Needle roller bearings are roller bearings with cylindrical rollers, which are both thin and long relative to their diameter. This type of roller is called a rolling needle. Despite having a small cross-section, the bearing still has a high load-bearing capacity. Needle roller bearings are equipped with thin and long rollers (roller diameter D ≤ 5mm, L/D ≥ 2.5, L is the length of the roller), so the radial structure is compact. When the inner diameter size and load capacity are the same as other types of bearings, the outer diameter is the smallest, making them particularly suitable for support structures with limited radial installation dimensions. According to different usage scenarios, bearings or needle and cage components without inner rings can be selected. At this time, the journal surface and shell hole surface that match the bearing directly serve as the inner and outer rolling surfaces of the bearing. To ensure the same load capacity and operating performance as bearings with rings, the hardness, machining accuracy, and surface quality of the shaft or shell hole raceway surface should be similar to the raceway of the bearing ring. This type of bearing can only withstand radial loads.