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Tube bending is a mechanical process used to deform tubes using a bending process that transforms straight tubes into curved shapes. Although straight tubes are beneficial for certain applications, in many cases, to meet specific requirements, tubes need to be bent, shaped, and stressed.
Bent tubes are more useful than straight ones and are an integral part of trombones, stair railings, handles, furniture frames, automotive parts, and air conditioning equipment Pipes and tube fittings are types of bent tubes used to change the direction of conduits for fluids and gases in exhaust systems, hydraulic lines, and pipelines.
The two processes used to bend tubes are hot bending and cold bending where hot bending includes heating the metal tubing above room temperature to be shaped while cold bending is performed at room temperature or slightly above room temperature. Hot and cold bending are two general categories that are used to describe the wide array of bending processes that include press bending, rotary bending, heat induction bending, sand packing and hot slab forming, and ring roll bending to name a few.
Any type of metal tubing can be bent including aluminum, stainless steel, mild steel, brass, and titanium that are changed into a wide variety of shapes and configurations each of which is designed for a specific purpose. The most common types of bend shapes are L bend, U bend, S bend, and coil bend, which are shapes that are created by applying force that stresses and reconfigures straight metal tubing.
At the beginning of a bending operation, the metal tube is secured by a clamp die and pressure die after which a rotating die, roller, or press bends it. The process can be form bound or free form. Tensile and compressive forces act on the tubing material as the tooling advances it through the tubing die. The results of the bending are dependent on the type of tubing material, tooling, the amount of pressure applied, lubrication, and the bending geometry.
Mechanical tube bending is a collection of processes that are used to make an assortment of products and assemblies out of a straight tube. Aside from tube bending, other mechanical processes include cutting and deburring, slotting, notching, and welding.
Before selecting the right die for a specific type of tube bending, it is beneficial to know the geometry of a bend. The following terminologies are used in tube bending:
Center-Line Radius. Center-line radius (CLR) refers to the distance from the center of the curvature to the centerline (axis) of the tube. It can be equal to the radius of the die, depending upon the extent to which the die is forced on the tube. For tubes with the same radial dimensions and material, the length of the curvature increases as CLR becomes larger. CLR is oftentimes referred to as the bend radius.
Outside Diameter. In hollow cylinders like tubes, the outside diameter is the distance between two points on the outermost edges of the tube‘s cross-section which passes through the centerline.
Inside Diameter. Inside diameter is the distance of the innermost edges of the tube‘s cross-section which passes through the centerline. It is the size of the tube‘s hole.
Wall Thickness. Wall thickness is the difference between the outside and inside diameters of a tube. It is the width of the tubing material, usually measured by calipers for precision. The outside diameter and the wall thickness of the tube are the most important considerations when choosing a die for a tube bending method.
Degree of Bend. The degree of bend is the angle formed by bending the tube which is measured in degrees. It is the "sharpness" of the bend; the tubes with smaller bend angles formed have shaper curvatures. The complementary angle of the degree of bend is called bend angle.
Tubes and pipes appear to be the same and can be subjected to the same bending techniques. The terms, tubes and pipes, are often used interchangeably but actually refer to two distinct and different components. Tube is a general term used to describe round, square, rectangular, or oval hollow shapes that are used for mechanical and structural applications, pressure equipment, and instrumentation systems.
Pipes are a conveyance system used for moving liquids, gasses, cold and hot water, and other liquids. Their sizes are represented by Nominal Pipe Size (NPS) and schedule numbers. NPS is the North American set of standards to designate diameters and the wall thicknesses of pipes used for high or low pressures and temperatures. The schedule number is a dimensionless value that refers to the wall thickness of a pipe. Tube sizes are represented by the outside diameter with the wall thickness expressed in Birmingham Wire Gauge (BWG) terms.
There are several physical changes per area that the tube experiences during bending, depending on the bending technique used and the properties of the tubing material.
Wall Factor. The wall factor is the relative wall thickness. It is the ratio of the outside diameter of the tube and its wall thickness. The resulting value determines whether a tube is "thick-walled" or "thin-walled".
The wall factor is used to assess the difficulty of making any kind of bend. Tubes with lower wall factors are easier to bend because less material is required to stretch. Tubes with higher wall factors require more sophisticated dies and mandrels to support the tube.
