You’ve probably seen them: YouTube videos of forklift operators accidentally spilling their loads or upending their trucks. While they often poke fun at the hapless operator, they are no laughing matter. These types of accidents—including longitudinal (forward or backward) or lateral (sideways) tipping—not only result in damage to the load, but they also put the life of the operator and nearby pedestrians at risk. On average, there are 70 to 100 forklift-related fatalities and many thousands of nonfatal forklift-related injuries in the United States each year.
Although the number of reported incidents involving forklifts has steadily declined since lift truck operator training and certification were mandated by the Occupational Safety and Health Administration (OSHA) decades ago, some industry observers say they’re noticing more such incidents, including tipovers. There are no data available to confirm those anecdotal observations, but one likely factor may be the high rate of turnover among warehouse workers. According to the U.S. Bureau of Labor Statistics, roughly 50% of warehouse operators left their jobs in 2021—and reports of much higher turnover rates abound. That means warehouse managers are “constantly training new people … [and] in some cases, an operator may not have the necessary level of training or experience for a situation they find themselves in, such as handling heavy loads or high lift heights,” says Alex Sakowski, new products business manager for Yale Lift Truck Technologies.
A second, related, factor may be “the explosion in the number of e-commerce warehouses and DCs,” says Martin Boyd, vice president, product planning and solutions for the Hyster Co. These facilities are high-speed and high-volume, with a lot of forklifts, warehouse robots, and/or people moving around. While he has not seen verifying data, Boyd believes that the proliferation of such warehouse facilities combined with ongoing labor shortages in e-commerce might be contributing to a potential increase in incidents involving forklifts.
All that suggests that it’s a good time for warehouse and fleet managers to pay special attention to preventing tipovers. The list of potential causes of these dangerous accidents is long indeed. The most common ones fall into four general categories:
- Speed—driving too fast, especially without a load or over uneven surfaces; accelerating or stopping too quickly; turning a corner too quickly or at a sharp angle; traveling too fast with an oversized or oddly shaped load.
- Load and weight—carrying a load that is too heavy for the truck, is not evenly balanced by weight on the forks, or is unstable; allowing a load suspended from the forks to swing; failing to take into account the weight of attachments.
- Travel—turning or improperly lifting or lowering on a ramp or slope; traveling with the forks or a heavy attachment raised; driving with a wheel over the edge of a dock plate or a dropoff; hitting an overhead obstruction; traveling in wet or slippery conditions.
- Mast control—tilting the mast too far forward or backward, or too quickly in either direction; failing to tilt the mast slightly backward when traveling; tilting an elevated load forward except when depositing it on a rack or stack.
Many potential causes means many ways to prevent tipovers. Forklift safety experts point to three key elements for improving safety: an understanding of “forklift physics,” effective operator training, and support from safety-enhancing technology.
STAY CENTERED
While lift truck operators are unlikely to be physics experts, they do need to understand the physical forces that affect the stability of the forklift they drive, whether it’s a traditional sit-down counterbalanced truck or a standup model. This is a complex topic that can only be properly addressed through OSHA-compliant training; the explanation offered here is just a brief summary of considerations gathered from forklift manufacturers, dealers, and training firms. In particular, we relied on the descriptions and illustrations for counterbalanced lift trucks in the online article “How to Avoid Forklift Tip Overs” by Mitsubishi Logisnext Americas. (Note: Always consult the operations manual for instructions specific to a particular forklift model.)
Forklift stability—both longitudinal and lateral—depends on several factors. One is the balance between the weight of the load on the forks and the weight of the truck, with the front axle functioning as the fulcrum. Another is the “center of gravity,” or the point of an object where the weight is evenly distributed.
