Factors Affecting the Friction Between Surfaces

7/27/2019 Factors Affecting the Friction Between Surfaces

1/12

Factors affecting the friction between surfaces

Dry surfaces

1. For low surface pressures the friction is directly proportional to the pressure between thesurfaces. As the pressure rises the friction factor rises slightly. At very high pressure the frictionfactor 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 aminimum 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

Please refer to... Surface Friction Notes

Static Coefficient of Friction

The static friction coefficient () between two solid surfaces is defined as the ratio of thetangential force (F) required to produce sliding divided by the normal force between thesurfaces (N)

= F /N

For a horizontal surface the horizontal force (F) to move a solid resting on a flat surface

F= x mass of solid x g.

If a body rests on an incline plane the body is prevented from sliding down because of thefrictional resistance. If the angle of the plane is increased there will be an angle at whichthe body begins to slide down the plane. This is the angle of repose and the tangent of thisangle 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 thesurfaces begins to slide relative to each other. In the case of a body resting on a flatsurface the body starts to move. The sliding frictional resistance is normally different tothe static frictional resistance. The coefficient of sliding friction is expressed using thesame formula as the static coefficient and is generally lower than the static coefficient of

friction..
http://www.roymech.co.uk/Useful_Tables/Tribology/class_Friction.htmlhttp://www.roymech.co.uk/Useful_Tables/Tribology/class_Friction.html


2/12

Friction Coefficients

A table below shows approximate friction coefficients for various materials. All values areapproximate and are only suitable for guidance only. The sliding/lubricated values mustbe used with extreme care. The only way to determine the accurate coefficient of frictionbetween two materials is to conduct experiments.

Coefficients of friction are sensitive to atmospheric dust and humidity, oxide films, surfacefinish, velocity of sliding, temperature, vibration, and extent of contamination. In manycases the degree of contamination is perhaps the most important single variable.. LinkTable of Coefficients of Friction

The friction values provided are obtained by different test methods under different ambientconditions. This factor can also affect the results. LinkTest Methods

Rolling Friction

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

center>F = f x W/R

f is the coefficient of rolling friction and has the same unit of length as the radius R -in theexample below m (metres)

Typical values for f are listed below

Note: Values for rolling friction from various sources are not consistent and the followingvalues 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 on steel f = 0,002m
http://www.roymech.co.uk/Useful_Tables/Tribology/co_of_frict.htm#coef%23coefhttp://www.roymech.co.uk/Useful_Tables/Tribology/co_of_frict.htm#method%23methodhttp://www.roymech.co.uk/Useful_Tables/Tribology/co_of_frict.htm#coef%23coefhttp://www.roymech.co.uk/Useful_Tables/Tribology/co_of_frict.htm#method%23method


3/12

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 frictionPlain Bearing Friction Values

Rolling Bearing Friction

For values of rolling bearing frictionRolling 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 bolttensile forces and stress and in calculating the resulting friction between the connectedsurfaces. Below are provided a small number of values showing approximate values offriction coefficients to be used for steel screw fastened connections. The values are onlyrepresentative values and should be confirmed against other sources of information andpreferably testing.

Coefficient of Friction for screw threads

Female Thread -Nut or Tapped Hole insteel(untreated)

Male screwFriction

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 PlatedSteel

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 insteel(Galvanised)

Male screwFriction Coeffient

(Dry)Friction

Coefficient(Lub.)

Untreated Steel 0,14 - 0,2 0,12 - 0,18


Cadmium PlatedSteel

0,1 - 0,16 0,09 - 0,15


Degreased steel 0,19 - 0,25

Coefficient of Friction Nut/Bolt Face against Clamped surface

Clamped Surface = Steel

Bolt/Nut Mat'lFriction Coeffient

(Dry)Friction

Coefficient(Lub.)
http://www.roymech.co.uk/Useful_Tables/Tribology/Plain_Bearing%20Friction.htmlhttp://www.roymech.co.uk/Useful_Tables/Tribology/Plain_Bearing%20Friction.htmlhttp://www.roymech.co.uk/Useful_Tables/Tribology/Bearing%20Friction.htmlhttp://www.roymech.co.uk/Useful_Tables/Tribology/Bearing%20Friction.htmlhttp://www.roymech.co.uk/Useful_Tables/Drive/Brake_Clutch_mat.htmlhttp://www.roymech.co.uk/Useful_Tables/Drive/Brake_Clutch_mat.htmlhttp://www.roymech.co.uk/Useful_Tables/Tribology/Plain_Bearing%20Friction.htmlhttp://www.roymech.co.uk/Useful_Tables/Tribology/Bearing%20Friction.htmlhttp://www.roymech.co.uk/Useful_Tables/Drive/Brake_Clutch_mat.html


