Accumulating strip before the mill
An introduction to entry end equipment
Strip can be stored in a deep pit, where it will hang in a loop.
In simple terms, a strip processing line consists of a device to hold a supply coil of strip and a device to perform an operation to convert the flat coiled strip into a product.
In tube mill operations, a supply coil is positioned in the coil holding device, the end of the coil is fed into the tube mill, and the mill operates until the supply coil is depleted. The total operation then stops until a new supply coil is positioned and the leading end is again fed into the mill. Line operation is intermittent, possibly running 60 percent of the time.
Needless to say, this is not an ideal situation, as a tube mill line represents a significant capital investment that produces a high-value product. Because of these factors, many companies place a great deal of emphasis on the continuous operation of the mill.
Most tube mills can justify the added cost of entry-end equipment that will permit them to operate continuously. Noncontinuous operation resulting from intermittent strip supply to the mill can be eliminated by the addition of a strip accumulator and a coil joining machine.
The Accumulator's Role
A strip accumulator provides a continuous line of strip to a tube mill from a supply of noncontinuous coils of strip. It does this by accumulating or temporarily storing strip in a manner that permits the strip to be withdrawn to a mill line from the accumulator, while strip is simultaneously added to the accumulator at a higher speed.
The rest of this article describes the various types of strip accumulators that are available.
Pit or Tower
Strip can be stored in a deep pit where it will hang in a loop (see Figure 1).
If the pit is "filled" so that the strip loop is at position A, the mill can then be started and operated continuously. If the supply from the coils is added at the same speed at which the tube mill removes strip, the loop will stay at position A, and the accumulator will remain full.
If the supply end is stopped because the supply coil has been depleted, and the tube mill continues its strip removal, the loop will rise to position B as a new supply coil is end-joined to the trailing end of the depleted coil. This joining can readily be accomplished because the entering strip is not in motion.
As soon as the end joining has been completed, the supply to the accumulator can be restarted and strip can be entered at a higher speed than that at which the strip is being removed by the tube mill. Under this condition, the loop will return to position A as the accumulator fills.
The length of strip in the pit between positions A and B represents the capacity of this accumulator and is equal to or greater than the length of the strip removed during the time that is required to perform the end joining operation.
For example, if the tube mill runs at 100 feet per minute (FPM) (30 meters per minute [MPM]) and it requires four minutes to complete the end joining operation, 400 feet (122 meters) of strip will be used by the mill while end joining occurs. The accumulator must therefore have a capacity of at least 400 feet (122 meters).
A tower can also be used. Rather than requiring a deep hole in the ground, a tower system requires a high structure. Storage is limited by the practical height of the tower.
Pits and towers are typically used on low-speed lines.
The festoon accumulator combines several vertical strands.
A variation of the pit is the festoon, which combines several vertical strands. It can extend either upward or downward from ground level (see Figure 2).
A festoon has a number of fixed rolls and a moving structure containing a number of rolls that move together vertically. Strip accumulation occurs as the moving rolls move away from the fixed rolls in a vertical direction. A large accumulation can be obtained from a much smaller total vertical movement.
These accumulators are capable of storing more strip than the pit-type accumulators but not as much as the rotary free loop or rotary horizontal accumulators.
On a loop accumulator, the strip is looped horizontally rather than vertically.
A variation of the pit is a loop, either overhead or underground, on which the strip is looped horizontally rather than vertically (see Figure 3).
The moving drum carriage can move horizontally between positions A and B. In position A, the accumulator is full; in position B, it is empty. The drum carriage moves from B to A when supply strip is entered at a speed higher than the mill speed.
This type of accumulator requires a great deal of space above or below the mill to operate. It is most commonly used on lower-speed lines.
In the stuffing box, strip is driven into and removed from a long narrow box barely wider than the strip.
In the stuffing box (also known as a tiger cage), strip is driven into and removed from a long narrow box, just barely wider than the strip, to prevent sideways movement. No internal orientation device is used (see Figure 4).
