# Large Press Brake Crowning: Introducing a New Mechanical Crowning Solution

The length of a sheet metal bend greatly affects its bending accuracy. The longer the sheet metal, the greater the bending force required, leading to larger equipment inclinations and ram deformations, making accuracy harder to ensure. This bending accuracy, including the total bending length, is referred to as “straight line accuracy.”

Without effective measures, inconsistent amounts of concave die entering the full length direction of the bending upper die can cause the bending part to have a “boat belle” effect. To address this issue, a finite element simulation method was used to analyze the ram’s force and deformation displacement. The deflection compensation curve was extracted and modified, and combined with empirical data to design and manufacture a new mechanical deflection compensation device.

The linear accuracy of large-size press brake machines can be improved by using a driving motor or manual adjustment to compensate for deflection in the whole or part of the length.

## Characteristics analysis of the ram load

Modeling

The press brake ram is made of steel plates of various shapes. During the modeling process, only the main structure of the ram is considered, while details that have little impact on the results are ignored. The main body dimensions are 8000mm x 2500mm x 120mm.

The elastic modulus is set to 2 x 105 MPa, Poisson’s ratio to 0.27, and density to 7.8 x 103 kg/m3. Given the structural characteristics of the ram, a solid95 element defined by 20 nodes was selected for the analysis.

This element has the capability to adapt to curved boundary models and accurately analyze the elastic deformation of the ram, as it has arbitrary 3D orientation.

## Load and constraint application of the ram block

(1) Constraints

In real-world conditions, the ram is always in motion. However, to perform a static analysis of the ram, it is necessary to simplify and approximate the constraints of the ram. To do this, symmetrical constraints are imposed on the nodes located on the middle symmetry plane of the ram.

The ram is fixed by connecting the guide rail on the frame to its back, where a full constraint is applied. This ensures that the ram remains in a fixed position during the analysis.

The surface load is applied to the contact area between the bottom of the hydraulic cylinder and the ram block. As the vertical deformation of the ram block is small compared to its total length, it is considered to be small elastic deformation. As a result, a uniform load is applied to the stress surface at the bottom of the ram block in the model.

To ensure that the force is evenly transmitted from the ram block to the upper die, the bottom of the ram block is connected to the upper die by a connecting block. This ensures that the load is distributed evenly and does not cause any imbalances in the system.

Extraction and analysis of simulation results

The displacement diagram of the ram block under load is shown in Figure 1. The path is set in ANSYS for result processing, and the deformation deflection curve of the stress surface at the bottom of the ram is extracted and shown in Figure 2.

As seen in the figure, the maximum displacement appears at the center of the ram and decreases gradually towards both sides in a parabolic shape. At the same time, the deformation displacement at any position along the bending length direction can be obtained, providing data support for designing wedges with different array angles to form the deflection curve.

## Mechanical deflection compensation device

The analysis shows that when a press brake machine is loaded, its stress surface on the ram produces parabolic deflection deformation due to its own structure, resulting in inconsistent bending angles of the workpiece along its full length. Additionally, local wear on the bending die also affects the straightness of the bent workpiece.

Currently, there are two common methods to address this issue. The first method is to install a hydraulic top cylinder at an appropriate position on the upper ram or lower worktable of the bending machine and control the ejection height of each top cylinder to compensate for the deformation. The second method is to use a mechanical deflection compensation device on the lower worktable, which compensates for deformation by adjusting the wedge blocks with different angles.

The hydraulic top cylinder method is easy to operate and meets the general accuracy requirements of bending production. However, for large-size and high-accuracy bending parts, the mechanical deflection compensation method is primarily used.

The traditional method for mechanical crowning involves manual adjustment of the compensation block or adding a gasket at worn areas, which is time-consuming, labor-intensive, and not very efficient, making it difficult to guarantee accuracy.

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More advanced press brake machines, on the other hand, have automatic or semi-automatic deflection compensation mechanisms, such as the commonly used wedge type and pull rod type compensation devices. The wedge type device can ensure consistent angles and improve bending accuracy, but it requires a lot of manual labor and is not very efficient. The pull rod type device, on the other hand, easily compensates for deflection along the whole length, but it does not solve the issue of local wear.

Figure 3 (a) and (b) depict two types of deflection compensation devices.

Fig. 3   Common mechanical deflection compensation device

### New deflection compensation device

To address the issue of local wear, our mold company designed a four-piece wedge deflection compensation device. This device not only automatically compensates for the entire deflection of the workpiece, but also allows for manual adjustment to compensate for local wear of the die.

Figure 4 is a two-dimensional sectional view of the device, and its 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
• 10 – bearing pedestal
• 11 – bearing
• 12 base
• 13 nut

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

(2) In each group of wedges, the lower pair, Wedge I and Wedge II, form a local adjusting mechanism. The inclined planes of each pair are matched correspondingly and arranged in a transverse direction.

Screw holes are set in the middle of the front and rear seat walls of the base corresponding to the big end of Wedge I. Adjusting bolts are installed on the outside of the base wall and each one extends into the base to connect with Wedge I.

To achieve local compensation, the bolt can be adjusted manually to move Wedge I forward and backward (transversely), thereby adjusting the upper cover plate and causing the worktable to move up and down.

(3) The upper pair, Wedge III and Wedge IV, form an integral adjusting mechanism. They are set longitudinally in each group and form an integral adjusting inclined wedge device.

Each pair of Wedges III are matched with the inclined plane of Wedges IV, with the largest inclination located in the middle of the rectangular groove on the base. The inclination gradually decreases towards the left and right sides of the groove. When the Wedges III move equidistantly along the length direction, the middle lift is substantial, forming a curve that adjusts the deflection based on the movement of the Wedges. This realizes the overall deflection compensation.

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The short axis of each Wedge IV is symmetrically arranged on the front and rear side walls. A vertical notch groove is arranged on the upper part of the front and rear side walls of the rectangular groove of the base, corresponding to the short axis. The short axis of each Wedge IV slides in each notch groove, allowing only up and down movement and ensuring 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 the screw holes are set on the other Wedge III. A hollow spacer sleeve is installed between each pair of adjacent Wedge III. A pull rod is installed in each Wedge III and hollow spacer sleeve. The right end of the pull rod is threaded into the Wedge III at the right end. An adjusting screw is installed at the right part of the screw hole of the Wedge III at the right end, and a motor is installed at the end of the adjusting screw to start the motor, which can achieve automatic overall deflection compensation.

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

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

## Wrap it up

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

Based on the experience data, a four-piece wedge deflection compensation device has been designed. It not only automatically adjusts the overall deflection compensation of the processed parts but also allows for manual adjustment of local die wear compensation.

The device has a well-designed structure, is convenient and reliable to use, improves the quality and production efficiency of sheet metal bending parts, and provides a new solution for large precision bending compensation.