What Is Coefficient of Friction? (And Why It Runs Your Whole Bolt-Up)
- Gregrey Majors
- 19 hours ago
- 3 min read
If you’ve ever asked yourself, “Why did that bolt hit torque but still feel wrong?”, congrats. You’ve just met the coefficient of friction.
In plain English: coefficient of friction (CoF) is a dimensionless number that tells you how much two surfaces resist sliding against each other. Lower number = slipperier. Higher number = grippier. It’s not “good” or “bad” by itself — it’s just the truth of what’s happening between materials.
Now here’s the part most people miss: in industrial bolting, friction is the silent boss. It decides whether the torque you apply becomes bolt stretch (clamp load)… or gets burned up as heat and drag.
In Bolting, You Don’t Have “One” Friction - You Have Two
When you tighten a nut/bolt assembly, friction shows up in two main places:
Thread friction (between male and female threads)
Underhead friction (under the nut face or bolt head where it’s rotating against the joint surface/washer)
ISO 16047 (a torque/clamp force test standard for fasteners) literally breaks friction out this way and shows methods for determining:
coefficient of friction between threads
coefficient of friction between bearing surfaces
and “total friction” for comparison under defined test conditions
So if you change anything, lubricant, coating, washer type, surface finish, plating, even cleanliness, your friction changes, and your clamp load changes even if you use the exact same torque value.
The Torque Lie: Most of Your Torque Doesn’t Stretch the Bolt
This is the part that makes people mad (because it’s true):
When you torque a fastener, a big chunk of that effort is consumed by friction in the threads and under the turning surface, not by creating clamp load. That’s why torque-only tightening can have large preload scatter when friction varies.
Bolted Nut Guidance: the “nut factor” (torque coefficient) is basically wrapping friction effects into one messy value, and friction is hard to estimate accurately in real life.
Translation: torque is not clamp load. Torque is just torque. Friction decides what clamp load you actually got.
Coefficient of Friction vs K-Factor (Nut Factor): Don’t Mix These Up
You’ll hear people throw around K-factor like it’s the same thing as coefficient of friction. It’s not.
Coefficient of friction (µ) describes the sliding resistance between surfaces (threads/bearing faces).
K-factor / nut factor (K) is a simplified torque-to-tension shortcut that lumps friction + geometry into one practical number. ISO 16047 even calls out that K-factor is simpler to measure but limited to specific dimensions and geometry.
K is essentially accounting for friction effects in threads + under-head/nut contact.
HEX Technology has a solid breakdown that K-factor and coefficient of friction aren’t the same thing.
So if someone says “our friction is 0.18” and someone else says “use K = 0.18”… that’s not automatically the same reality. (That confusion has ruined more bolt-ups than bad coffee.)
Why CoF Matters in the Real World (a.k.a. How Stuff Actually Fails)
Here’s what friction swings can do on the job:
1) Under-tension (too little clamp load)
You hit target torque, but friction was high → torque got eaten by drag → bolt didn’t stretch enough → joint loosens, leaks, or moves.
2) Over-tension (too much clamp load)
Friction was lower than expected (fresh lube, different coating, smoother washer) → more torque turned into stretch → you overshoot load → yield, neck, snap, or crush gasket/joint members.
3) Inconsistent preload across the pattern
If friction varies bolt-to-bolt, clamp load varies bolt-to-bolt. That’s how you get “one bolt feels fine, the next one feels sketchy”, because it is.
NASA notes torque tightening uncertainty can be significant, and friction inconsistencies are a major driver of preload scatter.
How Professionals Control CoF Instead of Guessing
If you want bolt-up results that don’t feel like gambling, this is the playbook:
Control the surfaces
Specify washer type / bearing surface
Specify coatings and plating
Keep contact faces clean and consistent
Control the lubrication (or the lack of it)
Same lube, same amount, same application method
Don’t “mix and match” lubes mid-job like it’s a playlist
Measure it when it matters
ISO 16047 describes torque/clamp force testing to determine tightening characteristics including friction coefficients (threads and bearing surfaces). That’s the grown-up version of “send it and pray.”
Use better methods than torque-only when the joint is critical
Torque + angle
Direct tension indicating methods
Load measurement (ultrasonic, load cells, etc.) NASA calls out experimental torque-preload relationships and instrumented approaches as alternatives when accuracy matters.
Old-school bolt-up is fine. But old-school with measurement is undefeated.
The Bottom Line
Coefficient of friction is the hidden variable that determines whether torque becomes clamp load or wasted effort. In bolting, you’re managing friction at the threads and under the turning surface. If you don’t control it, it controls you.
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