The short answer
A spiral accumulator is the strip storage unit in the entry section of an ERW (high-frequency welded) tube mill. It sits after the shear and butt welder and before the forming mill. Before a coil change, it stores a length of strip. While the uncoiler is stopped for the new coil and the welder cuts and joins the strip ends, the mill keeps pulling strip out of the accumulator, so the tube mill does not have to stop. After the joint is made, the feeder runs faster than the mill and refills the accumulator.
InGen MillTek builds two designs: the horizontal (rotating-disc) spiral accumulator and the floor-type spiral accumulator. Both are configured for the strip width, thickness and line speed of the project. In our catalogue, horizontal accumulators range from 2 to 6.5 m in diameter and floor-type accumulators from 4 to 20 m. The final size depends on strip width, thickness, material and required storage, and is confirmed in the technical proposal.
Other names you may see are strip accumulator and coil accumulator. A spiral accumulator is a strip storage machine for straight-seam ERW lines. It is not the same as a spiral welded (SSAW) pipe mill.
Where the accumulator sits in the line
Following the strip, a typical entry section for an ERW tube mill is arranged in this order. The machines before the accumulator pay off the coil and join the coils into one continuous strip; the tube mill comes right after it.
- Uncoiler: holds the coil and pays off the strip.
- Leveler: added when the material strength, thickness and strip shape require it; our engineers advise whether a separate leveler is needed.
- Shear and butt welder: cuts the tail of the old coil and the head of the new coil square, then welds them together.
- Flat support stand and adjustable twist stand: turn the strip step by step from flat to on-edge.
- Feeder: pushes the strip into the accumulator; its pinch rolls are mounted vertically.
- Spiral accumulator: stores strip and releases it to the mill.
- High turnover stand: turns the strip from on-edge back to flat and guides it into the tube mill.
What happens during a coil change
One coil change runs in three stages:
- Filling before the change: the feeder speeds up and fills the accumulator to the required length.
- Coil change and joining: the uncoiler stops, the new coil is loaded and the welder joins the strip ends. No strip enters during this time, but the mill keeps pulling stored strip from the center of the accumulator.
- Refilling after the joint: the feeder runs faster than the mill and puts back the strip that was used, ready for the next change.
Two conditions the accumulator must meet
First, enough storage: the usable strip length in the accumulator must cover one complete coil-change cycle. Second, fast enough refill: after the joint, the accumulator must be full again before the next coil change. When both are met, the mill keeps running through the coil change. If either one is not met, the mill has to stop and wait.
Why the strip stands on edge
Inside a spiral accumulator, the strip is wound on edge (standing upright) into a spiral of loops. That is why the entry needs a flat support stand and an adjustable twist stand to turn the strip upright, and the exit needs a high turnover stand to lay it flat again before the mill. For the same reason, the feeder pinch rolls are vertical.
The strip bends as it is wound into loops. At the same winding diameter, thicker strip is strained more at its surfaces, and if the diameter is too small the strip takes a permanent bend. So thicker strip needs a larger winding diameter. For high-strength strip, a larger diameter is also usually chosen in practice: the higher the strength, the greater the bending force and springback, and the heavier the load on the rollers and drive. Our engineers confirm this for each material. This is one reason the accumulator diameter is chosen from the maximum strip thickness and the material strength, not from the strip width alone.
Two designs: horizontal and floor type
Both designs store strip on the same principle. The main difference is the drive:
- Horizontal (rotating-disc) spiral accumulator: one main motor turns the whole disc through a reducer and a large open gear, while the outer ring stays fixed. The strip enters tangentially at the outer ring, forms loops, makes a U-turn at a ring of upright rolls into the inner ring, and is pulled up and out through an inclined exit roller at the center. Compact, with a small footprint.
- Floor-type spiral accumulator: the base does not rotate. The outer horizontal rollers are driven in groups by geared motors, with one group usually left undriven, and the inner horizontal rollers are idle. For heavy, thick strip, one large motor with a central drive can be used instead. It suits wider and thicker strip or larger storage, and is built as either a fixed accumulator or a variable accumulator.
Which design should you choose?
As a general rule, narrower and thinner strip with moderate storage needs points to a horizontal accumulator; wider or thicker strip, or a large storage requirement, points to a floor-type accumulator. There is no single cut-off value that fits every project. Our engineers decide from strip width, thickness, material strength, line speed and the storage length you need. For a side-by-side comparison, see our guide Horizontal vs. Floor-Type Spiral Accumulator: How to Choose.