4/12

Untreated Steel 0,10 - 0,18 0,08 - 0.15



Clamped Surface -Galvanised Steel

Bolt/Nut Mat'lFriction

Coefficient (Dry)Friction

Coefficient (lub)

Untreated Steel 0,10 - 0,18 0,08 - 0,15Phosphated Steel 0,10 - 0,18 0,08 - 0,15


Coefficient of friction between surfaces clamped by bolts /screws.These values allow calculation of the shear force necessary to cause slip between surfaceswhen clamped by bolts.

Contact Surfacesslip

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 efficiencycalculations..

1) Screw Thread Friction values (s)

(Friction factors apply mainly for screw thread friction (s) - can be applied to collar

friction(c)

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 (c)

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 connectionsbetween a shaft and hub using only a tight fit. Methods of achieving this type ofconnection include the engineered interference fit, the taper lock bush and hydraulic fitbush. These keyless shaft/hub connections all transfer torque by friction.

The coefficient of friction used for designing these types of connections is dependent on theinterface pressure, materials, surface condition, surface coatings etc. The coefficient offriction is also dependent on the method of installation. A different value result if the shaftis forced into the hub (force fit) compared to the value if the assemble is completed by


5/12

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 productsare based on a coefficient of friction of 0,12 for lightly oiled connections and 0,15 for dryassemblies. These companies can provide surface coating fluids containing particles toincrease the coefficient of friction i.e. coefficient of friction to 0,25 to 0,3. (ref links 1 below)

The American Gear Manufactures Association (AGMA) recommends a value of between0,12-0,15 for hydraulically expanded hubs and 0,15-0,20 for shrink or press fit hubs.

When calculated the torque to be transmitted it is generally sufficient to use the simple

equation

T= ..d2.L.Pc/2

d= the shaft diameterL is the length of the interference joint.The surface pressure Pc is calculated typically using lame's equation.

Calculators are available for obtaining the transmitted toque very conveniently.Tribology -abcEngineers edge - press fit calculatgor

Testing Methods

There are a number of test methods for coefficient of frictions as some of which are listedbelow

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 referencesshould be used. For any specific application the ideal method of determining thecoefficient of friction is by trials. A short table is included above the main table to illustratehow the coefficient of friction is affected by surface films. When a metal surface is perfectlyclean in a vacuum , the friction is much higher than the normal accepted value and seizurecan easily occur.

......The links below the tables provide further information.
http://www.tribology-abc.com/calculators/e6_2.htmhttp://www.engineersedge.com/calculators/machine-design/press-fit/press-fit-calculator.htmhttp://www.tribology-abc.com/calculators/e6_2.htmhttp://www.engineersedge.com/calculators/machine-design/press-fit/press-fit-calculator.htm


6/12

Effect of oxide film etc on coefficient of static friction

MaterialCleanDry

ThickOxideFilm

SulfideFilm

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 anexample. 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 -0,23sliding - for steel 1032? on steel 1032?).. The same table includes a value for mild steel onmild steel of 0,62 sliding.

Material 1 Material 2

Coefficient Of Friction

TestmethodDRY Greasy

Static Sliding Static Sliding

Aluminum Aluminum1,05-1,35

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 FOFCopper-Lead Alloy Steel 0,22 -

Diamond Diamond 0,10,05 -0,1

Diamond Metal 0,1 -0,15 0,1

Glass Glass 0,9 - 1,0 0,4 0,1 - 0,60,09-0,12

Glass Metal 0,5 - 0,7 0,2 - 0,3


7/12

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 0,160,12 -

0,14

Hard Carbon Steel 0,140,11 -0,14

Iron Iron 1,00,15 -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 Nylon0,15 -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)0,25-0,75

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


8/12

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 0,570,09-0,19

Steel (Mild) Steel (Mild) - 0,62 FOR

Steel(Hard) Steel (Hard) 0,78 0,420,05-0,11

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



0,36 0,30 FOF


Bronze 0,36 0,27 FOF

Tungsten Carbide Tungsten Carbide 0,2-0,25 0,12

Tungsten Carbide Steel 0,4 - 0,60,08 -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 FrictionTest

methodDRY LUBRICATED

Static Sliding Static Sliding

FOR = Flat against rotating Cylinder, FOF = Flat against flat, POF = Pin on flat, IS = inclinedsurface,SPOF Spherical end pin on flat.

