Hydraulic Stop-to-Cut Shear PLC Sequence — Full Logic, Timing Compensation & Troubleshooting

1. Introduction — How Stop-to-Cut Shear Systems Work

In many roll forming machines, especially roofing and cladding lines, the most common cutting system is the hydraulic stop-to-cut shear.

Unlike flying shear systems, stop-to-cut machines temporarily stop the strip before the cut occurs.

This simplifies control but introduces other engineering challenges such as:

• deceleration timing
• hydraulic delay compensation
• encoder synchronization
• mechanical stopping accuracy

The PLC must coordinate several machine systems to produce accurate panel lengths.

These systems include:

• main drive motor (VFD)
• encoder length measurement
• hydraulic power unit
• shear cylinder
• shear position sensors

This article explains the complete PLC sequence used to control hydraulic stop-to-cut shears and how to diagnose common problems.

2. Components of a Stop-to-Cut Shear System

A typical hydraulic shear cutting system consists of several key components.

PLC Controller

Controls the cutting sequence and monitors machine status.

Encoder

Measures strip length and signals when the cut position approaches.

Main Drive (VFD or Servo)

Moves the strip through the roll forming machine.

Hydraulic Power Unit

Provides pressure for the shear cylinder.

Hydraulic Cylinder

Moves the cutting blade down and up.

Limit Switches or Sensors

Detect the blade position.

Typical sensors include:

• shear up position sensor
• shear down position sensor

These sensors confirm that the blade has completed its movement.

3. Basic Stop-to-Cut Sequence Overview

The cutting process occurs in several stages.

Typical sequence:

1. Strip feeds forward through roll former
2. Encoder counts strip movement
3. Target length reached
4. PLC commands drive stop
5. Machine decelerates
6. Strip stops moving
7. PLC activates hydraulic shear
8. Blade moves down and cuts strip
9. Blade returns to up position
10. PLC restarts drive

This sequence repeats for each panel.

4. Length Detection and Cut Trigger

The encoder provides real-time position feedback.

The PLC counts encoder pulses to determine strip length.

Example:

Encoder scaling:

2 pulses per mm

Target panel length:

6000 mm

Total pulses required:

6000 × 2 = 12,000 pulses

When the pulse count reaches the trigger point, the PLC initiates the stop sequence.

However, the trigger point must account for machine deceleration.

5. Deceleration Compensation

Stopping the strip is not instantaneous.

The strip continues moving during drive deceleration.

Example:

Line speed = 60 m/min
= 1000 mm/sec

Drive deceleration time = 0.4 sec

Distance traveled during deceleration:

1000 × 0.4 = 400 mm

Therefore the PLC must trigger the stop command before reaching the final panel length.

If the target panel length is 6000 mm:

Stop command must occur at approximately:

5600 mm

Fine adjustment is then performed during commissioning.

6. PLC Logic Structure for Stop-to-Cut

The PLC program typically uses a sequence-based control structure.

Simplified logic stages include:

Stage 1 — Run Mode

Conditions required:

• machine run command active
• safety interlocks satisfied
• encoder counting active

During this stage, the strip moves continuously.

Stage 2 — Length Reached

Condition:

Encoder count ≥ trigger value

PLC commands:

• stop main drive
• prepare shear activation

Stage 3 — Strip Stopped

PLC waits for confirmation that motion has stopped.

This may include:

• drive speed feedback = zero
• timer delay after stop command

Once confirmed, the PLC activates the hydraulic valve.

Stage 4 — Shear Down

PLC energizes hydraulic valve to move blade downward.

The PLC monitors:

• shear down limit switch

If the down signal is not received within a defined time, a fault is triggered.

Stage 5 — Shear Up

After the cut is completed, the hydraulic valve retracts the blade.

The PLC waits for:

• shear up sensor signal

Once confirmed, the cut cycle is complete.

Stage 6 — Reset and Restart

The PLC performs several actions:

• reset encoder count
• reset cut flags
• restart main drive

Material begins feeding again.

