A rolling bearing is composed of four parts: the inner ring, the outer ring, and the rolling element retaining cage. It is precisely because there are "rolling elements" between the bearing rings that the bearing can achieve "rolling motion". 

From this fundamental perspective, the rolling elements are the prerequisite for the existence of rolling bearings. The original intention of using rolling elements was to replace sliding bearings, in order to significantly reduce friction and make the operation more smooth and flexible. 

If the number and size of the rolling elements are determined by the bearing design and are thus optional, then for the rolling elements themselves, in terms of load capacity and service life, in addition to fully meeting the specified quality requirements such as materials and manufacturing, they generally also need to satisfy some specific technical requirements: 

1. Enhance surface hardness. Just like the raceway of the race ring, the rolling elements generally need to reach a surface hardness of above 58 HRC after conventional quenching and tempering to ensure the basic rated load capacity of the bearing. Among them, the hardness matching between the rolling elements and the raceway has a significant impact on the bearing life. Tests have shown that when the surface hardness of the rolling elements is 1 to 2 HRC higher than that of the raceway, the bearing life is longer. This is because the precision of the rolling elements is usually higher than that of the raceway. If the surface hardness is also higher, during the operation of the bearing, "precise cold rolling expansion" can be performed on the raceway, improving the raceway precision and forming compressive stress on the surface, which is beneficial to the improvement of the fatigue life of the bearing. In addition, simultaneously increasing the surface hardness of the rolling elements and the raceway can also significantly improve the anti-pollution life of the bearing. Therefore, within the required surface hardness range of the rolling elements, the heat treatment process control should generally be as close as possible to the middle or upper limit level. 

2. Surface strengthening treatment. Surface strengthening treatment is also known as pressurization. It involves applying a similar high-pressure shot peening treatment to the surface of the rolling elements within the elastic and plastic deformation range of the material. This process increases the surface hardness and creates a uniform compressive stress layer on the surface, thereby further enhancing the bearing's lifespan. The general requirements for surface hardness after surface strengthening are as follows: the surface hardness should be increased by 1 to 3 HRC; the hardness difference between the same rolling elements should not exceed 0.5 HRC; the maximum difference in hardness between rolling elements should not exceed 1 HRC. The principle requirements for the surface compressive stress are: it should be evenly distributed and should follow a certain gradient distribution from the surface inward. 

3. Perform convexity shaping on the rollers. During the operation of the bearing, the rolling contact between the rolling elements and the raceways of the rings occurs more frequently than that between the rotating rings. Therefore, the probability of contact fatigue failure is relatively high, which is an inevitable phenomenon. Under the condition that the bearing load and the number of rolling contacts remain unchanged, reducing the contact stress, especially the phenomenon of stress concentration, is one of the effective measures to reduce the probability of fatigue failure. Therefore, for straight-line contact rollers (cylindrical, needle rollers) that are prone to stress concentration at the end face, convexity shaping must be carried out. Among various convexity shapes such as arcs, arc correction lines, logarithmic curves, etc., the logarithmic curve convexity is considered the best, as it can achieve uniform distribution of contact stress and be insensitive to stress concentration. Therefore, logarithmic curve convexity shaping should be attempted as much as possible. In the current international standards, the calculation coefficient for the rated dynamic load of roller bearings is determined based on the assumption that the rollers have convexity shaping, which can ensure the uniform distribution of contact stress. It is clearly stated that the presence of convexity on the rollers is a conventional requirement, while straight-line rollers are an exceptional case. That is, the rollers used in the bearing must have convexity. Additionally, for the spherical base surfaces of the tapered rollers and some end faces of the cylindrical rollers, since they need to form a sliding contact working surface with the ring flange, they also require a certain shape (such as a spherical surface) to prevent burnout failure during the bearing operation.