Understanding Bearing Internal Clearance: The "Goldilocks" of Machinery

Why Your Bearings Need "Wiggle Room" to Survive

If you pick up a brand-new ball bearing and give the inner ring a wiggle, you might feel a tiny bit of movement. Is it defective? Is it low quality?

Likely, it is exactly as it should be. That movement is called Bearing Internal Clearance, and it is one of the most critical—and most misunderstood—factors in bearing selection. Getting it wrong is a leading cause of premature machinery failure.

In this guide, we’ll break down what internal clearance is, why "C3" is such a common term, and look at the specific micron values that define these fits.

What is Bearing Internal Clearance?

Simply put, internal clearance is the total distance one bearing ring can move relative to the other when there is no load applied. Think of it as the "wiggle room" inside the bearing.

It is divided into two types:

1. Radial Internal Clearance (RIC): The movement of the inner ring up and down (perpendicular to the shaft).

2. Axial Internal Clearance: The movement of the inner ring back and forth (parallel to the shaft).

Crucial Distinction: Clearance is NOT precision. A bearing can have "Normal" precision (standard manufacturing tolerances) but "C3" clearance (extra internal space). Do not confuse the quality of the bearing with the amount of space inside it.

The "Goldilocks" Principle: Why Clearance Matters

Bearing clearance follows the Goldilocks principle: it has to be "just right." However, there is a catch. The clearance you feel when the bearing is in your hand (Initial Clearance) is not the clearance the bearing will have when it is running (Operating Clearance).

1. Too Little Clearance (Tight)

If a bearing operates with zero or negative clearance (preload) without being designed for it, friction skyrockets.

• The Result: Rapid heat generation, thermal runaway, and eventually, the bearing seizes (locks up) catastrophically.

2. Too Much Clearance (Loose)

If the bearing is too loose during operation, the load is not distributed evenly across the rolling elements.

• The Result: Excessive vibration, noise, and "skidding" of the balls/rollers, which significantly shortens the fatigue life of the metal.

Decoding the Designations: C2, CN, C3, C4

Manufacturers use a standardized code system (ISO 5753-1) to define clearance.

• C2 (Tight): Less clearance than Normal. Used for high precision and low vibration where temperature is stable.

• CN (Normal): The standard. Often not marked on the bearing (e.g., a "6205" is implied CN).

• C3 (Loose): Greater clearance than Normal. The industry standard for electric motors.

• C4 (Extra Loose): Greater clearance than C3. Used for vibrating screens or very hot environments.

Cheat Sheet: Clearance by the Numbers (Microns)

Maintenance teams often ask: "How much difference is there really?"

Below is a comparison of Radial Internal Clearance values for standard Deep Groove Ball Bearings (e.g., 62xx, 63xx series).

(Values in Microns, μm)

Bore Diameter (mm)	Bearing Code (Last 2 Digits)	C2 (Tight)	CN (Normal)	C3 (Loose)	C4 (Extra Loose)
10 – 18 mm	00, 01, 02, 03	0 – 9	3 – 18	11 – 25	18 – 33
18 – 24 mm	04	0 – 10	5 – 20	13 – 28	20 – 36
24 – 30 mm	05, 06	1 – 11	5 – 20	13 – 28	23 – 41
30 – 40 mm	06, 07	1 – 11	6 – 20	15 – 33	28 – 46
40 – 50 mm	08, 09	1 – 11	6 – 23	18 – 36	30 – 51
50 – 65 mm	10, 11, 12, 13	1 – 15	8 – 28	23 – 43	38 – 61
65 – 80 mm	14, 15, 16	1 – 15	10 – 30	25 – 51	46 – 71
80 – 100 mm	17, 18, 19, 20	1 – 18	12 – 36	30 – 58	53 – 84

Note: These ranges represent the ISO standard. As you can see, the overlap is minimal. A high-end Normal bearing is still tighter than a low-end C3 bearing.

From Box to Shaft: Where Does the Clearance Go?

You start with the clearance in the table above, but you lose it quickly.

1. Interference Fits: Pressing a bearing onto a shaft stretches the inner ring, reducing clearance by up to 80% of the interference fit.

2. Thermal Expansion: In operation, the inner ring (shaft) is usually hotter than the outer ring (housing), causing it to expand and eat up the remaining clearance.

The Formula for Failure:

If you select a "Normal" clearance bearing (CN), press it onto a shaft tightly, and run it hot, you will likely end up with Negative Operating Clearance. The bearing will overheat and fail. This is why C3 is the safe bet for most electric motors and pumps.

Summary

Don't ignore the suffix on your bearing part number. Replacing a 6310-C3 with a standard 6310 might save you a few dollars today, but it could cost you thousands in downtime next week when the machine seizes.

Key Takeaways:

• Clearance is the internal "play" in the bearing.

• Clearance reduces once the bearing is mounted and running.

• C3 is looser than Normal; it is the industry standard for applications involving heat or tight press fits.

• Always match the clearance spec of the bearing you are replacing.