D of the Bend. The "D of the bend" is a technical term used by tube fabricators which refers to the ratio of the CLR of the bend to the tube‘s outside diameter. This value tells how difficult the tube is to form tight radii. The higher D of the bend, the easier it is to form bends with tighter radii. In an ideal unsupported bend, the tube must have a combination of low wall factor and high D of the bend. Otherwise, it may result in a flat tone bend. This results when the outside wall of the bend collapses because it is not thick enough to support itself. Elongation refers to the extent to which a material can stretch before a fracture occurs. The higher D of the bend, the more material required to stretch it to produce bends with a tighter radius. Elongation is not only dependent on the D of the bend, but also on the material‘s property (i.e., stainless steel has a higher percentage of elongation than mild steel).
Springback. When a tube is bent to a certain degree, it tends to return to its original flat shape which results in a slightly smaller bend angle. To compensate for this phenomenon, an operator will slightly "overbend" to make up for the angular difference to meet the desired bend angle.
The movement of a tube in an attempt to return to its original shape during bending is referred to as springback. When a bend in a tube is created, an uneven molecular density from the shrinking and stretching of the material occurs. The inner region of the bend is compressed while the outer region is stretched. The tensile forces on the stretched region are greater than the compressive forces, which causes the bent tubing to return to its original shape.
In the tube industry, a common term used to describe the radius of the bending of a tube is the center line radius or CLR, which is the radius down the center of the tube. For most dies, the CLR will be larger than that of the die, especially where a mandrel is not used, and there isn’t any push. As the CLR increases, the more variability there will be in springback between each bent piece. Such variations have a direct impact on the position of the bend.
An increase in the CLR is caused by springback. As the tubing is springing back, the radius is springing open. In the case of most bender dies, the CLR equals the size of the die multiplied by 1.042. This formula can be used when a die does not permit a 180° bend.
Springback is influenced by several factors such as the material‘s stiffness, tensile strength, and wall thickness, type of tooling, and bending technique used. Harder materials and smaller CLR produces a greater springback. The bend angle is always augmented by a springback factor which is derived from performing several test bends. Springback factor is not constant for all materials and changes for varying wall thickness and diameters.
Tube bending techniques may be form-bound or freeform bending. In form-bound bending, forming is dependent on the geometry of the die, such as press bending and rotary draw bending. In freeform bending, forming is reliant on the movement of the tube through the tooling, such as roll bending. Tube bending techniques may also be classified as cold tube bending or hot tube bending. Cold tube bending is done at room temperature. The most common cold tube bending techniques are the following:
Press bending is the oldest industrial tube bending technique. In this method, the tube is fixed at two points and the ram (or the bend die) is forced against the tube to conform to the shape of the bend. The external dimensions of the cylindrical ram give the characteristics of the bend to be enforced to the tube.
Press bending is a quick bending method for symmetrical parts and requires no lubrication and cleaning. However, it is difficult to make a smaller degree of bend using this method. It offers no support on the tube internally; hence it is prone to deformation in the internal and external curvatures. It often produces an oval cross-section, depending on the tube‘s wall thickness. This method of bending is difficult to control and is only used when a consistent cross-section is not required.
Rotary draw bending is a suitable method to create precise bends with constant CLR and constant diameter, giving minimal ovalization. Some of the applications of this technique are found in pipe fittings, instrument tubing, handrails, automotive and aerospace parts. This method is also used for hollow sections with different cross-sectional shapes (e.g., square, oval). A smooth and aesthetically-pleasing bend is produced from the right tooling used matched to the application.
In this method, bending is done by a set of interlocking dies, and the tube is supported internally by a mandrel.
Bend dies are an essential part of rotary draw bending. They are designed to control the form and shape of the tube during the bending process and determine the radius of the bent tube. There are an endless number of dies that include very simple dies and extremely complex ones each of which is engineered to serve the functions of an application.
Tube bending dies are chosen in accordance with the type of bend required by the design parameters. For example, when the height of a bend is greater than its width, a pedestal and flange mount bend die will be used, which has a platform for increased stability.
Although the type of bend is a key factor in determining the type of bending die, the type of metal also influences the choice of die. The main metals used to produce tubing are steel, stainless steel, brass, copper, and aluminum. Engineers select metal tubing with a sufficient amount of wall thickness to be able to withstand the stress of the bending process to avoid any complications.
