Coefficient of Friction
Quark top tip cone
The ultra-high performance Quark spinning top uses a tungsten ball pressed into this aluminum cone and spins on a glass surface. Although glass and tungsten do not have very low coefficients of friction their hardness results in a very low contact radius which is an overriding factor.
Anyone who has examined or taken apart an electric motor has likely noticed that the shaft is often steel and the bushings are often brass or bronze. This combination of materials is often chosen because such materials slide easily together - they have a low coefficient of friction.
The coefficient of friction is the ratio of the friction force to
normal force. For example, if it takes 1 lb of force to slide a 10 lb
weight along a level surface the coefficient of friction is 0.1. The
coefficient depends on the two materials involved. Although the coefficient of friction is usually specified for material
pairs, there are other factors that are involved such as temperature,
velocity, atmosphere, shape, etc. Thermoplastics such as nylon, HDPE and PTFE are often used for low
friction bearings as their coefficient of friction declines with
increasing loads.
Static coefficient of friction is the ratio when the objects are stationary relative to each other.
Dynamic coefficient of friction is the ratio when the objects have relative movement. Usually the dynamic coefficient of friction is less than the static coefficient although in some cases such as Teflon on Teflon they are essentially the same.
The charts below gives the static coefficient of friction for several combinations of materials in a dry and clean state:
For steel against:
For other pairs of materials:
- Zinc - Cast iron - 0.85
- Copper - Cast iron - 1.05
- Copper - Glass - 0.68
- Glass - metal - 0.5 - 0.7
- Diamond - metal - 0.1 - 0.15
- Wood - metal - 0.2 - 0.6
For material to same material:
- Steel - 0.80
- Teflon - 0.04
- Glass - 0.94
- Aluminum - 1.35
- Copper - 1.0
- Iron - 1.0
- Silver - 1.4
- Zinc - 0.6
- Glass - 0.9 - 1.0
- Diamond - 0.1
- Polystyrene - 0.5
- Nylon - 0.15 - 0.25
- Wood - 0.25 - 0.5
- BAM (made of boron, aluminum and magnesium) - 0.02
Reducing friction
There are many method of reducing friction. For example, wheels and ball bearings change sliding friction to rolling friction which dissipates much less energy.
Lubricants, such as oil, water or grease, placed between surfaces, can reduce the coefficient of friction substantially.
