A Practical 3-Step Guide: How Do You Measure a Bearing for Replacement in 2025?

Abstract

The accurate measurement of a rolling bearing is a foundational requirement for sourcing a correct replacement, directly impacting machinery performance, longevity, and operational safety. This text provides a comprehensive exploration of the methodologies for determining a bearing's critical dimensions. It focuses on the three primary measurements: the inside diameter (bore), the outside diameter, and the width. The process necessitates the use of precision instruments, with a digital caliper being the most common and accessible tool for most applications. The discussion extends beyond simple dimensional checks to include the identification of specific features like seals, shields, and internal clearances, which are often denoted by suffixes in the bearing's part number. An improper measurement or misinterpretation of these features can lead to incorrect bearing selection, resulting in premature failure, costly downtime, and potential damage to associated components. This guide systematically breaks down the procedure, addressing common errors and offering solutions for challenging scenarios, such as measuring damaged or unmarked bearings, to ensure a successful and reliable replacement.

Key Takeaways

• Use a digital caliper for accurate bore, outside diameter, and width measurements.
• Always clean the bearing thoroughly before taking any measurements to ensure precision.
• Take multiple measurements at different points and average them for a reliable result.
• Knowing how do you measure a bearing correctly prevents costly downtime and equipment damage.
• Record part numbers and suffixes, as they provide vital data about seals and clearance.
• Compare your measurements to standard bearing size charts to confirm the part.
• When in doubt, consult a bearing specialist for critical or complex applications.

Table of Contents

• The Foundational Importance of Precise Bearing Measurement
• Step 1: Accurately Measuring the Bearing's Inside Diameter (Bore)
• Step 2: Determining the Outside Diameter (OD) with Confidence
• Step 3: Finalizing with the Width and Other Key Features
• Beyond the Basics: Advanced Measurement Scenarios and Bearing Types
• From Measurement to Selection: Using Your Data to Find the Right Replacement
• よくあるご質問
• Conclusion
• References

The Foundational Importance of Precise Bearing Measurement

The world of machinery, from the simplest electric motor to the most complex industrial gearbox, operates on a principle of motion. At the heart of this motion, one often finds the bearing, a component designed to reduce friction and guide moving parts. When a bearing reaches the end of its service life, its replacement seems, on the surface, a straightforward task. Yet, the success of this operation hinges entirely on a single, foundational practice: precise measurement. A failure in this initial step can cascade into a series of mechanical failures, transforming a routine maintenance task into a significant operational and financial setback. The act of measuring a bearing is not merely a procedural step; it is an exercise in mechanical empathy, an attempt to understand the exact spatial requirements the component must fulfill to perform its duty flawlessly.

Why a "Close Enough" Measurement Is a Recipe for Failure

The temptation to accept a "close enough" measurement is a common pitfall, born from haste or a lack of proper tools. Imagine trying to open a high-security lock with a key that is just a fraction of a millimeter off. The key might slide in, it might even feel close to turning, but it will not open the lock. Worse, it might jam, creating a more significant problem. A bearing is much the same. A replacement that is slightly too small on its outside diameter will spin in its housing, generating heat, wear, and destructive vibration. A bearing with a bore that is fractionally too small will require excessive force to mount onto a shaft, potentially damaging the shaft, the bearing's inner ring, or its delicate rolling elements. Conversely, a bore that is too large will result in a loose fit on the shaft, leading to fretting corrosion and a loss of rotational precision. These are not minor inconveniences; they are the direct precursors to catastrophic failure, unscheduled downtime, and costs that far exceed the price of the bearing itself. The pursuit of precision is, therefore, an economic imperative.

Understanding the Language of Bearings: Key Dimensions Defined

To measure something effectively, one must first understand what is being measured. A standard rolling bearing has three primary dimensions that form its universal identification code. The consideration of these dimensions is a fundamental aspect of the process outlined by industry leaders (NSK Ltd., 2024).

