Loop Pit Underfill Tension Spike – Causes, Inspection, Repair & Prevention Guide for Roll Forming Machines

Loop Pit Underfill Tension Spike

Roll Forming Machine Troubleshooting & Repair Guide

Loop pit underfill tension spikes occur when the strip loop in the loop pit becomes too small or disappears completely during production. Loop pits are designed to act as a material buffer between upstream equipment (such as decoilers or feeders) and downstream equipment (such as roll formers, punching systems, or flying shears).

When the loop pit becomes underfilled, the downstream equipment begins pulling the strip directly from the upstream system without a buffer. This creates a sudden increase in strip tension, commonly referred to as a tension spike.

If this issue is not corrected, it may lead to several production problems including:

• Sudden strip tension increases
• Strip stretching or deformation
• Feeding instability at entry guides
• Encoder measurement errors
• Increased wear on feeding equipment
• Potential strip breakage

For manufacturers producing metal roofing panels, cladding sheets, structural decking, purlins, trims, and other roll formed components, maintaining proper loop pit strip levels is essential for stable tension control and smooth production.

This guide explains the causes of loop pit underfill tension spikes, inspection procedures, corrective actions, and preventative maintenance strategies used by experienced roll forming technicians.

Causes of Wear or Failure

Loop pit underfill tension spikes typically occur when downstream equipment consumes strip faster than upstream equipment supplies it.

Upstream Equipment Running Too Slowly

If the decoiler or feeder operates slower than the roll forming machine, the loop pit may gradually empty.

Loop Control System Misconfiguration

Incorrect loop control parameters may fail to maintain adequate strip accumulation in the loop pit.

Loop Sensor Malfunction

Loop height sensors detect strip position in the pit.

Faulty sensors may not signal upstream equipment to increase feed speed.

Servo Feed Speed Variations

If a servo feeder accelerates unexpectedly, it may consume strip faster than the loop pit can replenish.

Temporary Downstream Speed Increase

Production speed changes may cause downstream equipment to pull material faster than normal.

Strip Snagging in Loop Pit

If the strip catches on pit structures or debris, the loop size may decrease unexpectedly.

Why It Happened and What Caused It

From a mechanical standpoint, the loop pit functions as a dynamic tension buffer.

During normal operation:

1. The strip enters the loop pit from upstream equipment.
2. A controlled loop of material accumulates.
3. Downstream equipment pulls material from the loop.

This system isolates the upstream and downstream equipment so that small speed variations do not affect strip tension.

However, when the loop pit becomes underfilled:

• The buffer disappears.
• Downstream equipment pulls directly on the upstream equipment.
• Strip tension increases rapidly.

These tension spikes can damage the strip and destabilize production.

How to Inspect the Problem

Inspection Procedure

Proper inspection helps determine whether loop pit underfill is causing strip tension spikes.

Step 1 – Observe Loop Pit During Operation

Watch the loop pit while the line is running.

Signs of underfill include:

• Very small strip loop
• Loop disappearing completely
• Strip pulled tightly through the pit

Step 2 – Monitor Strip Tension

Observe strip behavior between machines.

Signs of tension spikes include:

• Sudden strip tightening
• Strip vibration
• Feeding instability

Step 3 – Inspect Loop Control Sensors

Verify that loop sensors accurately detect strip height.

Dirty or damaged sensors may fail to trigger upstream speed adjustments.

Step 4 – Inspect Upstream Equipment Operation

Check whether the decoiler or feeder is supplying strip fast enough.

Step 5 – Inspect Loop Pit for Obstructions

Ensure the strip can move freely through the pit without catching on surfaces.

Step-by-Step Technician Guide – How to Fix

Several corrective actions can restore proper loop pit function and prevent tension spikes.

Method 1 – Adjust Loop Control Settings

Reconfigure loop control parameters to maintain a larger buffer loop.

Method 2 – Increase Upstream Feed Speed

Ensure the decoiler or feeder supplies strip fast enough to maintain the loop.

Method 3 – Repair or Replace Loop Sensors

Replace faulty sensors that fail to detect loop height.

Method 4 – Remove Obstructions from Loop Pit

Clear debris or damaged strip that may restrict strip movement.

Method 5 – Synchronize Equipment Speeds

Ensure upstream and downstream equipment operate within compatible speed ranges.

Preventative Maintenance Tips

Routine monitoring helps prevent loop pit underfill problems.

Inspect Loop Sensors Regularly

Clean and test sensors to ensure accurate loop detection.

Monitor Loop Pit Levels

Operators should observe loop height during production startup and speed changes.

Maintain Equipment Speed Synchronization

Ensure upstream and downstream equipment remain properly coordinated.

Keep Loop Pit Clean

Remove debris or damaged strip from the pit regularly.

Train Operators on Loop Control Systems

Operators should understand how loop pits regulate strip tension.

FAQ Section

What causes loop pit underfill?

Underfill occurs when downstream equipment consumes strip faster than upstream equipment supplies it.

Why is loop pit underfill a problem?

It removes the tension buffer and causes sudden strip tension spikes.

How can loop pit underfill be corrected?

Adjusting loop control settings and synchronizing equipment speeds usually resolves the issue.

Can tension spikes damage roll forming machines?

Yes. Excessive tension may damage strip material and feeding equipment.

Should loop pits always maintain a visible strip loop?

Yes. A properly sized loop helps stabilize strip tension.

Can faulty sensors cause loop pit underfill?

Yes. Sensors that fail to detect loop height may disrupt loop control systems.

Machine Matcher Technical Support

If your roll forming line is experiencing strip tension spikes, loop control instability, or feeding problems, Machine Matcher technical specialists can help diagnose and resolve the issue.

Support services include:

• Roll forming machine troubleshooting
• Loop control system diagnostics
• Strip tension and feeding analysis
• Production line performance evaluation
• Remote technical assistance

Submit your issue through the Machine Matcher Technical Support Desk:

Include the following information:

• Machine type
• Coil width, thickness, and material type
• Photos or videos showing the loop pit issue
• Production speed and loop control settings

Our technicians will review the information and provide step-by-step recommendations to restore stable production.