7. Hydraulic Timing Considerations

Hydraulic systems introduce delays that must be considered.

Typical delays include:

• valve activation delay
• cylinder movement delay
• pressure build-up time

These delays affect cut timing.

If compensation is not applied, panels may become longer than expected.

Commissioning engineers typically adjust delay values within the PLC program.

8. Position Sensors for Shear Control

Shear systems typically use two position sensors.

Shear Up Sensor

Indicates blade is in safe position.

Machine cannot start unless blade is fully raised.

Shear Down Sensor

Confirms blade reached cutting position.

Used to verify that the cut occurred.

If the sensor fails to activate, the PLC generates a fault.

9. Typical PLC Fault Conditions

Several faults may occur during the cutting sequence.

Shear Down Timeout

Occurs when the blade fails to reach the down position within the allowed time.

Possible causes:

• hydraulic pressure loss
• stuck valve
• sensor failure

Shear Up Timeout

Occurs when the blade fails to return to the up position.

Possible causes:

• hydraulic blockage
• cylinder seal failure
• sensor malfunction

Drive Stop Failure

Occurs when the strip continues moving after stop command.

Possible causes:

• incorrect deceleration ramp
• drive fault
• mechanical inertia issues

10. Common Cut Length Problems

Length errors often occur due to incorrect timing compensation.

Common issues include:

Panels Too Long

Possible causes:

• insufficient deceleration compensation
• hydraulic delay not accounted for

Panels Too Short

Possible causes:

• excessive compensation
• encoder scaling error

Random Length Variation

Possible causes:

• encoder noise
• measuring wheel slip
• inconsistent hydraulic response

11. Troubleshooting Stop-to-Cut Systems

Troubleshooting should follow a structured process.

Step 1 — Verify Encoder Accuracy

Confirm encoder scaling and signal stability.

Step 2 — Inspect Measuring Wheel

Check for slippage or contamination.

Step 3 — Check Hydraulic Pressure

Ensure pressure is stable during cutting.

Step 4 — Verify Sensor Operation

Confirm shear up and down sensors operate correctly.

Step 5 — Review PLC Timing Parameters

Adjust:

• deceleration compensation
• hydraulic delay compensation

Small adjustments often correct length errors.

12. Commissioning Stop-to-Cut Shear Systems

Commissioning procedure typically includes:

1. Run machine at low speed
2. Produce several test panels
3. Measure panel lengths
4. Adjust compensation values
5. Repeat testing

Once stable results are achieved, speed can be increased gradually.

13. Maintenance for Hydraulic Shear Systems

Regular maintenance improves reliability.

Recommended inspections include:

Monthly:

• inspect hydraulic hoses
• verify pressure levels
• test shear sensors

Quarterly:

• inspect blade wear
• check cylinder seals

Annually:

• inspect hydraulic pump
• replace hydraulic oil if necessary

Proper maintenance reduces shear faults and length variation.

6 Structured FAQ — Hydraulic Stop-to-Cut Shear Systems

1. Why does the machine stop before cutting?

In stop-to-cut systems the strip must stop moving so the shear blade can cut accurately without damaging tooling.

2. Why do panels become longer when production speed increases?

Higher speeds increase the distance traveled during deceleration and hydraulic delay. If compensation is not adjusted, panels will become longer.

3. What causes shear down timeout faults?

These faults occur when the blade fails to reach the down position within the allowed time, often due to hydraulic pressure loss or sensor failure.

4. Why is encoder accuracy important for stop-to-cut systems?

The encoder determines when the PLC initiates the stop sequence. Incorrect measurements lead to inaccurate panel lengths.

5. What is deceleration compensation?

Deceleration compensation adjusts the trigger point for stopping the strip to account for the distance traveled while the machine slows down.

6. Why must the shear be fully raised before the machine starts?

The blade must be in the up position to ensure the strip can move freely and to prevent tool damage when feeding material.