The Inner Ring

The smaller of the two rings, the inner ring is also manufactured of through-hardened bearing steel. It has a highly finished groove on its outside diameter to form a pathway for the balls. The inner ring is usually the rotating element. The bore diameter of the inner ring is generally finished to fit the shaft.

In the U.S., the bore is usually measured in inches. At NTN, the bore size designation, in an inch series insert bearing, is coded numerically: the first of the three numbers is the full inch size, the second two numbers indicate the fractional inch in sixteenths. For example, a bore measuring 1" would be designated as 100; a 15/16" bore would be 015; and, a 1 15/16" bore would be 115. In the previous example used to classify outer ring configurations, the part number UCS206-104D1NR indicates that the bore size is 1 4/16", or 1 1/4".

The inner ring of the insert bearing is wider than the inner ring of a standard radial ball bearing. This width difference plays a role in how the two are mounted on the shaft. The standard radial ball bearing is press fit onto a machined shaft, requiring special tools for installation. The standard insert bearing, however, is mounted to a shaft that is typically slightly smaller than the bearing’s bore diameter. The insert bearing may be slip fit onto the shaft. The looser fit of the standard insert bearing, however, necessitates a locking device to hold it securely to the shaft. The inner ring of the bearing is extended to one side to accommodate the locking device. In addition, the width of the insert bearing’s inner ring may vary according to the type of the bearing.

Narrow inner ring inserts

Narrow Inner Ring, AS-type with set screw

Wide Inner Ring, UC-type with set screw

The narrow inner ring, or single extended inner ring, of some inserts features an inner ring that extends beyond the outer ring on one side in order to accommodate the locking device. The NTN designations for these types of insert bearings are AS, AEL, AR and JEL. This type of insert (and/ or a bearing unit featuring this insert) is generally a lower cost product.

Wide inner ring inserts

The wide inner ring, or double extended inner ring insert features an inner ring that extends beyond the width of the outer ring on both sides of the insert. The extension on one side accommodates a heavy duty seal. The other side is extended even further to accommodate both a heavy duty seal and the locking device.

The wide inner ring insert bearing also distributes the load over the larger surface area created by the wider inner ring. These bearings are generally used for heavier duty applications and are designated by NTN as UC, UEL, UK, UR or REL.

Some manufacturers create inserts that fall between narrow and wide inner ring inserts. By combining features of both they create a more economical "will-fit" that is neither a true narrow or wide inner ring.