Although this general arrangement is in use with tube mills, it is more commonly found on radiator tubing lines processing thin (.016 inch [0.4 millimeter] and less), narrow (1-1/2 to 2 inches [38 to 51 millimeters]), and low-yield-strength strip. Such thin strip is coiled in lengths of several thousand feet, so the accumulator is used only at the end of the coil. Therefore, the strip runs along path B for most of the coil.
Near the end of the coil, the entering strip is accelerated to a speed greater than the line speed, and the strip falls in a random pattern A within the constraints of the stuffing box. It lies like ribbon candy and is prevented from twisting and tangling by the constraining sides of the box.
Strip can be damaged on this type of accumulator, however, if it is bent beyond its yield, or if it is scratched or sustains edge damage, so the use of this accumulator is not recommended if the tubing finish is important.
A typical spiral accumulator.
In a spiral accumulator (see Figure 5), a loop forms at position B, and the incoming strip enters at a speed higher than that of the tube mill. Loop B progresses in a guided spiral path (roller guides) to position A to fill the accumulator. When entering strip is stopped to end join a new coil, the accumulator is depleted, and the loop progresses from A toward B.
Spiral accumulators are suitable for use with light- and medium-gauge (.118 inch [3 millimeters] and less), narrow (2 to 12 inches [50 to 305 millimeters]) strip with a low yield strength.
Horizontal Accumulator, Type 1
Another method of accumulating strip is to form it in a large-diameter coil positioned horizontally with the coil axis pointing upward. The entry to this coil can be stopped while strip is withdrawn, reducing the coil diameter but maintaining the same number of wraps. This accumulator is shown full and empty in Figure 6.
The amount of strip accumulation equals the number of wraps multiplied by the difference in the average coil circumference from full to empty.
This kind of accumulator is suitable for wide, heavy-gauge strip (such as .350 inch [8.9 millimeters] thick and 30 inches [762 millimeters] wide) or very light-gauge, narrow strip (less than .028 inch [.71 millimeter] thick and less than 2 inches [51 millimeters] wide).
The strip is supported on edge, since the coils are formed in a horizontal plane. Also, the strip must be twisted 90 degrees when entering and exiting the machine.
Horizontal Accumulator, Type 2
The horizontal accumulator type 1 is shown here both full (top) and empty (bottom).
The second type of horizontal accumulator (see Figure 7) differs from the first type in three major ways:
1. The feed pinch rolls are located outside of the machine to aid threading and maintenance.
2. The strip is fed into the machine on the outside coil rather than the inner coil.
3. The support for the strip is provided by a driven rotary table that is synchronized with the pinch roll, rather than by driven support rollers.
The principle of operation is simple. The strip is fed into the accumulator by a set of pinch rolls. As in all horizontal accumulators, the strip is turned 90 degrees before it enters the pinch rolls of the machine.
The pinch rolls are driven by a direct current (DC) motor at two to three times line speed. As the strip enters the accumulator, it is supported immediately by a rotary table. The rotating table is driven by a separate DC motor, and the OD is synchronized with the pinch roll.
Therefore, the entering strip has no resistance, and the driven support table helps pull and guide the strip into the accumulator. This reduces the chances of strip cobbling, jam-ups, marking, tension, etc. The inside wrap of the accumulated coil is pulled to the center takeout arbor by the mill.
The drive has three modes of operation: stopped, line synchronized, and high-speed fill. Once the accumulator reaches the full condition, the drive automatically goes into the synchronized mode, and the accumulator stays full until the supply of strip from the uncoiler is depleted.
A horizontal accumulator type 2.
The operator or an automatic end detector senses the end of the coil, and the pinch roll stops pulling strip in. The tube mill then feeds from the accumulator by pulling the inner wrap of the outer coil around the takeout arbor and into the mill.
When the end weld is complete, the operator signals the pinch rolls to start filling. The accumulator then fills at two to three times line speed until it is full. At this time, the drive is signaled to go into synchronized mode until the new coil is depleted.
This accumulator is simple in design, has few moving parts, and stores a lot of strip in a small space. However, it also takes a lot of floor space because the strip must be turned 90 degrees before entering the machine and another 90 degrees after exiting.
