April 11, 2005
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.
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.
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:
Regardless of the size, shape, and wall thickness, tension is important in keeping the line running steadily and at a reasonable speed.
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.
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:
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.
Several factors contribute to tension control, including the number of drives, the root diameter progression, the roll tooling design, and the setup.
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.
A good setup is crucial to manufacturing good tube.
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.
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