Tackling tough terrain

July 8, 2009
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Rough terrain forklifts often have an oscillating — or free-moving — rear axle that allows the vehicle to better negotiate changing grades. This added movement, combined with the uneven terrain of a construction site, plus the heavy weight and unusual shape and bulk of construction loads, makes operating the rough terrain forklift difficult. Here are seven safety tips:

1— Weight
In order to lift the load, the base of the vehicle must be stable enough to avoid tipping over as the load is raised or extended in the air. Rough terrain forklifts weigh thousands of pounds more than any automobile. In order to lift and move heavy loads, a counterweight is mounted on the unit to keep everything in balance. The mass of the vehicle itself provides counterweight for extension and lifting.

A greater weight must be at the base of the lever in order to raise a heavy load into the air. Forklifts are basically levers supported on wheels, requiring force on one side to lift the other side.

2— Steering
Operators must approach and negotiate turns carefully. Steering and stability go hand in hand. Rough terrain vehicles have the following different steering modes:
  • Select steering allows the operator to select which wheels do the steering.
  • Two-wheel steering allows either the front or back wheels to steer the forklift. Front-wheel steering enables the forklift to operate like an automobile and move at a faster speed over longer distances. Back-wheel (rear-wheel) steering allows for increased maneuverability in tight cornering situations.
  • Four-wheel steering allows the rear wheels to always follow the front ones and provides the tightest turning circle.
  • Crab steering allows for responsive steering that aids in picking up an inconveniently positioned load. This type of steering can turn the wheels in the same direction, enabling the machine to move sideways or at a diagonal.
3— Suspension, stability triangle, center of gravity
Today’s rough terrain forklifts are being designed with four-point suspension, but many continue to be made using a three-point suspension system where the vehicle is supported at three points.

An imaginary line drawn between these three points — the two front tires and the pivot point of the rear axle — makes up what is known as the stability triangle. The stability triangle is a reference point; as long as the center of gravity remains within this imaginary area, the vehicle will be stable. Unloaded and parked, a forklift’s center of gravity will remain centered in the stability triangle.

Once the vehicle is moving, acceleration, braking, and turning can affect this center of gravity. That’s why operators should avoid accelerating quickly and should always take corners slowly. Keeping the forks or attachments at the lowest possible point for traveling maximizes this center of gravity. If the center of gravity is shifted to the point outside the stability triangle, the vehicle could tip over.

When the forks are loaded there is even more to keep track of. Besides the center of gravity of the heavy vehicle itself, each load has its own center of gravity. This is called the combined center of gravity. Because the load is being constantly moved, the combined center of gravity is almost always shifting.

When moving a load, the forklift’s stability also depends on knowing where the line of action is. The line of action is an imaginary vertical line that extends through the combined center of gravity. If the line of action exits the triangle of stability, the unit will become unstable and tip over. The line of action is even more important when picking up a load when parked on an incline.

4— Load handling and attachments
Before any load is handled, the operator must know that the forklift can safely move, lift, or lower it. This requires the employee to know the weight of the load and the capacity of the vehicle. This information can be found in several places — the identification plate, operator’s manual and the load chart that comes with the vehicle.

5— Load charts
The design and weight of the forklift determines its load and lifting capacity. Since manufacturers design their vehicles for specific applications, different weights may make it necessary for the operator to use a different forklift or split up the load.

6— Load center
The load center is the center of gravity of the load. It’s expressed as the horizontal distance from the edge of the load at the front of the carriage, to the center of gravity of the load.

Because construction loads are often irregularly shaped with uneven centers of gravity, load centers will vary depending on the size of the forks or attachments. The stability of the load decreases the farther the load center is from the face of the carriage. Make sure the load center is appropriate for the length of the fork or attachment.

7— Frame leveling
Always level the vehicle before raising the boom. If the operator attempts to tilt the frame with a raised load, the forklift could tip over. Adjust the frame slowly until the ball is centered in the frame level indicator for maximum stability.

If the surface grade is too steep or uneven to level the frame, do not attempt to raise the boom. It will be necessary to move the vehicle to a location that provides better stability.

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