The special sealed and clean bearing for gearbox developed by NSK is a bearing with a special sealing ring, which can prevent foreign matter from mixing in the gearbox and greatly improve the fatigue life of the bearing. In the actual gearbox durability test, this type of bearing has proved that the durability life is 6 to 10 times that of the standard ball bearing. The special sealing ring prevents the intrusion of harmful small foreign objects floating in the gear oil of the gearbox, which greatly reduces the raceway indentation caused by the intrusion of foreign objects. To this end, the bearing fatigue characteristic curve is replaced from the surface fatigue characteristic curve to the internal fatigue characteristic curve based on the original fatigue life of the bearing to prolong the bearing life. Moreover, it is not affected much by the recent trend of gear oil viscosity reduction, which is superior to open type bearings. These special bearings for gearboxes are commonly known as TM ball bearings. They have the following four characteristics: (1) They adopt design standards that can meet the requirements of special bearings for gearboxes; (2) In order to assist initial lubrication, they are filled with gear oil that is compatible with gear oil. (3) The sealing lip structure that prevents the intrusion of foreign objects but allows the lubricating oil to flow in is adopted (Fig. 16); (4) The friction torque is lower than that of the conventional contact type sealing ring bearing. In addition, this type of TM ball bearing has been serialized (see table 17), and can be replaced with the same nominal size as the currently used 62 and 63 series open bearings.