# Coefficient of Friction

### 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