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Forklift Attachments and Forklift Stability

Forklift attachments greatly enhance a lift truck’s usefulness, but they also can affect visibility, carrying capacity and stability. Of these 3, visibility is the most easily identified. The dangers associated with carrying capacity and stability only become apparent when an operator goes beyond safe limits and an accident occurs. A better understanding of the relationship between forklift attachments and forklift stability can help prevent those accidents from occurring.


The Forklift Stability Triangle

Understanding the forklift stability triangle is the key to understanding how forklift attachments affect stability. Take a look at the illustration below and notice how the forklift has 4 wheels, yet has only 3 point stability. This is important to remember, because the stability triangle applies to all forklifts.

 

the forklift stability triangle

 

Now notice how forklifts are designed to have an unloaded center of gravity in the center of the carriage, equidistant from the hubs of the wheels. That center of gravity shifts forward when the forklift is carrying a load and when carrying a maximum load is in the middle of the front axle. The front axle then becomes a fulcrum point, not unlike the fulcrum point of a seesaw. In a counterbalanced forklift, the rear of the forklift is weighted to counteract the weight of a load and the maximum carrying capacity of the lift truck is determined in part by the counterbalancing weight.


In the above illustration, the word “theoretical” is in parentheses under “Maximum Load.” It is a very important word because a maximum load is only theoretically placed at the back of the tines near the mast and the load is only theoretically an optimally stacked 4X4 pallet. The illustration below shows how drastically a forklift’s carrying capacity changes when a less-than-ideal load is placed on a forklift.


illustration of forklift carrying different types of loads

 

This illustration shows a 4’X4’X4’ load and a 2’X4’X6’ load. The first load can be safely carried at the forklift’s maximum carrying capacity of 4,000 lbs., while the second load, whose weight center is at 36”, can only be 2,666 lbs. Now imagine having forklift fork extensions on the forklift. The great thing about fork extensions is that they help make a load more stable. They do not, however make the forklift more stable. With 6’ fork extensions, the maximum carrying capacity of the rectangular load would still be 2,666 lbs.

 

Another popular forklift attachment is the telescoping boom. Boom attachments allow you to lift odd-shaped loads and lift them up and over obstacles. However, a telescoping high rise boom can be as long as 70” when fully extended. What would a forklift’s carrying capacity be if the weight of the load was beyond the tips of the forks?

 

The maximum carrying capacity of 2,666 lbs. in the example above was arrived at by a fairly simple calculation.

 

  1. The load center as stated on the data plate was divided by the actual load center (24/36=.6666).

  2. This result was multiplied by the rated load capacity (4000X.6666=2666.6).

 

For argument’s sake, let’s say you want to lift something with a boom attachment extended 48” beyond the blade tips. The load center remains 24”, but you divide that by 96”, since the center of the load will be directly beneath the end of the boom attachment: 24/96=0.25. Multiply 4000x0.25 and you get 1,000 lbs. That’s still a heavy load, but only a quarter of the forklift’s “theoretical” maximum load carrying capacity.

 

The height at which a load is lifted and carried also affects its stability. Notice the relationship between the loaded forklift’s load center of gravity at ground level and its center of gravity when the load is fully raised. An elevated load reduces a forklift's stability both longitudinally (forward and backwards) and laterally (sideways). This is why OSHA regulations stipulate moving with the load lowered to near ground level. forklift carrying load at height

 

Forklift Stability Tips

 

When operating a forklift on a sloping surface, always drive in reverse both up and down the slope. When driving an unloaded forklift, drive forward only. using a forklift on a sloping surface

 

Almost half of forklift fatalities occur when forklifts tip over sideways. This is usually because:

  • The operator turns too sharply at too high a speed.

  • The operator tries to turn on a sloping surface, making the forklift tilt sideways.

  • A load is off balance.

The danger of tipping sideways increases if the load is being carried at height instead of near the ground.

 

Here are 8 simple forklift stability tips to remember:

  1. Never exceed the forklift’s maximum carrying capacity.

  2. Get manufacturer’s approval before using forklift attachments that affect its stability.

  3. Carry a balanced load at the rear of the forks.

  4. Tilt the mast backwards for even greater stability and carrying capacity.

  5. Carry the load as low to the ground as possible.

  6. Accelerate and brake smoothly.

  7. Turn slowly and smoothly.

  8. Never turn on a sloping surface.

 

Note: This article is for informational purposes only and is not intended to replace OSHA approved forklift training materials. The illustrations and much of the information in this article comes from OSHA's Powered Industrial Trucks Load Composition page. More extensive information is available there.


Further Reading:

What You Need to Know about Forklift Load Capacities

Forklift Stability Basics

Forklift Accessories - It's a Matter of Balance

Determining Your Forklift Carrying Capacity