The critical concept: force, energy and the press tonnage curve

A stamping press has a rated tonnage, but that rated tonnage is not available at every point in the stroke. On a mechanical eccentric press, the full nominal force is available only in the final portion of the stroke, in the vicinity of bottom dead centre (BDC). Higher up in the stroke, the available force is significantly lower. This is the rated tonnage point concept: the nominal capacity of the press is defined at a specific height above BDC (typically expressed in millimetres), and the force curve drops away sharply above that point.

 

The practical consequence is important: an operation that requires force to be applied high in the stroke (for example, a deep drawing operation where significant deformation occurs throughout the travel, or a bending operation with a long flange) may require a press with a much higher nominal rating than a pure blanking operation at the same force level. For deep drawing in particular, it is also necessary to check the energy available per stroke; the flywheel of a mechanical press stores a finite amount of kinetic energy, and very deep or long draws can require more energy than a lighter press can deliver without excessive flywheel speed loss.

 

Hydraulic and hydropneumatic presses, by contrast, deliver their rated force throughout the full stroke; which is one reason they are better suited to deep drawing operations that require sustained force over a long working travel.