Tension control in tube and pipe production

On a tube or pipe mill, the incoming strip is formed by about 24 pairs of tool stands. To help ensure such a line runs smoothly, the strip must be pulled between every pair of stands. The parameter that indicates if or how much the strip is pulled is tension.

Traditionally, tension is controlled by the root diameter progression and by adjusting the drive ratio between the fin and breakdown passes or between the fin and the sizing passes. However, viewing tension as a characteristic that links together the breakdown, fin, weld, sizing, and Turk's head passes lets setup personnel and mill operators see how all these mill sections work together and influence each other.

Tension Control

The tube and pipe forming process is continuous—a flat strip goes through a series of pairs of forming stands that gradually form the strip into a tubular shape. Between every two adjacent stands the strip is either pulled or pushed. When the strip is pulled, the tension is considered positive; when compressed, the tension is negative.

Tension control encompasses the drive system, the roll design, and the setup to ensure the tension is positive throughout the line.
Several examples can describe the role of tension in a tube mill.

• 3/16-inch Stainless Tubing Line. This tubing line is characterized by its long weld shoe, narrow strip, and zero fin reduction. A fault in this line causes the strip edges to turn to one side after the cluster side roll but before the first fin pass. The welding process is not steady. The diagnosis is negative tension before the weld box and the first fin pass. The long weld shoe and limited driving capacity of the fin rolls are the main reasons for this fault.
• 1- by 2-in. Rectangle Reshap-ing Line. The strip is formed round, welded, and reshaped into rectangle in the sizing section. The tube folds up in the quench tank between the weld box and the sizing section. The fin rolls have to be very loose, and the line runs slowly. The diagnosis is that the reshaping driven rolls are not properly designed or set up and thus cannot drive the tube. As a result, the tube is under negative tension between the weld box and the sizing section.
• 3/16- and 1/2-in. Double-walled Brazed Tubing Line. The strip, which is less than 1/64 in. thick, is formed 720 degrees into a double-walled tubular. Then it goes through passes of round-profiled rolls with mandrels inside. The strip folds up in the middle of the forming zone. The diagnosis is negative tension because of limited driving force from the last two passes and big resistance force from the mandrel.
• 0.540- by 0.014-in. Aluminum Tubing Line. Because the material is so thin and soft, it often buckles in a mill that has a typical drive. In addition, as material becomes thinner, it becomes more difficult to align the edges with each other—that is, edge misalignments are more likely on material that is 0.014 in. thick than on material that is 0.250 in., for instance. The solution is to develop positive tension everywhere in the line by adopting a pull-through drive.

These four examples demonstrate that negative tension can cause strip edges to turn to one side; strip or tube to fold up; unsteady welding; difficult setup; and low line speed.

Several guidelines are helpful when analyzing tension:

• Thin tubing is more sensitive to tension than thick tubing.
• Small-diameter tubing is more sensitive to tension than large-diameter tubing.
• Squares or rectangles are more sensitive to tension than round tubing is.

Regardless of the size, shape, and wall thickness, tension is important in keeping the line running steadily and at a reasonable speed.

Fundamentals of Tension

Figure 1 The roll pressure point and driving points vary among the breakdown, fin, and sizing passes. The driving point of the top breakdown roll is lower than its root point, not at its desirable point on the top roll profile.

The forming stands in a tube mill can be classified into two types: driven rolls and idle rolls.

The idle rolls include all the side roll stands, the seam guiding rolls, weld rolls, and Turk's head rolls. All of them drag the strip back.

The driving force to the strip comes from the breakdown, the fin, and the sizing driven rolls.

When a line runs steadily at a constant speed, the driving force from the driven stand balances the resistance from the idle rolls.

Forward Slip and Backward Slip. Tube and pipe rolls are not flat—they have a profile depth. When the rolls spin at a given number of revolutions per minute (RPM), the surface speed is different at various points on the roll's profile. The surface speed is lowest where the diameter is smallest (at the root), and highest where the diameter is greatest (at the rim). However, the strip has only one speed at all the points. Thus, the roll surface speed equals the tube's speed at only one specific point on the roll profile, called the driving point. The corresponding roll profile diameter is called the driving diameter.