1. Bore Diameter (d): This is the measurement of the hole in the center of the bearing. It is arguably the most important dimension, as it dictates the fit of the bearing onto its corresponding shaft. It is universally abbreviated with a lowercase 'd'.

2. Outside Diameter (D): This is the measurement across the full exterior of the bearing. This dimension determines the fit of the bearing within its housing or enclosure. It is abbreviated with an uppercase 'D'.

3. Width (B) or Height (H): This is the measurement of the bearing's thickness. For radial bearings (like most ball and roller bearings), it is called the Width and abbreviated with an uppercase 'B'. For thrust bearings, which handle axial loads, this dimension is referred to as Height and is abbreviated with an uppercase 'H'.

These three dimensions, expressed in the order d x D x B, form the fundamental language for identifying a majority of bearings worldwide.

The Tools of the Trade: Selecting Your Measurement Instrument

The quality of a measurement is inextricably linked to the quality of the tool used. While a simple tape measure or ruler might suffice for rough estimations in carpentry, it is wholly inadequate for the tolerances required in mechanical engineering. The necessary precision demands a more sophisticated instrument.

Instrument	Best For	Precision Level	Ease of Use	Typical Cost
Digital Caliper	General purpose (ID, OD, Width, Depth)	High (0.01mm – 0.03mm)	Very Easy	Low to Moderate
Vernier Caliper	General purpose (ID, OD, Width, Depth)	High (0.02mm – 0.05mm)	Moderate (Requires reading a scale)	Low
Micrometer	Single dimension (e.g., OD or ID)	Very High (0.001mm – 0.01mm)	Moderate to High (Specific to type)	Moderate to High

For the vast majority of bearing replacement tasks, the digital caliper represents the ideal balance of precision, versatility, and ease of use. It provides a clear digital readout, eliminating the potential for misreading a vernier scale. It can measure inside diameters, outside diameters, and widths with a single, affordable tool. Micrometers offer superior precision but are less versatile; a different micrometer is needed for internal versus external measurements, and each has a limited measurement range. For the purposes of identifying a standard replacement bearing, the precision of a quality digital caliper is more than sufficient.

Step 1: Accurately Measuring the Bearing's Inside Diameter (Bore)

The journey of measurement begins with the bore, the bearing's interface with the shaft. This dimension dictates the very foundation of the fit. An error here can compromise the entire assembly before it even begins to turn. The process requires a methodical approach, a steady hand, and an appreciation for the subtleties that can influence the final reading.

Preparing the Bearing for Measurement

Before a caliper ever touches the bearing, the component must be properly prepared. Any dirt, grease, or rust on the measurement surfaces will introduce errors.

First, thoroughly clean the bearing. For a sealed or shielded bearing, wipe the exterior surfaces and the bore with a clean, lint-free cloth. A mild solvent can be used if necessary, but take care not to flush the grease out of a sealed bearing that you might need to reuse temporarily. For an open bearing, a more thorough cleaning might be possible.

Second, perform a visual and tactile inspection of the inner ring. Run a finger around the bore. Are there any burrs, nicks, or areas of raised metal caused by a difficult dismounting process? These imperfections can catch the jaw of a caliper and give a false, smaller reading. If small burrs are present, they can sometimes be carefully removed with a fine file or deburring tool, though one must be exceedingly careful not to remove any of the original parent material. The goal is to measure the bearing's original manufactured dimension, not a dimension altered by damage.

The Caliper Technique for Bore Measurement

With a clean and inspected bearing, the measurement can begin. Using a digital caliper, the process is systematic.