Ultra-thin wall mandrel. Ultra thin wall mandrels are used for very thin walled tubes with a wall factor of 200 or more. The unique design of ultra thin wall mandrels makes it possible for them to create the tightest radius bends. The close placement of the ball segments of ultra thin wall mandrels makes them the most flexible of the various types of mandrels due to the smaller size of their connected parts.
Ultra thin wall mandrels and thin wall mandrels are weaker by design. Any attempt to use them to bend tubes with thicker walls will result in the mandrels breaking.
Links. Links are an essential part of mandrel construction since they connect the balls of a mandrel to the shank. The type of links in a mandrel determine the length of its usefulness. When the balls of a mandrel are placed closer together, as in thin walled and ultra thin walled mandrels, their strength and firmness can be damaged.
Compression bending is cheaper than rotary draw bending due to its simpler set-up. However, it is limited to circular hollow sections. The set-up does not allow the use of a mandrel to support the inner diameter and may cause the outside surface to flatten slightly. It cannot be used for bending tubes to a small CLR because the tube may break or buckle. This method is commonly used in bending symmetrical workpieces and electrical conduits for structural application.
The roll bending method is used for creating bends with large CLR for large tubing components. It consists of two stationary rotating rollers and a moving roller which is positioned in a triangular pattern. The stationary rollers rotate in the counter direction of the moving roller. The bend radius is gradually formed as the tube moves back and forth on the rotating rollers.
The roll bending method is used for workpieces in structural applications, powder transfer systems, and much more. It is also used to bend the tube into spirals, as the operator can position the tube after one revolution to produce a continuous coil.
Bending springs are suitable for household tube bending for softer workpieces with small diameters such as PVC pipes and soft copper pipes (0.6 – 0.9 inches). A strong and flexible spring, which has a diameter slightly less than the inside diameter of the tube to be bent, is inserted inside the tube walls from its end to the center of the bend radius. A wire can be attached to the ends of the spring to aid in its positioning and removal. The spring provides support during manual bending. Manual bending is made by fixing the tube at a single point and gently pulling the ends of the spring until a desired degree of bend is made. It is one of the simplest tube-bending techniques but has low accuracy and repeatability.
The following are the hot tube-bending techniques that use heat energy to enhance the tube‘s plastic deformation during bending. Hot tube bending is commonly used for bending polymeric tubes such as PVC, CPVC, and ABS.
Heat induction bending can produce bends with a wide range of CLR and does not require compression dies and mandrels. It can accommodate a wide range of pipe sizes and wall thicknesses, which results in minimal wall thinning and ovality. The accuracy of the heat induction bending technique serves many applications in the petrochemical, energy, and mining industries. However, this method has a high operating cost.
Sand packing hot-slab bending is a flexible hot bending process that begins by filling a tube with a fine granular sand. The sand is packed tightly into the metal tubing with both of its ends sealed. The packed tube is heated in a furnace at a temperature exceeding 870° C. The heated pipe is then placed in a slab with pins set on it where mechanical force is applied to bend the tube around the pins. The packed sand assists in maintaining the bent shape.
Hydroforming involves the use of a two half die into which a straight metal tubing is placed. It is a mechanical metal forming procedure that uses the application of a pressurized liquid, usually either, to deform and shape the metal tubing. Although several shapes of tubing can be used in the process, round is the most common shape since it offers the ability to produce several design options.
Once the metal tubing is positioned between the two halves of the die, the pressurized fluid is injected into the metal tubing. As the pressure on the liquid increases, it deforms the walls of the metal tubing to match the shape of the die. The halves of the die are then separated, and the newly shaped part or component is removed.
The process of hydroforming produces shapes with exceptional quality and aesthetic appeal. The enhanced hydrostatic pressure guarantees that the completed piece will not experience spring back and will be rigid and reliable.
Application of lubricant is necessary before the insertion of the dies to reduce friction, prevent premature wear, and extend the life of the tooling. Lubricants are supplied as a paste or gel and have unique formulations for different tubing materials such as steel, aluminum, copper, and titanium. Areas for lubricant application are inside and outside of the tube, bending mandrels, contact point of wiper dies, and bending springs. More concentrated lubricant application is required for heavier duty bending (i.e. thicker walls and tight radii). After the bending operation, the remaining lubricant is cleaned on the surface of the tube and dies.
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