In order to insulate and to provide cooling to the windings we use transformer oil. Transformer oil has very high breakdown voltage so that it does not breakdown inspite of having high voltage coils around it. When transformer works on high load its winding temperature increases hence oil temperature also increases which then cooled through the cooling tubes having contact with outside air.
Cooling purpose , transformer oil is insulation oil also
Ball bearing, roller bearing, bush bearing and collar bearing. Ball bearing is used where minimal contact is required and alignment is not the issue. Roller bearings are used in case of alignment issues and high pressure applications.