1. Power On and Zero: Turn on the caliper. Close the jaws completely and press the "ZERO" or "ORIGIN" button. This ensures your measurement starts from a true zero point.
2. Position the Jaws: Use the smaller, outward-facing jaws at the top of the caliper, which are designed specifically for internal measurements. Insert these jaws into the bearing's bore.
3. Find the Diameter: Gently slide the thumb roller to open the jaws until they make light contact with the inner surfaces of the bore. The key here is to ensure you are measuring the true diameter, not a shorter chord. To do this, lightly rock the caliper back and forth in one direction while holding it steady in the other. You are looking for the maximum possible reading—this will be the true diameter. Simultaneously, ensure the jaws are inserted deep enough into the bore to be parallel with the bearing's central axis, not tilted at an angle.
4. Take the Reading: Once you are confident you have found the maximum reading, lock the jaws in place if your caliper has a locking screw. Read the dimension from the digital display.
5. Repeat and Average: Do not trust a single measurement. Rotate the bearing 90 degrees and repeat the process. Then, take a third measurement at a 45-degree angle to the first two. If the bearing is perfectly round and undamaged, these readings should be nearly identical. If there is slight variation due to wear, calculating the average of these three readings will give you the most representative dimension.

This repetition is not redundant; it is a vital cross-check that accounts for potential out-of-roundness from wear or damage, ensuring a more robust and reliable final number.

Common Pitfalls in Bore Measurement

Several common errors can undermine the accuracy of a bore measurement. Being aware of them is the first step toward avoidance.

• Measuring at an Angle: If the caliper is not held perpendicular to the face of the bearing, the jaws will be tilted within the bore. This will result in measuring an ellipse rather than a circle, giving a reading that is larger than the actual diameter. Imagine slicing a cylinder at an angle—the resulting cut surface is an oval, which is wider than the cylinder's diameter.
• Applying Excessive Force: Digital calipers are precision instruments, not clamps. Pushing too hard with the thumb roller can cause the slender internal jaws to flex slightly. This flexing will result in a measurement that is larger than the true bore diameter. The contact should be firm but light.
• Forgetting to Zero: It is a simple mistake, but failing to zero the caliper with the jaws closed before starting will add or subtract an error from every subsequent measurement. Make it a habit to zero the tool every time you turn it on.
• Measuring Debris: A small chip of metal or a patch of hardened grease can easily throw off a measurement by several hundredths of a millimeter, which is more than enough to specify the wrong bearing. The importance of cleaning cannot be overstated.

Special Considerations for Different Bearing Types

While the fundamental technique remains the same, some bearing types introduce unique challenges. For example, when examining large-diameter 旋回リングベアリング, the sheer size may preclude the use of standard calipers. In such cases, specialized large-format internal micrometers or even measurements of the mating shaft might be necessary.

A particularly common variation is the tapered bore. Some bearings are designed to be mounted on a tapered shaft or used with a tapered adapter sleeve. These bearings will have a different inside diameter at each face. When you encounter a tapered bore, you must measure both the small-end diameter and the large-end diameter. Additionally, the bearing's width becomes a critical component of defining the taper itself, which is often expressed as a ratio (e.g., 1:12 or 1:30). Identifying that a bore is tapered is the first step; measuring both ends is the necessary follow-through.

Step 2: Determining the Outside Diameter (OD) with Confidence

After securing the bore dimension, the focus shifts to the bearing's exterior. The outside diameter (OD) is just as vital as the bore, as it governs how the bearing seats within its housing. A loose fit can lead to spinning and wear, while an overly tight fit can compress the bearing, reducing its internal clearance, increasing friction and heat, and leading to a rapid demise.

The Standard Procedure for OD Measurement

Measuring the OD is typically more straightforward than measuring the bore, as it utilizes the main, larger jaws of the caliper, which are less prone to flexing. The principles of precision, however, remain the same.

