Going Straight: How a Tube Straightener can Affect the Curves in a Tubular Profile
The need for straightening a tube is all throughout the tube fabrication industry. You may have a coil in bulk form that feeds a forming process. You may have purchased a straight tube, as defined by your supplier, and you need it straighter. In both cases, a tube straightener may be of interest. Choosing the right type straightener can make all the difference.
Tube straighteners are nothing new. They have been around for years. The US Patent Office documents various straightening devices going back decades. The need has always been there. The problem is simple: take a bent metal tube and straighten it. Sounds straight forward. Sometimes it is….sometimes you may get a curve.
When forming a tube, tube fabricators have to take into account spring back. For example, if you take a straight tube and bend it in a rotary draw bender to say 90°, after unclamping the tube will spring back to a say 86°. To bend a tube to 90°, you may have to bend it first to about 90° + 4°. Then when you release the tube from the tooling the tube will relax. In this case the metal that forms the tube has memory and tries to return to its previous relaxed shape. This, in a nut shell, is the spring back.
Taking this concept forward, to straighten a tube that is bent requires that the tube be bent once again in a direction past the point of straightness. That’s right, you have to bend the tube past of the point of straightness in order to get the tube to spring back to a straight condition.
Which brings up the next point: how do you define straightness? Well, there are several ways. One way involves rotating a fixed length of tubing between two V-blocks and noting the run out between the V-blocks; see Figure 1.
Figure 1 above shows the run-out being measured between to V-blocks.
A straightening operation is a bending operation. You are just trying to get the tube to take a set in a new position.
The 2D Problem
The 2D problem is often found when trying to straighten a tube that starts out in a bulk spool. Being that the tube is predominately bent in a single plane, the problem is considered 2D. For example: a dead soft bulk spool of copper that feeds into a cut-to-length line. In this case as the copper is pulled from the bulk spool it has a consistent bow in it from being formed around the bulk spool itself. Running this bowed tube thru a series of precision rollers goes a long way to straightening the tube; see Figure 2.
Figure 2 above shows a tube being forced thru a series of roll dies.
A bonus here is having the copper dead soft. This means that the copper has very little spring back and sending it thru a series of roll dies can quickly remove a vast amount of run-out; see Figure 1.
Now, I say this is a 2D problem. In truth as the tube is pulled from a bulk spool, the tubing has a tendency to untwist about its long axis; now it is a 3D problem. The twist is a result of the stored energy in the bulk spook. Think of the bulk spook as one big torsion spring. When the untwisting condition exists, it can be gradual; say 5° over a 15 foot span. This gradual twist can propagate thru the straightener and can cause a fluctuation in the output. This is because the straightening dies are set/fixed as the tube twists gradually thru the straightener dies. The output fluctuation is spring back dependent. The more spring back in the tubing, the more impact the twist can have.
The straightening process describe above works well when the system is used to remove a consistent bow in a long length of tubing. However, if the tube is say 10 feet long and looks like a piece of spaghetti, then another straightening process should be considered. The process involves a couple of V-blocks and a manual press located between the V-blocks. Look at Figure 1. In your mind’s eye, replace the dial indicator with a press and you now have a sectional tube straightener. In the case where the bows in the tube are many and no two bows look the same, then the straightening process may involve straightening the tube one section at a time.
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