3-in-1 Uncoiler Straightener Feeder Composite Tension Out-of-Control Fault

Case Study of 3-in-1 Uncoiler Straightener Feeder Composite Tension Out-of-Control Fault: Root Cause Analysis, Repair Scheme and Preventive Maintenance Specification

This case is based on a stamping production cell equipped with a 200T servo precision punch press and HCK series three-in-one uncoiler straightener feeder. The equipment is used for continuous stamping of new energy battery stainless steel shell, raw material is SUS304 stainless steel strip with thickness 0.6 mm and width 180 mm. The whole production line adopts two-shift continuous production mode, and the equipment has been put into use for 18 months, with average monthly stroke reaching 1.8 million strokes. The three-in-one feeder integrates hydraulic expansion reel, multi-roll leveling mechanism, tension detection unit and servo feeding roller. The material reel is equipped with pneumatic friction brake, and the closed-loop tension control is realized through tension sensor + PLC + brake torque adjustment. The rated working speed is 80 strokes per minute, and the target stable tension range is set as 12~20 N for this specification strip.

The fault phenomenon first appeared in the night shift production. The operator found slight edge wave on the edge of stainless steel strip. At that time, the customer suspected that the raw coil material had poor flatness, so they replaced a new coil and continued production. However, the edge wave defect still existed after coil replacement. Three days later, the strip began to appear irregular arching and local buckling, and fine scratch marks appeared on the material surface. On the third day of fault occurrence, two strip breakage accidents happened successively. After each breakage, the production line needed to stop, re-thread the strip and adjust the material path, consuming more than 40 minutes each time. Meanwhile, unstable tension caused stretching deformation of local material, and the size of stamped battery shell exceeded tolerance, resulting in a sharp rise of reject rate. In order to avoid more material breakage and die collision risk, the production team reduced the working speed from 80 spm down to 45 spm, and the production capacity decreased significantly. The customer contacted our after-sales department and applied for on-site troubleshooting service.

The first step is to eliminate raw material quality factors. We sampled multiple sections from the current coil and standby spare coil, and carried out flatness inspection on the inspection platform. The test result showed that the raw coil itself had no obvious edge wave and internal stress abnormality. Therefore, the strip flatness defect was generated during the material conveying and leveling process of three-in-one feeder, rather than inherent defect of raw coil. Then we recorded the fault characteristics: the edge wave of the strip is unstable, sometimes appears on left edge, sometimes on right edge; buckling occurs randomly, and the probability increases when the line speed rises; scratch marks are distributed along the feeding direction on the strip surface.

We lifted the coil and emptied the material reel, then manually rotated the reel to check rotation smoothness. Obvious jamming feeling was found during rotation, and the rotation resistance was not uniform. After disassembly, we found that the support bearing of the reel had partial wear and insufficient lubrication, leading to unstable rotation resistance. Then inspect the pneumatic friction brake assembly: the friction pad was worn seriously, the thickness was close to the replacement limit. The friction surface had partial carbon deposition and uneven wear. When the brake works, the braking torque could not be output stably. In the feeding cycle, the feeder roller pulls the strip forward, the reel may slip and release material excessively, causing loose material and low tension; when the brake suddenly grabs, the tension rises instantly, pulling the strip sharply and leading to fracture risk.

We checked the tension sensor installed at the outlet of leveling section. Long-term stamping workshop environment made the sensing surface accumulate a large amount of metal powder and lubricating oil sludge, which covered the sensing area. The signal feedback had serious delay and distortion, the real strip tension value could not be transmitted to PLC accurately. The closed-loop control logic failed, and the system could not automatically adjust the brake torque in real time according to the actual tension.

For the leveling mechanism, we measured the gap between upper and lower leveling rollers on left and right ends with precision feeler gauge. The left-right gap deviation reached 0.12 mm. The strip bore asymmetric extrusion force in the leveling process, so one side of the strip was over-leveled while the other side was insufficiently leveled, forming edge wave defect. At the same time, there were metal debris adhered to the surface of individual material guide rollers, which produced linear scratches on the strip surface during conveying.

We logged into the equipment control system to view historical tension data. The tension value fluctuated violently between 8 N and 42 N during operation. The original tension upper and lower limit threshold set by customer was too narrow, and the proportional gain of brake response was too small. When tension changed, the adjustment speed of brake torque was slow, which could not restrain tension fluctuation in high-speed feeding cycle. The original program did not enable automatic speed reduction function under tension abnormality, so the equipment kept running at set speed even if tension exceeded safe range, increasing material breakage risk.

