Powder metallurgy bearings are made by pressing, sintering, sizing and oil impregnation of metal powder and other anti-friction material powders. They have a porous structure. After being soaked in hot oil, the pores are filled with lubricating oil. During operation, due to the suction effect of the rotation of the journal and the heat generated by friction, the metal and the oil expand when heated, squeezing the oil out of the pores, and then lubricating the friction surface. After the bearing cools down, the oil is sucked back into the pores.