Indicators of Roll Failure and Weld Roll Design Engineering

Roll design can affect how tooling wears. A set may not be properly designed to handle the material to be run. Many tube producers try to run everything their customer demands on one set of tooling. Sometimes it can be done, sometimes not. When a roll set is sent in for rework or ordered to be made new, all parameters should be explained to the roll supplier. This will enable them to determine if a set requires a design analysis. All sets of tooling should be examined for the following areas:

1)  Proper strip formation. Are all the rolls on the mill doing their job? A badly designed set can cause wear and make setup difficult.

2)  Are the side rolls forming properly? The side rolls are there to support and present the strip between driven passes. If they are not designed or used correctly, problems can occur.

3)  Are the breakdowns cleared for the maximum gauge? If the top breakdowns are not cleared for the correct gauge, it can cause many problems. If the strip to be formed is thicker than the rolls are designed for, the result will be thinning, walking of the strip, and excess wear to the first pass. If the top breakdowns are not cleared for the lighter end, flat spots may occur that can cause welding problems and buckling.

4)  Are the fin passes designed for the type of welder and material? The fin passes are a critical area of the roll set. If they are not designed for the correct type of welder or material, it may be impossible to weld.

5)  Determine the range of materials to be run. The roll set must be made to handle the maximum gauge and highest tensile strip to be run. All other materials will be a compromise that may or may not be compatible.

6)  Will this set of breakdowns be used to run more than one size? Most tube producers will try to cut down setup time and save tooling costs by running more than one size on a set of breakdowns. Mild steel is very forgiving, and this is perfectly acceptable, with certain limitations. A suggested maximum is 80 percent of tube size. Example: 1.0000.0. breakdowns to run down to .8000.0. Note: Stainless steel and other high-tensile materials cannot be formed correctly this way and usually require a dedicated set of breakdowns.

7)  Mill configuration. Has there been a pass added or removed? Mill configuration plays a crucial part in how and what materials can be formed.

To run light-gauge and high-strength materials, proper breakdown design is a must. There are four types of forming:

• Conventional or single-radius design is good for mild steel with a TID ratio between 3 percent and 8 percent.
• Standard edgeform is the most widely used, because it forms mild steel, higher tensile, and a greater gauge range very well.
• Versatile edgeform works very well on light-gauge and higher tensile material. It forms more of the strip with the smaller edgeform radius in several passes. This gives greater stability, less buckling, and provides a rounder tube when entering the fin passes.
• “W-style’ or reverse bend works very well on high-tensile, light- or heavy-gauge material. It can be combined with a versatile design for maximum results. Whatever design is used, it is important to have the correctly cleared top rolls for the gauge to be run. The strip memory must be worked out with the proper radius and pressure. The final tube use must be examined also, to ensure the correct design is used.

A reverse bend works very well in forming the strip but, with the increased amount of cold work, can raise the hardness of the tube. However, if bending, expanding, drilling, or other operations are to be done to the tube, reverse bend may not be the answer.