Static calibration test was carried out after finishing the above inspection. The feeder ran without coil material, simulated feeding movement, and the tension sensor reading jumped irregularly, which was consistent with the fault characteristics observed in actual production. The test confirmed that this was a composite fault formed by mechanical wear, sensor pollution and unreasonable control parameters.

After systematic inspection and test, we sorted out four layers of root causes for this tension out-of-control fault:

1. Mechanical root cause: Long-term high-cycle operation caused wear of pneumatic friction brake pads and poor lubrication of reel support bearing. The reel rotation resistance was unstable, and the braking torque output was discontinuous, which was the core reason for tension fluctuation.
2. Detection and signal cause: The tension sensor was covered by oil sludge and metal dust, the sensing function degraded, signal feedback was delayed and distorted. The closed-loop tension control lost accurate feedback source, and the system could not realize real-time dynamic adjustment.
3. Process adjustment cause: The parallelism of leveling rollers was out of tolerance, left-right gap was inconsistent. The strip received uneven leveling force, which induced edge wave and buckling defects. Foreign matters on guide rollers caused surface scratch of strip.
4. System operation cause: The tension control parameters were not updated according to component aging. The brake response gain was unreasonable, and the tension abnormal protection function was not activated. The equipment lacked automatic early warning and speed limiting mechanism when tension deviated, which made small fault evolve into material breakage accident.

The repair and optimization work was implemented step by step according to the root cause:

1. Disassemble the reel brake assembly, remove old worn friction pads, clean carbon deposition on friction disc, install brand new matched brake pads, adjust brake clearance, test the braking torque output repeatedly to ensure smooth and stable torque. Replace the worn reel support bearing, inject high-temperature lubricating grease, make the reel rotate smoothly with uniform resistance.
2. Dismantle the tension sensor completely, clean metal powder and oil contamination on sensing surface with special cleaning agent, carry out zero-point calibration and linearity test of the sensor, confirm that the signal feedback delay is less than 20 ms. Clean each material guide roller, remove metal debris, replace damaged anti-scratch rubber sleeve.
3. Adjust the parallelism of upper and lower leveling rollers, fine tune the gap at left and right ends with feeler gauge until the gap deviation is controlled below 0.02 mm. Carry out trial leveling test on scrap strip to verify flatness improvement effect.
4. Re-optimize the closed-loop tension control parameters on HMI. Reset upper and lower tension limit, adjust PI gain of brake torque response, accelerate the dynamic response speed of brake. Activate the tension abnormal alarm and automatic speed reduction protection function. Once tension exceeds preset safe interval, HMI pops alarm and the feeder reduces speed automatically to avoid strip fracture.
5. Complete equipment assembly, and carry out multi-stage test: no-load test, low-speed material threading test, medium-speed trial stamping, and finally full-speed continuous verification. The full-speed test lasts 150 minutes, completing 12,000 strokes continuously, and record tension data every 200 strokes for trend analysis.

表格

Q1: Why is brake pad wear the most common mechanical factor leading to tension instability of three-in-one feeder? A1: The friction brake pad is the core wearing part of the uncoiler reel. In each feeding cycle, the brake needs to engage and release repeatedly to control the releasing speed of coil material. Under continuous two-shift production, the friction pad is worn gradually. With the decrease of thickness, the contact pressure and friction coefficient change, so the braking torque cannot keep stable. Once the torque fluctuates, the pulling force of strip will change accordingly, resulting in tension out-of-control. It is a typical gradual failure, which deteriorates slowly with running time and is easy to be ignored in daily inspection.

Q2: How to distinguish whether the tension fluctuation fault comes from sensor failure or brake mechanical wear? A2: Isolation test is the most effective method. Fix the strip static, apply known standard tension manually, observe the reading change of tension sensor. If the sensor reading cannot follow the change of actual tension or the reading jumps randomly, the fault source is the tension sensor and signal circuit. If the sensor reading can reflect manual tension normally, but tension fluctuates obviously during reel rotation, the problem lies in brake pad wear, bearing jamming and other mechanical parts of the reel. This method can avoid blind replacement of expensive sensors.