A lift truck and a load each has its own center of gravity. When the forklift picks up the load, the newly combined unit now has a new, combined center of gravity (CG). The CG moves forward and backward as the mast is tilted in those directions, and it moves up and down as the mast is raised and lowered. Thus, the CG is affected by the size, weight, shape, and position of the load; the height of the lifted load; the degree of tilt; the forces generated by accelerating, braking, or turning; and the condition or grade of the surface where the lift truck operates. In addition, any attachment operation, such as moving a side shifter or rotating a roll clamp, will change the CG, especially at height or if the clamp isn’t centered on the roll. In short, everything an operator does affects the center of gravity.
For a forklift to remain stable, the center of gravity must stay within a “stability triangle”—an imaginary triangle that draws a line between the front wheels and stretches to the center point of the rear axle. (See illustration at left.) This triangle applies to both four-wheeled and three-wheeled lift trucks. While it might appear that a four-wheeled model would have a rectangular base, it actually does not. Unlike the front axle, which remains in place, with only the wheels turning, the rear axle pivots on a pin located at the center point of the axle. The pivot point is the third point of the triangle.
If the center of gravity moves forward of the front axle, then the lift truck will tip forward. If it moves outside of the triangle on either side, then the lift truck will tip sideways. When the forks are kept low, especially when carrying a load, the lift truck is more stable. Raising the forks high—with or without a load—makes it easier to tip over. In addition, exceeding the forklift’s rated capacity or the load center (the allowable distance from the front face of the forks to the load’s center of gravity), both of which appear on the forklift’s capacity plate, can also cause tipping.
The above information is just the tip of the iceberg (or maybe the tip of the forks?) when it comes to maintaining stability. The specifics will vary depending on the forklift class and model, so be sure to consult your local forklift dealer or other qualified provider of operator training for guidance.
SHOW, DON’T JUST TELL
Good safety training programs should teach operators how to avoid all of the errors mentioned at the beginning of this article. But training operators on how to avoid situations that could lead to tipovers comes with some special challenges for trainers.
First, they have to overcome the human tendency on their students’ part to assume that accidents happen to other people and convince them to take the risk seriously. Operators are more likely to understand how serious lateral and longitudinal tipovers are if trainers “teach people in a way they can relate to,” says Tony Parsons, regional operator training manager for Wolter Inc., which represents forklift makers Linde and Doosan throughout the Midwest. For example, to help operators visualize the number of forklift-related accidents and injuries reported in the U.S. each year, he often compares that statistic to the capacity of a local sports arena or stadium.
A wire model with moveable sinker can be used to demonstrate how a truck's center of gravity changes during common forklift operations.Second, trainers must teach in a way that is effective while also minimizing or eliminating the chances of accidents during training sessions. One way to do that is to reinforce verbal explanations and diagrams with physical props designed to demonstrate “forklift physics” principles. “There’s a much greater likelihood [operators] will understand center of gravity and stability if they can see with their own eyes” the impact of load weights and operator behaviors, Parsons says. To do this safely, many trainers use accurate, scale models of the various classes of lift trucks. Parsons and others also favor a simple wire model (see photo) that uses a lead sinker hanging on a wire to demonstrate how the center of gravity changes and may leave the stability triangle as a load moves horizontally and vertically, when an unloaded truck travels with raised forks, and when a mast tilts forward and backward.
And third, they must make sure operators are trained and certified on each type of forklift they will use in their job because each has unique operating requirements and will respond differently to changes in the center of gravity. “You can’t train operators on a sit-down counterbalanced truck and then expect them to safely operate a pantograph reach truck or an order selector that elevates over 400 inches high,” Boyd says, adding that the sit-down and standup trucks also have completely different operator stations and controls. “Operators must be trained on the specific pieces of equipment they plan to use.”
Virtual training, which allows operators to apply what they’ve learned in various scenarios in a safe, controlled environment, is quickly gaining fans. Virtual training includes simulation, using desktop simulators that are similar to video games; and virtual reality (VR) systems, where learners wear VR headsets while at the controls of an actual (but immobile) forklift or a simulated forklift “dashboard.” Both are interactive; i.e., the scenarios respond to users’ actions just as they would in real life. Simulation and VR systems can expose learners to potential hazards like tipping and rollovers virtually, so they can learn how to recognize, prevent, or react to them without putting people and products at risk. The trainer, who is able to see what the student is doing, can provide immediate feedback and correction.
