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Tube bending is a mechanical process designed to curve tubes from their straight, original form. Many applications use straight tubes effectively, but bending is frequently necessary to create unique designs, fit specific constraints, or meet particular functional requirements.
Curved tubes provide more flexibility and are crucial in numerous uses such as musical instruments like trombones, stair handrails, furniture frames, handles, vehicle parts, and HVAC systems. Additionally, bent tubes and fittings are utilized to redirect fluid and gas paths in systems like exhaust systems, hydraulic networks, and pipeline infrastructures.
Two main methods are used to bend tubes: hot and cold bending. Hot bending involves heating the metal tubing above room temperature for shaping, while cold bending occurs at or slightly above ambient temperature. These processes include various techniques such as rotary bending, press bending, heat induction bending, ring roll bending, sand packing, and hot slab forming, among others.
A wide range of metal tubing, such as aluminum, brass, mild steel, stainless steel, and titanium, can be transformed into different shapes and configurations, each serving a specific purpose. The most prevalent bending shapes include L bends, U bends, S bends, and coil bends, achieved by applying force that reshapes and stresses the metal tubing.
During a bending operation, the metal tube is clamped by a pressure die and clamp die before a rolling die, roller, or press bends it. The process may be free form or form bound. While the tooling moves the tubing through the tubing die, tensile and compressive forces are applied to the tubing material. The end result of bending hinges on factors like the tubing material, tooling type, pressure level, lubrication, and the bending design.
Mechanical tube bending includes a variety of techniques used to turn straight tubes into diverse products and structures. Besides bending, other related mechanical processes involve cutting, slotting, deburring, welding, and notching.
Understanding the geometry of a bend is crucial before choosing the appropriate die for tube bending. The following terms are commonly used in the tube bending process:
Center-Line Radius (CLR). The center-line radius is the measurement from the center of the curvature to the centerline (axis) of the tube. This radius may match the die radius based on how the die interacts with the tube. For tubes with identical radial dimensions and materials, a larger CLR results in a longer curvature. CLR is commonly known as the bend radius.
Outside Diameter. For hollow tubes, the outside diameter is the measurement between two points on the outer edges of the tube's cross-section, extending through the centerline.
Inside Diameter. The inside diameter refers to the measurement between the inner edges of a tube's cross-section, extending through the centerline. It represents the size of the tube’s internal opening.
Wall Thickness. Wall thickness is determined by subtracting the inside diameter from the outside diameter of a tube. It represents the thickness of the tube's material and is typically measured with calipers for accuracy. When selecting a die for tube bending, the outside diameter and wall thickness are crucial factors to consider.
Degree of Bend. The degree of bend refers to the angle created when the tube is bent, measured in degrees. It indicates the "sharpness" of the bend: tubes with smaller bend angles exhibit sharper curves. The angle complementary to the degree of bend is known as the bend angle.
Although tubes and pipes may seem similar and can be bent using comparable techniques, they actually represent distinct components. The term "tube" is broadly used to describe hollow shapes in round, square, rectangular, or oval forms, which are employed in mechanical and structural applications, as well as pressure equipment and instrumentation systems.
Pipes function as systems for transporting liquids, gases, and both cold and hot water. They are sized according to Nominal Pipe Size (NPS) and schedule numbers. NPS is a North American standard for specifying the diameters and wall thicknesses of pipes designed for various pressure and temperature conditions. The schedule number indicates the wall thickness of a pipe as a dimensionless value. In contrast, tube sizes are defined by their outside diameter, with wall thickness typically measured using Birmingham Wire Gauge (BWG) standards.
During bending, a tube undergoes various physical changes in different areas, which can vary based on the bending method employed and the characteristics of the tubing material.
The outer side of the bend receives tensile forces, which results in the elongation and thinning of the wall.
The inner side of the bend receives compressive forces, which results in the wrinkling and thickening of the wall.
The tube‘s cross-section experiences a phenomenon called ovality. Ovality is the distortion of the tube‘s cross-section from the original round shape after bending. It results from unbalanced forces acting on the bend, especially when the tube internal is unsupported. Ovality of the tube is acceptable in some applications, but some industries require precise dimensions of the bend where ovality must be controlled.
Wall Factor. The wall factor represents the relative wall thickness of a tube, calculated as the ratio of the tube's outside diameter to its wall thickness. This value helps determine whether a tube is classified as "thick-walled" or "thin-walled."
The wall factor helps evaluate the complexity of bending a tube. Tubes with lower wall factors are simpler to bend because they require less material to stretch. Conversely, tubes with higher wall factors need more advanced dies and mandrels to properly support and shape the tube.
