Coefficient of Friction in Sheet & Roll Forming

1. Definition

In forming analysis, the coefficient of friction μ usually means the Coulomb factor relating friction shear stress to normal contact pressure: τ ≈ μ σ_n (with limits so friction does not exceed shear yield). It is a contact-system property, not a single number stamped on the steel grade.

2. Coulomb Model and Limits

Constant Coulomb friction is the industry default in many FEA decks because it is simple. Reviews and shop studies show real μ varies with contact pressure, sliding velocity, lubricant amount/type, tool and sheet roughness, strain, temperature, and coatings. Using one constant can produce large force/thinning errors in drawing simulations.

3. What Changes μ

4. Measurement Concepts

Strip drawing tests clamp a strip under known normal force and pull it at controlled speed. Friction force over normal force yields Coulomb μ. Report the pressure, speed, lubricant, and materials with every number.

5. CAE Practice

Options range from constant μ, to pressure- or velocity-dependent Coulomb enhancements, to virtual tribology packages. Constant μ remains common for early roll-form studies; critical automotive stampings increasingly need advanced models.

6. Roll Forming Specifics

• Roll–strip contact needs enough traction to feed without excessive slip.
• Dry coated strip raises marking and load; correct lube or roll polish helps.
• Changing lubricant brand mid-run can move springback and motor current.
• Do not confuse rising amps from hard coil (yield) with rising amps from dry friction—check both.

7. Boundaries

• μ ≠ viscosity (related via lubrication regime).
• μ ≠ wear coefficient.
• A textbook μ=0.1 is not a universal truth for every stand.

Buyer / Engineer FAQ

What μ should I put in a roll-forming simulation?

Start from measured or vendor-typical values for your lubricant/coating pair, then sensitivity-study. Blindly copying 0.15 from a stamping tutorial is risky.

Does higher friction always hurt?

Excess friction raises loads, marking, and heat. Some traction is required to drive strip through stands without slip.

Can I eliminate friction with more oil?

Over-oiling can cause slip, messy housekeeping, and coating issues. Optimize—do not flood blindly.

Why did CAE miss the split when μ was “standard”?

Constant μ ignores pressure/velocity dependence. Enhanced friction models often improve force and thinning predictions.

Is friction on galvanized the same as on CRC?

No. Coating chemistry and roughness change the tribological system.

How do plants measure μ?

Strip-draw / flat-die friction tests record tangential and normal forces to compute Coulomb μ under controlled pressure and speed.

Plant Practice Notes

Lead RFQ conversations with measurable acceptance criteria, not adjectives. Document whether a number comes from a mill certificate, a plant trial, or a published standard table.

When two heats of the same grade behave differently, pull certificates and process logs before rebuilding tooling. Most mystery forming issues are heat-to-heat property or lubrication shifts.

• Keep units consistent (MPa vs ksi; mm vs in).
• Record test methods when citing formability indices.
• Attach heat numbers to scrap photos and CAPA files.
• Separate design minimums from actual forming windows.
• Update SOPs when a new grade or coating family is introduced.
• Train receiving to stop on out-of-window certificates.

1. Define the decision this page supports (buy / form / inspect / redesign).
2. List the three certificate or process fields that matter most.
3. List shop symptoms that should trigger a re-read of this page.
4. Name the next specialist encyclopedia page to open.
5. File the lesson learned after each coil-related incident.

Auditors respond better to traceable certificate fields than to verbal grade nicknames. Keep EN 10204 type and heat mapping visible at receiving.

After any coil change that moves yield by a large step, expect springback and load changes even when thickness is identical.

Do not invent universal machine kW or m/min values on materials encyclopedia pages.

Extended Notes

Treat μ as a process variable, not a material constant. Record lubricant type/dilution, strip speed, roll finish, and coating when you cite a friction value in CAE or setup sheets.

Catalog Coulomb μ from textbooks rarely matches wet roll-forming contact. Re-prove load and surface marks after any oil change or dry-vs-wet switch.

• Do not blame “high μ” for every motor-load rise—check yield, thickness, and roll gap first.
• For coated outer fiber, separate friction-driven galling from paint-crack limits.
• Archive the friction assumption with the flower/CAE deck so later heats are comparable.