Factors affecting the friction
between surfaces
Dry surfaces
- For low surface pressures the friction is directly proportional to
the pressure between the surfaces. As the pressure rises the
friction factor rises slightly. At very high pressure the friction
factor then quickly increases to seizing
- For low surface pressures the coefficient of friction is independent
of surface area.
- At low velocities the friction is independent of the relative
surface velocity. At higher velocities the coefficent of friction
decreases.
Well lubricated surfaces
- The friction resistance is almost independent of the specific
pressure between the surfaces.
- At low pressures the friction varies directly as the relative
surface speed
- 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.
- The friction is not so dependent of the surface materials
- The friction is related to the temperature which affects the
viscosity of the lubricant
Surface
Friction Notes
Static Coefficient of Friction
The static
friction coefficient (μ) between two solid surfaces is
defined as the ratio of the tangential force (F) required to produce
sliding divided by the normal force between the surfaces (N)
μ = F /N
For a horizontal surface the horizontal force (F) to move a solid
resting on a flat surface
F= μ x mass of solid x g. If a body rests
on an incline plane the body is prevented from sliding down because of the
frictional resistance. If the angle of the plane is increased
there will be an angle at which the body begins to slide down the
plane. This is the angle of repose and the tangent of this angle is
the same as the coefficient of friction. .
Sliding Coefficient of Friction
When the
tangential force F overcomes the frictional force between two surfaces
then the surfaces begins to slide relative to each other. In
the case of a body resting on a flat surface the body starts to move.
The sliding frictional resistance is normally different to
the static frictional resistance. The coefficient of sliding
friction is expressed using the same formula as the static coefficient and
is generally lower than the static coefficient of friction..
Friction Coefficients
A table
below shows approximate friction coefficients for various materials.
All values are approximate and are only suitable for guidance
only. The sliding/lubricated values must be used with
extreme care. The only way to determine the accurate
coefficient of friction between two materials is to conduct
experiments.
Coefficients of friction are sensitive to
atmospheric dust and humidity, oxide films, surface finish, velocity of
sliding, temperature, vibration, and extent of
contamination. In many cases the degree of contamination is
perhaps the most important single variable.. Link Table
of Coefficients of Friction
The friction values provided are obtained by different test
methods under different ambient conditions. This factor can
also affect the results. Link Test
Methods
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 converted to force, or the force between the cylinder and the
flat surface, 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 -in the example below m (metres)
Typical values for f are listed below Note: Values for rolling
friction from various sources are not consistent and the following values
should only be used for approximate calculations.
- Steel on Steel f = 0,0005m
- Wood on Steel f = 0,0012m
- Wood on Wood f = 0,0015m
- Iron on iron f = 0,00051m
- Iron on granite f = 0,0021m
- Iron on Wood f = 0,0056m
- Polymer of steel f = 0,002m
- Hardrubber on Steel f = 0,0077m
- Hardrubber on Concrete f = 0,01 -0,02m
- Rubber on Concrete f = 0,015 -0,035m
Plain Bearing Friction
factors
For values of rolling bearing friction Plain
Bearing Friction Values
Rolling Bearing Friction
For
values of rolling bearing friction Rolling
Bearing Friction Values
Clutch - Brake Friction
Values
The coefficient of friction value is important in the
design and brakes and clutches. Various values are
provided on the following linked page Clutch/Brake
Materials
Friction coefficient Bolted
Joints
The coefficient of friction is required in calculating
tightening torques and resulting bolt tensile forces and stress and in
calculating the resulting friction between the connected
surfaces. Below are provided a small number of values showing
approximate values of friction coefficients to be used for steel screw
fastened connections. The values are only representative
values and should be confirmed against other sources of information and
preferably testing.
