Friction

Friction, also referred to as friction, is the inhibition of movement occurring between contacting solid bodies or particles. A distinction is made between external friction between contacting interfaces of solids and internal friction between adjacent particles in the deformation of fluids and solids. In physical models frictional forces are often neglected when they are relatively small and / or difficult to measure quantitatively. With regard to energy loss occurring in the friction ( their conversion into heat to increase the entropy) is referred to as the dissipation, particularly in the case of kinetic energy, but also in general terms for non- mechanical processes.

Instead of " friction force " and the term is friction needed. With the scientific study of friction processes dealt tribology ( tribology ).

Types of friction

External friction

External friction is also referred to as solid friction because it occurs between the contact surfaces of the contacting solids. It is divided into static friction and sliding friction, both of which are referred to in honor of the physicist Charles Augustin de Coulomb as Coulomb friction. You do not always strictly separated from each other on. You can also or alternately occur; For example, the stick -slip effect is a periodic transition between static and dynamic friction. On the application -oriented concepts are rolling friction, drilling friction and belt friction.

The frictional force FR increases as the normal force FN, often approximately linear and independent of the size of the contact surface (see Amontonssche laws):

The coefficient of friction μ, depending on the nature of the surfaces. The coefficient of adhesion is generally greater than that for sliding. Its value is determined experimentally.

Stiction

In many cases, adhesion between contacting bodies is desirable. Without static friction of everyday life would not work. Furniture would not stay in place, parked vehicles on the road ( the wheels blocked ) could be propelled solely by the wind. You could not get their foot "fixed" on the ground, all driven wheels would " spin ", ie allow no traction. In industrial applications, a technically generated pressure between the contact surfaces is used except for the most acting weight force, for example by means of tensioned springs in a friction clutch.

Stick is a state of rest, so that neither wear nor energy losses occur. It is a combination of form-fitting on a small scale by roughness as shape deviation 3rd to 5th order, which would be destroyed in motion, and molecular adhesion on a small scale by molecular attraction forces, ie adhesion.

Sliding friction

Sliding friction occurs at the contact surfaces between bodies that move relative to each other. For some material combinations occurs a creep, so that the friction force against the Amontonsschen law is speed-dependent. The sliding friction is usually less than the static friction force with the same normal force.

Rolling friction

Rolling friction occurs when a body rolls on a backing. In the model, the rolling friction can be explained by the deformation of a non- ideal rigid body. The rolling friction is described by the dimensionless rolling friction coefficient. This is defined as the ratio of the length and the radius of the rolling friction of the roller body:

Rolling friction

Overlap sliding and rolling friction, it is referred to as rolling friction.

Drilling friction

Drilling friction occurs at the contact point of a rotating about the vertical axis of the body on a plane. The coefficient of the drilling friction mB is interpretable as the radius of the apparent support disc, ie as the resulting CG of the area moments. Since the drilling friction acts in a rotating motion, the drilling friction torque is given as:

Rope friction

The Euler Eytelwein formula describes the friction of a set to a round body rope forces act on both sides of that, and specifies the conditions under which the liability of the rope.

Internal friction

Internal friction causes the toughness of materials and fluids and has an influence on deformation and currents. In addition to the movement of the particles in a material, the internal friction also describes the frictional resistance of bodies moving in fluids, as well as the attenuation of sound waves. Typically, the internal friction in the gas (viscosity) with temperature, and from fluids. In simple cases it is possible with the tools of statistical physics, a quantitative description.

At temperatures close to the zero temperature some liquids lose their internal friction completely (see superfluidity ).

Unlike in mechanics, is neglected as long as possible in the friction is internal friction in the standard theory of hydrodynamics, the Navier -Stokes equations include fixed. These nonlinear equations are solved numerically in general. For the case of small Reynolds number Re, ie when the advection of momentum compared to the momentum transport by viscosity can be neglected, exist for simple geometries and Newtonian fluids closed-form solutions:

This applies, for example, a thin layer of lubricant between surfaces moving against one another. The friction is then proportional to the shear rate, that is to the velocity v. The same situation is for the case of a small sphere in a viscous fluid before, refer to the Stokes' law. In contrast, the dominant Impulsadvektion dissipation is proportional to the square of the velocity, see flow resistance.

The plastic deformation of solids is generally highly non-linear, and thus not be described well by the viscosity. Even with smaller forces or stresses, there are deviations from the ideal elasticity as another type of internal friction in the solid state, but also not easy can be understood as viscosity. According to the equation of internal friction and viscosity is restricted to fluids.

Friction conditions in lubrication technology

The optimization of friction processes is the subject of tribology.

In the solid friction, the surfaces sliding against each touch. This surface elevations are leveled (abrasion or wear). With unfavorable material combination and large surface pressure, the surfaces weld together (adhesion). Solid friction occurs, for example when using dry lubricants (graphite, PTFE ) when no lubricant is used or failed lubrication. This friction condition is therefore also referred to as dry friction and can be significantly reduced by linear ball bearings.

The mixed friction can occur with insufficient lubrication or lubrication at the beginning of the movement of two friction partners. Here, the sliding surfaces touching certain points. The friction force in the mixed friction range is speed dependent and can be observed on plain bearings. It takes the friction force / friction torque with increasing sliding speed, until pure liquid friction occurs and the friction surfaces separated. With further increase in sliding speed then increases the friction force / friction torque again. In mixed friction area of the wear increases similar from the friction torque until the sliding speed has reached the wear-free fluid friction. The mixed friction is therefore in continuous operation always undesirable, but is sometimes unavoidable or its avoidance is so complex that the cost of wear and tear repairs are taken into account.

The liquid friction occurs when forming a permanent lubricating film between the sliding surfaces. Typical lubricants are oils, but water and gases (see air bearing ). The sliding surfaces are completely separated. The resulting friction based on the fact that the lubricant molecules slide over one another. In order for these shear forces lead only to a portable temperature rise of the lubricant, the generated heat has to be dissipated in a suitable manner. Fluid friction is the desired state of friction in bearings and guides, if durability, high sliding speed and high capacity are required. An important example is the pressure oil lubrication of the bearings between the crankshaft and connecting rod in the car engine ( Hydro Dynamic bearing).

The transition from the mixed friction to fluid friction is represented by the Stribeck curve, the minimum of friction force / torque curve marks the transition to a pure fluid friction.

Examples of friction processes

• The internal friction of bulk material is defined by the angle of repose.
• Rheology deals with friction in complex fluids, such as polymers and dispersions, are not sufficient to describe the linear Navier-Stokes equations.
• Non- linear is the friction that occurs during deformation in solids, such as the tidal force in astronomy and the modeling of relativistic effects.
• Also, for the description of metal forming processes constitutive laws are used that take into account the internal friction. This material laws are used in the plastomechanics.
• By friction of a match on the relevant designated painting area at the Steichholzschachtel can ignite by friction heat ( with the participation of ignition accelerators ) the wood.