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taffy and producing tube
Proper tension is a key to success for both processes
By W.B. “Bud” Graham, Contributing Writer
whether making taffy or welded tube, requires attention to tension’s role. friction develops between the strip and the rolls. Turning the stand adjustment screws increases or decreases the roll gap. The resulting contour gap between thebreakdown tools should be parallel and should match the gauge being formed. • The strip is flat as it enters the mill. The contours of the rolls force the strip to bend slightly to conform to the roll profile. The strip’s tendency to resist changing shape develops friction. The resistance to bending across the strip’s width in the breakdown passes and the resistance to girth reduction in the fin,weld, and sizing passes creates friction by the development of two separating forces (see Figure 1). The principal separating force is directly related to the strip’s width, thickness, and strength. The secondary separating force is related to the speed at which the strip passes through the mill. In other words, this is the torque required. The sum of these two forces creates the combined stress forthe individual driven roll shafts. If the stress exceeds the mill design limits, the roll shafts fatigue and fail. Conflict Among the Forming Passes. The motor or motors that drive the tube mill develop torque,
October/November 2003 tpj
A TPA Publication

o you remember going to the county fair and watching candy makers make taffy? As a child I often would watch the whirling motion of the taffypull machine as it whipped and pulled and whipped and pulled again and again until the candy was the right consistency, texture, and color. As long as the machine speed was constant or accelerating, everything went well. When an inexperienced operator was working, onlookers might be lucky to see the train wreck that followed the wrong speed selection and the embarrassed looks on the candymakers’faces as they scurried about picking up the shredded results. Tension was at play in a successful folding of the finished taffy, much like the tension needed in tube forming. Although differences exist (taffy is a solid structure whereas tube is hollow), many of the same rules apply to both processes. Forces in the processes bring the material to a plastic state to form the finished product.Achieving desired results,

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Tension and Friction
Two complementary forces—tension and friction—are created at various places on a tube mill. Understanding and controlling these forces can help you control the strip as it passes through the mill. The rotation of the roll shafts and the tooling mounted on them creates the drawing power, or tension, needed to pull the strip into the tube mill andprogressively form it into the desired tubular section. Tension is achieved only when the drive friction is sufficient to transmit the rotational force, or torque, from the rotating rolls to the strip. Friction is resistance to motion. In welded tube production, friction develops between the strip as it is being formed and the rolls providing the work (rotation) to move the strip through the mill.Friction is the direct result of a load being forced to slide over a surface. What Develops Friction? In a tube mill, friction is developed by the roll gap and the contours of the successive rolls. • When the roll gap (the space between the rolls) is set correctly,

producingresults Forces Developed in a Tube Mill
a) Closure force is generated by adjusting upper roll chock adjustment jacks orscrews. (purple arrows) b) Torque is provided by the mill’s drive system. (red arrows)

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a

a
c) The mill bearings are supported in bearing blocks on each stand. (not shown) d) The combination of compression and torque generates separating forces. (blue arrows)

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d

e) An ideal strip has a consistent thickness across its width.

Direction of Travel

b
f) Typical...