Determination of press tonnage during bending
Determining the tonnage of a bending press is a crucial step in bending process design, directly impacting production safety and economic efficiency. The press tonnage must be greater than the maximum force required during the bending process (including bending force, ejection force, and pressing force). Factors such as mold structure, material property fluctuations, and operational errors must also be considered, allowing for a safety margin. Accurately determining the press tonnage can prevent press overload, mold damage, or poorly formed parts due to insufficient tonnage, as well as energy waste and equipment idleness caused by excessive tonnage. The determination of press tonnage requires a combination of theoretical calculations, empirical data, and practical verification to ensure scientific and rationality.
Bending force is the primary basis for determining press tonnage. It includes both free bending force and corrected bending force, and the appropriate calculation formula must be selected based on the bending method. The free bending force calculation formula is: F self = (0.6 × B × t² × σb) / (r + t) for V-shaped parts, and F self = (0.7 × B × t² × σb) / (r + t) for U-shaped parts. The corrected bending force calculation formula is: F corrected = K corrected × σs × A, where B is the width of the bent part, t is the material thickness, σb is the tensile strength, r is the bend radius, K corrected is the correction factor (1.2-1.5), σs is the yield strength, and A is the corrected projected area. For example, for a V-shaped part (r=5mm) made of low-carbon steel (σb=450MPa) with a thickness of 3mm and a width of 100mm, the free bending force Fself=(0.6×100×3²×450)/(5+3)=(0.6×100×9×450)/8=243000/8=30375N≈30.4kN; if correction bending is adopted, the correction area A=100mm×50mm, σs=235MPa, Fcorrection=1.3×235×100×50=1.3×1175000=1527500N≈1528kN.
The ejector force and the pressing force must be included in the total tonnage calculation. The ejector force is generally 5%-20% of the bending force, and the pressing force is generally 10%-30% of the bending force. The total force Ftotal = Fbend + Flift + Fpress. For example, for the free-bend part mentioned above, the ejector force is 10% of Fbend = 3.04kN, and the pressing force is 20% of Fbend = 6.08kN. The total force Ftotal = 30.4 + 3.04 + 6.08 ≈ 39.5kN. For the corrected-bend part, the ejector force is 10% of Fcorrection = 152.8kN, and the pressing force is 20% of Fcorrection = 305.6kN. The total force Ftotal = 1528 + 152.8 + 305.6 ≈ 1986.4kN. When calculating the total force, it should be noted that the ejection force and the pressing force do not always reach the maximum value at the same time. The actual total force may be slightly less than the sum of the three, but for safety reasons, it is usually calculated as the sum.
When selecting press tonnage, a safety factor should be considered. Typically, a factor of 1.2-1.5 times the total calculated force is used to account for factors such as material property fluctuations (e.g., measured σb or σs values exceeding the designed values), uneven die clearance, and improper operation. For example, if the total free bending force is 39.5 kN and the safety factor is 1.3, the required press tonnage is 39.5 × 1.3, which equals 51.4 kN. A 63 kN press would be selected. If the total corrected bending force is 1986.4 kN and the safety factor is 1.2, the required tonnage is 1986.4 × 1.2, which equals 2383.7 kN. A 2500 kN press would be selected. The safety factor should be adjusted based on the production batch size and part importance. A higher value (1.4-1.5) is recommended for high-volume production or critical parts, while a lower value (1.2-1.3) is recommended for low-volume production or simple parts.
The press stroke and closing height must match the mold. The closing height of the bending die (the distance from the upper surface of the upper die base to the lower surface of the lower die base) must be between the press’s maximum and minimum closing heights, typically 5-10mm less than the maximum closing height to ensure sufficient travel space. For example, if the press has a maximum closing height of 300mm and a minimum closing height of 200mm, the mold closing height should be designed to be approximately 290mm. The press stroke must be greater than the combined height of the part being bent and the depth of the punch entering the die. For V-bends, the stroke is typically 2-3 times the part height, and for U-bends, 3-4 times, to ensure the material is fully formed.
In actual production, the determination of press tonnage requires verification through mold trials. Test bends are performed to observe the press’s operating conditions. Any vibration, unusual noise, or poor part formation indicates insufficient tonnage and requires replacement with higher-tonnage equipment. A press load factor below 50% indicates excessive tonnage and, in this case, energy conservation measures. For example, a U-shaped part was calculated to require a 2000kN press. During mold trials, it was discovered that a 1600kN press would suffice (because the material’s actual strength was lower than the design value). Ultimately, the 1600kN press was selected, reducing equipment costs. Furthermore, parameters such as the press’s worktable size and slide speed must be considered to ensure compatibility with the mold size and bending process, ensuring efficient and safe production.