D of the Bend. The term "D of the bend" is used by tube fabricators to describe the ratio of the center-line radius (CLR) of a bend to the tube's outside diameter. This ratio indicates how challenging it is to achieve tight radii bends. A higher D of the bend value makes it easier to create tighter bends. For an ideal unsupported bend, a tube should ideally have both a low wall factor and a high D of the bend. If not, it can lead to a flat tone bend, where the outer wall collapses due to insufficient thickness. Elongation, or the material's ability to stretch before breaking, also influences bending. A higher D of the bend means more material needs to stretch to form tighter radii. Additionally, elongation is affected by the material's properties; for example, stainless steel typically has greater elongation compared to mild steel.
Springback. Springback refers to the tendency of a tube to revert to its original straight form after being bent, resulting in a bend angle that is slightly less than intended. To counteract this effect, operators often "overbend" the tube slightly to achieve the desired bend angle after accounting for the expected springback.
Springback describes the tendency of a tube to revert to its original shape during bending. This occurs because bending causes uneven molecular density: the material in the inner part of the bend is compressed, while the outer part is stretched. The tensile forces on the stretched outer region exceed the compressive forces on the inner region, leading the tube to partially straighten back to its initial form.
In the tube industry, the radius of a tube's bend is often referred to as the center line radius (CLR), which is the radius measured along the tube's centerline. Typically, the CLR is larger than the die radius, especially in cases where a mandrel is not used and no additional force is applied. As the CLR increases, the variability in springback between each bent piece also increases. These variations can significantly affect the accuracy of the bend's positioning.
Springback results in an increase in the center line radius (CLR) as the tube tries to return to its original shape, causing the radius to expand. For most bending dies, the CLR is approximately 1.042 times the size of the die. This formula is useful when the die does not allow for a complete 180° bend.
Springback is affected by factors such as the material’s stiffness, tensile strength, wall thickness, type of tooling, and the bending technique employed. Materials with higher hardness and smaller center line radii (CLR) tend to exhibit greater springback. The bend angle is adjusted for springback by applying a springback factor, determined through test bends. This factor varies with different materials, wall thicknesses, and tube diameters.
Tube bending methods can be categorized into form-bound or freeform bending techniques. Form-bound bending relies on the shape of the die, as seen in methods like press bending and rotary draw bending. In contrast, freeform bending depends on the tube's movement through the tooling, such as in roll bending. Additionally, tube bending techniques are divided into cold and hot bending. Cold tube bending is performed at room temperature. Some of the most commonly used cold tube bending techniques include the following:
Press bending is one of the earliest industrial tube bending techniques. In this process, the tube is secured at two points while a ram (or bend die) is pressed against it to shape the bend. The shape and dimensions of the cylindrical ram determine the characteristics of the bend applied to the tube.
Press bending is a fast method suitable for symmetrical parts and does not require lubrication or cleaning. However, it struggles with creating smaller bend angles and lacks internal support for the tube, making it susceptible to deformation in both internal and external curves. This technique can often result in an oval cross-section, especially depending on the tube’s wall thickness. Due to its difficulty in controlling the bend precisely, press bending is typically used only when a uniform cross-section is not critical.
Rotary draw bending is ideal for producing accurate bends with a consistent center line radius (CLR) and diameter, resulting in minimal ovalization. It is commonly used in applications such as pipe fittings, instrument tubing, handrails, and components for automotive and aerospace industries. This technique is also effective for hollow sections with various cross-sectional shapes, such as square or oval. Properly matched tooling ensures a smooth and visually appealing bend.
This method involves bending the tube using a set of interlocking dies, with internal support provided by a mandrel.
Bend dies are crucial components in rotary draw bending, used to shape and control the form of the tube during bending. They define the radius of the bend and come in a wide range of designs, from simple to highly complex, each tailored to meet specific application needs.
Tube bending dies are selected based on the specific bending requirements of the design. For instance, if the height of the bend exceeds its width, a pedestal and flange mount bend die is used, which provides a stable platform for better support during the bending process.
While the type of bend is crucial in selecting the appropriate bending die, the choice of metal also plays a significant role. Common metals used for tubing include steel, stainless steel, brass, copper, and aluminum. Engineers must choose tubing with adequate wall thickness to endure the stresses of the bending process and prevent potential issues.
The clamp die secures the tube by gripping its outside diameter and holding it against the bend die. Its main role is to stabilize the tube during the bending process. As the bend die rotates to create the bend, the clamp die rotates in alignment with the curvature and moves in and out to facilitate tube feeding. Proper clamping pressure is crucial: insufficient pressure can lead to tube slippage, while excessive pressure may cause wrinkling or collapse of the tube.
