The principle is to have a film of oil under pressure between the shaft and liner bore which allows no contact between these 2 surfaces. To establish this pressure in the film of oil, it is necessary to have surface convergence and sufficient speed. The pressure also depends on the viscosity of the lubricant, generally oil.
The advantage of using these types of parts, is that they can combine the characteristics of friction and anti friction with the mechanical characteristics of the steel body. The thin layers, more resistant to creep, assure tribological qualities and the support, underlying, carry the weight and guarantee tightness.
Whilst with hydrodynamic bearings, it's the speed that creates the pressure in the film of oil, with hydrostatic bearings, the pressure is established by external means , such as a pump. Both types of pressure can be usefully combined when starting or stopping heavy rotors.
Also called friction layer, lead coating favors sliding. This layer is applied by electroplating, and its thinness allows the conservation of the load properties of the anti friction layer in the copper-lead that it covers. Moreover this layer has a power of accommodation that favorites the adjustment of the shaft in its housing.
The main antifrictions used are copper lead, aluminum with a lead coating and Regulus Whilst the first two are applied uniquely on a steel body Regulus can also be applied on a bronze support and sometimes cast iron.
We can distinguish 3 modes of functioning : limited where friction is dominant, hydrodynamic where the two surfaces are separated by a film of oil , and mixed where the 2 precedent modes coexist. It's the permanent hydrodynamic mode that is sort after for optimal functioning. In these conditions, we eliminate the contacts metal/metal and the theoretical working life of the components becomes infinite.