1. Prepare the Surface: Just as with the bore, the outer surface of the bearing's outer ring must be clean and free from burrs, rust, or damage. Wipe it down thoroughly.
2. Position the Jaws: Open the main (external) jaws of your zeroed digital caliper so they are wider than the bearing. Bring the jaws into contact with the outer surface of the bearing.
3. Find the Diameter: Ensure the caliper is positioned across the widest point of the bearing. You can do this by gently wiggling the caliper back and forth as you close the jaws, feeling for the point of maximum dimension. The faces of the jaws must be flat against the bearing's outer ring, and the beam of the caliper should pass directly over the center of the bearing.
4. Take the Reading: With light but firm pressure, close the jaws and take the reading from the display.
5. Repeat and Average: Just as with the bore, one measurement is not sufficient. Rotate the bearing 90 degrees and measure again. Take a third measurement for confirmation. Averaging these readings mitigates errors from minor imperfections or out-of-roundness.

Navigating Challenges: Damaged or Worn Outer Rings

What happens if the outer ring is heavily corroded, pitted, or visibly damaged? Measuring a compromised surface will not yield the bearing's original dimension. In such situations, a different approach is required.

The most reliable alternative is to measure the housing where the bearing was seated. Using the same internal measurement technique as for the bearing's bore, carefully measure the inside diameter of the housing. This dimension should correspond directly to the bearing's original OD. This method is often more accurate than measuring a damaged bearing, as the housing is typically a more protected surface. However, one must also inspect the housing for wear or damage. If the old bearing was spinning in the housing, the housing itself might be worn to a larger diameter, which must be addressed before installing a new bearing.

How OD Affects Fit and Function

The relationship between the bearing's OD and the housing's bore is known as the "fit." There are three general categories of fits:

• Clearance Fit: The bearing's OD is slightly smaller than the housing's bore. This allows the bearing to be easily slid into place by hand. This is common in applications where the outer ring does not need to be rigidly fixed.
• Interference Fit (Press Fit): The bearing's OD is slightly larger than the housing's bore. This requires a press or thermal expansion (heating the housing or cooling the bearing) to install. This creates a tight, rigid connection that prevents the outer ring from spinning under load.
• Transition Fit: This fit falls between the other two, where depending on the exact tolerances of the specific bearing and housing, the fit could be either a small clearance or a small interference.

The required fit is determined by the application's design. Your job in measuring is to determine the original dimension so that a replacement with the correct standard OD can be selected to achieve the intended fit. An accurate OD measurement is the key to this entire process.

A Look at Flanged Bearings

Some bearings feature a flange on the outer ring. This flange acts as a shoulder to help locate the bearing axially and simplify housing design. When you encounter a flanged bearing, you must take additional measurements beyond the standard OD.

• Body OD: Measure the outside diameter of the main body of the bearing, as described above.
• Flange OD: Measure the overall diameter across the widest part of the flange.
• Flange Width: Measure the thickness of the flange itself.

These additional dimensions are critical for ensuring the replacement unit will fit correctly into the machined recess in the housing. A standard ボールベアリング catalog will list these dimensions separately for flanged varieties.

Step 3: Finalizing with the Width and Other Key Features

With the two most challenging dimensions, the bore and the OD, successfully recorded, the final measurement is typically the width. While often the simplest to take, it is no less important for ensuring the bearing fits correctly within its designated space and aligns properly with other components. Beyond this, a visual inspection for other features can provide the final pieces of the puzzle.

Measuring the Bearing Width (or Height)

The width of a bearing (designated 'B') is the distance between its two faces. For a thrust bearing, this same dimension is called the height ('H').

The measurement is straightforward with a digital caliper:

1. Ensure the faces of the bearing are clean.
2. Use the main external jaws of the caliper.
3. Place the bearing between the jaws and gently close them until they make contact with the flat faces.
4. Ensure the caliper is sitting flat and not on any raised lettering or part of the seal that protrudes from the face.
5. Take the reading.
6. As always, it is good practice to take measurements at several points around the bearing to check for consistency.

This dimension is vital for ensuring the bearing does not have excessive side-to-side movement (end play) and that any retaining clips or nuts will align correctly on the shaft or in the housing.

Identifying Radii and Chamfers

Look closely at the corners of the bearing, where the faces meet the bore and the OD. You will notice they are not sharp 90-degree angles. They are either rounded (a fillet radius) or beveled (a chamfer). These features are not just for show; they are engineered to accommodate the corresponding fillet radius on the shaft shoulder or in the housing shoulder where the bearing will seat.

