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

Troubleshooting of 3-in-1 Uncoiler Straightener Feeder Tension Out-of-Control Fault Resulting in Strip Buckling, Edge Wave and Material Breakage on High Speed Punch Press Stamping Line

1.1 News Lead

In automated stamping production, the 3-in-1 uncoiler-straightener-feeder is the front-end core equipment of the whole stamping cell. It integrates coil loading, leveling and feeding functions, and its stable material tension control directly determines the surface quality of raw strip material and the continuity of stamping production. The coil material is released from the material reel, passes through multiple leveling rollers for flattening, and is sent to the stamping die under coordinated tension control. Once the tension loses balance, even a tiny fluctuation will bring a series of bad phenomena: strip edge wave, middle arching, local buckling, scratch on material surface, and even sudden material fracture. Material fracture will directly stop the whole production line; besides wasting raw material, it also needs operators to spend a lot of time threading material again, greatly reducing production efficiency. In severe cases, loose material will stack into the die cavity, causing die damage and bringing huge economic loss to customers.

Our after-sales engineering department received an emergency service ticket from a precision metal parts manufacturer in Eastern China. The customer’s production line adopts a 200T servo punch press matched with HCK series three-in-one uncoiler straightener feeder, which is used to continuously stamp stainless steel shell parts for new energy battery components. The equipment has been running for about 18 months under two-shift working mode. In the early stage of the fault, operators occasionally found slight edge waves on the stainless steel strip. At first, the customer judged it as raw material problem and continued production. Within three working days, the fault gradually deteriorated: the strip appeared intermittent arching and buckling, obvious scratch marks on the material surface, and two strip breakage accidents occurred in succession. Each breakage forced the line to shut down for more than 40 minutes for re-threading. Meanwhile, unstable material delivery caused inconsistent forming size of stamped workpieces, the reject rate rose rapidly, and the customer had to reduce the production speed from the rated 80 strokes per minute down to 45 strokes per minute to temporarily ease the material breaking risk, resulting in nearly 44% reduction of production capacity.

1.2 On-Site Diagnosis

After arriving at the customer workshop, our service engineers carried out layered inspection from mechanical structure, pneumatic control, tension detection sensor and HMI parameter setting, and adopted step-by-step isolation method to eliminate irrelevant influencing factors.

First of all, we checked the raw coil material to exclude material defects. Multiple coil sections were sampled for flatness inspection, and no inherent edge wave or internal stress abnormality of the raw material was found, which proved that the bad strip shape was generated in the conveying process of the three-in-one feeder rather than coming from the original coil.

The second step was to check the mechanical parts of the uncoiler reel. We found that the friction brake pad of the material reel had obvious wear and thinning. When the reel rotates and releases material, the braking torque cannot be maintained stably. When the feeder pulls the strip forward, the reel sometimes releases material too fast, leading to material accumulation and loose tension; when the brake suddenly engages, the tension rises instantly, pulling the strip tightly and easily causing stretching deformation and fracture. At the same time, the supporting roller bearing of the uncoiler reel has slight jamming, and the rotation resistance fluctuates randomly in operation, further aggravating the instability of material pulling force.

The third step was to inspect the leveling section and tension detection mechanism. The tension sensor installed at the outlet of the straightener had accumulated thick oil dust for a long time, the sensing surface was contaminated, and the feedback signal was delayed and distorted. The actual tension value of the strip could not be accurately transmitted to the PLC control system. The control system could not make real-time adjustment of the brake torque according to the real material tension, forming a closed-loop control failure. Besides, the gap between upper and lower leveling rollers was uneven on left and right sides, which made the strip bear asymmetric extrusion force in the leveling process, and finally produced edge wave defects.

The fourth step was to verify the control parameters on HMI. The original tension threshold parameters set by the customer were not adjusted according to the attenuation of brake pad performance. The upper and lower limit values of tension were too close, and the response gain of brake adjustment was unreasonable. When tension fluctuated, the system could not respond quickly enough, which amplified the vibration of material tension in high-speed operation.

Static test was carried out after preliminary fault positioning. Under no-load state, the system was commanded to run material conveying at fixed speed, and the tension data collected by the sensor jumped up and down continuously, with fluctuation range from 8 N to 42 N, while the standard stable tension fluctuation should be controlled within 12 N ~ 20 N for this stainless steel strip specification. This test result confirmed that the fault was a composite failure including mechanical wear, sensor pollution and unreasonable control parameters, instead of a single component damage.

1.3 Repair and Verification

The repair work was divided into four core links to solve the problems one by one, and then carry out combined commissioning.

First, disassemble the uncoiler reel brake assembly, replace the worn friction brake pads, clean the brake friction disc, check and replace the stuck reel support bearing, adjust the clearance of brake assembly, so that the braking torque output of the reel is smooth and continuous without sudden clamping or slipping phenomenon.

Second, disassemble and clean the tension detection sensor, remove oil sludge and metal powder on the sensing surface, calibrate the sensor zero point, test the signal output linearity, ensure that the tension feedback signal can be transmitted to PLC accurately and in real time. Then re-adjust the parallelism of upper and lower leveling rollers, finely tune the left and right roller gaps according to the thickness and material characteristics of stainless steel strip, eliminate asymmetric extrusion force in the leveling section.

Third, re-optimize the tension closed-loop control parameters on HMI. Reset the upper and lower limit threshold of material tension, adjust the proportional-integral gain of brake torque response, optimize the dynamic response speed of the system, avoid excessive or insufficient braking force during high-speed feeding, so that the tension can be automatically stabilized within the target interval during operation. Meanwhile, set up tension abnormal alarm function, once the tension exceeds the preset safe range, the HMI will pop up alarm prompt in advance, and slow down the feeding speed automatically to prevent material breakage.

Fourth, clean the material guide rollers and anti-scratch rubber sleeves along the material path, remove metal debris attached to the roller surface, to avoid secondary scratch on the strip surface during conveying.

After the repair work was completed, continuous running verification was implemented. The equipment ran at full rated speed of 80 strokes per minute for 150 minutes continuously, completing 12,000 strokes stably. The tension fluctuation of the strip was controlled steadily within the standard range. There was no edge wave, buckling or scratch on the strip surface, and no material breakage occurred during the whole test period. The dimensional detection of stamped battery shell workpieces showed that all parts met drawing requirements, and the reject rate dropped back to the normal production level. The customer’s stamping production line successfully restored full capacity on the same day after service. This maintenance case shows that the tension out-of-control fault of three-in-one feeder involves the coordination of machinery, sensor detection and PLC control system. Only by comprehensive inspection and composite repair can we completely eliminate hidden dangers instead of simply adjusting a single parameter.

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