BENDING TUBE WITHOUT A MANDREL

What are the effects of deformation on the tube structure?

In today's competitive environment, there is a relentless push toward greater efficiency. As a result, the tube industry demands that parts be bent faster, cleaner, easier, and at a lower cost, while at the same time improving the quality of the finished product.The first area to consider is wall thinning. In a mandrel bend, the neutral axis is typically about one-third of the way outward of the inside radius.

Therefore, material inside of the neutral axis is in compression, and material outside of the neutral axis in in tension. In other words, the outer wall is being stretched and thinned.

Because of the tube diameter reduction and the absence of the internal support, the tube can be boosted by the machine to a greater degree. The neutral axis will shift outward. This shift is generally toward the centerline radius (CLR), or even outward to the outer half of the tube. In some instances, the shift is nearly all the way out to the outer wall.

Any shift in the neutral axis is going to directly and proportionally affect wall thinning.

The next area to consider is ovality of the bent tube structure. As a result of reshaping the tube during bending, the outer wall is caused to be popped out to minimize outer wall flattening.

In the past, the greatest area of flattening could be measured 90 degrees from the tooling parting line. With nonmandrel bending, however, the area with the greatest ovality loss is typically measured about 45 degrees from the tooling parting line. This is a result of the tube being reshaped to a structural form.

Ovality loss due to the restructuring of the tube varies with each application. However, it is somewhat predictable. Calculating that loss will be examined later in this article.

The final area to consider is how favorably nonmandrel bending will stand up to the issue of finished product quality. Generally, the nonmandrel-shaped bend is just as strong and stable as mandrel bends.

Two other areas should be examined regarding quality. First is the issue of flow. Nonmandrel bends, as a result of the diameter reduction, are also going to experience a proportional reduction in flow.

Second, as a result of the tube's structural deformation, bent tubes will have a certain amount of out-of-roundness detectable to the eye. This out-of-roundness, in some of the more generous applications, can be minimized to some degree, but not eliminated.