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Basic Tube and Pipe Bending Concepts Commonly Discussed

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This topic is going to involve some basic bending concepts that we at Baileigh Industrial discuss with our customers and prospective customers on a daily basis. I have collected my information from a variety of resources. The primary one is the day-to-day dealing with customers and their applications and attempting their applications on our machines as they come about. There are many great online resources and libraries for bending applications and I will cite some of them as I think of them. We have a basic library at our main site under the Library section and will refer to that section on occasion.

There are 4 different types of bending tube and pipe that are typically known in industry as a standard. The first is compression bending, which is also known as rotary draw bending. The second is Press Bending and the third is mandrel bending. Finally there is roll bending. Most applications involve several die components. These die components are typically specific to the actual outside diameter of a tube or pipe and it is specific to what radius the material will be bent upon. How these components interact and how they are driven by a bending machine determines what type of bending is being dealt with.

I will give a brief review of each type of bending application.

  1. Compression Bending / Rotary Draw Bending – Compression bending employs a bend die and counter die. The bend die remains in a static position while the counter die rotates around the bend die.
  2. Press Bending – Press bending typically employs a hydraulic ram that is fitted with a bend die on the end of the ram. The tube is mounted in front of the ram and the ram moves forwards in to the counter dies. There are two counter dies employed. These counter dies move in opposite directions around the bend die.
  3. Mandrel Bending – Mandrel benders are much like a compression bender, except that they allow for the bending of materials with much thinner walls. Mandrel machines do this by providing support to the inside of the material while the bend is being completed.
  4. Roll Bending – Roll Bending is typically used for rolling large center line radii. These machines are typically setup much like press benders except that they will roll the material between 3 rolls oriented in a pyramid to create a bend.

Basic Bending Concepts

A very basic and critical concept for any bending application is the center-line radius or as it will be referred to from here on out, CLR (An explanation of center-line radius (CLR) is demonstrated above. This is a very common everyday question that people ask when inquiring about our benders. What CLR can be accomplished is a function of the physical properties of the tube or pipe but how the tube bending machine and its respective tooling is engineered and designed has a definite impact and will allow you to bend tighter more clean bends. What CLR a tube or pipe can be bent to is determined by several factors. The two main factors that determine what CLR a tube or pipe can be bent on is 1) nominal outside diameter of the tube or pipe and the wall thickness. A quick standard to gauge what CLR can be achieved on a particular tube or pipe is 4 times (or 4D) the nominal outside diameter of the material. So for example, if you had 1 inch schedule 40 pipe, the actual outside diameter of 1 inch pipe is 1.315″, so by the quick-standard your could “guesstimate” that you could achieve a 5.26 inch (5 ¼ inch) CLR. Please keep in mind this is a “quickie” guideline and most applications can be bent to a tighter CLR with the Baileigh Industrial rotary draw bender than what the quickie guideline would indicate. When making suggestions to customers using 1 inch schedule 40 pipe when they request the “tightest CLR” I will often times suggest a CLR between 2.5 inches and 3 inches. Depending on the application a CLR tighter than 2.5 inches may be achieved.

Another simple analytical tool is to figure out the wall factor. From the standpoint of evaluating an application from the standpoint of using the same outside diameter tube, on the same CLR, but with a different wall thickness and determining the probability of it working, this is a good tool. You can figure the wall factor by dividing the wall thickness of the tube or pipe into the actual outside diameter of the tube or pipe. If you have a 1 inch tube with a 13 gauge wall (.109 inches) your wall factor would be 9.17. A CLR of 1.75 to 2 inches should be a safe application. Now if you want to use the same outside diameter tube but use an 18 gauge wall thickness (.049 inches) you wall factor jumps up to 20.41. So, you know that the 1 x .109 inch on a CLR of two will work and it has a wall factor of 9.17. Taking the same application and applying a tube with a wall factor of 20 plus indicates that there is a very good chance that it will not work in the same CLR as the tube with a wall factor of around 9. So, CLR and wall factor are directly related. The larger the wall factor the larger the CLR that is needed.

Other factors to consider when bending.

  • Springback- the tendency of material to return to its original position once bent.
  • Material type and its tensile strength.
  • Alloy of Material
  • Design of the machine and tooling

Once again these are some common topics discussed with our customers on a daily basis and some basic bending concepts. Please feel free to contact us with any questions or feel free to add any facts or figures that you feel are missing.

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