May 16, 2002
The two basic types of structurals are load-bearing and architectural; the two bending methods are cold and induction. Understanding bending criteria is the key in determining which bending method is best for the application at hand.
Structurals come in a variety of shapes and configurations, and they are categorized by groups within sections, from small to large. They may be in the form of angle, channel, I and wide-flange beams, square and rectangular tubing, solid bars, flat stock, railroad rails, and T's.
But, structurals fall into two primary types, and fabricators have two primary ways to bend them.
Structural shapes generally fall into one of two main categories, load-bearing or architectural. In both cases, product quality is critical. Bend and material integrity is the principal consideration for load-bearing structurals, and physical appearance is most important for architectural shapes.
A typical load-bearing application is wide-flange beams used in road overpasses, whereas shaped profiles often are found in buildings and entrances to recreational areas and amusement parks and in sculptures.
As you might expect, both types of applications have challenges. For structural material—angle, channel, T's, beams, and rails—the challenge begins with tight radii. When a radius is tight, the material tends to deform easily, eventually reaching a point at which further deforming will destroy it.
For example, you can't roll-bend a structural on a tight radius without kinking it at the tangent points. Although you can attain the required radius, after bending the material you must remove the kinked portion and the straight section preceding it. To prevent wall collapse, or at least keep it to a minimum, it may be necessary to fill hollow materials with sand.
Of the two types of work, architectural shapes present by far the greater challenges. Architects and designers seem to delight in conceiving weird shapes (often in aluminum) that have strangely shaped openings, or "legs," and then want the material to be bent into an impossible configuration.
Just as certain types of machines are designed to bend tube and pipe, special types of machines perform structural bending. Because many structural shapes have a leg, sometimes two, facing in or out from the bend radius, it isn't cost-effective to make dies for all the different configurations and dimensions. So the rolls, both drive and forming, are essentially flat rolls that can adjust for varying thicknesses and spacing between legs.
For example, bending a channel with legs and then a T with no legs requires just a roller adjustment rather than a complete roller changeout. This equipment capability is vital for bending large structural shapes.
The equipment you choose will depend on the shape to be bent and any special end-use requirements.
For example, a draw bender can bend squares and rectangular tubing. However, if the required radius is small, you need mandrel tooling and, in some instances, split dies.
A mandrel helps prevent wall collapse, particularly in rectangular tubing. Split dies enable removal of the work after you've completed the bend. Both mandrels and split dies generally are expensive. In most cases, induction bending is an affordable alternative.
The most practical, accepted, and economical way to make large-radii bends generally is to cold-roll the material. Many variables have a bearing on the type of equipment you should choose, including wall, flange, and leg thickness.
For large-radii bends, cold rolling achieves the desired radius with a minimum number of passes. Because of their size, wide-flange beams can be cold-rolled as well. The larger the radii, the easier cold rolling is (see Figure 1).
A 36-inch wide-flange beam can be bent with minimal distortion. Of course, it requires a heavy piece of equipment to make these bends. In addition, it is possible to camber beams up to 44 in. (see Figure 2).
Sometimes you may find it more economical or necessary to bend some sections, particularly heavier sections, by induction. The type of material and the configuration required will dictate whether the job is suitable for induction bending. This also is true for large rectangular and square sections of tube.
In induction bending, an electrical coil conforming to the shape of the material being bent is wound around the workpiece. This coil is specially built, and a different shape is required for each different material shape.
With only a minimum amount of clearance between the coil and the section, uniform heat is induced into the workpiece. This heat is obtained from an electrical unit capable of producing high ranges of kilowatts. The larger the unit and bending machine capacity, the more kilowatts are needed to produce enough heat to bend the workpiece.
This heated area is controlled so that the heat coil induces only a narrow band of heat into the workpiece. This narrow band is where the actual bend takes place. A coil mounted around the workpiece delivers a water spray to cool the material immediately as it is bent (see Figure 3).
The heated bending area then is the only malleable portion of the piece, so it is the only area that will bend. An air coil is needed to remove the water in an even flow to control the heat in the bent area uniformly. Some of the parameters you must maintain during the induction process are temperature, cooling water, airflow, and speed (inches per minute, IPM).
The machine has a bend arm that is adjusted to the required radius. This arm is clamped to the end of the workpiece. The workpiece extends through the coils where it is clamped to the pushing device. Normally, this is a hydraulic push ram, although some machines are equipped with a push clamp that is gear- and chain-driven. As the hydraulic ram pushes the workpiece through the heated area, the clamping arm pivots, swinging the piece around to the desired preset radius.
Induction bending can produce low-distortion bends in material that cold bending would destroy, such as large tubes. Several types of induction machines are on the market, although relatively few are in the U.S. These machines vary widely in their capabilities, with some able to bend very large sections.
An important factor to consider when bending structural shapes, especially large ones with a large radius, is supporting the material on both sides.
You can't expect to run a 40- or 60-ft. length of material being bent with a 100-ft. radius through rollers and not support it equally on both sides. It's necessary to have strong support for the material throughout its length on both sides so that it rolls freely into and out of the machine.
Another point to consider is measuring the bend radius. You have several ways to do this. One, of course, is to bend a length of small pipe to the required inside radius and use it to check the bend. Another way is to use a length of rigid straight material to measure the chord over a given degree of bend. Of course, you can always do a floor layout, which is often done for pipe bends, but this is seldom practical.
One other challenge to consider is shipping. For large wide-flange beams with a large radius, it may be possible to get only one bend on a truck, and an escort may be necessary because of road clearances.
However, structural bending can be less challenging than pipe bending, particularly if you have the right type of machinery and equipment to do the work.
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