While precisely measuring a small radius is difficult without specialized tools like a radius gauge or an optical comparator, it is an important feature to note. If a bearing with too small a corner radius is forced against a shaft with a larger fillet radius, the bearing will not seat properly. This will leave a gap, preventing the bearing from being secured tightly against the shoulder and potentially leading to misalignment and failure. When selecting a replacement, ensure its corner radius (often listed in catalogs as 'r') is appropriate for the shaft and housing it will be mounted on. A visual comparison with the old bearing is often sufficient for standard applications.

Decoding the Numbers: What If the Bearing Has Markings?

Often, the most reliable way to identify a bearing is not by measurement alone, but by reading the part number stamped or etched onto it. Before you even begin measuring, you should always thoroughly inspect the bearing's outer ring face, inner ring face, and sometimes the seals for any alphanumeric codes.

These numbers are the bearing's identity. A code like "6204-2RS C3" tells a complete story.

• "6204" is the basic part number, which defines the bearing series (62xx – Deep Groove Ball Bearing) and the bore size (04 = 20mm). This number directly corresponds to the dimensions: 20mm bore x 47mm OD x 14mm width.
• "2RS" is a suffix that indicates the bearing has a rubber seal on both sides.
• "C3" is another suffix indicating the bearing has a greater-than-normal internal clearance.

If you can find a part number, even a partial one, your task becomes much simpler. You can enter this number into an online search engine or a supplier's database to find the exact specifications, including the dimensions. This can then be used to confirm your own measurements. The process of how do you measure a bearing becomes a verification step rather than a primary identification method.

Putting It All Together: The d x D x B Formula

Once you have your three primary measurements, you should record them in the standard format: inside diameter (d) x outside diameter (D) x width (B).

For example, if you measured:

• Bore (d) = 25 mm
• Outside Diameter (D) = 52 mm
• Width (B) = 15 mm

You would record this as 25x52x15. This standardized format is understood globally and is the key you will use to search for a replacement bearing, such as the common 6205 series bearing. Comparing your measured values to a standard bearing chart is the final confirmation. If your measurements are very close (within a few hundredths of a millimeter) to a standard size, you have likely identified your bearing correctly.

Beyond the Basics: Advanced Measurement Scenarios and Bearing Types

The three-step process of measuring bore, OD, and width forms the bedrock of bearing identification. However, the vast world of bearings includes many specialized designs that require additional knowledge and measurement techniques. An understanding of these variations is what separates a novice from an experienced technician. The principles of bearing selection are complex, and a deep knowledge of different types is invaluable (SKF, n.d.-b).

The Challenge of Tapered Roller Bearings

Tapered Roller Bearings are masterpieces of engineering, designed to handle a combination of both radial and axial loads. Unlike a simple ball bearing, they are separable. The inner ring, rollers, and cage form a single assembly called the cone. The outer ring is a separate piece called the cup. They cannot be measured as a single unit.

To measure a tapered roller bearing, you must measure the components individually:

1. Cone Bore (d): Measure the inside diameter of the cone, following the standard procedure for bore measurement.
2. Cup OD (D): Measure the outside diameter of the cup, following the standard procedure for OD measurement.
3. Cone Width (B): Measure the width of the cone assembly across its face.
4. Cup Width (C): Measure the width of the cup across its face.

Often, these bearings are identified by two different part numbers, one for the cone and one for the cup. For example, a Set-4 bearing might consist of an LM11949 cone and an LM11910 cup. It is essential to identify both to ensure you get a matching set. The overall assembled width of the bearing is also a key parameter, but it is a resultant dimension determined by the specific cone and cup pairing.

