Hydrodynamic (HD) lubrication is sometimes called fluid-film or thick-film lubrication. As a bearing with convergent shape in the direction of motion starts to spin (slide in the longitudinal direction) from rest, a thin layer of fluid is pulled through because of viscous entrainment and is then compressed between the bearing surfaces, creating a sufficient (hydrodynamic) pressure to support the load without any external pumping agency.

This is the principle of hydrodynamic lubrication, a mechanism that is essential to the efficient functioning of the self-acting journal and thrust bearings widely used in modern industry. A high load capacity can be achieved in the bearings that operate at high speeds and low loads in the presence of fluids of high viscosity.

Fluid film can also be generated only by a reciprocating or oscillating motion in the normal direction (squeeze), which may be fixed or variable in magnitude (transient or steady state). This load-carrying phenomenon arises from the fact that a viscous fluid cannot be instantaneously squeezed out from the interface with two surfaces that are approaching each other.

It takes time for these surfaces to meet, and during that interval¾because of the fluid's resistance to extrusion¾a pressure is built up and the load is actually supported by the fluid film. When the load is relieved or becomes reversed, the fluid is sucked in and the fluid film often can recover its thickness in time for the next application.

The squeeze phenomenon controls the buildup of a water film under the tires of automobiles and airplanes on wet roadways or landing strips (commonly known as hydroplaning) that have virtually no relative sliding motion. HD lubrication is often referred to as the ideal lubricated contact condition because the lubricating films are normally many times thicker (typically 5-500 ¹m) than the height of the irregularities on the bearing surface, and solid contacts do not occur.

The coefficient of friction in the HD regime can be as small as 0.001. The friction increases slightly with the sliding speed because of viscous drag.

The behavior of the contact is governed by the bulk physical properties of the lubricant, notable viscosity, and the frictional characteristics arise purely from the shearing of the viscous lubricant.

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