On a free loop accumulator, two concentric coils are formed about a common centerline, connected by a free reverse loop from the inside wrap of the outer coil to the outside wrap of the inner coil.
Free Loop Accumulator
On a free loop accumulator, two concentric coils are formed about a common centerline and connected by a free reverse loop from the inside wrap of the outer coil to the outside wrap of the inner coil. The coils can be formed in either a vertical or horizontal plane, or in an inclined plane (see Figure 8).
In operation, strip is driven into a free loop and constrained between a set of ID basket rolls and a set of outside diameter (OD) basket rolls. In the example shown, the free loop travels in a clockwise direction when the entry speed exceeds the mill speed.
As accumulation takes place, an outer coil is formed within the outer basket rolls, and an inner coil is formed over the inner basket rolls. Strip is transferred from the inside of the outer coil to the outside of the inner coil via the free loop. The strip is then removed to the tube mill from the inside of the inner coil through a spiral path around the takeout arbor.
The outer basket rolls must move outward as entering strip is wrapped on the rotating outer coil, and the inner basket rolls must move outward as strip is removed from the inside of the inner coil. The implication of these roll movements is that there is no relative movement between wraps of strip, so no strip marking occurs.
When no strip is entering, the mill runs from the accumulated strip, and the free loop moves in a counterclockwise direction.
On a Sendzimir spiral looper accumulator, strip is driven by pinch rolls to the outside of the upper coil and pulled from the outside of the lower coil.
The G-accumulator, a variation of the free loop accumulator, involves a sheave to drive the free loop and a rotating outer basket roll assembly over which the outer coil is formed.
This design is suitable for narrow, very light-gauge strip because such strip can better be pulled into the accumulator (in this design) than pushed.
Sendzimir Spiral Loop Accumulator
The Sendzimir spiral loop accumulator is similar in operation to the free loop accumulator, except that the two storage coils are formed in a horizontal plane, one above the other, and the connection is through an S curve from the inside of the upper coil to the inside of the lower coil.
Strip is driven by pinch rolls to the outside of the upper coil and pulled from the outside of the lower coil (see Figure 9).
With an all-day coil system, the outside wrap of each coil in a horizontal stack is end-joined to the inside wrap of the coil immediately above it.
Other Accumulator Styles
Two other methods of increasing uptime or approaching continuous line operation should be mentioned. One is the big wheel, which involves the off-line assembly of a number of smaller coils into one very large coil, or big wheel. Downtime between individual big wheel assemblies is less than the total downtime between individual coils.
A second method is the all-day coil system. With this system, several coils are preassembled off-line on a special mandrel. The outside wrap of each coil in a horizontal stack is end-joined to the inside wrap of the coil immediately above it (see Figure 10).
In operation, strip is removed from the top coil, then the next lower coil, and so on until all coils are expended. A special roll stand is used to direct the strip from each coil to the mill. This roll stand indexes to the next lower coil each time a coil is depleted.
In most operations, enough coil can be preassembled to satisfy the requirement for one shift, so mill operation is continuous for at least one shift.
These accumulation methods allow for more production between stoppages; however, they do not allow for continuous operation.
The trend in the tube and pipe industry in the last 10 to 15 years has been toward mills that can run continuously at higher speeds over a greater range of thicknesses and widths with less maintenance and greater flexibility. The strip accumulator is one of the most important machines in the line because the line's productivity is often a function of its ability to run nonstop.
When comparing the relative advantages of any type of accumulator, tube producers must consider all of the relevant costs associated with its purchase and maintenance, as well as its effects on productivity.
If an accumulator is chosen based only on the lowest price, the apparent savings may be misleading. Remember, just a few stops in production to make end welds can offset any savings that might have been gained in purchasing a less costly accumulator.
This article was supplied by Kent Corp., 9801 York Alpha Drive, North Royalton, Ohio 44133, phone 216-582-3400, fax 216-237-5368. Kent Corp. manufactures machinery for the tube and pipe mill industry.
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