Measuring Complex Assemblies: The Bearing Unit

A Bearing Unit is a pre-assembled component that consists of a bearing insert (typically a deep groove ball bearing) mounted within a housing. The most common types are pillow blocks (where the mounting surface is parallel to the shaft) and flanged units (where the mounting surface is perpendicular to the shaft).

When you need to replace a bearing in a Bearing Unit, the question becomes: are you replacing the entire unit or just the insert?

• Replacing the Insert Only: If the housing is in good condition, you can often just replace the insert bearing. In this case, your primary task is to measure the insert itself. You will need its bore (d), OD (D), and width (B). A key feature to note is that the OD of an insert bearing is often spherical, allowing it to pivot within the housing to accommodate misalignment. You will also need to identify the locking mechanism used to secure the insert to the shaft, which is commonly either setscrews or an eccentric locking collar.
• Replacing the Entire Unit: If you are replacing the whole assembly, you still need the shaft size (bore diameter). Additionally, you will need to measure the housing's critical mounting dimensions: the bolt hole spacing, the bolt hole size, and the height from the base to the shaft centerline (for pillow blocks).

An Examination of Plain Bearings

A Plain Bearing, often called a bushing or journal bearing, operates on a principle of sliding motion rather than rolling motion. They consist of a single, smooth sleeve. Despite their simpler construction, the measurement principles are analogous.

You will need to measure three key dimensions:

1. Inside Diameter (ID): The bore of the bushing.
2. Outside Diameter (OD): The external diameter of the bushing.
3. Length (L): The overall length of the bushing.

The measurement process is identical to that for rolling bearings, using a caliper for the ID, OD, and length. However, clearances are of particular importance in plain bearings, as they rely on a film of lubricant (like oil or grease) to separate the shaft and the bearing surface. The material is also a primary identifier—is it bronze, plastic, or a composite material? Some plain bearings are flanged, requiring the same additional flange measurements as a flanged rolling bearing.

The Unique Case of Linear Bearings

A Linear Bearing is designed for motion along a straight path (a shaft) rather than rotation. The most common type is the linear ball bushing. Measuring a linear bearing for replacement involves a similar set of dimensions.

• Inside Diameter (d): This must precisely match the diameter of the hardened and ground shaft it will run on.
• Outside Diameter (D): This determines the fit into the linear bearing block or housing.
• Length (L): This defines the overall length of the bearing.

In addition to these basic dimensions, you must observe the configuration. How many circuits of balls are there? Is the bearing open (with a section removed to accommodate a supported shaft) or closed? Are there seals on the ends? For complete linear motion systems, like profile rail guides, the measurements become far more complex, involving the rail width, block height, and mounting hole patterns.

A Word on Ball Bearings

The term Ball Bearings encompasses a vast family of the most common rolling bearing type. Deep groove ball bearings, angular contact ball bearings, and self-aligning ball bearings all fall under this category. The fundamental three-step measurement process—bore, OD, width—is the primary method for identifying all of them. The structural differences between them (as detailed in resources from manufacturers like NTN Corporation (2024)) dictate their load-carrying capabilities but do not fundamentally change how their basic dimensions are determined. The key is to recognize that after measuring, you may need to visually identify the type (e.g., the two-part ring of an angular contact bearing) to select the correct functional replacement, even if the core dimensions are identical to a simpler deep groove ball bearing.

From Measurement to Selection: Using Your Data to Find the Right Replacement

Having meticulously measured your bearing and noted its features, you are now armed with the data needed to find a replacement. This final phase of the process involves translating your raw numbers into a specific part number. This is where your careful work pays off, allowing you to navigate supplier catalogs and technical charts with confidence.

Translating Millimeters to Inches (and Vice Versa)

The vast majority of rolling bearings are manufactured to metric (millimeter) dimensions, even those used in machinery built in countries that traditionally use the imperial (inch) system. It is highly likely that your measurements in millimeters will correspond to clean, round numbers (e.g., 25.00 mm, 52.00 mm, 15.00 mm).

