Friction Coefficient, Dry Steel on Steel
| Value | 0.6 — |
| Status | Measured: ± 0.2 — (0.33 relative) |
| Source | Machinery's Handbook, 31st ed. / ASM Handbook, Vol. 18 (Friction & Wear) |
| Categories | Material PropertiesEngineering & Trade |
| part per trillion | 600,000,000,000 ppt |
| part per billion | 600,000,000 ppb |
| part per million | 600,000 ppm |
| per cent mille | 60,000 pcm |
| basis point | 6,000 bp |
| per mille | 600 per mille |
| percent | 60 % |
| ratio | 0.6 — |
Learning zone
This is the least reproducible number a mechanical engineer uses. Published values for dry steel on steel run from 0.4 to 0.8 — and genuinely clean, outgassed steel in vacuum can cold-weld with an apparent coefficient above 1.0. What you actually measure depends on surface finish, oxide films, adsorbed moisture, contact pressure, sliding speed and how recently the surfaces were cleaned. Two nominally identical test rigs can differ by 50 %.
Treat it as an order of magnitude, and design so that the answer is not sensitive to it. Where friction is what holds the assembly together — a slip-critical bolted connection, a friction clutch, a shrink fit — codes specify a surface condition (AISC Class A mill scale, µ = 0.30; Class B blast-cleaned, µ = 0.50) and require it to be verified, rather than trusting a handbook value. Where friction is a loss, assume the unfavourable end of the range. And remember the Amontons idealisation behind the coefficient: friction force independent of contact area and of speed is a useful approximation, not a law of nature.