Friction is categorized by the nature of the surfaces in contact and the conditions under which they are interacting. There are many different types of friction, some of which are listed below.
1. Dry Friction
Dry friction, also called Coulomb friction, is a force which appears between two solid surfaces in contact. This force is distributed over the contact area and always acts in whichever direction opposes relative motion between the surfaces.
Depending on the details of the situation, dry friction is divided into:
Static Friction: hold the object in equilibrium “between non-moving surfaces”.
Kinetic Friction: retard but not prevent motion “between moving surfaces”.
Kinetic friction is also further subdivided into two categories:
Sliding Friction: The friction which is generated between the surfaces of any two objects when they are in a sliding motion against each other.
Rolling Friction: the friction which prevents the motion of any spherical object like a wheel or a ball, etc. It is also the weakest type of friction among all the other types of frictional forces.
2. Wet Friction
Most machines are lubricated in order to decrease the effects of friction. But in most cases, this is not sufficient and significant friction effects remain. There are three regimes of lubrication in a system with grease or oil Each of these regimes contribute to the dynamics of the system.
Boundary Friction: Force that resists relative motion between to solid bodies whose opposing surfaces are wetted by a lubricant but barely separated by the lubricant film.
Mixed-film Friction: Force that resists relative motion between to solid bodies whose opposing surfaces are partially separated by a full fluid.
Fluid Friction: Force that resists the flow of liquids or gases. Such a force opposes the sliding action, one over the other, of the molecular layers of the fluid. (Shearing)
Stribeck Curve
The different regimes of wet friction are often characterized by use of so-called Stribeck Curve. Friction coefficient is plotted as a function of speed. At the Stribeck curve while the ordinate is always the coefficient of friction, the abscissa can be speed, Hersey number.
Hersey Number
=
Viscosity x Speed
Load
=
Z N
P
Friction coefficient gradually drops, passes through a minimum, and then increases. As shown in the chart, the Stribeck curve involves three distinctive features:
Boundary Friction:
Surfaces are partially in contact, and the coefficient of friction of the system is a function of the combined properties of the fluid and surfaces. When dry-film (solid) lubricants are used, the friction coefficient of the system may be related to the properties of the dry-film lubricant slipping on itself or to those of an uncoated solid on the lubricant film. Lubrication with graphite, molybdenum disulfide, or fluorocarbons is an example of this.
Mixed-film Friction:
Friction drops with increasing speed up to a transition lift-off point; where sufficient fluid film pressure first develops with increased speed to completely support the load.
Fluid Friction:
The sliding members are fully separated by a film of lubricant, the friction coefficient of the system is essentially the friction coefficient of the fluid.
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