The approximate driving diameters of the breakdown rolls, fin rolls, and sizing rolls are shown in Figure 1.

The driving diameter of the bottom breakdown rolls typically is bigger than the root diameter. However, the driving point of the top roll is typically lower than the root point. The driving diameters of the fin rolls and the sizing rolls are at about a 45-degree orientation.

The roll profile beyond the driving point is called the forward slip zone, where the roll surface speed is faster than the tube's speed and thus slips forward relative to the tube. The roll profile within the driving point is called the backward slip zone, where the roll surface speed is slower than the tube's speed and thus slips backward relative to the tube.

The forward slip zone provides a driving force that pulls the tube forward, whereas the backward slip zone develops resistance that hinders the tube's forward motion. The balance of the forward slip zone's driving force against the backward slip zone's resistance force is the net driving force that the roll tooling provides.

Not all the driven rolls drive the tube. Some of the driven rolls also can drag the tube back. This happens if the backward slip zone dominates the forward slip zone.

Typically, all the breakdown sections' top rolls drag the tube back. So in most cases, the driven shafts for the breakdown top rolls can be idled to lower the resistance, reduce the horsepower consumption of the line, and reduce the mill building cost. However, in certain cases, the top breakdown rolls must be driven to establish proper tension in the strip.

Roll Pressure Distribution. In addition to the forward and backward slip zones, the contact pressure distribution on a roll profile also influences the roll's driving force.

Figure 2 The tension increases throughout the breakdown section (stands BD 1-6). The tension decreases in the fin section (stands Fin 1-3 in the center). It then increases dramatically before dropping off in the sizing section (stands Sizing 1-3).

The contact pressure distribution typically can be viewed as uniform on the fin rolls and the sizing rolls. However, the contact pressure distribution on the breakdown rolls usually is not uniform. It is determined not only by the strip and roll design, but also by the setup. If the operator presses the top roll harder, the center pressure increases. In contrast, the edge pressure is determined only by the strip and the roll design.

Tension Distribution. The strip, roll tooling, motor drive configuration, and setup together make up a tension distribution along the whole mill line (see Figure 2). Typically, it starts from zero before the breakdown section and gradually goes up, although it still can be low before the cluster rolls.

The tension is lowest before the weld roll. It picks up in the sizing section, but then finally decreases to zero.

The line has several critical points. The first is before the weld. If the sizing section does not develop enough tension, especially when reshaping, the tension before the weld roll can be negative, which is harmful to edge presentation and, thus, to welding.

The second point is before the cluster. If the strip is loose in the fin rolls, the fin rolls cannot pull enough. Thus, the tension before the cluster rolls, especially when there are three or four pairs, can be negative.

The third point is the cooling tank, which is located between the weld box and the sizing section. A long distance usually separates the weld box and the sizing section, and the tube is not supported well in this area. Without proper design, when reshaping squares in a flat orientation or rectangles in either a flat orientation or diamond orientation, the reshaping rolls cannot provide enough tension to pull the tube, and the tube has to rely on the fin rolls to push. In this case, the tension at the cooling tank is negative. This causes the tube to fold up in the cooling tank.

The tension distribution can be divided into three phases:

1. Sizing section. This section includes the weld roll, the sizing driven and side rolls, and the Turk's head rolls. Because the tube must be pushed out of the Turk's head rolls, the tension before these rolls will be negative. Often this is not a problem. The most critical factor is that the sizing section must be able to pull the tube out of the weld rolls without any assistance from the fin section. If it can do this, the tension before the weld roll will be positive.

   Positive tension before the weld roll usually is not a problem during the manufacture of round tube or pipe. However, it can be a problem during the reshaping of rectangles and squares in the flat orientation.
   
   

2. Fin section. This section includes the cluster side rolls and the fin passes (both the driven and side rolls of the fin passes). The fin tooling must be able to pull the tube out of the cluster rolls without any assistance from the breakdown section.
   
