Ball Bearing Precision Grades: Differences Between Metal and Plastic Bearings
The precision of a ball bearing is generally divided into two main categories: Dimensional Accuracy and Rotational Accuracy.
● Dimensional Accuracy
This refers to precision in a stationary state.
It includes tolerances (permissible deviations) for dimensions such as outer diameter, inner diameter, width, chamfers (R), and more.
● Rotational Accuracy
This refers to precision during operation.
It includes the permissible deviation (tolerance) of radial and axial runout of the inner and outer rings while the bearing is rotating.
(Radial = perpendicular to the shaft / Axial = parallel to the shaft)
Each of these categories includes various specific criteria with clearly defined tolerance ranges, which are grouped into precision grades. These grades are standardised internationally, allowing one to understand the bearing’s precision even when comparing products from different manufacturers.
● About Ball Bearing Standards
Ball bearing precision grades are defined by standards such as JIS (JIS B 1514) in Japan and ISO internationally. Other standards include DIN (Germany) and ABEC (USA). While U.S. standards have slightly different tolerance values, they are generally comparable.
Let’s take a look at JIS, the most commonly used standard in Japan.
● JIS Grades for Ball Bearings
JIS grades include: Grade 0, 6, 5, 4, and 2 — with Grade 0 being the least strict and Grade 2 the most precise.
By default, when ordering metal ball bearings without specifying a grade, the product is typically supplied as Grade 0 (standard grade). However, even Grade 0 does not imply a loose tolerance — it still reflects high precision.
Let’s take a practical example using dimensional accuracy for better understanding:
■ Example: Deep Groove Bearing #6005 (Nominal size: Outer diameter 47 mm / Inner diameter 25 mm / Width 12 mm)
● Grade 0 tolerances:
- Outer diameter: 0 to -11 µm
- Inner diameter: 0 to -10 µm
Converted to millimetres, this means:
- Outer diameter must be between 47.000 mm and 46.989 mm
- Inner diameter must be between 25.000 mm and 24.990 mm
This gives you an idea of how strict the dimensional tolerances are, along with radial and axial runout values for both inner and outer rings.
To those familiar with metal machining, this might seem standard — but achieving this level of precision in mass-produced products is truly impressive.
● Grade 2 tolerances (highest precision):
- Outer diameter: 0 to -4 µm
- Inner diameter: 0 to -2.5 µm
Converted values:
- Outer diameter: 47.000 mm to 46.996 mm
- Inner diameter: 25.000 mm to 24.9975 mm
At this level, the bearing is manufactured to such high roundness that it ranks among the most precisely produced components in the world.
Grade 2 and Grade 4 bearings are typically used in spindles of machine tools or other applications requiring high-speed rotation and minimal runout.
However, to take full advantage of this precision, the surrounding components — such as shafts and housings — must also be manufactured to very tight tolerances. Additionally, because extremely high-precision components can be diverted for military use, export restrictions (such as ITAR) may apply.
As a result, Grade 0 bearings are sufficient for most general industrial applications.
■ Precision in Plastic Bearings
Now that we’ve explained the grading system for metal bearings, let’s talk about plastic ball bearings.
We are often asked:
“What grade are your plastic ball bearings? Are they standard grade?”
To which we usually answer:
“Unfortunately, they cannot be classified using the traditional grading system.”
Plastic bearings do not fit within the tolerance ranges defined for metal bearing grades — precisely because they are plastic. In fact, they are often deliberately manufactured outside of these grades based on the material’s properties.
Why Plastic Bearings Can’t Be Graded Like Metal Bearings
The primary reason lies in the material characteristics of plastic.
Plastics have very different thermal expansion coefficients and water absorption rates compared to metals.
Even if a plastic bearing is manufactured to the equivalent of Grade 0 at the time of production, changes in temperature and moisture over time will alter its dimensions, pushing it outside the strict tolerance ranges used for metal bearings.
Let’s look at some examples of thermal expansion coefficients (per °C):
- SUJ2 steel (standard for metal bearings): 12 × 10⁻⁶
- Ultra-high-molecular-weight polyethylene (UHMWPE): 10–19 × 10⁻⁵
- POM: 10 × 10⁻⁵
- PPS: 5 × 10⁻⁵
As you can see, plastic materials expand or contract much more significantly with temperature. In addition, many plastics absorb moisture, further affecting dimensions.
Deliberate Deviation from Precision Grades
Another reason plastic bearings aren’t graded is intentional design.
Bearings consist of an inner ring, outer ring, and balls between them. There is a very slight clearance between these components, known as internal clearance or “play.”
To increase precision, both dimensional and rotational accuracy must be improved, which naturally reduces this internal clearance.
However, plastic bearings are more susceptible to environmental changes (e.g. temperature, moisture, self-heating during rotation). If the internal clearance becomes too tight, it can result in seizing or poor rotation.
Therefore, at Kashima Bearings, we manufacture our plastic bearings with different clearances depending on the material:
Around 50 µm for PEEK and PPS
Around 200 µm for UHMWPE
Dimensional Stability Ranking of Common Plastic Materials
Here is a general ranking of commonly used bearing materials, from most to least dimensionally stable:
Carbon > PPS > PEEK > PET > POM > Phenolic Resin > PP > Filled PTFE > UHMWPE > PTFE
(Note: the order may vary depending on the application environment.)
Conclusion
This is why plastic bearings cannot be assigned a standard precision grade like metal bearings.
Their performance is defined not by strict tolerances, but by careful material selection and design adapted to their specific operating environments.
