(Flying Shear Physics, Blade Metallurgy & Dynamic Load Control)
Roll forming is continuous.
Cutting is instantaneous.
That transition is where most instability is introduced.
At the moment of cut:
• Strip tension changes
• Drive torque spikes
• Frame experiences shock load
• Hydraulic pressure peaks
• Torsional oscillation increases
Poorly engineered cutting systems cause:
• Length inaccuracy
• Profile deformation
• Gear wear
• Bearing fatigue
• Frame micro-movement
• Noise spikes
This section explains the physics and control behind cutting systems.
Hydraulic systems in roll forming power:
• Flying shears
• Stop-cut shears
• Punch presses
• Notching systems
A hydraulic system must provide:
• Adequate force
• Controlled speed
• Repeatable stroke
• Pressure stability
• Thermal control
Hydraulic power:
P=p×QP = p × QP=p×Q
Where:
• p = pressure (Pa)
• Q = flow rate (m³/s)
Power in kW:
P(kW)=p(bar)×Q(L/min)600P(kW) = \frac{p(bar) × Q(L/min)}{600}P(kW)=600p(bar)×Q(L/min)
Example:
Pressure = 180 bar
Flow = 40 L/min
P=180×40600=12kWP = \frac{180 × 40}{600} = 12 kWP=600180×40=12kW
Hydraulic motor must exceed this requirement.
Cutting force estimation:
F=τ×AF = \tau × AF=τ×A
Where:
• τ\tauτ = shear strength
• A = shear area
For mild steel:
Shear strength ≈ 0.8 × yield strength
Example PBR:
Yield = 350 MPa
Shear strength ≈ 280 MPa
Material thickness = 0.75 mm
Profile width = 914 mm
Shear area:
A=0.00075×0.914=0.0006855m2A = 0.00075 × 0.914 = 0.0006855 m²A=0.00075×0.914=0.0006855m2
Force:
F=280,000,000×0.0006855F = 280,000,000 × 0.0006855F=280,000,000×0.0006855
F≈191,940N≈192kNF ≈ 191,940 N ≈ 192 kNF≈191,940N≈192kN
Hydraulic cylinder must exceed 200 kN with safety margin.
Process:
• Line stops
• Shear actuates
• Line restarts
Advantages:
• Simpler
• Lower cost
Disadvantages:
• Speed limitation
• Profile distortion at restart
• Production inefficiency
Process:
• Shear moves with strip
• Synchronizes speed
• Cuts while moving
• Returns
Advantages:
• Continuous production
• Higher speed
• No stop marks
Disadvantages:
• Complex motion control
• Higher cost
• Servo synchronization required
Flying shear must:
Accelerate to strip speed
Match velocity
Perform cut
Decelerate
Return
Motion equation:
v(t)=atv(t) = a tv(t)=at
If strip speed = 40 m/min = 0.666 m/s
If shear acceleration = 2 m/s²
Time to reach speed:
t=0.6662=0.333st = \frac{0.666}{2} = 0.333 st=20.666=0.333s
Total cut window often < 1 second.
Control must be extremely precise.
Cutting introduces shock load into frame.
Dynamic load multiplier:
Fdynamic=Fstatic×KdF_{dynamic} = F_{static} × K_dFdynamic=Fstatic×Kd
Where KdK_dKd often 1.3–1.8 depending on blade geometry.
For 192 kN static:
Worst case dynamic:
192×1.5≈288kN192 × 1.5 ≈ 288 kN192×1.5≈288kN
Frame and shafts must absorb this.
Shear blades operate under:
• High compressive stress
• Impact loading
• Abrasion
• Micro-chipping risk
Common blade materials:
• D2
• H13
• SKD11
• Tungsten carbide inserts (high production lines)
Roofing:
58–60 HRC
Heavy deck:
60–62 HRC
Too hard:
• Brittle fracture
Too soft:
• Rapid edge rounding
Proper blade clearance:
Clearance≈5–10Clearance ≈ 5–10% \text{ of material thickness}Clearance≈5–10
For 0.75 mm:
Clearance = 0.037–0.075 mm
Too tight:
• Burr
• Excess load
Too loose:
• Tearing
• Distorted edge
Advantages:
• Robust
• Cost effective
• Low maintenance
Disadvantages:
• Slower torque response
• Less precise position control
• Speed drift under rapid load change
Best for:
• Medium-speed lines
• Stop-cut systems
Advantages:
• Instant torque response
• High precision speed matching
• Better flying shear sync
• Reduced torsional oscillation
Disadvantages:
• Higher cost
• More complex control
• Requires tuning
Best for:
• Flying shear
• High-speed stud lines
• Punch integration
Induction motor torque response delay:
50–150 ms
Servo motor response:
< 10 ms
At 40 m/min:
In 100 ms strip travels:
0.0666 m = 66 mm
That difference directly affects cut accuracy.
Mechanical systems have natural frequencies.
Natural torsional frequency:
fn=12πKJf_n = \frac{1}{2\pi} \sqrt{\frac{K}{J}}fn=2π1JK
Where:
• K = torsional stiffness
• J = inertia
If motor excitation frequency matches fnf_nfn:
Resonance occurs.
Effects:
• Gear noise
• Shaft fatigue
• Dimensional fluctuation
Increasing shaft diameter increases K → raises natural frequency → reduces resonance risk.
Helical gears reduce harmonic excitation vs spur gears.
If natural frequency = 22 Hz
Motor electrical frequency at certain RPM = 22 Hz
Resonance risk occurs.
Engineering solution:
• Change gear ratio
• Increase shaft stiffness
• Add damping
• Adjust motor ramp profile
Hydraulic oil must:
• Maintain film strength
• Resist oxidation
• Dissipate heat
• Prevent cavitation
| Operating Temperature | Recommended ISO Grade |
|---|---|
| 20–40°C | ISO 32 |
| 40–60°C | ISO 46 |
| 60–80°C | ISO 68 |
| Heavy load / hot climate | ISO 68–100 |
Too low viscosity:
• Film breakdown
• Wear
Too high viscosity:
• Pump inefficiency
• Heat buildup
Heat generated from:
• Pressure drops
• Flow restriction
• Pump inefficiency
Temperature target:
40–55°C oil temperature
Cooling methods:
• Air cooler
• Oil-water heat exchanger
Overheating causes seal degradation.
For 36” PBR at 35 m/min:
• 250 kN hydraulic cylinder
• ISO 46 oil (moderate climate)
• Servo-driven flying shear
• 30 kW main motor
• 50:1 gearbox
• Harmonic damping via helical gears
• Blade clearance 0.05 mm
This configuration ensures:
• ±1 mm length accuracy
• Minimal burr
• Controlled dynamic load
• Reduced torsional oscillation
Hydraulic and cutting systems are the highest instantaneous force zones in roll forming.
They determine:
• Length accuracy
• Dynamic stability
• Gearbox lifespan
• Frame fatigue
• Profile edge quality
Servo integration, harmonic control, blade metallurgy, and oil selection must be engineered together.
Cutting is not just slicing metal.
It is a controlled energy transfer event inside a synchronized mechanical system.
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