How to get a good leading edge
April 11, 2006
Three-roll bending with a variable axis offers an advantage over four-roll bending for bending conical sections in that it allows for a good leading edge and rolling without having to lift the plate to pivot it with a bridge crant.
|The U.S wind industry ended its most productive year in 2005, installing nearly 2,500 MW, or more than $3 billion worth of new generating equipment in 22 states, according to the Washington, D.C.-based American Wind Energy Association (AWEA). There are now commercial wind turbine installations in 30 states. Because Congress extended the wind energy production credit, the wind industry is looking forward to growth in 2006 and several record-breaking years in a row, according to the association. Photo courtesy of Vestas Wind Systems A/S.|
The use of wind power has been growing over the last several years. Its success lies in its place in the field of clean and renewable, environmentally friendly energy. Three-roll plate benders are suitable for roll bending the conical plate sections used in the construction of wind towers. Here's how it is done with this type of bender.
First, the plate is placed on a supporting, motorized rollaway and introduced into the three bending rolls (see Figure 1).
First, the plate is placed on a supporting, motorized rollaway and introduced into the three bending rolls. In this example, the material is fed into the right side of the machine.
The plate is aligned with the help of two hydraulic side arms. One is positioned on the opposite side of the material infeed. The other hydraulic arm acts like a rollaway.
Because the plate will become a circular section with a ring shape, the plate side that will become the smaller diameter of the cone must have a smaller amount of curvature. Therefore, the plate must be aligned at its central axis and fed into the rolls at a slight angle, rather than fed parallel to the rolls (see Figure 2).
This conical section is where three-roll bending with a variable axis offers an advantage over four-roll bending. The four-roll configuration requires that the material always be presented parallel to the rolls for the entire leading edge to be prebent. If the plate were presented to a four-roll bender at an angle, the corner of the plate would not get prebent. With a three-roll machine with variable axis, the material can be presented to the rolls on an angle—not parallel to the top roll axis.
| Figure 3
Then, the plate is prebent. It is taken through the rolls' rotation, up to the center of the top and left side rolls. At the same time the top roll comes down, pressing and bending the plate. The top roll presses the leading edge of the plate, leaving the remaining metal plate in a horizontal position on the rollaway.
Next, the leading edge of the plate is prebent. The plate is taken through the rolls' rotation, up to the center of the top and left side rolls. At the same time the top roll comes down, pressing and bending the plate (see Figure 3). The top roll presses the leading edge of the plate, leaving the remaining metal plate in a horizontal position on the rollaway. To allow the plate to slide, the side rolls must be tilted at the cone angle.
The prebending stage is another area in which three-roll bending with variable axis offers an advantage over four-roll bending for wind tower sections. With a three-roll bender, the prebending can be done on the tilted edge of the plate. In contrast, a four-roll machine uses the top and central rolls to pinch the plate, while its side roll lifts the plate to perform the prebending. Because the plate for a wind tower is very long, a bridge crane is needed to lift and reposition the plate.
Once the plate is prebent, it continues its clockwise rotation. The left hydraulic side arm with double articulation supports a medium- to large-diameter pipe and leads it up to the hydraulic central arm (see Figure 4). To get a good bend, the pipe should be closed for 80 percent to 90 percent of its final diameter during this first cycle.
Once the plate is prebent, it continues its clockwise rotation. The left hydraulic side arm with hydraulic double articulation supports a medium- to large-diameter pipe and leads it up to the hydraulic central arm.
To facilitate and speed up all operations, three-roll benders can be equipped with a control that shows the position of the rolls and the lateral roll's tilting. The panel helps the operator move the rolls. In addition, it retains memory of the operation so that it can repeat the operation with pipes that have the same dimensions. The panel may give other information as well, such as the presence of anomalies or alarms, diagnostics of all electrovalves, and monitoring of automatic lubrication.
The trailing edge of the plate is prebent in the same way as the leading edge (see Figure 5). While the plate ends its rotation, the top roll presses on the last section of the plate to prebend it. The lateral rolls are in the same position they were in for the leading edge; just inverted.
The second prebend is performed in the same way as the first. When the rotation of the plate is at the end of its stroke, the top roll presses on the last section of the plate to make the prebend. The two side arms with double articulation and the central arm provide support for the plate. Here an operator checks the degree of curvature.
Next, the pipe is closed by rotating it counterclockwise.
Next, the pipe is closed by rotating it counterclockwise (see Figure 6). The two side arms with double articulation and the central arm provide support for the plate.
The cone is complete and is extracted from the roll bender.
This phase is important because it helps the internal plate tensions to stabilize, allowing good diameter tolerance and eliminating the need to calibrate the pipe again in the machine after it has been welded.
During calibration the pipe is rotated clockwise once again. Altogether the pipe is rotated three times from one side to the other. While bending can be performed in just one step, the pipe sometimes can have an inconsistent tolerance because of the different mechanical features of the material and the internal tensions caused by strains. Therefore, either the pipe needs to be reinserted into the machine for recalibration, or there will be a long task of welding the two ends.
The cone is complete and is extracted from the roll bender (see Figure 7).