Large Press Brake Crowning: New Mechanical Crowning Solution

The bending length of sheet metal has a great influence on the bending accuracy. The longer the sheet metal is, the greater the bending force is required.

Therefore, the inclination of the equipment and the deformation of the ram become larger, which makes it more difficult to ensure the accuracy.

Generally, the accuracy of the bending angle including the total bending length is called “straight line accuracy”.

If we don’t take effective measures, the amount of concave die entering in the full length direction of the bending upper die is inconsistent, which makes the bending part appear “boat belle” effect.

In this paper, the finite element simulation method is used to analyze the force and deformation displacement of the ram, extract and modify the deflection compensation curve, combined with empirical data, a new mechanical deflection compensation device is designed and manufactured.

The linear accuracy of large-size press brake machine is improved by using driving motor or manual adjustment to realize deflection compensation in the whole length or part.

Characteristics analysis of the ram load


The ram block of press brake is made of different shape steel plates.

In the process of modeling, the details that have little influence on the results are ignored, and only the main structure of the ram is retained.

The main body size is 8000mm × 2500mm × 120mm

Set the elastic modulus E = 2 × 105MPa; Poisson’s ratio = 0.27; Density = 7.8×103 kg/m3

According to the structural characteristics of the ram, solid95 element defined by 20 nodes is selected.

This element has arbitrary 3D orientation, and can adapt to the curve boundary model, and can analyze the elastic deformation of the ram more accurately.

Load and constraint application of the ram block

(1) Constraints

In actual conditions, the ram is always in motion.

Because of the static analysis of the ram, it is necessary to simplify and approximate the constraints of the ram, and impose symmetrical constraints on the nodes on the middle symmetry plane of the ram;

The ram is fixed by connecting the guide rail on the frame with the back of the ram, where full constraint is applied.

(2) Load condition

The surface load is applied to the contact part between the bottom of the hydraulic cylinder and the ram block. Because the vertical deformation of the ram block is small relative to the total length, it belongs to small elastic deformation, so the uniform load is applied to the stress surface at the bottom of the ram block in the model; The bottom of the ram block is connected with the upper die by a connecting block to ensure that the force is evenly transmitted from the ram block to the upper die.

Extraction and analysis of simulation results

The displacement nephogram of ram block under load is shown in Figure 1.

The path is set in the ANSYS result processing, and the deformation deflection curve of the stress surface at the bottom of the ram is extracted, as shown in Figure 2.

It can be seen from the figure that the maximum displacement appears in the center of the ram and gradually decreases to both sides in a parabolic shape. At the same time, the deformation displacement at any position along the bending length direction can be obtained, which provides data support for the design of wedge with different array angles to form deflection curve.

Mechanical deflection compensation device

It can be seen from the above analysis that when the press brake machine is loaded, due to its own structure, the stress surface of the ram will produce parabolic deflection deformation, resulting in the bending angle of the workpiece in the full length direction can not guarantee consistency.

In addition, the local wear of the bending die also affects the straightness of the bending workpiece.

At present, there are two common ways to solve this problem.

One is to install a hydraulic top cylinder at the proper position of the upper ram or (and) the lower worktable of the bending machine, and to achieve the purpose of compensation by controlling the ejection height of each top cylinder;

The other is to use the mechanical deflection compensation device in the lower worktable, that is, to achieve the purpose of compensation by adjusting the wedge blocks of different angles.

Hydraulic top cylinder method is easy to operate, and can meet the general accuracy requirements of bending production. For large size and high accuracy bending parts, the mechanical deflection compensation method is mainly used at present.

Traditional deflection compensation device

The traditional mechanical crowning method is to manually adjust the compensation block or add a gasket at the worn place, but this method is time-consuming and laborious, low efficiency and difficult to guarantee the accuracy.

The more advanced press brake machine is equipped with automatic or semi-automatic deflection compensation mechanism, such as the commonly used wedge type deflection compensation device and pull rod type deflection compensation device.