Coefficient of Friction for screw threads
Female Thread -Nut or Tapped Hole in
steel(untreated) |
Male screw |
Friction Coefficient (Dry) |
Friction Coefficient (lub) |
Untreated Steel |
0,12 - 0,18 |
0,10 - 0,17 |
Phosphated Steel |
0,12 - 0,18 |
0,10 - 0,17 |
Cadmium Plated Steel |
0,09 - 0,14 |
0,08 -0,23 |
Galvanised steel |
0,14 - 0,23 |
0,12 - 0,2 |
Degreased steel |
0,19 - 0,25 |
|
Female Thread -Nut or Tapped Hole in
steel(Galvanised) |
Male screw |
Friction Coeffient (Dry) |
Friction Coefficient(Lub.) |
Untreated Steel |
0,14 - 0,2 |
0,12 - 0,18 |
Phosphated Steel |
0,14 - 0,2 |
0,12 - 0,18 |
Cadmium Plated Steel |
0,1 - 0,16 |
0,09 - 0,15 |
Galvanised steel |
0,14 - 0,25 |
0,12 - 0,2 |
Degreased steel |
0,19 - 0,25 |
|
Coefficient of Friction Nut/Bolt Face against Clamped surface
Clamped Surface = Steel |
Bolt/Nut Mat'l |
Friction Coeffient (Dry) |
Friction Coefficient(Lub.) |
Untreated Steel |
0,10 - 0,18 |
0,08 - 0.15 |
Phosphated Steel |
0,10 - 0,18 |
0,08 - 0,15 |
Galvanised steel |
0,10 - 0,2 |
0,09 - 0,18 |
Clamped Surface -Galvanised Steel |
Bolt/Nut Mat'l |
Friction Coefficient (Dry) |
Friction Coefficient (lub) |
Untreated Steel |
0,10 - 0,18 |
0,08 - 0,15 |
Phosphated Steel |
0,10 - 0,18 |
0,08 - 0,15 |
Galvanised steel |
0,16 - 0,22 |
0,09 - 0,18 |
Coefficient of friction between surfaces clamped by bolts
/screws. These values allow calculation of the shear force necessary to
cause slip between surfaces when clamped by bolts.
Contact Surfaces |
slip coefficient |
Steel On Steel- No treatment |
0,15- 0,25 |
Steel On Cast Iron- No treatment |
0,18 - 0,3 |
Steel On Steel- Machined (Degreased) |
0,12- 0,18 |
Steel On Cast Iron- Machined (Degreased) |
0,15 - 0,25 |
Grit -Sandblasted surfaces |
0,48 - 0,55 |
Friction Factors for Power
Screws
The following factors are typical friction factors for
power screw torque and efficiency calculations..
1) Screw Thread
Friction values (μs) (Friction factors apply
mainly for screw thread friction (μs) - can be
applied to collar friction(μc)
Screw Material |
Nut Material |
Steel |
Brass |
Bronze |
Cast Iron |
Steel(Dry) |
0,15-0,25 |
0,15-0,23 |
0,15-0,19 |
0,15-0,25 |
Steel (Lubricated) |
0,11-0,17 |
0,10-0,16 |
0,10-0,15 |
0,11-0,17 |
Bronze (Lubricated) |
0,08-0,12 |
0,04-0,06 |
- |
0,06-0,09 | 2)
Thrust collar Friction values (μc)
Surface Combinations |
Moving |
Starting |
Soft Steel on Cast Iron |
0,12 |
0,17 |
Hard Steel on Cast Iron |
0,09 |
0,15 |
Soft Steel on Bronze |
0,08 |
0,10 |
Hard Steel on Bronze |
0,06 |
0,08 |
Press Fit Mechanical Joints
In
mechanical engineering rotary motion can be transferred by mechanical
connections between a shaft and hub using only a tight fit.
Methods of achieving this type of connection include the engineered
interference fit, the taper lock bush and hydraulic fit bush.
These keyless shaft/hub connections all transfer torque by
friction.
The coefficient of friction used for designing these
types of connections is dependent on the interface pressure, materials,
surface condition, surface coatings etc. The coefficient of
friction is also dependent on the method of installation. A
different value result if the shaft is forced into the hub (force fit)
compared to the value if the assemble is completed by heating the hub or
freezing the shaft prior to assembly (shrink fit)...
Various
values of relevant coefficients of friction are provided below;
- Steel Hub , Steel Shaft unlubricated - force fit ...C. of Friction =
0,07 to 0,16
- Steel Hub , Steel Shaft greased - force fit ...C. of Friction = 0,05
to 0,12
- Steel Hub , Steel Shaft unlubricated - Shrink fit ...C. of Friction
= 0,15 to 0,25
- Steel Hub , Steel Shaft greased - Shrink fit ...C. of Friction =
0,08 to 0,16
The manufacturers of the proprietary keyless
hub/shaft systems indicate that their products are based on a coefficient
of friction of 0,12 for lightly oiled connections and 0,15 for dry
assemblies. These companies can provide surface coating
fluids containing particles to increase theie coefficient of friction to
0,25 to 0,3. (ref links 1 & 2 below)
The American Gear
Manufactures Association (AGMA) recommends a value of between 0,12-0,15
for hydraulically expanded hubs and 0,15-0,20 for shrink or press fit
hubs.