Source of above values.... The values are checked against a variety of internet andliterature sources including the links below eg Link 6-Page 16. I have referred to booksincluding Machinerys Handbook Eighteenth edition, Kempes Engineers Year Book 1980,


9/12

Concise Metals Handbook by J.R.Davis ASM - (Good source of referenced data) and KurtGiecks 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 thesimplest 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 BlockFrictioncoefficie

nt

TestMethod

StaticSlidin

g

Silver (Ag) Silver (Ag) 0,5InclinePlane

Gold(Au) 0,53InclinePlane

Copper(Cu) 0,48 InclinePlane

Iron(Fe) 0,49InclinePlane

Aluminium(Al) Aluminium(Al) 0,57InclinePlane

Titanium (Ti) 0,54InclinePlane

Gold(au) Silver (Ag) 0,53InclinePlane


Cadmium(Cd) Cadmium(Cd) 0,79

Incline

Plane


Cobalt(Co) Cobalt(Co) 0,56InclinePlane

Chromium(Cr) 0,41InclinePlane

Chromium(Cr) Cobalt(Co) 0,41InclinePlane


Copper(Cu) Cobalt(Co) 0,44Incline

Plane


Copper(Cu) 0,55InclinePlane


Nickel(Ni) 0,49InclinePlane


10/12

Zinc(Zn) 0,56InclinePlane

Iron(Fe) Cobalt(Co) 0,41InclinePlane



Maganese(Mg) 0,51InclinePlane

Molybdenum(Mo) 0,46InclinePlane

Titanium(Ti) 0,49InclinePlane

Tungsten(W) 0,47InclinePlane


Indium(In) Indium(In) 1,46 InclinePlane

Maganese(Mg) Maganese(Mg) 0,69InclinePlane

Molybdenum(Mo) Iron(Fe) 0,46InclinePlane

Molybdenum(Mo) 0,44InclinePlane

Niobium(Nb) Niobium(Nb) 0,46InclinePlane

Nickel(Ni) Chromium(Cr) 0,59InclinePlane

Nickel(Ni) 0,50

Incline

Plane

Platinum(Pt) 0,64InclinePlane

Lead(Pb) Silver (Ag) 0,73InclinePlane


Copper(Cu) 0,55InclinePlane


Iron(Fe) 0,54Incline

Plane

Lead(Pb) 0,90InclinePlane

Platinum(Pt) Nickel(Ni) 0,64InclinePlane

Platinum(Pt) 0,55InclinePlane

Tin(Sn) Iron(Fe) 0,55InclinePlane


11/12

Tin(Sn) 0,74InclinePlane

Titanium(Ti) Aluminium(Al) 0,54InclinePlane

Titanium(Ti) 0,55FlatSliding

Tungsten(W) Copper(Cu) 0,41InclinePlane


Tungsten(W) 0,51InclinePlane

Zinc(Zn) Copper(Cu) 0,56InclinePlane



Table of friction Values associated with civils and structures

Notes : Friction is lower when one of the materials is wet

Experimental results in the published literature show that at low normal stresses, asinvolved in civils design,the shear stress required to slide one rock over another varieswidely between experiments. This is because at low stress rock friction is stronglydependent on surface roughness.ref. link to "Friction of Rocks" below

Material 1 Material 2Friction

coefficientSliding

Rubber Paving 0,7 -0,9

Masonry Masonry 0,7 -0,9

Masonry Earth 0,5

Earth on Earth 0,25 -1,0

Concrete Soil / Rock 0.3

Concrete Steel 0.45

Brick Moist clay 0.33

Brick Dry clay 0.5

Brick Sand 0.4

Brick Gravel 0.6Brick Brick 0.7

Brick Rock 0.75

Granite Granite 0.6

LimestoneLimestone onLimestone

0.75

Cement Cement Blocks 0.65

Cement Dry Clay 0.4


12/12

Cement Wet Clay 0.2

Cement Wet Sand 0.4

Cement Dry Sand 0.50 - 0.60

Cement Dry Gravel 0.50 - 0.60

Cement Dry Rock 0.60 - 0.70

Cement Wet Rock 0.5

Brick on Brick Brick 0.65

Wood on Wood Wood 0.48

Documents

Factors Affecting the Friction Between Surfaces