TAKE ADVANTAGE OF TECH
While effective operator training is fundamental to preventing tipovers, technology can lend a helping hand. For example, Parsons of Wolter Inc. notes that forklift telematics software can be programmed to limit truck speeds in specific areas of a facility, preventing the excessive speed that can lead to accidents. He emphasizes, though, that such technology is not a substitute for operators’ own decision making. “It’s there to remind them that they should be driving at appropriate speeds,” he says. “The software provides positive reinforcement of good driving habits to limit risk.”
Technology that detects imbalances and enhances stability is designed to help trained operators reduce lateral and longitudinal tipping. One example is Toyota Forklift’s patented System of Active Stability (SAS). Sensors take over 3,000 readings per second to detect instability. For four-wheel models, the system locks the rear steer axle in place, converting the forklift’s stability “footprint” from a triangle to a rectangular pattern to reduce the risk of a lateral tipover. For three-wheel forklifts, which can be more prone to lateral tipping when cornering at excessive speeds, SAS limits the drive speed when cornering. When risk of a longitudinal tip is detected, SAS reduces the extended mast’s forward or reverse tilt speed as appropriate for the weight of the load. Front and back angle control helps prevent forward or backward tipping that could cause a load to fall off the forks.
Yale Lift Truck Technologies’ Yale Reliant system continually maintains the combined center of gravity while taking into account the weight of the load, the lift truck’s weight and capacity, its travel speed and acceleration, whether the mast is tilted forward or back, and whether the forks are raised or lowered. If the system detects a condition that could cause instability, it proactively deploys what Sakowski calls “prohibitors”: hydraulic and traction controls that temporarily override the operator’s manual controls to restore stability. For example, depending on the specific situation, Yale Reliant can take such actions as preventing lifting and lowering of loads that exceed weight limits, and reducing mast speed, tilt, and height, to name just two of many possible responses.
Tipovers can also happen when operators suddenly brake or swerve for a pedestrian or object in the travel path. Yale Reliant includes object and proximity sensing: When the system detects an obstruction, it takes into account the load weight, travel speed, and center of gravity to slow the truck safely. And because the system provides a visual alert on a display screen showing operators what their error is at the same time it is imposing restrictions on the truck’s operation, it can instill safe driving habits and help new operators avoid tipovers, Sakowski says.
The Hyster Dynamic Stability System (DSS) employs an array of sensors that monitor speed, mast tilt position, fork height, and steering angle and detect whether or not a load is on the forks. DSS is constantly monitoring all of those inputs dynamically, and if it senses instability, it will then—based on the complete picture of the lift truck’s condition—limit the operator’s control inputs to help maintain stability, Boyd explains. For example, when DSS detects a load at high height being tilted forward, it will limit both tilt speed and tilt angle to help maintain stability. Another example: When DSS detects a load that is beyond a certain height threshold, the system will limit top speed.
Boyd emphasizes that such technology is never a substitute for effective operator safety training; rather, it should be used to reinforce and supplement the training. DSS utilizes the truck’s display to alert the operator when mistakes are made and displays simple icons indicating what is happening and why. The system is able to wirelessly transmit event data through Hyster’s optional Tracker telemetry system, allowing fleet managers to connect those incidents to specific trucks and operators—opening the opportunity to provide extra training for operators who need reinforcement.
Even with the best training program and the most experienced forklift operators, it’s impossible to foresee every possible error or hazard that could lead to a tipover. By recognizing the potential causes of lateral and longitudinal tipping, and focusing on the three key safety factors—an understanding of “forklift physics,” effective operator training, and assistance from safety-enhancing technology—forklift fleets can make strides toward preventing these dangerous accidents.
