how high should a load be carried on a forklift

The Physics of Forklift Stability

A forklift is a counterbalance machine. Its stability is governed by the principles of physics, specifically center of gravity and the stability triangle. The goal is to keep the combined center of gravity of the forklift and its load within the stability triangle, an imaginary three-point zone formed by the front wheels' pivot points and the center of the rear axle.

The Center of Gravity (CoG)

The center of gravity is the point at which the entire weight of an object is concentrated. On a forklift, the CoG is constantly shifting. When unladen, the CoG is low and near the front of the machine. When a load is added, the combined CoG shifts forward and upward. The higher the load is lifted, the higher the combined CoG.

The Stability Triangle

The stability triangle is the forklift's base of support. As long as the combined CoG stays within this triangle, the forklift remains stable.  When the CoG moves outside the triangle, the forklift becomes unstable and can tip over, either sideways or forward.

A forward tip occurs when the combined CoG shifts too far forward, often due to an overloaded or improperly positioned load. A sideways tip is typically caused by turning with the combined CoG too high, especially on a ramp or an uneven surface.

Optimal Carrying Height: The "Sweet Spot"

The ideal carrying height for a load on a forklift is a balance between ground clearance and stability. It's often referred to as the "travel height." The general rule of thumb is to carry the forks as low as possible while still allowing for adequate ground clearance to avoid scraping the ground, obstructions, or irregularities.

Ground Clearance vs. Stability

Too Low: If the forks are too low, they can catch on uneven surfaces, debris, or small bumps, which can destabilize the load and the forklift. This can also damage the product or the floor.

Too High: If the forks are too high, the combined center of gravity of the forklift and its load is elevated. This significantly reduces the forklift's stability, making it prone to tipping, especially when turning, braking suddenly, or traveling over rough terrain.

The recommended travel height is typically between 4 to 6 inches (10 to 15 cm) off the ground. This height provides sufficient clearance for most indoor and outdoor surfaces while keeping the combined center of gravity as low as possible for maximum stability.

Factors Influencing Carrying Height

While the "4 to 6 inches" rule is a good starting point, the optimal carrying height can be influenced by several factors. A competent operator must assess each situation individually.

1. Type of Terrain

Smooth, indoor floors: On a clean, smooth warehouse floor, the forks can be kept at the lower end of the recommended range (around 4 inches). This is because there's less risk of hitting bumps or debris.

Rough or outdoor surfaces: When operating on gravel, asphalt, or uneven ground, a higher carrying height (closer to 6 inches) may be necessary to clear obstacles. However, this must be done with extreme caution, as the reduced stability is a significant risk.

2. Load Characteristics

Weight: Heavier loads elevate the combined CoG more significantly than lighter loads. For heavy loads, it's even more critical to keep the carrying height to an absolute minimum to maintain stability.

Dimensions: A wide or tall load, known as a high-cube load, also raises the CoG. A tall load carried too high is particularly dangerous as it can easily become unbalanced. The operator must ensure the load is tilted back against the mast to secure it.

Center of Load: The load's center should be as close to the forklift's front wheels as possible. This is achieved by having the load fully seated against the heel of the forks. The further the load's center is from the forklift's CoG, the more leverage it exerts, increasing the risk of a forward tip.

3. Operator Skill and Training

Experience: A well-trained and experienced operator understands the feel of the forklift and the subtle shifts in stability. They can instinctively adjust the carrying height based on the load and the environment.

Training: Proper training should include a thorough understanding of the stability triangle, the effects of load elevation, and practical exercises on various surfaces.

Practical Scenarios and Best Practices

Scenario 1: Traveling with a Load

Rule: Travel with the load tilted back and as low as possible.

Reason: Tilting the load back moves its CoG closer to the forklift, and keeping the load low maintains a low combined CoG. This is the safest way to transport a load.

Scenario 2: Turning with a Load

Rule: Always slow down and lower the load before making a turn.

Reason: When turning, centrifugal force pushes the load outward. This force, combined with an elevated CoG, can easily cause a sideways tip. Slowing down and lowering the load reduces the effect of centrifugal force and keeps the CoG within the stability triangle.

Scenario 3: Loading and Unloading at Height

Rule: Raise and lower the load only when the forklift is stationary.

Reason: The most unstable moment for a forklift is when a load is elevated. Any movement—even a small shift—can cause a tip. Therefore, all travel must cease before the load is raised or lowered to its final destination.

Scenario 4: Traveling on Ramps or Inclines

Rule:

With a load: Always drive forward when going up a ramp and in reverse when going down a ramp.

Without a load: Drive in reverse when going up and forward when going down.

Reason: This procedure ensures that the weight (the load or the forklift's own counterbalance) is always on the uphill side. This prevents the combined CoG from shifting too far downhill and causing a tip.

Scenario 5: Stacking and De-stacking

Rule:

Approach the stack with the forks at travel height.

Stop completely before raising the load.

Raise the load to the correct height, then slowly move forward to place it.

Never tilt the mast forward when the load is elevated.

Reason: Raising a load while in motion or tilting the mast too far forward when high can cause a forward tip.

The Role of Technology and Safety Features

Modern forklifts are equipped with a range of safety features that aid in stability and operation.

Load-sensing systems: Some advanced models can detect the weight and position of the load and provide real-time feedback to the operator.

Electronic stability control: This system can automatically adjust travel speed or limit mast functions to prevent a tip-over.

Overload protection: This feature prevents the operator from lifting a load that exceeds the forklift's rated capacity.

While these technologies are valuable, they should be seen as aids, not substitutes for proper operator training and adherence to safety protocols.

Conclusion: A Culture of Safety

The question of "how high should a load be carried on a forklift?" is not just a technical one; it's a fundamental safety question. The answer, in short, is "as low as safely possible." The precise height depends on a combination of physical laws, environmental conditions, and operator competence.

A safe workplace is one where operators are not only trained on the mechanics of the forklift but also understand the physics behind its operation. By consistently applying the principles of the stability triangle, respecting the "sweet spot" of travel height, and adhering to best practices, warehouses and logistics operations can significantly reduce the risk of accidents, protect valuable assets, and, most importantly, safeguard human lives.