The main purpose of lubrication is to form an oil film between the two surfaces, which has sufficient thickness and is resistant to operating conditions. It aims to transform the mechanical friction that causes loss of energy into internal friction that occurs in the movement of oil particles in the oil film. This oil film layer can separate the shaft and bearing from each other and prevent the contact of the rough parts on their surfaces. If such an oil film is formed between the two surfaces, the friction here will be reduced to the friction between the oil layers. All lubricant particles will spread on metal surfaces and adhere to shaft and bearing surfaces with a performance dependent on the type, chemical structure and additives of the lubricant.
One of the factors affecting the formation of this oil film is the oil inlet and the oil gallery. One or multiple oils are supplied to bearings. The oil inlet opening must be in the appropriate geometric form (usually circular) and size; because the oil inlet must be sufficient not to prevent the flow of oil. For this purpose, the oil inlets are generally designed in the form of countersinks and have suitable roundings. In order to facilitate the formation of an oil wedge between the surfaces of the bearing and to provide a better distribution of the oil within the bearing, usually a number of oil galleries are opened by metal-cutting method. Oil galleries are also available in circumferential, helical, axial and open ended axial forms according to ISO 4378. In practice, the most commonly used types are circumferential and axial oil galleries.
Another purpose of oil galleries is to provide an oil chamber in the bearing and adequate oil flow for adequate cooling effect. It is possible to cool a hot bearing by using a simple shaped oil gallery to increase the oil flow. An axial oil gallery in the no-load part of the bearing is usually sufficient for the transfer of unidirectional loads. With this oil gallery, it is possible to achieve three times more oil flow in a bearing construction where there is only one oil hole. For loads with variable directions, circumferential oil gallery is a more suitable construction. However, the circumferential oil gallery also divides the bearing into two smaller bearings. This may reduce the total load carrying capacity of the bearing. In principle, it is more convenient to design the bearing so that the oil inlet and oil galleries are located in an area where there is no hydrodynamic pressure. By doing this, better oil flow will be achieved. Otherwise, the hydrodynamic oil pressure distribution will be significantly impaired, which reduces the load capacity of the bearing.