In recent years, the operating temperatures of machines have been steadily polarizing between high-temperature and low-temperature.
The composition of both metal and plastic is sometimes changed by temperature. In the case of bearing steels (special steels out of which bearings are made), if annealing temperature is exceeded, the structure softens and the life span shortens while size changes. With plastic, adding heat causes melting and the plastic becomes unable to withstand even a slight external force, and high temperatures also cause such phenomena as fusion and carbonization to occur.
With common bearing steels, the limit is said to be approximately 120 °C, and with all-purpose engineering plastics, it is said to be around 100 °C.