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You’ve got a forklift that has a rated capacity of 4000 pounds. If that’s all you know, you don’t know enough, because there’s more to forklift load capacities than meets the eye.
In a previous post, we compared a forklift to a child’s seesaw and pointed out that “if 2 children of equal weight sit on each end of the seesaw, it will become balanced”, but if one of the children moves forward, the other child will become in effect “heavier,” and the seesaw will drop in their direction. The same thing is true of forklifts. When critical centers of balance shift, forklift load capacities are altered.
OSHA identifies the key factors of forklift capacity as:
- Load weight
- Weight distribution
- Load size
- Load shape
- Load position
More often than not, 2 or more of these factors work together to lower a forklift’s load capacity. The load weight is always an obvious factor, but its shape and size are often the factors that literally determine its “tipping point.”
Let’s say you own a typical 4000 pound capacity counterbalanced forklift. When carrying an ideally shaped 4’X4′ box against the back of the tines, the forklift will easily and safely lift those 4000 pounds. Change the shape or position of the load, however, and the capacity becomes reduced. Why? It’s because the load’s center of gravity moves forward.
A simple calculation demonstrates just how significantly the shape of a load can reduce a lift truck’s carrying capacity. The maximum load limit of a forklift is determined by a load whose center of gravity is 24″ from the vertical back of the forks. Move the center of gravity forward just 4″ and the forklift’s load capacity is reduced to just 3428 pounds. How did we arrive at that number?
- Divide the load’s ideal center of gravity (24″) by the actual center of gravity (in this case, 28″).
- Multiply that number by the forklift’s rated capacity (in this example, it’s 4000 lbs).
When you’re using forklift attachments like fork extensions or forklift booms, the load center of gravity often changes dramatically. At the same time, the forklift’s load capacity is drastically reduced. Let’s use another example and see how drastic this reduction can be.
You have attached a boom attachment to the forks and want to use it to lift an irregularly shaped 1000 pound load from beneath the fork tips. That brings the load’s center of gravity out from 24″ to 48″. Using the method above: 24/48 X 4000 = 2000 lbs. You’re still well within the forklift’s limits. Now extend the load another 100″ with a telescopic boom attachment. The calculations now look like this: 24/148 X 4000 = 648 lbs. You are now well beyond the safe load limit. Reduce the length of the boom to 90″ and you can safely lift that 1000 lbs.
Keep that simple calculation in mind whenever you’re in doubt about a load you want to carry, but even if you feel you can safely lift and move it, proceed with caution. Test the load first and then accelerate slowly, keeping the load close to the ground. Even better, read OSHA’s Load Composition article and buy forklift training materials that will give you and your forklift operators a thorough understanding of forklift load capacities and other important forklift safety information.