How much storage do you need? Calculate length, not tonnage
Check storage in meters: required storage length ≥ mill line speed × time for one complete coil change and joint × safety factor (usually 1.2 to 1.5). Count the whole cycle, from the moment the uncoiler stops feeding until strip feeding restarts. With a single-head uncoiler, loading the new coil falls inside this time, whether the coil is loaded by overhead crane or by coil car. A double-head uncoiler can load the next coil on its other arm while the current coil is running, so the cycle is mainly turning the uncoiler, threading the strip head, cutting, aligning, welding and resetting. Real times vary widely: aligning the strip ends at the shear and butt welder, the coil loading method and the number of operators all affect this time, and with manual welding and a single-head uncoiler, one coil change often takes several minutes or longer, so estimate it by timing real changes on site.
Then check the refill: net refill speed = feeder speed − mill speed, and refill time = strip length to be replaced ÷ net refill speed. The faster the mill, the more important it is to confirm that refilling finishes before the next coil change.
A storage figure in tons changes with strip width and thickness, so it does not tell you whether the accumulator covers your coil-change time. Storage should be checked as a length in meters.
Worked example (calculation method only; all inputs are assumptions, not the performance of any particular machine): at a line speed of 80 m/min, one coil lasts about 11 minutes, so the line needs a coil change roughly every 11 minutes. Assume one complete coil change and joint takes 5 minutes and use a safety factor of 1.3: the storage needed is about 80 × 5 × 1.3 ≈ 520 m. During the change the mill actually uses about 80 × 5 = 400 m, which must be put back before the next coil change. Of the roughly 11-minute coil cycle, 5 minutes go to the change, leaving about 6 minutes for refilling, so the feeder speed must be checked against that.
Does a small ERW pipe line need a spiral accumulator?
The accumulator keeps the mill running while coils are changed and joined. Without stored strip, the mill has to stop and wait at every coil change and joint. Whether you need one, and what size, depends on line speed, coil weight (which sets how often you change coils), the joining method and how much continuous running you require. Our catalogue range for horizontal accumulators starts at 2 m in diameter, so small sizes exist for narrow strip. Our engineers recommend a size for each project.
Information to send for an accumulator proposal
A spiral accumulator can be supplied as part of a complete entry section, or as a single machine to add to or replace on an existing tube mill. Pricing is configured to order and quoted separately. The more of this information you can send, the more accurate the proposal:
- Strip width and thickness range (maximum and most common), material grade, and yield and tensile strength
- Tube mill model, maximum and normal line speed
- Coil weight, and coil inner and outer diameter
- Joining method (integrated shear and manual welder, semi-automatic or fully automatic shear and welder) and the approximate coil-change time
- Mill pass-line (center) height and feed direction (left-hand or right-hand)
- Floor length and width available for the accumulator
- Local voltage and frequency, for example 380 V/50 Hz, 415 V/50 Hz or 440 V/60 Hz
- New line, or an accumulator to add to or replace on an existing mill
Frequently asked questions
- Does InGen MillTek build its own spiral accumulators? Yes. The spiral accumulator is our flagship product. We build horizontal and floor-type accumulators, together with their stands, adjustable twist stands and feeders, in our own workshop in Jiangyin, Jiangsu, China. YinJiang has focused on strip processing equipment since 1997, and in 2023 JiangSu InGen MillTek Co., Ltd. was recognized as a National High-Tech Enterprise.
- With an accumulator, will the mill never stop at a coil change? Only if two conditions are met: the stored length covers one complete coil-change cycle, and the refill finishes before the next change. When both are met, the mill can keep running.
- How are the accumulator drives controlled? Storage and feeding use variable-frequency drives (VFDs). Motors and controls are configured for your local grid voltage and frequency.
- Are spiral accumulators used only on tube mills? Tube mills are the main use. Spiral accumulators are also used on other continuous strip lines, such as slitting, pickling and cold-rolled galvanizing lines. Other lines are evaluated separately for their process and speed.
About this article
Prepared by the InGen Engineering Team. This is a general technical guide, not a specification or performance guarantee for any project. Final parameters are set in the project's technical proposal and technical agreement.