They are a familiar sight to all forklift operators: the little stickers that indicate the maximum load they can carry. But checking them repeatedly for each new load takes time. And for warehouse greenhorns, or any workers who rarely drive a forklift, the diagrams are more or less unintelligible hieroglyphics.
Basically, the stickers are physics for beginners – and are child’s play, pretty literally. You simply have to imagine a seesaw. The direction in which the seesaw tilts depends not just on the relative weights of the kids sitting on each side, but on their position. The further they are away from the center, or to be scientifically precise, the fulcrum, the greater the leverage.
The situation is similar with forklifts and their loads. As a truck should ideally never tip over, the side with the vehicle and the driver should always be far heavier than the side with the load. The front wheels are therefore the fulcrum. And if the load is lifted or moved forward, this changes the relative forces at work. The load’s center of gravity moves away from the vehicle’s center of gravity. In other words, the load moves towards the periphery of our imaginary seesaw, which means, if it were moved far enough, the whole thing would topple over.
To avoid this happening, each truck has its own load capacity diagram. If it is understood and applied correctly, then there is always sufficient counterweight on the vehicle/driver side. The correct side of our seesaw remains on the ground, and our truck remains stable.
But what does a diagram look like and how do you interpret it?
As the name says, the diagram gives the truck’s load capacity – and places it in relation to two further parameters: lift height and the load’s center of gravity. As long as you know two, you can easily identify the third on the diagram.
Load is given in kilograms, and lift height in millimeters. The center of gravity is trickier. It is only in the physical center of the load if its weight is evenly distributed. This would be the case with a barrel of oil, for instance. But a crate containing heavy material that is concentrated on one side is a very different kettle of fish, with a very one-sided center of gravity. And this needs to be taken into account when consulting the lower scale of the diagram: it indicates how far the assumed center of gravity is from the fork face. Again, the same physical principle applies as with a seesaw: the further the load’s center of gravity from the truck, the greater the leverage. And the less the maximum load capacity and safe lift height.
On our diagram, the permissible load capacity values are given on the left, the lift height values are presented on the right. The figures on the left increase, the figures on the right fall. Which is only logical – as the greater the weight, the more difficult it is to lift. A mathematician would say they are inversely proportional. And as already described, the center of gravity is provided on the scale at the bottom. The falling lines indicated that the permissible load continuously falls, the greater the distance between the load’s center of gravity and the fork face.
The examples below answer the two most important questions.
Example 1: What is my maximum lift height?
The load weighs 1,500 kilos, the load’s center of gravity is 600 millimeters from the fork face. Where the two lines meet is our initial reference point. We then trace the inclined line to the right where we find our maximum lift height: 5,500 millimeters. So we cannot lift our load higher than 5.5 meters, otherwise things could get dangerous.
Example 2: What is my maximum load capacity?
We intend lifting to a height of 5 meters, our load’s center of gravity is 500 millimeters from the fork face. On the right-hand scale, we identify a lift height of 5,000 millimeters, and then we follow the inclined line to the left until it intersects with the line giving the load’s center of gravity, i.e. 500 millimeters. And at this intersection, we can see the maximum load capacity: 2,100 kilograms. So, with our load’s center of gravity, we can lift a maximum of 2,100 kilograms into racking that is 5 meters high. If we want to lift a heavier load, we will have to distribute the load across several pallets or choose a lower storage position.
Unpredictable hazards: what you will not find on a diagram
Every warehouse is unique, with its own goods, routes and equipment. And trucks themselves often vary in terms of their configuration and attachments. This can influence load capacity. Even a very simple fork tine extension can shift the center of gravity. And it has a weight of its own that needs to be added to the load. Attachments such as safety cages, containers or arms have an even greater impact. And anything that moves, such as a tipping bucket or grippers, is even harder to gauge.
But the laws of physics can make things even more challenging. As soon as you actually start to drive your forklift, for example, the diagram loses its value. When cornering, for instance, centrifugal forces come into play that push the load to the outside. And the faster you are traveling, the greater the danger of tipping over. Then there is braking: in this instance, the entire weight of the truck and of the load is pushed forwards – and the forces exerted onto the fork and pushing the vehicle forward can suddenly become highly unpredictable.
Intelligent assistants instead of graphs and arithmetic
Interpreting load capacity diagrams, and especially working out “special cases”, takes time that hardly anyone has. But the price of poor guesswork and misreadings is high in terms of accidents. Assistance systems provide practical help, and minimize the risk of injury and damage. As soon as you lift a load, they indicate what is possible and what not.
Linde Material Handling has made the management of load capacity a priority, and automated it with the Linde Safety Pilot. There is also a comparable system for pallet stackers. These assistance systems give the operator an at-a-glance overview of the information needed to ensure the safe movement of goods. The screen displays the weight on the forks, the current and maximum lift height, and the angle of fork tilt.
Anyone who has used a system of this type no longer wants to use a load capacity diagram – and instead enjoys greater safety, more time and enhanced productivity.