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#047 Relationship Between Press Machine and Die (5) Torque Capacity of Press Machine

Category : Formed Materials
July 2, 2010

In machine presses such as crank presses, etc., the pressure generated changes depending on the position of the stroke. This point is a big difference from fluid presses such as hydraulic presses in which the pressing force does not change at any position of the stroke. Fig. 1 shows a graph of pressure versus stroke of a crank press.

Fig. 1

This indicates that the pressure generated becomes higher as the stroke position of the slide gets closer to the bottom dead point. Theoretically, it is possible to get infinite pressure. In the indication of the capacity of a press machine, the position of the bottom dead point is determined, and the capacity is indicated. This is called "Torque Capacity".

In press formation, the formation starts from a certain position above the bottom dead point, and ends at the bottom dead point. The change in the forming force between the starting point of product formation until the completion of formation is called "the curve of pressure of product formation versus stroke". Fig. 2 shows the pressure versus stroke curves for the product and the press machine superimposed on each other.

Fig. 2

Two curves A and B have been shown for product formation. The product formation curve A is inside the curve for the press machine. If the condition is like this, it indicates that there is no problem in press formation. However, the product formation curve B is intersecting the curve for the press machine. The part indicated by "Capacity exceeded" in the figure indicates that the torque capacity of the press machine has been exceeded. Since the possibility increases of abnormality occurring in the movement transmission parts such as the crank shaft, etc., of the press machine, it will be necessary to use a press machine with a higher capacity.

In actual practice, since it is difficult to know the pressure versus stroke curve of the product formation, the formation will have to be made by judging based the maximum formation pressure. While the maximum formation pressure occurs a little after starting the formation, it should be confirmed that the pressure generated by the press machine at the formation starting position is exceeding the product formation force. If this is done, it is possible to carry out the work with some leeway in the capacity. It is necessary to take particular care in drawing operations because very often the formation starting point comes above.

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