In maintenance and procurement environments, it is common to encounter bearings that have lost their original packaging or labeling. Identifying the correct part number from physical measurements alone is a practical skill that saves time and reduces the risk of ordering the wrong replacement. Fortunately, bearing manufacturers follow standardized dimension systems, which means that a set of accurate measurements will almost always point you directly to the right part number.
This article walks through how bearing numbering systems work, what measurements you need to take, and how to cross-reference dimensions to find the correct part, with the LM20UU linear bearing used as a worked example throughout.
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How Bearing Part Numbers Encode Dimensions
Bearing part numbers are not arbitrary. They follow standardized coding conventions established by ISO and JIS standards, and adopted by virtually all major manufacturers worldwide. For rotary deep groove ball bearings, the part number typically encodes the bearing series and the bore size. A 6204 bearing, to take a familiar example, has a 20mm bore, a 47mm outer diameter, and a 14mm width. The "04" at the end, multiplied by five, gives the bore diameter in millimeters for bearings in this size range.
Linear motion bearings follow a similar logic but use a different format. The LM series encodes the shaft diameter directly into the name. "LM" stands for Linear Motion, the number that follows is the bore diameter in millimeters, and any suffix describes the sealing configuration or length variant. This makes the naming system highly readable for engineers and procurement teams who work with these parts regularly.
What to Measure and How
To identify a bearing by its dimensions, you need three measurements: the inner diameter (bore), the outer diameter, and the width or length depending on the bearing type. For linear bearings, length is the relevant third dimension rather than width. Use a vernier caliper for all measurements and record values to the nearest 0.1mm. The bore is the most critical measurement because it determines shaft compatibility and narrows the part number to a specific size within a series.
Once you have all three measurements, compare them against a standard dimension table for the bearing type you have identified. Most bearing manufacturers publish free dimension charts, and ISO or JIS standard tables cover the majority of parts in common industrial use. If the bearing carries any stamped or etched markings on the outer race, check these first, as even a partial marking will confirm or accelerate the identification process.
The LM20UU as a Practical Example
The LM20UU is one of the most widely used linear bearings in automation and precision machinery. Breaking down the name: "LM" indicates a standard cylindrical linear bearing, "20" indicates a 20mm bore designed for a 20mm hardened linear shaft, and "UU" indicates that both ends are sealed with rubber contact seals to protect the internal ball retainer from dust and contamination.
In terms of the LM20UU size, the bearing has an outer diameter of 32mm and a body length of 42mm. These three dimensions, 20mm bore, 32mm outer diameter, and 42mm length, are exactly what you would arrive at by measuring an unlabeled part and cross-referencing a JIS B 1192 linear bearing table. The LM20UU sits at a practical midpoint in the LM series, offering meaningfully more load capacity than the smaller 8mm and 12mm variants while remaining compact enough for desktop CNC machines, laser systems, and pick-and-place gantries where a 20mm shaft is already specified.
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When Dimensions Alone May Not Be Sufficient
Standard dimension tables cover the vast majority of bearings in general industrial use, but there are cases where additional verification is needed. Some equipment manufacturers specify bearings to non-standard dimensions, particularly in older machinery from Japan and Europe. A bearing may share the bore and outer diameter of a standard part but differ in internal clearance classification, load rating, or cage material. Two bearings with identical external measurements can carry meaningfully different dynamic load capacities depending on the number of ball rows and the ball diameter.
Material variants also require attention. Standard LM series linear bearings use carbon steel construction, but stainless steel versions are available for corrosive or washdown environments. These carry the same dimensional designation but are not interchangeable in every application. When specifying a replacement for a production environment, confirming both the dimensional code and the material specification is the correct practice.
Summary
Finding a bearing part number from physical measurements follows a straightforward process: measure bore, outer diameter, and width or length; identify the bearing type; cross-reference a standard dimension table; and confirm any stamped markings if present. The logic built into bearing naming systems, including the LM series convention illustrated by the LM20UU, means that accurate measurements will lead you to the correct part number in most cases. Where application requirements are critical, always validate load ratings, material grade, and internal clearance against the manufacturer's datasheet before finalizing a specification.
