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Forklifts are rugged, heavy machinery than can weigh several tons. But when they are carrying heavier loads than they can handle, moving at overly rapid speeds, and turning too quickly, the risk of tipping over is increased.
Forklifts are characterized by a heavy counterweight that usually is attached to the rear of the vehicle. This is what provides stability for the forklift when it is lifting, moving and shifting heavy loads on its forks. This type of vehicle is known as a counterbalance forklift.
But the counterweight can also cause the forklift to tip over. There are four basic ways forklifts can tip over as a result of the counterweight: They can tip forward; They can tip to the side; Too high a level of braking can cause them to tip forward; They accelerate too quickly in the backwards gear, causing them to tip forward.
The Stability Triangle
There are three ways a counterbalance forklift can tip over forwards or sideways:
1. The vehicle can tip forward with the contact points of the drive axle tires and the ground acting as the pivit
2. The vehicle can tip sideways t the left with the tipover initially pivoting alng a line through the left front tire contact point and the pivot point of the rear steer axle
3. The vehicle can tip sideways to the right with tipover initially pivoting along a line through the ride front tirer contact point and the pivot point of the rear steer axle.
These three lines combined are what are known as the “stability triangle”.
In some cases, the rear steer axle has a limit to its travel around its pivot point, so that at some point the side tipover changes to pivot about the left or two right tires. At this stage, the vehicle sometimes stops tipping and falls back on its wheels. But in most cases, it will continue to tip over.
Center of Gravity
The forklift’s center of gravity changes depending on whether it is moving or stationary, whether it is carrying a load, and the weight of the load that is being carried.
When a forklift is stationary, the center of gravity is straight down through the stability triangle, so the forklift can’t tip over.
But when a load is placed on the forks, the center of the gravity moves toward the front axle. Once the load exceeds the vehicle’s maximum capacity, the forklift will tip forward. When a driver tries to lift a load that is behind the capacity of the vehicle, the steer axle will be lifted off the ground.
When a forklift is driving up or down a hill or when it is traveling around a corner at a higher than recommended rate of speed, or when the forklift brakes or accelerates too quickly, forces are developed that act on the forklift sideways or to the front of rear.
If the sum of all these forces cause the center of gravity to pas outside the stability triangle, the vehicle will tip either forward or backward. And when heavy loads are added to the forks, the center of gravity moves forward closer to the front axle and the vehicle becomes progressively less stable.
Raising the load reduces stability in all tip over directions. Smaller side, front or rear forces can cause forward or side tipovers as a result.