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Friction Factors

Some of the factors affecting the friction between surfaces are listed below:

    Dry surfaces

  1. For low surface pressures the friction is directly proportional to the pressure between the surfaces.  As the pressure rises the friction factor rises slighty. At very high pressure the friction factor then quickly increases to seizing
  2. For low surface pressures the coefficient of friction is independent of surface area.
  3. At low velocities the friction is independent of the relative surface velocity.  At higher velocities the coefficent of friction decreases.

    Well lubricated surfaces

  1. The friction resistance is almost independent of the specific pressure between the surfaces.
  2. At low pressures the friction varies directly as the relative surface speed
  3. At high pressures the friction is high at low velocities falling as the velocity increases to a minimum at about 0.6m/s.  The friction then rises in proportion the velocity 2.
  4. The friction is not so dependent of the surface materials
  5. The friction is related to the temperature which affects the viscosity of the lubricant

Rolling Friction

When a cylinder rolls on a surface the force resisting motion is termed rolling friction.  Rolling friction is generally considerably less than sliding friction.    If W is the weight of the cylinder, or the vertical force, and R is radius of the cylinder and F is the force required to overcome the rolling friction then.

F = f x W/R

.. f is the coefficient of rolling friction and has the same unit of length as the radius R.

    Typical values for f are

  • Steel on Steel f = .005m
  • Wood on Wood f = .06m
  • Iron on iron f = .02m
  • Iron on granite f = .085m
  • Iron on Wood f = .22m

Coefficient of Static Friction


Extreme care is needed in using friction coeffiecients and independent references should be used. The links below the table provide further information.
MATERIAL 1 MATERIAL 2 Coefficient Of Friction
DRY LUBRICATED
Aluminum Aluminum 1.35 0.3
Aluminum Bronze Steel 0.45 -
Brake Material Cast Iron 0.4 -
Brake Material Cast Iron (Wet) 0.2 -
Brass Steel 0.35 0.19
Brick Wood 0.6 -
Bronze Steel - 0.16
Cadmium Cadmium 0.5 0.05
Cast Iron Steel 0.4 0.21
Chromium Chromium 0.41 0.34
Copper Copper 1.0 0.08
Copper-Lead Alloy Steel 0.22 -
Diamond Diamond 0.1 0.05 - 0.1
Diamond Metal 0.1 -0.15 0.1
Glass Glass 0.9 - 1.0 0.1 - 0.6
Glass Metal 0.5 - 0.7 0.2 - 0.3
Graphite Graphite 0.1 0.1
Graphite Steel 0.1 0.1
Graphite (In vacuum) Graphite (In vacuum) 0.5 - 0.8 -
Hard Carbon Hard Carbon 0.16 0.12 - 0.14
Hard Carbon Steel 0.14 0.11 - 0.14
Iron Iron 1.0 0.15 - 0.2
Leather Wood 0.3 - 0.4 -
Leather Metal(Clean) 0.6 -
Leather Metal(Wet) 0.4 -
Leather Metal(greasy) 0.2 -
Magnesium Magnesium 0.6 0.08
Nickel Nickel 0.7 0.28
Nylon Nylon 0.15 - 0.25 -
Phos Bronze Steel 0.35 -
Platinum Platinum 1.2 0.25
Plexiglas Plexiglas 0.8 0.8
Plexiglas Steel 0.4 - 0.5 0.4 - 0.5
Polystyrene Polystyrene 0.5 0.5
Polystyrene Steel 0.3 -0.35 0.3 -0.35
Polythene Steel 0.2 0.2
Saphire Saphire 0.2 0.2
Silver Silver 1.4 0.55
Sintered Bronze Steel - 0.13
Solids Rubber 1.0 - 4.0 --
Steel Steel 0.8 0.16
Teflon Steel 0.04 0.04
Teflon Teflon 0.04 0.04
Tungsten Carbide Steel 0.4 - 0.6 0.1 - 0.2
Tungsten Carbide Tungsten Carbide 0.25 - 0.25 0.12
Wood Wood(clean) 0.25 - 0.5 -
Wood Wood (Wet) 0.2 -
Wood Metals(Clean) 0.2-0.6 -
Wood Metals (Wet) 0.2 -
Wood Brick 0.6 -
Wood Concrete 0.62 -
Zinc Zinc 0.6 0.04

    Links to Friction Information

  1. Friction Joint Theory... An informative article
  2. Carbide Depot.. Friction Coefficients
  3. NORD-SKG.. Download manual with various Calculations involving sliding and rolling friction

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Last Updated 06/11/2001