  • Wedge type can ensure the same angle and improve the bending accuracy, but it has high labor intensity and low efficiency.
  • The pull rod type can easily realize the deflection compensation in the whole length direction, but it can not solve the problem of local wear.

Fig. 3 (a) and (b) show two types of deflection compensation devices.

Fig. 3   Common mechanical deflection compensation device

New deflection compensation device

In order to solve the problem of local wear, our mold company designed a four piece wedge deflection compensation device.

The device can not only automatically adjust to compensate the whole deflection of the workpiece, but also manually adjust to compensate the local wear of the die.

Fig. 4 is a two-dimensional sectional view, and the working principle is described as follows:

Fig. 4   Four piece wedge type deflection compensation device

  • 1-upper cover plate
  • 2-wedge IV
  • 3-wedge III
  • 4-wedge II
  • 5-wedge I
  • 6-tie rod
  • 7-spacer
  • 8-right wedge
  • 9-lead screw
  • 10 – bearing pedestal
  • 11 – bearing
  • 12 base
  • 13 nut

(1) A rectangular groove is set on the base along the length direction (i.e. longitudinally). In this rectangular groove, odd groups of wedge mechanisms are evenly distributed longitudinally, and each group of wedge mechanisms are stacked with two pairs of four pieces of wedge, i.e. wedge I, II, III and IV, from bottom to top.

(2) In each group of wedges, wedge I and wedge II at the lower part are a pair to form a local adjusting wedge mechanism.

The inclined planes of each pair of inclined wedges I and II are matched correspondingly and are arranged in a transverse direction.

Screw holes are respectively set in the middle part of the front and rear seat walls of the base corresponding to the big end of the wedge I, and adjusting bolts are correspondingly installed on the outside of the base wall, and each adjusting bolt extends into the base to connect with each wedge I.

Manually adjust the bolt to drive the wedge I to move forward and backward (transversely), and adjust the upper cover plate locally as required, that is, the worktable moves up and down to realize the purpose of local compensation.

(3) Inclined wedge III and inclined wedge IV at the upper part are a pair, and each pair of inclined wedge III and inclined wedge IV is set longitudinally to form an integral adjusting inclined wedge device.

Each pair of wedges III is matched with the inclined plane of wedges IV, and the inclination of wedges III located in the middle of the rectangular groove of the base is the largest, while the inclination of wedges III located on the left and right sides of the rectangular groove of the base gradually decreases. When the wedges III move equidistant along the length direction, the middle lift is large, forming a curve that can adjust the deflection according to the moving distance of the wedges, The whole deflection compensation is realized.

At the same time, the short axis is symmetrically arranged on the front and rear side walls of each wedge IV, and a vertical notch groove is arranged on the upper part of the front and rear side wall of the rectangular groove of the base corresponding to the short axis. The short axis of each wedge IV is sliding in each notch groove, so that each wedge IV can only move up and down, but not left and right, so as to ensure the lifting effect of wedge IV.

(4) Longitudinal screw holes are set on wedge III at the right end, while longitudinal through holes with the same center line as screw holes are set on other wedge III. Hollow spacer sleeve is installed between two adjacent wedge III. Pull rod is installed in each wedge III and hollow spacer sleeve. The right end of pull rod is threaded into wedge III at the right end. The adjusting screw is installed at the right part of the screw hole of the wedge III at the right end, and the motor is installed at the extended end of the adjusting screw to start the motor, which can realize the automatic overall deflection compensation.

Figure 5 shows an 8-meter-long double pull rod four piece wedge deflection compensation device.

Fig. 5   8m double strut wedge type deflection compensation device

Wrap it up

In this post, the small elastic deformation of the ram of the press brake machine is simulated and analyzed, and the deflection deformation data of the stress surface at the bottom of the ram is extracted;

Combined with the experience data, a four piece wedge deflection compensation device is designed, which can not only automatically adjust the whole deflection compensation of the processed parts, but also manually adjust the local wear compensation of the die.

The device has reasonable structure design, convenient and reliable use, improves the quality and production efficiency of sheet metal bending parts, and provides a new device for large precision bending compensation.

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