How Forklifts Work

A forklift is a useful and powerful piece of machinery that allows one worker to lift and move heavy loads. Around the world it may be called a hi-lo, a jitney or a lift truck. OSHA (Occupational Safety and Health Administration) identifies them as powered industrial trucks.

The essence of how a small vehicle can lift such large loads is balance. With the correct balance the truck has great stability, but a truck out of balance can be quite unstable, therefore dangerous. The design of a forklift truck is based on critical elements of physics. Stability is created through a cantilever system similar to a teeter-totter – two sides balanced on a pivot point, the fulcrum. On a forklift the forward wheels are the fulcrum. The load on the forks must be counterbalanced by weight on the other side of the fulcrum, in other words, the truck itself and the counterweight that is built into it. Each forklift has a capacity rating that is listed on the data plate of the truck. The plate may define the appropriate load with a particular attachment.

An important factor when evaluating loads is the center of gravity, that is, the point where an object is balanced in all directions. Every object that might be lifted has a center of gravity (CG). The truck and the load together form a combined CG. It must stay within a triangular range between the drive wheels (fulcrum) and the steering axle in the rear of the truck. If the combined CG moves outside this range the truck will become unstable. For instance, if the combined CG were to be in front of the forward wheels the truck would tip forward. Likewise, if the combined CG was outside the triangle on either side the truck would tip to that side.

Quite a number of factors can affect the combined CG, causing it to move outside of the “stability triangle.” Tilting the load forward or back or raising the load while tilted can completely change the expected stability. Driving on any sort of incline or rough surface (possibly a railroad track) can lead to disastrous results unless the driver plans ahead and compensates for the differences. Even simple things like starts, stops, or turns can shift the combined CG and cause trouble. When a load is particularly wide or is raised on the tips of the fork instead of flush with the face problems may ensue. Finally, lifting the load changes the combined CG. Be sure to only lift to heights that are correctly defined for the truck.

Each forklift truck has a load center. One common load center is 24 inches. This information can be found on the data plate. The capacity of a truck is given in three factors on the data plate. First there is a capacity in pounds. Two more factors listed are the load center and the maximum height. The forklift’s overall capacity is determined by multiplying the pound capacity by the load center. For instance, a truck rated for 6000 pounds with a 24” center would have a maximum load of 144,000 inch-pounds. The load to be lifted also has a load center. It is the measurement of the load including any pallet it may rest on, divided in half. To determine if a load is appropriate for the truck multiply weight of the load by its load center. If the result is less than the maximum inch-pounds for that truck it is safe to lift the load.

Safety and stability are essential elements when using a forklift truck. It is clear that a degree of knowledge is necessary to safely operate one. Be certain that drivers have the required training to understand the complex physics of a forklift in order to prevent accidents that can damage goods or injure workers.

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