   
3. Breakdown section. The driven stands should be able to balance the resistance from the side rolls. Unlike the fin and the sizing sections, the breakdown section must drag back the tube a little bit to establish a positive tension before the first cluster roll. If the breakdown section pushes the tube before the cluster, the tension before the cluster roll will be negative, which means trouble.

Controlling the Tension

Several factors contribute to tension control, including the number of drives, the root diameter progression, the roll tooling design, and the setup.

• Number of drives. It is recommended to have a motor drive for each of the three phases of tension control.
• Roll diameter progression. When the mill is driven by three motors, the root diameter of the rolls does not need progression, because the relative driving ratio can be adjusted. When the mill is driven by one or two motors, progression is recommended.
• Roll tooling design. Proper sizing reduction is necessary for the sizing section to develop the necessary amount of driving force. The 2es criterion can be used to determine the amount of sizing reduction.¹

The weld resistance should be reduced when using welding shoes. Usually the weld rolls do not apply too much resistance to the tube. If weld shoes are used, however, as they sometimes are for stainless steel tubing, resistance can be a problem.

Proper fin reduction also is necessary for the fin rolls to drive the tube. Otherwise, unless the sizing section develops enough force, the tube will rely on the breakdown rolls to push, which causes negative tension before the cluster rolls. Floating fin blades can help to reduce the resistance force to the tube. A normal fin blade tries to hold the tube back.

The rim diameter of the top breakdown roll usually is a product of the bottom roll's root diameter multiplied by the gear ratio. In this case, the top roll tries to hold back the tube, so it does not need to be driven.

The top breakdown roll should be cleared more than the actual material thickness. This way, the contact pressure will be concentrated near the root, which in turn can help keep the driving diameter down near the root.

Importance of Accurate Setup

A good setup is crucial to manufacturing good tube.

• The bottom line must be flat. A tube with a bottom line that undulates all the way down the mill creates extra resistance.
• The driven shaft shoulders must be aligned properly. If the driven shaft shoulders are not lined up, especially if the top and bottom roll shoulder in the same pass are not lined up, the contact pressure distri-bution will be biased to one side. This influences the drive, and the tube might turn to one side.
• The gaps must be set correctly. In a normal setup, the breakdown driven rolls should be gapped to the material thickness and the side rolls gapped to the setup chart. The sizing section is gapped in the same way. However, the fin gaps on the setup chart are based on a specific material gauge. Before running a different gauge, the operator should fine-tune the fin pass gaps.
• The relative speed ratio among the breakdown, the fin, and the sizing motors must be determined. The sizing section should be 2 percent to 5 percent faster than the fin motor. The breakdown motor speed may be equal to the fin motor speed, or 0.5 percent to 1.0 percent slower.

Pulling It All Together

For the line to run steadily, the tension must be positive throughout the line. A three-motor drive gives mill operators more control than one- or two-motor drives. The tooling also influences tension in a mill. The fin and sizing reduction, root diameter distribution, and breakdown top roll pressure relief are the relevant tooling parameters that affect tension. Finally, the setup brings everything together.

To get the tension just right means starting with the setup. A precise alignment is the first step to achieving a good setup. After using the setup chart as a guideline, the operator may need to fine-tune the fin gaps for the material gauge and the breakdown passes' top rolls for pressure. After this, the operator can adjust the drive ratio control as needed until the proper tension is reached.

Dr. Yunjiang Li is senior project engineer and Harry Focht is president of Chicago Roll Co., 970 N. Lombard Road, Lombard, IL 60148, 630-627-8888, fax 630-629-8858, www.chicagoroll.com.

Note: 1. Y.J. Li,"2es Reduction: A Criterion for Tube Fin and Sizing Reduction," Tube & Pipe Technology, September/October 2000.

This article is adapted from "Tension Control of Tubing Line—A Key to Smooth & Steady Running," presented at the Tube & Pipe Making and Roll Forming Conference, June 5-6, 2002, Northbrook, Ill., sponsored by the Society of Manufacturing Engineers.