When considering capacity of potential attachments for your forklift, it’s crucial to review the forklift’s rating at its load center, which determines how much weight the machine can safely lift. This will also determine what is possible when attempting to build and implementing longer, high-capacity attachments like forklift fork extensions.
Since forklifts are rated at a load center, the further out you want to lift, the lower the capacity becomes. A 5,000 lb. capacity forklift will lift that much weight up to 48-inch forks (with a 24-inch load center) but going out to 60 inches (with a 30-inch load center), for example, drops the capacity to 4,000 lbs.
The weight capacity of a forklift attachment is tied to what is known as the Maximum Load Moment, which is when the load center distance increases, changing the weight distribution, and as a result, the amount of weight a truck can carry under those conditions. Increasing the load center distance too far can cause a forklift to tip.
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.
The maximum Load Moment for a truck is derived by multiplying the weight rating of the forklift by the center load distance. If we multiply the 5,000 lb. capacity of our theoretical forklift by its 24-inch center load, we arrive at a maximum Load Moment of 120,000 inch-pounds. However, when exceeding that 24 inches by use of a longer attachment, that capacity estimate must be revised. In order to discover the maximum load when the load center distance is greater than the distance stated on the data plate, one must divide the maximum Load Moment by the actual load center distance.
To determine what your forklift’s capacity will become when using an attachment, use the following formula:
Truck Capacity X Load Center (for its Load Moment) / New load center of desired attachment
You can also express this formula with an equation: (TC x LC)/NLC
So, in our 5,000 lb. forklift example, we’d multiply 5,000 by 24 (its load center), for a result of 120,000 inch-pounds as its maximum load moment. Then we divide 120,000 by 30, which is the load center for the desired 60-inch attachment. The end result is 4,000 pounds of maximum weight when using that specific attachment.
According to the Occupational, Safety & Health Administration (OSHA), the way a load is arranged on a forklift will also factor into load capacity. Most loads do not have their center of gravity exactly in the middle; so to whatever extent that the load differs from its theoretical centered load—like in instances when the load is irregularly shaped, has unbalanced weight distribution, or is not centered on the forks—capacity may be reduced further. If not factored into the calculated capacity, this can cause tipping, collisions, dropped loads and loss of steering control.
One way to address positioning concerns is to reduce the distance from the front wheels to the load center. For example, load a large rectangular box widthwise across the forks of the truck, instead of lengthwise, which causes the load center to shift forward and away from the front wheels, lifting the rear wheels off the ground. It is also helpful to load as close to the front wheels as possible, limiting the load center distance, and load the heaviest part toward the mast.