center distance of 24 inches, but tips over when the load center increases to 30 inches. Here's the calculation: 30 inches X 4,500 pounds = 135,000 inch-pounds
    24 inches X 4,500 pounds = 108,000 inch-pounds The load moment is increased by 27,000 inch-pounds.

    If the load center distance is 30 inches, the only way to keep the maximum allowable load moment within 108,000 inch-pounds is to limit the weight of the load to 3600 pounds:

    30 inches X 3600 pounds = 108,000 inch-pounds
     
  • Use extra caution when handling extra heavy loads that may approach the truck's maximum capacity. For example, when handling 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, there is no one rule for all situations.

  • Maintain control of the vehicle at all times. The operator is responsible for handling the truck. Drive slower when carrying a load near the maximum allowable.

  • Do not exceed the stated capacity of your truck. Know its mechanical limits.

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