The press's precision is influenced by the elastic deformation it undergoes when subjected to load, which manifests in three main forms: vertical displacement, bending, and lateral misalignment. These deformations directly impact the shape, size, and overall accuracy of the forged product, often leading to dimensional inaccuracies or misalignment. Additionally, the manufacturing and installation quality of the press, as well as the clearance between the guide rail and the column, play a significant role in determining the final forming precision. Therefore, the accuracy of the press mainly depends on three key factors: the vertical stiffness above the press, the angular stiffness between the table and the slider, and the amount of lateral misalignment perpendicular to the downward direction.
Vertical stiffness refers to the force required to cause a unit vertical deformation of the press. It can be represented as Cz = Fn / h (kN/mm), where Fn is the nominal force of the press, h is the vertical deformation of the mold height under this force, and the difference between the slider-to-table distance under load and without load. This parameter is crucial for maintaining consistent pressure during the forging process.
Angular stiffness, on the other hand, measures the resistance of the press to angular deformation under an eccentric load. It is expressed as Cθ, with the total angular deformation being the sum of initial deformation (such as from guide gaps) and elastic deformation due to the applied load. This factor is essential for ensuring that the press maintains its alignment and does not introduce unwanted angles into the workpiece.
In a closed combination fuselage press, the relationship between the columns and the tightening bolts is critical. When the press is operating, there should be no gaps or misalignments between the upper beam, base, and columns. To achieve this, the bolts must be pre-tightened to apply a certain compressive force, while the tension bolts are stretched accordingly. This creates a balance between the forces and deformations within the structure.
The pre-tightening force in the tension bolts significantly affects the press’s accuracy. As the preload increases, the vertical stiffness of the press changes slightly. For example, increasing the pre-tightening force from 760 kN to 1900 kN results in only a 4.3% reduction in vertical deformation and a corresponding 4.3% increase in stiffness.
Regarding the Y-direction bending deformation and angular stiffness, the effect of increased pre-tightening force is minimal. When the pre-tightening force rises from 760 kN to 1425 kN and then to 1900 kN, the angular deformation increases by 0.5% and 1.5%, respectively. Thus, the pre-tightening force has limited influence on these parameters.
However, the lateral misalignment of the slider is significantly reduced as the pre-tightening force increases. At 1425 kN, the lateral displacement decreases by 65%, and at 1900 kN, it drops by 72%. Beyond 1425 kN, further increases have diminishing returns, with only a 7% reduction when moving from 1425 kN to 1900 kN. This suggests that beyond a certain point, higher pre-tightening forces do not significantly enhance the press’s accuracy.
Analysis of the German KDH160/1250 closed-type combined fuselage press shows that the pre-tightening force in the bolts has a major impact on the press’s forming accuracy, particularly in the X-direction. It influences the bending deformation, angular stiffness, lateral misalignment, and the maximum gap between the slider and the column. Increasing the preload reduces these issues, improving overall accuracy. However, a pre-tightening coefficient of 3.5 (Fv=1425 kN) is optimal, as higher coefficients like 4.75 (Fv=1900 kN) provide little additional benefit.
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