As the name implies, tube bending is a fabrication process to bend tubes permanently. In many applications, bent tubes are required to fulfill certain tasks. So, tube bending is a necessary forming method for industries. Various instrument items use bent tubes as one of their component parts. Bent tubes are also found in stair railings, automotive parts, furniture frames, handles, air conditioning equipment, and many other places. In piping, plumbing, and pipeline industries pipe and tube fittings in the form of bent tubes are widely used whenever a flow direction change is required.
How does Tube Bending work?
To start the bending process, the tube is fixed at two points. The bending action is then enforced using a rotating press, roller, or die. The tube material experiences a combination of tensile and compressive force when the tool advances. The tube bending process is dependent on various parameters like:
- the material of the tube
- used tooling
- bending geometry required
- applied pressure for bending
- lubrication, etc
Depending on the bending technique employed, the tube undergoes several physical changes per area. The outer side of the bend experiences tensile forces and creates elongation and wall thinning whereas the inner side of the bend experiences compressive forces and creates wrinkling and thickening of the wall.
The tube‘s cross-section undergoes a phenomenon known as ovality which is defined as the distortion of the tube‘s cross-section from the original round shape after bending.
After the tube bending process, the product may undergo various other fabrication processes like cutting and deburring, slotting, notching, and welding.
Terminologies used in Tube Bending
Let’s learn some of the terminologies frequently used in tube bending:
- Center-Line Radius: It is the distance from the center of the curvature to the tube centerline.
- Outside Diameter: The outside diameter is the distance between the two outermost points on the pipe or tube‘s cross-section having the same centreline.
- Inside Diameter: Inside diameter indicates the size of the tube’s hole and is calculated as Outside diameter-2 X wall thickness.
- Wall Thickness: It is the difference between the outside and inside radius of a tube. When choosing a die for a tube bending method, tube thickness and OD are the most important considerations.
- Degree of Bend: It is the angle formed by bending the tube and is measured in degrees. The degree of bend indicates the “sharpness” of the bend.
- Wall Factor: It denotes the relative wall thickness and is defined as the ratio of the outside diameter of the tube and its wall thickness. Wall factor determines if a tube is “thick-walled” or “thin-walled”. Tubes having lower wall factors are easier to bend and vice versa.
- D of the Bend: The “D of the bend” is the term used by tube fabricators that denotes the ratio of the CLR of the bend to the tube‘s outside diameter. The higher the D of the bend, the easier is it to form bends with tighter radii.
- Springback: Springback is the tube’s tendency to return to its original shape while bending. Springback is influenced by several factors like the material‘s stiffness, tensile strength, wall thickness, type of tooling, and bending technique used. Harder materials and smaller CLR produces a greater spring back.
Types of Tube Bending
The tube bending process can be classified depending on various parameters. In general, the following types of tube bending are popular:
Based on the forming methodology, there are two types of tube bending processes:
- Form-bound tube bending where the die geometry influences the forming process and
- Freeform tube bending where the forming is dependent on the tube’s movement through the tooling.
Based on the working temperature condition tube bending process is grouped as
- Cold bending and
- Hot bending.
Hot tube bending uses heat energy and is performed at higher temperatures whereas cold tube bending is done at room temperatures.
Depending on the tools used during the tube bending, there are various types of tube bending processes as mentioned below:
In the press bending process, the tube is fixed at two points and a ram/die is forced against the tube to take the shape of the bend. The bend characteristics are governed by the external dimensions of the cylindrical ram.
Press bending is widely used for symmetrical parts. The process is quick and does not require lubrication and cleaning. But, making a smaller degree of bend is difficult in this method. As this tube bending method is very difficult to control, it is only employed where a uniform cross-section is not required.
Rotary Draw Bending
Rotary draw bending is a highly precise tube bending method giving minimal ovalization. This process is used in pipe fittings, instrument tubing, handrails, and automotive and aerospace parts. This method uses a set of interlocking dies. The tube is internally supported using a mandrel during the process.
A bend die that imparts the bend to the tube by rotating it around its outline determines the bend radius. Until the desired bend angle is obtained, the rotation is continued.
Compression bending has a simpler setup but is limited to circular hollow sections only. A mandrel is not used for supporting which may cause the outside surface to flatten slightly.
The roll bending method is suitable for creating bends with large CLR. Consisting of two stationary rotating rollers and a moving roller in a triangular pattern, the roll bending is used for large tube components and structural applications. The stationary rollers rotate in the opposite direction of the moving roller. As the tube moves back and forth on the rotating rollers, the bend radius is gradually formed.
Bending springs are used to bend softer workpieces having small diameters. A strong and flexible spring is inserted inside the tube walls from its end to the center of the bend radius. For positioning and easy removal, a wire is sometimes attached to the ends of the spring.
All the above-mentioned bending processes are performed at room temperature and hence fall into the cold tube bending category.
The heat energy used during hot tube-bending techniques enhances the tube‘s plastic deformation. Heat induction bending, Sand packing hot-slab bending, etc are examples of Hot tube bending which are normally used for bending polymeric tubes like PVC, CPVC, and ABS.
Heat Induction Bending
Heta induction bending is a hot tube bending process where the tube is supported on the front end and the bend clamp is placed in between the rotating arm and the rear end. After clamping the front end of the tube in the pivot arm, it is pushed gradually from the rear end. The tube passes through an induction coil where heat is supplied at a point tangent to the bend radius. The pivot arm is made approximately equal to the workpiece bend radius. Depending on the material of the tube, the working temperature can range from 430°C – 1,200°C. Once the bending is done, the tube piece is quenched in water spray or air.
Heat induction bending creates minimal wall thinning and ovality and can be applied for a wide range of pipe sizes and thicknesses. However, the operation is costly.
Sand Packing Hot-Slab Bending
This is one of the oldest hot tube bending methods that is still employed in many fabrication sites. In this technique, fine sand is filled inside the tube and then both tube ends are sealed. The sand-filled tube is then heated and at around 870° C, the pipe is placed in a slab with pins set on it. A mechanical force is applied to the tube to bend it around the pins. The sand inside the tube maintains its original cross-section.
To reduce friction, Lubricant must be used before the insertion of the dies. Proper lubrication prevents premature wear and increases the life of the tooling. Lubricants are available as a paste or gel. They have unique formulations for different tubing materials. Usually, lubrication is applied on the inside and outside of the tube, bending mandrels, contact point of wiper dies, and bending springs. For heavy-duty tube bending operation, a more concentrated lubricant application may be required for heavier-duty bending. At the end of the bending process, the remaining lubricant is cleaned from the tube and die surfaces.