The wiper die is employed to prevent wrinkling on the inside radius of the tube when a mandrel alone is inadequate. Positioned behind the bend die with its tip at the tangent point, the wiper die helps manage the bending process. Because it experiences frictional forces during bending, the material of the wiper die must be compatible with the tubing material to avoid issues like galling over time. Steel wiper dies are suitable for bending tubes made of steel, aluminum, copper, and bronze, while aluminum bronze wiper dies are used for stainless steel, titanium, and Inconel tubes. To minimize friction, hard chrome-plated steel wiper dies are often used.
The pressure die is positioned tangentially to the bend die and has two main functions. Firstly, it exerts the necessary force to bend the tube and maintains consistent pressure at the point of contact. Secondly, it helps to guide the straight tube around the bend, often with the assistance of a pressure die booster, which applies additional compressive force to counteract the elongation of the tube's outer wall. The length of the pressure die is adjusted based on the degree of bending required.
The mandrel provides internal support for tubing during the bending process to prevent collapsing, wrinkling, and ovalization. Part of the bending process involves choosing the correct die for the type of metal tubing being bent and formed. This factor also relates to the choice of metal for the mandrel since certain types of metal mandrels perform better with certain metals than others. For example, in the case of stainless steel tubing, aluminum bronze mandrels are used for the best possible performance and results.
Standard mandrel. Standard mandrels are the most commonly used type of mandrel and are capable of creating a wide range of bend characteristics. They are a very flexible mandrel that flexes as the bend is made. Standard mandrels consist of one ball or can be made from a few linked balls. They are the most durable of the flexible mandrels and use the largest links.
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.
Compression bending is a more cost-effective method compared to rotary draw bending due to its simpler setup. However, it is restricted to circular hollow sections. This setup does not support the use of a mandrel for inner diameter support, which can result in slight flattening of the tube's outer surface. It is also unsuitable for creating bends with a small center line radius (CLR), as the tube may risk breaking or buckling. This technique is frequently used for bending symmetrical workpieces and electrical conduits in structural applications.
Roll bending is employed to create bends with large center line radii (CLR) in large tubing components. This method involves two stationary rollers and one moving roller arranged in a triangular configuration. The stationary rollers rotate in the opposite direction of the moving roller. As the tube passes back and forth between the rotating rollers, the bend radius is gradually shaped.
Roll bending is used for various applications, including structural components, powder transfer systems, and more. It is also suitable for creating spiral shapes, as the operator can reposition the tube after each revolution to form a continuous coil.
Bending springs are ideal for home use and are designed for softer workpieces with smaller diameters, such as PVC and soft copper pipes (0.6 – 0.9 inches). A durable and flexible spring, slightly smaller in diameter than the tube's inner diameter, is inserted into the tube from one end to the center of the bend radius. To facilitate positioning and removal, wires can be attached to the spring’s ends. The spring provides support during manual bending, which involves securing the tube at one point and gently pulling the spring’s ends to achieve the desired bend. While this method is straightforward and easy to use, it offers limited accuracy and consistency.
The following are hot tube-bending techniques that utilize heat to facilitate the plastic deformation of tubes during bending. Hot tube bending is typically employed for bending polymeric tubes, including PVC, CPVC, and ABS.
Heat induction bending can create bends with various center line radii (CLR) without the need for compression dies or mandrels. This technique is versatile, accommodating different pipe sizes and wall thicknesses while minimizing wall thinning and ovality. It offers high precision, making it suitable for applications in the petrochemical, energy, and mining industries. However, it is associated with high operating costs.
Sand packing hot-slab bending is a versatile hot bending process where a tube is first filled with fine granular sand and tightly packed, with both ends sealed. The tube is then heated in a furnace to temperatures above 870°C. After heating, the tube is positioned in a slab with pins, and mechanical force is applied to bend the tube around the pins. The packed sand helps to support and maintain the shape of the bend.
Hydroforming is a metal forming process that uses a pair of dies to shape straight metal tubing. In this method, a pressurized liquid is applied to deform and form the metal into the desired shape. While various tube shapes can be used, round tubes are most commonly employed due to their versatility in producing a wide range of designs.
After placing the metal tubing between the two halves of the die, pressurized fluid is introduced into the tubing. As the pressure rises, the liquid pushes against the walls of the tubing, causing them to conform to the shape of the die. Once the desired shape is achieved, the die halves are separated, and the newly formed part or component is extracted.
Hydroforming results in shapes with excellent quality and visual appeal. The high hydrostatic pressure used in the process ensures that the finished piece retains its shape without springback, providing rigidity and reliability.
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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