How Motor Size Is Calculated in Roll Forming Machines

Required torque × rotational speed.

Engineering Guide to Roll Forming Drive Power

Motor size in a roll forming machine is determined by:

Required torque × rotational speed.

The basic relationship is:

Power (kW) = Torque × Angular Velocity

But determining required torque is where the engineering lies.

Motor power must overcome:

• ✔ Forming force
• ✔ Friction losses
• ✔ Bearing drag
• ✔ Gearbox losses
• ✔ Acceleration loads

If motor is undersized:

• ✔ Speed drops under load
• ✔ Overheating occurs
• ✔ Gearbox stress increases
• ✔ Production becomes unstable

If oversized:

• ✔ Higher initial cost
• ✔ Higher energy consumption
• ✔ Unnecessary capital expense

Correct motor sizing balances performance and efficiency.

1️⃣ Step 1 – Determine Forming Force

Forming force depends on:

• ✔ Thickness
• ✔ Yield strength
• ✔ Bend angle
• ✔ Number of bends
• ✔ Profile width

Higher thickness and higher grade steel require more force.

Forming force increases roughly proportionally with:

Thickness × Yield Strength.

Example:

0.6 mm G550 requires significantly more force than 0.6 mm G250.

2️⃣ Step 2 – Convert Forming Force to Torque

Torque at each stand depends on:

• Roll radius
• Friction coefficient
• Material resistance

Simplified relationship:

Torque = Force × Roll Radius

Larger roll diameters increase required torque.

Deep profiles often use larger roll diameters.

3️⃣ Step 3 – Account for Multiple Stands

Torque demand accumulates across the machine.

Total drive torque must overcome:

Sum of forming resistance at all active stations.

In practice:

Load distribution varies along the forming sequence.

Worst-case torque condition must be used for motor sizing.

4️⃣ Step 4 – Include Mechanical Efficiency

Drive system includes:

• ✔ Gearbox
• ✔ Chains or gears
• ✔ Bearings

Efficiency losses typically:

10–20%

Motor must provide:

Forming torque ÷ mechanical efficiency.

Example:

If required shaft power = 18 kW
And system efficiency = 85%

Motor required ≈ 21 kW

5️⃣ Step 5 – Consider Line Speed

Power = Torque × Speed

Higher line speed increases power requirement.

If speed doubles:

Power requirement roughly doubles (assuming torque constant).

High-speed lines require:

Significantly larger motors.

6️⃣ Step 6 – Add Safety Factor

Engineering safety factor typically:

1.2–1.5

Accounts for:

• Material variation
• Unexpected load spikes
• Acceleration forces
• Wear over time

Motor must handle worst-case condition reliably.

7️⃣ Example Calculation (Simplified)

Profile:

• 1000 mm trapezoidal
• 0.6 mm G550
• Industrial roofing
• Target speed: 25 m/min

Estimated forming power requirement:

~16–18 kW (base forming load)

After efficiency + safety factor:

Recommended motor:

22 kW

Same profile at 40 m/min may require:

30 kW+

Speed matters significantly.

8️⃣ Thickness & Grade Comparison

Light Roofing:

0.4–0.5 mm G250–G350
Typical motor: 11–15 kW

Industrial Roofing:

0.6 mm G350–G550
Typical motor: 18.5–22 kW

Structural Deck:

0.8–1.2 mm high strength
Typical motor: 30–45 kW+

Motor size scales rapidly with thickness and grade.

9️⃣ Why High-Strength Steel Demands More Power

Higher yield strength:

• ✔ Increases bending resistance
• ✔ Increases springback compensation force
• ✔ Raises friction load

G550 steel can require nearly double the torque of G250 at same thickness.

Motor must be sized for maximum grade, not average.

🔟 Acceleration & Startup Load

Motor must also handle:

Initial acceleration of coil mass.

Large coils (8–10 tons) increase startup demand.

Frequent start-stop operation increases peak load.

VFD (Variable Frequency Drive) helps manage acceleration smoothly.

1️⃣1️⃣ Overpowered vs Underpowered Machines

Underpowered machine:

• ✔ Slows under load
• ✔ Overheats
• ✔ Trips breakers
• ✔ Wears gearbox prematurely

Overpowered machine:

• ✔ Higher purchase cost
• ✔ Slightly higher energy consumption
• ✔ Better stability

Most professional designs slightly oversize motor.

1️⃣2️⃣ Gearbox Matching

Motor torque must match gearbox rating.

Incorrect gearbox selection causes:

• ✔ Gear wear
• ✔ Noise
• ✔ Heat buildup
• ✔ Premature failure

Motor sizing must be coordinated with gearbox torque rating.

1️⃣3️⃣ Speed vs Torque Trade-Off

• High torque + low speed
• or
• High speed + moderate torque

Machine designed for thick structural deck:

Often slower but high torque.

Machine designed for thin roofing:

Higher speed, lower torque.

Motor selection depends on production strategy.

1️⃣4️⃣ Real-World Failure Scenario

Machine rated:

15 kW
Designed for 0.5 mm G350

Customer runs:

0.6 mm G550

Result:

• ✔ Motor overload
• ✔ Gearbox overheating
• ✔ Reduced line speed
• ✔ Increased maintenance

Motor was not incorrectly built.

Material specification changed.

1️⃣5️⃣ What Buyers Should Ask

Before ordering machine, confirm:

• ✔ Maximum thickness at maximum grade
• ✔ Motor power rating
• ✔ Gearbox torque rating
• ✔ Efficiency assumption
• ✔ Production speed rating
• ✔ Safety factor used

If supplier cannot explain sizing logic:

Red flag.

1️⃣6️⃣ Energy Efficiency Consideration

Modern systems use:

• ✔ VFD control
• ✔ Soft start
• ✔ High-efficiency motors (IE3 / IE4)

Proper motor selection reduces:

• Energy waste
• Heat generation
• Electrical stress

1️⃣7️⃣ Interaction with Shaft Size

Motor power must align with:

Shaft structural capacity.

Oversized motor on undersized shaft:

Causes mechanical stress.

Drive system must be balanced.

1️⃣8️⃣ Engineering Summary

Motor size is calculated from:

• ✔ Forming torque
• ✔ Material thickness
• ✔ Yield strength
• ✔ Profile geometry
• ✔ Line speed
• ✔ Drive efficiency
• ✔ Safety factor

Power requirement increases with:

• Thickness
• Grade
• Speed
• Profile depth

Motor sizing is engineering — not guesswork.

FAQ Section

Does thicker material always require larger motor?

Yes, almost always.

Does higher steel grade increase motor power?

Yes, proportionally.

Can I run thicker steel at lower speed?

Sometimes, but structural limits still apply.

Is 15 kW enough for 0.6 mm G550?

Usually not for 1000 mm wide industrial profiles.

What happens if motor is undersized?

Overheating, speed drop, and premature failure.

Is bigger motor always better?

Only if matched to shaft and gearbox capacity.