Coefficient of Friction
The coefficient of friction, often symbolized by the Greek letter µ, is a dimensionless scalar value which describes the ratio of the force of friction between two bodies and the force pressing them together. The coefficient of friction depends on the materials used; for example, ice on steel has a low coefficient of friction, while rubber on pavement has a high coefficient of friction. Coefficients of friction range from near zero to greater than one.
The static coefficient of friction is also called the starting coefficient of friction, and has the symbol µo.
The sliding coefficient of friction is also called the kinetic coefficient of friction, and has the symbol µ.
| Mat’l | Description | Coefficients of Friction |
|---|---|---|
| steel on steel |
static-dry sliding-dry static-greased sliding-greased |
µo = 0.12~0.60 µ = 0.08~0.50 µo = 0.12~0.35 µ = 0.04~0.25 |
| wood on steel |
static-dry sliding-dry |
µo = 0.45~0.75 µ = 0.30~0.60 |
| wood on wood |
static-dry sliding-dry |
µo = 0.40~0.75 µ = 0.30~0.50 |
| polymer belt on steel |
static-dry sliding-dry |
µo = 0.25~0.45 µ = 0.25 |
| steel on plastic |
static-dry sliding-dry |
µo = 0.25~0.45 µ = 0.18~0.35 |
Although the dynamic friction coefficient for rolling bearings varies with the type of bearings, load, lubrication, speed, and other factors; for normal operating conditions, the approximate friction coefficients for various bearing types are listed in the table below.
| Bearing Type | Coefficient x 10¯³ (µ) |
|---|---|
| Deep groove ball bearings | 1.0 ~ 1.5 |
| Angular contact ball bearings | 1.2 ~ 1.8 |
| Self-aligning ball bearings | 0.8 ~ 1.2 |
| Cylindrical roller bearings | 1.0 ~ 1.5 |
| Needle roller bearings | 2.0 ~ 3.0 |
| Tapered roller bearings | 1.7 ~ 2.5 |
| Spherical roller bearings | 2.0 ~ 2.5 |
| Thrust ball bearings | 1.0 ~ 1.5 |
| Thrust roller bearings | 2.0 ~ 3.0 |
| Data from NTN Corporation | |
| Bearing Friction | Coefficient (µ) |
|---|---|
| Roller bearing | 0.001 ~ 0.005 |
| Friction bearing | 0.080 ~ 0.100 |
| Spindle (Acme greased) | 0.050 ~ 0.080 |
| Spindle (Acme dry) | 0.100 ~ 0.180 |
| Spindle (Ball) | 0.005 ~ 0.050 |
| Roller bearing supported wheels | 0.003 |
| Friction bearing supported wheels | 0.005 |
| Side guide rollers | 0.002 |