Roll Forming PLC, Electrical & Drive Engineering (Part 6): Control Logic, Gearbox Design & Vibration Modeling

Modern roll forming is not mechanical alone.

How a Roll Forming Machine Is Made — Part 6

Electrical System & PLC Control Engineering

(Control Logic, Encoder Accuracy, Safety Systems & High-Speed Stability)

Introduction — Where Mechanics Meets Intelligence

By this stage:

• • The frame exists
• • Tooling is manufactured
• • Shafts and bearings are sized
• • Drive torque is calculated

But without intelligent electrical control:

• • Length accuracy drifts
• • Punch timing fails
• • Shears misfire
• • Speed becomes unstable
• • Torque oscillation increases
• • Vibration amplifies

Modern roll forming is not mechanical alone.

It is a synchronized electromechanical system.

1. Electrical Architecture Overview

A modern roll forming line typically includes:

• • Main motor (AC induction or servo)
• • Gearbox
• • VFD (Variable Frequency Drive)
• • PLC (Programmable Logic Controller)
• • HMI (Human Machine Interface)
• • Encoder system
• • Safety relays
• • Hydraulic motor control
• • Lubrication pump control
• • Cooling system control

The electrical system must maintain:

• • Speed stability
• • Length accuracy
• • Torque stability
• • Safe operation
• • Thermal management

2. PLC Architecture & Control Philosophy

Roll forming requires deterministic control.

PLC scan cycle time matters.

2.1 PLC Scan Cycle

Typical industrial PLC scan time:

• 5–15 milliseconds

In high-speed roll forming:

A 10 ms delay at 40 m/min equals:

Speed = 40 m/min = 0.666 m/sec
In 0.01 sec:

0.666 × 0.01 = 6.66 mm

That means control lag could introduce ±6.6 mm error if improperly managed.

Therefore:

Critical length control must be interrupt-driven or high-speed counter based.

2.2 PLC vs Motion Controller

Standard PLC:
• Good for moderate speed lines

Dedicated motion controller:
• Better for:

• Flying shear systems

• High-speed stud lines

• Servo punch integration

High-speed roll forming requires real-time motion coordination.

3. Encoder Accuracy Engineering

Length accuracy depends on encoder resolution and strip behavior.

3.1 Encoder Resolution Calculation

Assume:

Measuring wheel circumference = 250 mm
Encoder resolution = 2,000 pulses/rev

Distance per pulse:

250 / 2000 = 0.125 mm per pulse

If target tolerance = ±1 mm

1 mm = 8 pulses

This is acceptable — but only if no slippage occurs.

3.2 Sources of Encoder Error

• • Measuring wheel slip on coated steel
• • Coil tension variation
• • Acceleration/deceleration
• • VFD response lag
• • Wheel wear changing circumference

Professional systems compensate via:

• • Tension control
• • Calibration routines
• • Servo-based measuring systems

4. High-Speed Length Control Modeling

Assume:

Speed = 50 m/min
Sheet length = 3 m

Pieces per minute:

50 / 3 ≈ 16.67

Time per piece:

60 / 16.67 ≈ 3.6 seconds

Cut must be synchronized within ±1 mm at 50 m/min.

Any delay in shear trigger causes length drift.

Control must predict deceleration and shear cycle time.

5. Gearbox Selection Deep Dive

Gearbox is the torque amplifier and stabilizer.

Improper gearbox selection causes:

• • Torque ripple
• • Backlash instability
• • Vibration
• • Premature gear wear

5.1 Gear Ratio Selection

Motor speed: 1,450 RPM
Desired shaft speed: 30 RPM

Required reduction:

1450 / 30 ≈ 48:1

This can be achieved with:

• • Helical gearbox
• • Multi-stage planetary
• • Bevel-helical combination

5.2 Gear Tooth Stress Modeling

Gear tooth bending stress (Lewis formula simplified):

σ=FtbmY\sigma = \frac{F_t}{b m Y}σ=bmYFt

• Where:
• • F_t = tangential force
• • b = face width
• • m = module
• • Y = geometry factor

High torque requires larger module and wider face width.

