Ball Bearing Part Numbering System

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NMB Miniature and Precision Ball Bearings Part Numbering System

The NMB Bearing Part Numbering System is unique to Minebea, and is comprised of the base part number and specificaitons of the bearing.

This system is designed to make it as simple as possible for you to find the exact bearing or bearing part number, that you need. If you don’t already know which part or part numbers you are looking for, the following guide should help. See what each segment of the bearing or part number means below.

NMB Part Numbering System PDF






Bearing Part Number

The first half of the bearing part number has three parts. The first part describes the ring and ball material, the second tells you what series bearing it is, and the third is the size reference number. This number may be followed by a suffix which tells you which additional or special features the ball bearing has.

Ring and Ball Material

The ring and ball material for the bearing may come in NMB developed stainless steel material, which falls within the 400 series Martensitic stainless steel grouping, or high carbon chromium steel. The high carbon chromium steel bearing is effective for almost any application, while the martensitic stainless steel bearing provides added protection against corrosion.

Bearing Series

The most common bearing series are R and L, with R representing a thick section and L representing a thin section. If the bearing is flanged, there will also be an F for the series designation, while if the bearing is measured by inches rather than metric, it will have an I after the first letter. You may also see NR, for a bearing with a snap ring on the outer ring.

Size Reference

Bearing sizes are measured along two main dimensions, the bore size, or the inner diameter, and the O.D., or outer diameter. This part of the bearing number is a composite of these two measurements, with the O.D. measurement first. Be sure to note whether you are looking at a bearing measured according to inches or millimeters.

Bearing Suffix

A suffix to the bearing part number will indicate special features for that particular bearing, specifically: enclosure, special feature/noise grade, and retainer. The following explains these features in slightly more detail.


Shields and seals can be a valuable addition to a bearing, depending on the application. These protective coverings can serve a twofold purpose. They protect the bearing itself from contaminants and protect the application that houses the bearing from lubrication leaks. This code indicates the type of shield or seal attached to that particular bearing. If there is a double letter, it means that both sides are enclosed, with a single letter meaning only one side of the bearing is enclosed. No code denotes open bearing.

Special Feature/Noise Grade

Some parts will be specially made for particular applications. This suffix indicates whether the part is wider or thinner than normal or has a special design. In addition, noise level can be an important factor when it comes to ball bearings. Moving parts and machinery can make an incredible amount of noise, and in areas and situations where noise sensitivity is an issue, you’ll need to consider this when selecting your part. An MT code indicates a highly noise sensitive application.


NMB offers three types of retaining cages for your bearings: the press steel ribbon (R) retainer, composed of two stamped steel parts. The balls are held between the two steel parts and the tabs of one of the steel parts are bent over to fasten them together. The Ribbon Retainer is the most common type of ball bearing retainer. The steel crown (H) is composed of a stamped steel part. The small difference in inner and outer diameters of the retainer allows them to be used for thin type and miniature ball bearings. The plastic retainer (MN) is composed of molded or machined plastic, including Polyamide, Polyacetal and others. It is used for high speed rotation and low noise level.

Bearing Spec Codes

The bearing spec codes provide information on the bearing’s dimensional accuracy and lubricant specifications. They indicate the precision grade, according to JIS or ABMA standards, the range of radial clearance, the type of lubricant (oil, grease, either oil or grease, or no lubricant), and the lubricant fill quantity.
This system should tell you everything you need to know about the bearing part you are researching, and should make it easy for you to find the high quality NMB part or parts that your design requires.