In the crank press which is used most widely in press forming, there is the pressing capacity (capacity to apply pressure) which is an indicator of the capacity of the press machine.

Pressing capacity is the maximum pressure that the press machine can generate safely. As shown in Table 1 in the case of a crank press, the pressure generated changes depending on the stroke position of the slide. So the pressing capacity indicates the pressure at a determined position above the bottom dead point. In the example shown in Table 1, it can be seen that a pressure of 980kN (980 kilo Newtons = 100 tons) is being obtained at a position of 5 mm above the bottom dead point.

While it is wrongly thought that the press machine is hard and does not get deformed even when the applied pressure is acting, even within the permissible range of the pressure applying capacity, the frame and the bolster plate of the press machine become elastically deformed as shown in Fig. 1. Even when there is deformation of the frame etc., safety is guaranteed if the load is less than the pressure applying capacity.

The pressure called pressing capacity assumes that uniformly distributed load is active on a predetermined region in the area of the bolster plate as shown in Fig. 1. Even within the permissible range of the pressing capacity it is not good to make a concentrated force act on a narrow region.

From the aspect of tool life of the die, if work is done at the maximum pressing capacity, the life of the die will become shorter due to the effects of deformations of the frame and the bolster plate of the press machine. When the tool life of the die is considered, it is necessary to carry out the work while having some leeway with respect to the pressing capacity. Generally, it is considered good for the life of the die to carry out work at pressures up to 60 to 70% of the pressing capacity.