However, some bearing series, particularly tapered roller bearings and certain types of ball bearings made for the North American automotive and agricultural markets, are made to inch-based dimensions. If your millimeter measurements do not seem to match any standard metric size, it is wise to convert them to inches.

• Conversion Factor: 1 inch = 25.4 millimeters.
• To convert millimeters to inches, divide by 25.4.
• To convert inches to millimeters, multiply by 25.4.

For example, if you measure a bore of 25.41 mm, it is almost certain that you have a 1-inch bore bearing. A measurement of 1.25 inches (31.75 mm) is another common inch-series size. Being comfortable with this conversion can resolve many identification puzzles.

Using Online Catalogs and Bearing Interchange Guides

With your d x D x B dimensions in hand, you can now turn to an online bearing supplier's website or a digital catalog. Most reputable suppliers have a parametric search function. You can input your measured bore, OD, and width, and the system will return a list of bearings that match those dimensions.

This is often the point where you will discover that multiple bearing types or series can share the same basic dimensions. For example, a 20x42x12 measurement could correspond to a 6004 deep groove ball bearing, but also potentially other types. This is where your visual identification of the bearing type (ball vs. roller, sealed vs. open) becomes critical for narrowing down the results.

A bearing interchange guide is another powerful tool. If you were able to retrieve a part number from the old bearing, but your local supplier stocks a different brand, you can use an interchange guide. These guides cross-reference part numbers from various manufacturers (e.g., SKF, NSK, NTN, Timken, FAG) to find functionally equivalent bearings.

Common Suffix	Meaning	Typical Application / Implication
2RS / 2RSR / LLU	Two Rubber Seals	Bearing is sealed on both sides to retain grease and keep out contaminants. Used in dirty or wet environments.
ZZ / 2Z	Two Metal Shields	Bearing is shielded on both sides. Offers better high-speed capability than seals but less protection against fine dust/liquids.
OPEN	No Seals or Shields	Bearing is open on both sides. Requires external sealing or operates in a clean, lubricated environment (e.g., an oil bath).
C3	Internal Clearance > Normal	Allows for more thermal expansion. Used in applications with high speeds, high temperatures, or press fits on both rings.
C4	Internal Clearance > C3	For even more extreme temperature or speed conditions. Common in high-performance engines and industrial machinery.
J / TN9	Stamped Steel / Polyamide Cage	Describes the material of the cage that separates the rolling elements. Steel is standard; polyamide can offer better performance in some high-speed or vibration scenarios.
P5 / P4	High Precision	Indicates the bearing is manufactured to a higher tolerance class (lower runout). Used in machine tool spindles and precision instruments.

Understanding Suffixes and Prefixes in Part Numbers

The string of letters and numbers after the basic part number are not random; they are a vital code that describes the bearing's specific features and capabilities. Misinterpreting or ignoring these suffixes is a common cause of premature bearing failure. The selection process involves a number of criteria beyond simple dimensions (SKF, n.d.-a).

Imagine replacing a "6205-2RS/C3" from an electric motor with a standard "6205". The standard 6205 may be open (lacking seals) and have normal clearance. In the motor, it would quickly become contaminated by dust, and the tighter clearance would cause it to overheat at operating speed, leading to failure. The "2RS" indicated it needed seals, and the "C3" indicated it needed extra internal clearance to accommodate the heat and fits. Understanding these suffixes is as important as getting the dimensions right. The table above provides a brief overview of some of the most common suffixes you will encounter.

When to Seek Professional Help

The skills outlined here will empower you to correctly identify and replace the vast majority of common bearings. However, there are situations where the stakes are too high or the application is too complex for a non-specialist.

If you are dealing with a bearing in a high-speed, high-precision application (like a CNC machine spindle), a piece of safety-critical equipment, or a very large and expensive industrial machine, it is always the wisest course of action to consult a bearing specialist or application engineer. They have access to advanced diagnostic tools, in-depth technical documentation, and the experience to analyze the failure mode of the old bearing, ensuring that the replacement is not just a component of the same size, but the optimal component for the job. The question of how do you measure a bearing is the first step; ensuring the longevity of the entire system is the ultimate goal.

