The Poznań University of Technology

Faculty of Engineering Management

Technique lead-in - „Friction” – exercises

Model Charlesa Coulomb’a (1785)

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first complete mathematical description, (so called elastic bending of unevenness connected with conept of Euler’s saw shape unevennesses concept),

static friction is not constant, variability of kinematic friction.

Charles Coulomb (1736–1806)

Types of friction

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Internal friction: • resistance between the elements of one body • Solids depend on damping materials and fluids from their viscosity, • Resistance is due to the existence of cohesive forces (forces of mutual attraction) and depend on the freedom of movement of these particles (liquid friction).

External friction: • the contact area of two solids in motion or run in motion without the presence of a lubricant, • In the case of contact of two bodies perfectly rigid and free of defects in shape or roughness, the external friction resistance is due to the effects of surface forces (adhesion), • In the case of friction of machine elements, the impact is of adhesive nature and mechanical impact due to inequalities of co-operating surfaces.

Types of friction

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Rolling resistance: •Resistance force acting when one body rolls over another (road tire, ball on flat surface, bearings), •usually much less than kinetic (sliding), •wide use in technology, •Turning is ALWAYS associated with surface deformation (even small), •rolling depends on the radius of the rolling body.

Types of friction

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Sliding friction:

• occurs at the contact of two solids (external friction) when the bodies move relative to each other or when the body rests with respect to each other and there is a force for moving the body,

Subtypes: • Stiction (static), • Sliding friction (kinetic).

Types of friction

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Stiction (static): • two bodies don’t move in relation to each other, • is equal to the power to be used to move one body relative to the other.

Sliding friction (kinetic): • Two bodies slipping or rolling one on another, • The frictional force is opposed to the movement, creating the friction resistance requiring appropriate forces • always causes the wear of the elements.

• Rolling: rolling bearings, gears, wheel-rail system • sliding (progressive), progressive, reciprocal, smooth.

Types of friction

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Friction types

type of contact

Dry

Liquid

terminal

assorted

Type of movement

Static

Kinetic

movement characteristics

Sliding

Rolling

Friction

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Friction (kinetic)

Vector of friction force is opposite to vector of velocity, friction force is directly proportional to the force of friction

Friction

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Kinetic friction

where: Kinetic friction forcr, Friction coeficient (Kinetic) (no unit), force of preassure (perpendicular to the surface).

Friction

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Static friction (stiction)

the body does not move - the force of static friction ( ) balances the force acting on the body (F)

The maximum value that a static friction force can achieve is equal to the force value to move the body.

Friction

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The model of friction

Inclined plane

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- friction force (F=T)

f - rolling resistance coefficient

r - radius

N - preassure force

Rolling resistance

Friction

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Examples of friction coefficients

The coefficient of friction depends on the type of material from which

the surfaces are made and their roughness

relation Frintion coefficient

relation Friction coefficient

steel – cast iron 0,18 rubber – concrete

1,0

steel – steel 0,1 glass – glass 0,94

steel – brass 0,15 ice – ice 0,1

cast iron – cast iron 0,16 steel – teflon 0,04

steel - wood 0,2 steel – granite 0,08

Friction

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przykłady współczynników tarcia tocznego f [cm]

The coefficient of friction depends on the type of material from which

the surfaces are made and their roughness

Relation Friction coefficient

hard steel ball on hard steel (as in the bearing) 0,0005 – 0,001

Steel ball on steel 0,003

Steel wheel (cylinder) on steel rail 0,005

Wooden wheel (cylinder) on steel rail 0,03

Wooden wheel (cylinder) on wood 0,06 – 0,15

Dependencies

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mechanical wear of machine parts due to friction • properties of the contact surfaces, • friction type (kinetic, static), • geometric shape of the cutting surface, • use or non-lubrication (dry, liquid, mixed, limit), • value of surface pressure, • the load causing the internal stresses of the material of the cutting parts, • slip speed or rolling speed, • ambient temperature, • presence of aggressive corrosive environment, • undesirable foreign bodies penetration between underside surfaces, • quality of operation (use and operation).

Dependances

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In mechanical devices the basic trunk parts are: • Plain bearing in rotary motion: sliding friction, • Transverse plain bearing in rotary-swing: sliding friction, • Guides in reciprocating motion: sliding friction, • Gearboxes normally operating with sliding-and-rolling friction, • Roller bearings with different profile of turning and different direction of transmission (longitudinal, transverse): sliding friction, • Camshafts operating on rolling or sliding friction. The durability of machine components depends on the effect of rolling or

sliding friction.

rolling bearing

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Main types of rolling bearings Plain bearing,

Angular contact bearing,

Double row ball berring, adjustable

Roller bearing,

tapered roller bearing , cone bearing,

Barrel bearing,

Longitudinal ball bearing, needle roller baring.

19 http://www.pg.gda.pl/~wlitwin/pkm/1_09.pdf

Plain bearing

Dry bearing (periodicaly lubricated or never): teflon,

Air bearing (The distance forms the air bag supplied by the compressor),

Fluid lubrication bearings (filled with oil): hydronamic and hydrostatic.

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