Testing Methods
There are a number of
test methods for coefficient of frictions as some of which are listed
below
- Flat block pressed against a OD of rotating ring (FOR)
- Flat block against another flat block (FOF)
- Flat block sliding down an inclined runway(IS)
- Pin pressed against a OD of rotating ring (POR
- Reciprocating loaded spherical end pin pressed on a flat
surface(RSOF)
It is clear that the different test methods
provide different friction results..
Coefficient of Friction
Extreme care is needed in using friction coefficients and
additional independent references should be used. For any
specific application the ideal method of determining the coefficient of
friction is by trials. A short table is included above the
main table to illustrate how the coefficient of friction is affected by
surface films. When a metal surface is perfectly clean in a
vacuum , the friction is much higher than the normal accepted value and
seizure can easily occur.
......The links below the
tables provide further information.
Effect of oxide film etc on coefficient of
static friction |
Material |
Clean Dry |
Thick Oxide Film |
Sulfide Film |
Steel-Steel |
0,78 |
0,27 |
0,39 |
Copper-Copper |
1,21 |
0,76 |
0,74 |
The level of uncertainty of the
information below is indicated by using steel on steel as an
example. Various reference sources provide values similar to
the values below.(0,74 Static- 0,42 sliding) Gieck( 7th ed)
provides values of (0,15...0,30 Static - 0,10...0,30 sliding). Concise
Metals Data Handbook by J.R. Davis (table 14,1) includes values (0,31
static -0,23 sliding - for steel 1032? on steel 1032?).. The same table
includes a value for mild steel on mild steel of 0,62 sliding.
Material 1 |
Material 2 |
Coefficient Of Friction |
Test method |
DRY |
Greasy |
Static |
Sliding |
Static |
Sliding |
Aluminum |
Aluminum |
1,05-1,35 |
1,4 |
0,3 |
|
|
Aluminum |
Mild Steel |
0,61 |
0,47 |
|
|
|
Brake Material |
Cast Iron |
0,4 |
|
|
|
|
Brake Material |
Cast Iron (Wet) |
0,2 |
|
|
|
|
Brass |
Cast Iron |
|
0,3 |
|
|
|
Brick |
Wood |
0,6 |
|
|
|
|
Bronze |
Cast Iron |
|
0,22 |
|
|
|
Bronze |
Steel |
|
|
0,16 |
|
|
Cadmium |
Cadmium |
0,5 |
|
0,05 |
|
|
Cadmium |
Mild Steel |
|
0,46 |
|
|
|
Cast Iron |
Cast Iron |
1,1 |
0,15 |
|
0,07 |
|
Cast Iron |
Oak |
|
0,49 |
|
0,075 |
|
Chromium |
Chromium |
0,41 |
|
0,34 |
|
|
Copper |
Cast Iron |
1,05 |
0,29 |
|
|
|
Copper |
Copper |
1,0 |
|
0,08 |
|
|
Copper |
Mild Steel |
0,53 |
0,36 |
|
0,18 |
|
Copper |
Steel |
|
0,8 |
|
|
SPOF |
Copper |
Steel (304 stainless) |
0,23 |
0,21 |
|
|
FOF |
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 |
0,4 |
0,1 - 0,6 |
0,09-0,12 |
|
Glass |
Metal |
0,5 - 0,7 |
|
0,2 - 0,3 |
|
|
Glass |
Nickel |
0,78 |
0,56 |
|
|
|
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 |
|
|
Lead |
Cast Iron |
|
0,43 |
|
|
|
Lead |
Steel |
|
1,4 |
|
|
SPOF |
Leather |
Wood |
0,3 - 0,4 |
|
|
|
|
Leather |
Metal(Clean) |
0,6 |
|
0,2 |
|
|
Leather |
Metal(Wet) |
0,4 |
|
|
|
|
Leather |
Oak (Parallel grain) |
0,61 |
0,52 |
|
|
|
Magnesium |