Q3: Why do we need to re-calibrate the tension sensor after cleaning or replacement? A3: The tension sensor relies on strain gauge to convert material pulling force into electrical signal. After cleaning or disassembly, the zero point of sensor may drift. Without zero calibration, the feedback reading will have fixed offset. The PLC will adjust brake torque according to wrong tension value, so the closed-loop control cannot work normally. Zero point calibration and linearity test are essential procedures after sensor maintenance, which cannot be omitted.

Q4: What electromagnetic or environmental interference factors will affect the tension sensor signal in stamping workshop? A4: The main interference sources include the electromagnetic signal of punch press servo drive, high-current power cable parallel to sensor signal cable, vibration impact generated by punch stamping. Besides, oil mist, metal powder and high temperature environment in workshop will contaminate the sensing surface and accelerate the aging of signal cable. The sensor cable should adopt shielded twisted pair wire, separate wiring from power cable, and regularly clean the sensing surface.

Q5: What maintenance cycle is suitable for three-in-one feeder under two-shift high-speed stamping production? A5: Daily: visual inspection of strip surface and real-time tension trend on HMI; check whether there is abnormal noise of reel during operation. Monthly: inspect the thickness of brake pads, check reel rotation flexibility, clean material guide rollers. Quarterly: clean tension sensor surface, check signal cable and connector, test tension alarm function. Semi-annually: measure leveling roller gap and parallelism, backup HMI tension parameters, inspect lubrication condition of reel bearing. Annually: full disassembly inspection of brake assembly, full calibration of tension closed-loop system and long-time full-speed verification test.

Q6: What economic loss may be caused if this tension out-of-control fault is not handled timely? A6: If the fault continues to deteriorate, frequent strip breakage will cause a large amount of raw material waste and long production downtime. Worse still, excessive loose material may pile up into die cavity and cause die collision. The repair cost of this battery shell die is high, and the delivery delay will bring penalty risk for customer order. Even before die damage, the reject rate of 7.2% and 44% capacity reduction already caused huge loss of output and raw material cost in each production shift. Timely composite repair restored full production capacity in one service day, protecting die assets and ensuring order delivery.

Combined with this on-site maintenance case, we formulate standardized preventive maintenance specification for tension control system of HCK three-in-one uncoiler straightener feeder:

1. Daily Maintenance: Visual check strip flatness and surface scratch; observe tension value trend on HMI; listen for abnormal noise from reel and brake during operation; record abnormal tension fluctuation log.
2. Monthly Maintenance: Measure residual thickness of pneumatic friction brake pads, replace pads once reaching wear limit; check rotation smoothness of reel, supplement lubricating grease for bearing; clean surface of guide rollers, remove metal debris.
3. Quarterly Maintenance: Disassemble and clean tension sensor sensing surface; check shielded signal cable and connector fastening status; trigger simulation test of tension abnormal alarm and automatic speed reduction function.
4. Semi-annual Maintenance: Use precision feeler gauge to detect parallelism and gap of leveling rollers, adjust if deviation exceeds standard; backup all tension closed-loop parameters in PLC and HMI; inspect cable routing and grounding condition.
5. Annual Maintenance: Full disassembly inspection of reel brake assembly; zero point calibration and linearity test of tension sensor; comprehensive optimization of tension PI parameters; 12,000-stroke full-speed continuous verification test.
6. Spare Part Inventory Requirement: Keep spare friction brake pads, tension sensor, reel support bearing, sealing rings and guide roller rubber sleeves in customer warehouse, to reduce mean time to repair when wearing parts fail.

This maintenance case shows that the tension out-of-control fault of three-in-one uncoiler straightener feeder is usually a composite fault containing mechanical wear, detection signal distortion and unreasonable control parameters, rather than damage of a single component. The fault develops gradually, starting from slight edge wave, then turning into buckling and scratch, and finally leading to strip breakage and production shutdown. If ignored, it may further trigger die collision accident and heavy economic loss.

By adopting layered isolation diagnosis method, our after-sales engineers eliminated raw material interference, checked reel brake, tension sensor and leveling mechanism one by one, found all root causes, and implemented composite repair including wearing parts replacement, sensor calibration, mechanical gap adjustment and control parameter optimization. After maintenance, the strip tension is stable, strip flatness meets production requirements, the reject rate returns to normal level, and the production line restores full rated speed. The preventive maintenance specification summarized from this case is applicable to HCK three-in-one feeder in high-cycle stamping production, which helps customers discover hidden tension faults in advance, reduce unplanned downtime and protect expensive stamping dies and raw material assets.