Types of friction

    Dry friction

    The metal surfaces rub directly against each other without a lubricating film. Friction resistance and wear and tear are both high. Very high local temperatures can build up, which can lead to the jamming and destruction of touching parts. This rubbing effect can occur in a lubricated engine only in extreme cases, for example in case of failure of the supply of lubricant to the location of friction.

    Dry friction

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    Mixed friction

    There is no continuous lubricating film between the metal surfaces, and different roughness peaks can touch each other. This condition always occurs in slide bearings at start-up and departure.

    Continuous reduced mixed friction occurs in the engine in the upper and lower dead point area between the piston rings and the cylinder lining, in the area of valve control, at the cam and tappet, at the valve guides and in all machines at the tooth flanks. The lubricants must be therefore capable of forming protective and reactive layers with the help of additives on the sliding surfaces and thus keep friction and wear and tear as low as possible.

     

    Mixed friction

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    Liquid Friction

    Both metal surfaces are separated completely by a lubricating film; friction (power loss) is low and wear and tear is equal to zero. This is the ideal condition. The following conditions must be met to make a complete load-carrying liquid film:

    • a narrowing lubrication gap;
    • sufficient sliding speed;
    • sufficient viscosity of the lubricant.
       

    A lubrication layer pressure is hereby created (hydrodynamic pressure) in the range of 1000 bar and more. The oil pressure created by the oil pump of up to 6 bar only serves as transport pressure and it has nothing to do with the carrying capacity of the lubricating film.

    Liquid friction

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