Magnesium |
0,6 |
|
0,08 |
|
|
Nickel |
Nickel |
0,7-1,1 |
0,53 |
0,28 |
0,12 |
|
Nickel |
Mild Steel |
|
0,64; |
|
0,178 |
|
Nylon |
Nylon |
0,15 - 0,25 |
|
|
|
|
Oak |
Oak (parallel grain) |
0,62 |
0,48 |
|
|
|
Oak |
Oak (cross grain) |
0,54 |
0,32 |
|
0,072 |
|
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 |
|
|
Rubber |
Asphalt (Dry) |
|
0,5-0,8 |
|
|
|
Rubber |
Asphalt (Wet) |
|
0,25-0,0,75 |
|
|
|
Rubber |
Concrete (Dry) |
|
0,6-0,85 |
|
|
|
Rubber |
Concrete (Wet) |
|
0,45-0,75 |
|
|
|
Saphire |
Saphire |
0,2 |
|
0,2 |
|
|
Silver |
Silver |
1,4 |
|
0,55 |
|
|
Sintered Bronze |
Steel |
- |
|
0,13 |
|
|
Solids |
Rubber |
1,0 - 4,0 |
|
-- |
|
|
Steel |
Aluminium Bros |
0,45 |
|
|
|
|
Steel |
Brass |
0,35 |
|
0,19 |
|
|
Steel(Mild) |
Brass |
0,51 |
0,44 |
|
|
|
Steel (Mild) |
Cast Iron |
|
0,23 |
0,183 |
0,133 |
|
Steel |
Cast Iron |
0,4 |
|
0,21 |
|
|
Steel |
Copper Lead Alloy |
0,22 |
|
0,16 |
0,145 |
|
Steel (Hard) |
Graphite |
0,21 |
|
0,09 |
|
|
Steel |
Graphite |
0,1 |
|
0,1 |
|
|
Steel (Mild) |
Lead |
0,95 |
0,95 |
0,5 |
0,3 |
|
Steel (Mild) |
Phos. Bros |
|
0,34 |
|
0,173 |
|
Steel |
Phos Bros |
0,35 |
|
|
|
|
Steel(Hard) |
Polythened |
0,2 |
|
0,2 |
|
|
Steel(Hard) |
Polystyrene |
0,3-0,35 |
|
0,3-0,35 |
|
|
Steel (Mild) |
Steel (Mild) |
0,74 |
0,57 |
|
0,09-0,19 |
|
Steel (Mild) |
Steel (Mild) |
- |
0,62 |
|
|
FOR |
Steel(Hard) |
Steel (Hard) |
0,78 |
0,42 |
0,05 -0,11 |
0,029-,12 |
|
Steel |
Zinc (Plated on steel) |
0,5 |
0,45 |
- |
- |
|
Teflon |
Steel |
0,04 |
|
0,04 |
0,04 |
|
Teflon |
Teflon |
0,04 |
|
0,04 |
0,04 |
|
Tin |
Cast Iron |
|
,32 |
|
|
|
Titanium Alloy Ti-6Al-4V(Grade 5) |
Aluminium Alloy 6061-T6 |
0,41 |
0,38 |
|
|
FOF |
Titanium Alloy Ti-6Al-4V(Grade 5) |
Titanium Alloy Ti-6Al-4V(Grade 5) |
0,36 |
0,30 |
|
|
FOF |
Titanium Alloy Ti-6Al-4V(Grade 5) |
Bronze |
0,36 |
0,27 |
|
|
FOF |
Tungsten Carbide |
Tungsten Carbide |
0,2-0,25 |
|
0,12 |
|
|
Tungsten Carbide |
Steel |
0,4 - 0,6 |
|
0,08 - 0,2 |
|
|
Tungsten Carbide |
Copper |
0,35 |
|
|
|
|
Tungsten Carbide |
Iron |
0,8 |
|
|
|
|
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 |
|
|
Zinc |
Cast Iron |
0,85 |
0,21 |
|
|
|
Material 1 |
Material 2 |
Coefficient Of Friction |
Test method |
DRY |
LUBRICATED |
Static |
Sliding |
Static |
Sliding | FOR = Flat against rotating
Cylinder, FOF = Flat against flat, POF = Pin on flat, IS = inclined
surface,SPOF Spherical end pin on flat.
Source of above values....
The values are checked against a variety of internet and literature
sources including the links below eg Link 6-Page 16. I have
referred to books including Machinerys Handbook Eighteenth edition, Kempes
Engineers Year Book 1980, Concise Metals Handbook by J.R.Davis ASM - (Good
source of referenced data) and Kurt Giecks Engineering Formulas 7th
Edition.. 1980, etc etc
Table of friction Values for elements
I provide the table below as a consistent set of values for
simple elements using the simplest of test methods. It can be seen that
values are generally different to the values in the table
above...
