Payload

Rated capacity and safe working load describe the same thing: the greatest mass a unit may lift. That includes everything hanging from it – workpiece, gripper, spreader beam and slings. The net payload is the part left over for the workpiece.

Confusing the two is the most common mistake in enquiries. Anyone who wants to lift a 100 kg component and orders a unit with a 100 kg rated capacity has forgotten the gripper. The calculation is always: net payload equals rated capacity minus the dead weight of the load handling attachment.

The starting point is the heaviest workpiece that will ever hang from the unit – not the typical one. To this is added the load handling attachment with all its fittings. The total is rounded up to the next available capacity step.

An example with assumed figures: the heaviest component weighs 85 kg, the gripper with rotary unit 22 kg. The required capacity is 107 kg; a unit of the next step is chosen, say 125 kg. Conditions that add weight have to be considered too: filled containers, adhering coolant, packaging.

The capacity of a workstation is never higher than that of its weakest link. This includes the hoisting unit, the structure, the track, the load handling attachment, the sling and the fixing to building or floor. Each of these parts has its own capacity, and each has to be marked.

On units with a jib the capacity may depend on the outreach. The figure then applies to a certain position, and a capacity plate or chart shows what is still permitted further out.

The EU Machinery Directive requires the maximum working load to be marked legibly and indelibly on machinery for lifting loads. From a working load of 1,000 kg or an overturning moment of 40,000 Nm, such machinery must also be fitted with devices that warn the operator and prevent dangerous movements in the event of overloading.

Below that, overload protection is not required in every case, though many units have it. It does not replace knowing the weight of the load.

Lifting equipment is built with safety coefficients and tested with loads above the rated load. That is the manufacturer’s proof, not an invitation to the operator. The reserve covers wear, dynamic forces and scatter. Anyone who uses it in operation has none left.