The press's precision is influenced by elastic deformation that occurs when it is subjected to load. This deformation manifests in three primary forms: vertical displacement, bending, and lateral misalignment. These types of deformation directly impact the shape, size, and overall accuracy of the forged product. As a result, the workpiece may end up with dimensional inaccuracies or misalignment issues.
Moreover, the manufacturing and installation quality of the press, along with the clearance between the guide rail and the column, also play a significant role in determining the forming precision. Therefore, the key factors affecting the press's forming accuracy are: the vertical stiffness above the pressure point, the angular stiffness between the table and the slider, and the amount of lateral misalignment perpendicular to the downward direction of the force.
Vertical stiffness refers to the resistance of the press to vertical deformation under a given load. It can be represented as Cz = Fnh (kN/mm), where Fn is the nominal force of the press, h is the vertical deformation of the mold height when the press is under Fn, and hâ‚€ is the distance between the slider and the table when no load is applied.
Angular stiffness, on the other hand, measures the resistance of the press to angular deformation under an eccentric load. It is expressed as Cθ, and the total angular deformation is calculated as K = Ka + Ke, where Ka represents initial angular deformation due to factors like guiding gaps, and Ke accounts for the elastic deformation caused by the eccentric load.
In a closed combination fuselage press, the relationship between the columns and the tightening bolts is crucial. When the press is operating, there should be no gap or misalignment between the upper beam, base, and columns. To ensure this, bolts must be tightened to apply pre-tensioning forces, which compress the structure and elongate the bolts accordingly.
The pre-tightening force in the tension bolts significantly affects the press’s accuracy. As the pre-tightening force increases, the vertical stiffness of the press improves slightly. For example, increasing the preload from 760 kN to 1900 kN results in only a 4.3% reduction in vertical deformation and a similar increase in stiffness.
However, the effect of the pre-tightening force on bending deformation and angular stiffness in the Y-direction is minimal. Even with increased pre-load, the angular deformation only increases slightly, indicating that the pre-tightening force has limited influence on these parameters.
In contrast, the pre-tightening force has a more pronounced effect on lateral misalignment. Increasing the preload from 760 kN to 1900 kN reduces the lateral displacement of the slider by 65% and 72%, respectively. However, beyond a certain point—such as when the preload increases from 1425 kN to 1900 kN—the improvement becomes marginal, suggesting that excessive pre-tightening offers diminishing returns.
Analysis of the KDH160/1250 closed-type combined fuselage press in Germany highlights that pre-tightening force has a major impact on the press's forming accuracy, especially in the X-direction. It influences bending deformation, angular stiffness, lateral misalignment, and the maximum gap between the slider and column. Increasing the pre-tightening force enhances the press's accuracy, but only up to a certain limit.
The optimal pre-tightening factor is around 3.5 (Fv = 1425 kN). Beyond that, further increases have little effect on precision. A pre-tightening factor of 4.75 (Fv = 1900 kN) is considered excessive and does not significantly improve performance. Thus, finding the right balance in pre-tightening is essential for maximizing the press’s accuracy and efficiency.
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