Why Do Spherical Roller Bearings Keep Losing Rollers?

Recently, a customer asked me, why do spherical roller bearings always drop rollers? In response to this question, China Bearing Network (abbreviated as: Huazhou Network) summarizes the reasons for the rollers dropping of spherical roller bearings based on its understanding of bearing knowledge, and shares them as follows.

First of all, we need to understand that there are three common structures of spherical roller bearings, two of which are shown in Figure 1 below. The inner ring 02 has small ribs, distributed on both end faces, and each has a roller notch, 180 degrees apart. The function of the notch is to facilitate the installation of suitable rollers 04. The cage 46 or 66 of this structure is generally made of brass. After the finished product is assembled, the bearing rotation flexibility needs to be checked, but after turning the outer ring 01, it is found that the rollers fall off the cage, or the rollers are pulled out of the bearing along the notch with a little force.

Figure 1 Spherical roller bearing structure

Analysis of roller falling reasons:

General assembly process of spherical roller bearing: Put the cage 46 or 66 on the outer diameter of the inner ring 02, install the appropriate roller 04 from the roller installation gap of the inner ring small rib to form the inner ring assembly, but leave one or two cage pockets symmetrically in each row without installing the rollers first. Put the inner assembly into the outer ring 01, and install the same group of rollers in the cage pockets from the gap. When checking the rotation flexibility, some rollers fall off from the cage, or the rollers fall out of the notch by hand with a little force. For this reason, it is necessary to analyze the relevant parts of the bearing one by one. The analysis is summarized as follows:

1. The influence of bearing ring processing

① The influence of small flange machining of the inner ring of the bearing

The outer diameter of the small flange machining of the inner ring of the bearing is “-” tolerance according to the product drawing requirements, usually -0.35～-0.15mm, and the depth dimension of the designed notch is generally only the basic dimension without tolerance. The actual processing is controlled within the range of ±0.2mm according to the current machine tool accuracy tolerance, and the notch depth size measurement is measured based on the outer diameter of the small flange (as shown in Figure 2 below). The depth tolerance of the oil groove is controlled at ±0.15mm according to the current processing industry standard. For a small rib with a shallow depth (the distance from the outer diameter of the small rib to the center of the oil groove), the depth dimension usually does not exceed 3mm. In actual processing, there may be a cumulative tolerance superposition effect (the outer diameter of the rib and the notch depth dimension are the minimum dimensions, and the oil groove depth is the maximum dimension), which ultimately results in the smallest contact area between the roller end face and the rib, and sometimes the contact part is even close to the oil groove. Therefore, a little force is applied at the notch to push the roller out along the notch. For a deeper rib, due to its large contact area with the rib, the tolerance effect is relatively small, and sometimes it can even be ignored.

Figure 2 Notch depth measurement method

② Influence of grinding of small rib of bearing inner ring

The small rib of the inner ring of the bearing is ground by the fixed-stroke method. Sometimes it is found that some small ribs have black skin after grinding. Due to the lathe processing, the grinding allowance of the small rib is small or the grinding process is not equal, resulting in black skin after grinding. Usually, the black skin can be removed and moved down normally, which causes the thickness of the small rib to be too small. For the small rib with a shallow depth, the contact area between the roller end face and it is small. After the small rib is over-grinded, the distance of the roller along the axial direction of the inner ring increases, making it easier for the roller to slip out along the notch, as shown in Figure 3 below.

Figure 3 Over-grinding of the small rib

③ The influence of grinding of the bearing ring raceway

When grinding the outer ring of the bearing, it is sometimes found that some black skin still exists after grinding. Due to lathe processing and heat treatment, the grinding allowance is too small or the outer raceway is deformed, resulting in black skin after grinding. Usually, the outer raceway is over-grinded, and the black skin can be removed and moved down normally. In this way, the inner raceway needs to be under-grinded to achieve normal fitting in the assembly process. This causes the center diameter of the roller group to be biased outward as a whole, so that the contact part between the roller and the rib is biased outward. After the notch is biased outward, the actual contact area is small, making the roller easier to slip out along the notch.

2. Influence of cage processing

Cage processing is usually done on a drilling machine, and the drilling process is performed by positioning the equally divided mold. Since the mold itself is not processed by a dedicated machine tool, the manufacturing accuracy is low. In addition, sometimes the flatness of the cage base surface during the flat end process is out of tolerance, resulting in inconsistent pocket processing depth dimensions. Generally, the machine tool is adjusted to reposition the drilling, resulting in a deeper pocket and a larger pocket diameter. After the roller is installed, the gap between the roller and the cage is large, and the axial movement along the cage is large. Finally, after the roller is fully loaded, the overall gap between the roller and the cage is large. It is easy for the roller to fall off the cage during the test bearing rotation.

In addition, the center diameter of the cage is measured. By measuring the distance control of the measuring column, the force of the caliper is different, and the degree of skewness of the pillar is also different (the position of the pillar in the pocket hole). Therefore, the final measured size can only be used as a reference and cannot effectively reflect the actual measurement value of the center diameter. For medium and large bearings, because their flanges are generally deeper, this measurement method has less impact. For small bearings, the impact is greater because the ribs are shallow. Sometimes the actual center diameter is larger, but the measurement is qualified. This situation often occurs. When the center diameter is larger, the center of the roller group is biased to the outside, causing the contact part between the roller and the small rib to be biased to the outside. When the gap is biased to the outside, the actual contact area is smaller, and finally the roller falls out of the gap.