Friction tests in air at room temperature. (50% relative
humidity)
Fixed Surface |
Moving Block |
Friction coefficient |
|
Test Method |
|
|
Static |
Sliding |
|
Silver (Ag) |
Silver (Ag) |
0,5 |
|
Incline Plane |
|
Gold(Au) |
0,53 |
|
Incline Plane |
|
Copper(Cu) |
0,48 |
|
Incline Plane |
|
Iron(Fe) |
0,49 |
|
Incline Plane |
Aluminium(Al) |
Aluminium(Al) |
0,57 |
|
Incline Plane |
|
Titanium (Ti) |
0,54 |
|
Incline Plane |
Gold(au) |
Silver (Ag) |
0,53 |
|
Incline Plane |
|
Gold(Au) |
0,49 |
|
Incline Plane |
Cadmium(Cd) |
Cadmium(Cd) |
0,79 |
|
Incline Plane |
|
Iron(Fe) |
0,52 |
|
Incline Plane |
Cobalt(Co) |
Cobalt(Co) |
0,56 |
|
Incline Plane |
|
Chromium(Cr) |
0,41 |
|
Incline Plane |
Chromium(Cr) |
Cobalt(Co) |
0,41 |
|
Incline Plane |
|
Chromium(Cr) |
0,46 |
|
Incline Plane |
Copper(Cu) |
Cobalt(Co) |
0,44 |
|
Incline Plane |
|
Chromium(Cr) |
0,46 |
|
Incline Plane |
|
Copper(Cu) |
0,55 |
|
Incline Plane |
|
Iron(Fe) |
0,50 |
|
Incline Plane |
|
Nickel(Ni) |
0,49 |
|
Incline Plane |
|
Zinc(Zn) |
0,56 |
|
Incline Plane |
Iron(Fe) |
Cobalt(Co) |
0,41 |
|
Incline Plane |
|
Chromium(Cr) |
0,48 |
|
Incline Plane |
|
Iron(Fe) |
0,51 |
|
Incline Plane |
|
Maganese(Mg) |
0,51 |
|
Incline Plane |
|
Molybdenum(Mo) |
0,46 |
|
Incline Plane |
|
Titanium(Ti) |
0,49 |
|
Incline Plane |
|
Tungsten(W) |
0,47 |
|
Incline Plane |
|
Zinc(Zn) |
0,55 |
|
Incline Plane |
Indium(In) |
Indium(In) |
1,46 |
|
Incline Plane |
Maganese(Mg) |
Maganese(Mg) |
0,69 |
|
Incline Plane |
Molybdenum(Mo) |
Iron(Fe) |
0,46 |
|
Incline Plane |
|
Molybdenum(Mo) |
0,44 |
|
Incline Plane |
Niobium(Nb) |
Niobium(Nb) |
0,46 |
|
Incline Plane |
Nickel(Ni) |
Chromium(Cr) |
0,59 |
|
Incline Plane |
|
Nickel(Ni) |
0,50 |
|
Incline Plane |
|
Platinum(Pt) |
0,64 |
|
Incline Plane |
Lead(Pb) |
Silver (Ag) |
0,73 |
|
Incline Plane |
|
Gold(Au) |
0,61 |
|
Incline Plane |
|
Copper(Cu) |
0,55 |
|
Incline Plane |
|
Chromium(Cr) |
0,53 |
|
Incline Plane |
|
Iron(Fe) |
0,54 |
|
Incline Plane |
|
Lead(Pb) |
0,90 |
|
Incline Plane |
Platinum(Pt) |
Nickel(Ni) |
0,64 |
|
Incline Plane |
|
Platinum(Pt) |
0,55 |
|
Incline Plane |
Tin(Sn) |
Iron(Fe) |
0,55 |
|
Incline Plane |
|
Tin(Sn) |
0,74 |
|
Incline Plane |
Titanium(Ti) |
Aluminium(Al) |
0,54 |
|
Incline Plane |
|
Titanium(Ti) |
0,55 |
|
Flat Sliding |
Tungsten(W) |
Copper(Cu) |
0,41 |
|
Incline Plane |
|
Iron(Fe) |
0,47 |
|
Incline Plane |
|
Tungsten(W) |
0,51 |
|
Incline Plane |
Zinc(Zn) |
Copper(Cu) |
0,56 |
|
Incline Plane |
|
Iron(Fe) |
0,55 |
|
Incline Plane |
|
Zinc(Zn) |
0,75 |
|
Incline Plane | |