For heavy PBR torque 6,750 N·m:

Tangential force at 100 mm pitch diameter:

Ft=2TdF_t = \frac{2T}{d}Ft=d2T

=2×6,7500.1= \frac{2 × 6,750}{0.1}=0.12×6,750

=135,000N= 135,000 N=135,000N

Gear teeth must withstand this continuously.

Undersized gearbox leads to:

• • Tooth pitting
• • Surface fatigue
• • Backlash increase
• • Torque oscillation

6. Dynamic Torsional Vibration Modeling

Torque in roll forming is not constant.

It fluctuates due to:

• • Bending cycles
• • Material thickness variation
• • Punching impact
• • Shear impact
• • Gear backlash

Torsional oscillation equation:

Jθ′′+Cθ′+Kθ=T(t)J \theta'' + C \theta' + K \theta = T(t)Jθ′′+Cθ′+Kθ=T(t)

• Where:
• • J = rotational inertia
• • C = damping
• • K = torsional stiffness
• • T(t) = time varying torque

If excitation frequency matches natural frequency:

Resonance occurs.

Resonant torsional vibration causes:

• • Gear noise
• • Shaft fatigue
• • Bearing wear
• • Dimensional instability

Increasing shaft diameter increases torsional stiffness.

Helical gears reduce torque ripple compared to spur gears.

7. Lubrication System Engineering

Lubrication affects:

• • Gear wear
• • Bearing life
• • Temperature control
• • Efficiency

7.1 Gearbox Lubrication Types

• • Splash lubrication
• • Forced oil circulation
• • Oil bath
• • Grease-packed (small gearboxes only)

Heavy industrial roll formers require oil circulation.

7.2 Lubrication Film Thickness

Elastohydrodynamic lubrication (EHL) film thickness:

h∝(ηUE′)0.7h \propto \left(\frac{\eta U}{E'}\right)^{0.7}h∝(E′ηU)0.7

• Where:
• • η = oil viscosity
• • U = rolling speed
• • E' = reduced modulus

Higher speed increases film thickness.

Too thin film → metal contact → pitting.

Too thick oil → energy loss and heat.

7.3 Bearing Lubrication

Grease vs oil:

• Grease:
• • Simple
• • Limited heat removal

• Oil:
• • Better cooling
• • Required at high speed

In high-speed lines (>50 m/min):

Oil lubrication preferred.

8. Electrical Noise & EMI Control

High power VFD systems generate:

• • Harmonics
• • Electrical noise
• • Encoder signal interference

Solutions:

• • Shielded encoder cables
• • Grounding strategy
• • Separate power and signal conduits
• • Line reactors
• • Harmonic filters

Poor grounding causes:

• • Length drift
• • PLC faults
• • Random stops

9. Thermal Management

Electrical cabinets generate heat.

Components sensitive to temperature:

• • PLC modules
• • VFD
• • Servo drives
• • Safety relays

Cabinet temperature target:

< 40°C internal

Cooling methods:

• • Forced air
• • Heat exchangers
• • Air conditioning

Overheating reduces component lifespan.

10. Safety Circuit Engineering

Modern lines require:

• • Dual-channel E-stop
• • Safety relays
• • Interlocked guarding
• • Overload protection
• • Emergency brake systems

Safety circuits must be separate from standard PLC logic.

Compliance depends on region (CE, UL).

11. Integrated System Stability Example (PBR Line)

For 36” PBR:

• • 30 kW motor
• • 50:1 gearbox
• • 80 mm shafts
• • Oil-lubricated gearbox
• • 2,000 pulse encoder
• • PLC scan time < 5 ms
• • Shielded wiring

This configuration allows:

• • Stable 35–40 m/min
• • ±1 mm length accuracy
• • Reduced torsional vibration
• • 10+ year gearbox life

Final Engineering Summary

Electrical and control systems determine:

• • Length precision
• • Speed stability
• • Torque smoothness
• • Gearbox lifespan
• • Bearing life
• • Safety compliance

Mechanical strength alone is not enough.

A roll forming machine is a synchronized electromechanical system.

When gearbox design, torsional stiffness, encoder resolution, PLC logic, and lubrication engineering align — production becomes stable and predictable.

When they do not — vibration, drift, and wear accelerate.