よくあるご質問

What if I can't read any numbers on the old bearing? If the part numbers are worn off or corroded, you must rely entirely on measurement. Follow the three-step process meticulously: measure the bore (d), outside diameter (D), and width (B). Then, use these dimensions to search an online bearing catalog. Pay close attention to visual details like seals, shields, or the type of rolling element to narrow down the options.

Can I use a regular ruler to measure a bearing? No, a ruler or tape measure is not precise enough. Bearings are manufactured to tolerances of hundredths of a millimeter (or thousandths of an inch). A ruler lacks the necessary resolution and can lead to selecting the wrong size. A digital or vernier caliper is the minimum requirement for accurate measurement.

How do I know if I need a high-temperature bearing? Look for suffixes on the original bearing, such as "C3" or "C4" for higher internal clearance, which accommodates thermal expansion. Also, consider the application. If the bearing operates inside an engine, oven, or high-speed motor, it likely requires special grease and potentially C3/C4 clearance. If the original bearing failed due to discoloration (blue or brown), it was likely overheating, indicating a high-temperature solution is needed.

My measurement is slightly different from the standard size. Why? This can happen for a few reasons. The bearing might be worn, causing the dimensions to change slightly. There could be a small amount of rust or a burr on the measurement surface. It could also be an inch-series bearing, and your millimeter reading will seem like an odd number until you convert it (e.g., 25.4mm = 1 inch). Always compare your reading to the nearest standard metric or inch size.

What does the "C3" on my bearing mean? "C3" is a very common suffix that denotes the bearing has a radial internal clearance greater than the "Normal" or "CN" standard. This extra room allows the bearing to accommodate heat expansion, slight misalignment, or press fits without becoming tight and generating excessive friction. It is commonly found in electric motors and other applications that generate heat.

Is it better to measure the bearing or the shaft/housing it fits into? Ideally, you should measure both. Measuring the bearing gives you the direct part information. Measuring the shaft and housing tells you what size the bearing should be and also helps diagnose wear. If the old bearing is badly damaged, measuring the shaft (for the bore) and the housing (for the OD) is often the more reliable method.

How do you measure a bearing without a caliper? While highly discouraged, if you are in an emergency situation with no caliper, you can try to get a rough estimate. You could try fitting drill bits of a known diameter into the bore to find the closest fit. For the OD, you could use an adjustable wrench, tighten it onto the bearing, and then measure the jaw opening with a ruler. These methods are highly inaccurate and should only be used as a last resort to get a "ballpark" figure before acquiring a proper tool.

What are the most common mistakes when measuring bearings? The most frequent errors are: not cleaning the bearing properly, using an inaccurate tool like a ruler, applying too much force and flexing the caliper jaws, measuring at an angle instead of across the true diameter, and ignoring suffixes on the part number.

Conclusion

The act of determining a bearing's dimensions transcends a simple mechanical chore; it is the foundational dialogue between a worn component and its successor. The precision of this dialogue, conducted with the right tools and a methodical mindset, dictates the future reliability and health of the machine. By diligently measuring the bore, outside diameter, and width, and by paying close attention to the subtle language of part numbers and suffixes, one moves from a position of uncertainty to one of informed confidence. This disciplined approach ensures that the replacement bearing is not merely a part that fits, but the correct part that performs. It transforms a moment of repair into an act of restoration, safeguarding the seamless motion that underpins our mechanical world.

References

NSK Ltd. (2024). Rolling bearings for industrial machinery. NSK.

NTN Corporation. (2024). Ball and roller bearings technical explanation. NTN Global.

SKF. (n.d.-a). Selection criteria. SKF Group. Retrieved February 28, 2025, from

SKF. (n.d.-b). Principles of rolling bearing selection. SKF Group. Retrieved February 28, 2025, from