How Shaft Size Is Determined in Roll Forming Machines
Shaft diameter is not chosen randomly.
The Engineering Behind Roll Forming Shaft Diameter
Shaft diameter is not chosen randomly.
It is determined by:
- ✔ Forming torque
- ✔ Bending load
- ✔ Material strength
- ✔ Profile width
- ✔ Roll spacing
- ✔ Deflection tolerance
- ✔ Fatigue life
Undersized shafts cause:
-
Profile inconsistency
-
Roll misalignment
-
Bearing failure
-
Surface waviness
-
Premature gearbox damage
Shaft sizing is structural engineering.
1️⃣ What the Shaft Actually Does
In a roll forming machine, shafts:
- ✔ Carry roll tooling
- ✔ Transmit torque
- ✔ Resist bending force
- ✔ Maintain alignment
Each forming station applies:
Vertical forming force + rotational torque.
Shaft must resist both simultaneously.
2️⃣ Two Primary Design Forces
Shaft design considers:
1. Torsional Stress (Torque)
From motor transmitting rotational force.
2. Bending Stress
From roll pressure deforming steel.
Both must stay below allowable stress limits.
3️⃣ Torsional Stress Calculation (Simplified)
Torque (T) depends on:
- Material yield strength
- Thickness
- Profile width
- Number of bends
Basic torsion formula:
τ = T × r / J
Where:
- τ = shear stress
- T = torque
- r = shaft radius
- J = polar moment of inertia
Larger shaft diameter increases J significantly.
Small diameter = large stress increase.
4️⃣ Bending Deflection Is Often the Real Limiting Factor
Even if torsional stress is acceptable:
Excessive bending deflection causes:
- ✔ Roll gap inconsistency
- ✔ Uneven profile thickness
- ✔ Oil canning
- ✔ Dimensional variation
Deflection formula (simplified beam theory):
Deflection ∝ Load × Length³ / (E × I)
Where:
I (moment of inertia) increases rapidly with diameter⁴.
This means:
Small increase in shaft diameter drastically reduces deflection.
5️⃣ Why Diameter Matters More Than You Think
Example comparison:
- 70 mm shaft
- vs
- 80 mm shaft
Moment of inertia increase is significant because:
I ∝ D⁴
Even 10 mm increase greatly improves stiffness.
That’s why serious industrial machines use larger shafts.
6️⃣ What Determines Required Shaft Diameter
1. Material Thickness
Thicker steel increases forming force.
2. Yield Strength
G550 requires much more pressure than G250.
3. Profile Width
Wider sheet increases total load across rolls.
4. Rib Height
Deep profiles create higher bending forces.
5. Station Spacing
Longer span between bearings increases deflection.
6. Production Speed
Higher speed increases dynamic load.
All of these influence shaft size.
7️⃣ Typical Shaft Diameter Ranges
Light Roofing (0.3–0.5 mm, G250–G350):
60–70 mm
Industrial Roofing (0.6 mm, G350–G550):
75–90 mm
Structural Deck (0.8–1.2 mm):
90–120 mm+
These are general engineering ranges, not marketing numbers.
8️⃣ Undersized Shaft Symptoms
Common signs of insufficient shaft size:
- ✔ Oil canning increases under load
- ✔ Panel width fluctuates
- ✔ Roll marks appear
- ✔ Bearings wear quickly
- ✔ Machine vibrates at higher thickness
Often blamed on “material quality” — but actually structural deflection.
9️⃣ Bearing Selection Interaction
Larger shafts allow:
Larger bearings.
Larger bearings:
- ✔ Distribute load better
- ✔ Increase lifespan
- ✔ Reduce heat buildup
Shaft diameter directly affects bearing capacity.
🔟 Profile Complexity & Shaft Size
Complex profiles with:
- ✔ Sharp bends
- ✔ Return lips
- ✔ Deep ribs
- ✔ Standing seam locks
Require more forming pressure.
More pressure → larger shaft required.
Geometry drives load.
1️⃣1️⃣ Why Cheap Machines Use Smaller Shafts
To reduce cost:
- Less material
- Smaller bearings
- Lower machining cost
But this leads to:
- Higher long-term maintenance
- Lower material range
- Reduced machine life
Shaft diameter is often where machines are under-engineered.
1️⃣2️⃣ Dynamic vs Static Load
At higher speed:
Dynamic load increases.
Repeated load cycles cause:
Fatigue stress.
Shaft must be designed for:
Long-term fatigue resistance — not just peak load.
1️⃣3️⃣ Material of the Shaft
Common materials:
- Medium carbon steel
- Alloy steel
- Hardened & ground surfaces
Higher quality shafts improve:
- Wear resistance
- Fatigue life
- Dimensional stability
Diameter is not the only factor — material matters too.
1️⃣4️⃣ Deflection Tolerance Requirements
Architectural profiles:
Require tight tolerance.
Small deflection causes visible panel waviness.
Industrial roofing may tolerate slightly more variation.
Precision requirement influences shaft sizing.
1️⃣5️⃣ Practical Engineering Example
Profile:
- 35 mm trapezoidal
- 1000 mm width
- 0.6 mm G550
Forming load high.
Recommended shaft:
80–90 mm range.
If 70 mm shaft used:
- Higher deflection
- Greater roll misalignment
- Potential bearing overload
1️⃣6️⃣ Interaction with Frame Design
Even large shaft in weak frame:
Will not solve deflection.
Frame stiffness + shaft stiffness must match.
Machine is a structural system.
1️⃣7️⃣ How Buyers Should Evaluate Shaft Size
When reviewing machine proposal, ask:
- ✔ Shaft diameter?
- ✔ Maximum thickness at G550?
- ✔ Bearing size?
- ✔ Distance between bearing supports?
- ✔ Frame type?
If shaft seems small for thickness and grade:
Investigate further.
1️⃣8️⃣ Engineering Summary
Shaft size is determined by:
- ✔ Torsional stress
- ✔ Bending deflection
- ✔ Material thickness
- ✔ Yield strength
- ✔ Profile width
- ✔ Geometry complexity
- ✔ Speed
Because stiffness increases with diameter⁴:
Even small diameter increase significantly improves performance.
Shaft size is one of the most critical structural decisions in roll forming machine design.
FAQ Section
Is shaft diameter more important than motor power?
Often yes — structural stiffness controls profile consistency.
Can small shafts run thick material?
Temporarily maybe — long-term reliability suffers.
Why do some machines advertise only thickness?
Because grade and geometry are often ignored.
Does shaft material matter?
Yes — alloy and hardness affect fatigue life.
Can shaft be upgraded later?
Not easily — requires major redesign.
What is the biggest risk of undersized shafts?
Deflection causing inconsistent profile and accelerated wear.