Z1–Z4 are bearing vibration-acceleration (noise) grades. The scale works like this: the higher the number, the quieter the bearing — Z4 is the quietest grade. Motor and appliance buyers specify it constantly: "this batch must be Z3" or "keep noise under 32 dB". This guide covers the grade definitions, the dB limit table by bore size, how to choose, and how to write inspection requirements that survive a dispute.
What Z1–Z4 actually measures
"Bearing noise" is not judged by ear in a factory — it is measured on a vibration tester (BVT). The instrument measures vibration acceleration while the inner ring rotates at a standard speed, reported in dB. Deep groove ball bearings are graded into four classes:
| Grade | Level | Typical applications |
|---|---|---|
| Z1 | Standard noise | General transmission, power tools, ordinary machinery |
| Z2 | Low noise | Standard electric motors, fans, appliances |
| Z3 | Extra-low noise | Air conditioner motors, washing machines, precision motors |
| Z4 | Ultra-low noise | Premium appliances, silent equipment, precision instruments |
The dB limit table (grouped by bore)
The same Z3 grade allows different absolute values for different bearing sizes — larger bearings tolerate more vibration. Check the bore column first, then read across. Values below are deep groove ball bearing vibration acceleration limits in dB:
| Bore d (mm) | Z1 (dB) | Z2 (dB) | Z3 (dB) | Z4 (dB) |
|---|---|---|---|---|
| 3–6 | 36 | 32 | 28 | 24 |
| >6–10 | 38 | 34 | 30 | 26 |
| >10–18 | 41 | 36 | 32 | 28 |
| >18–24 | 43 | 38 | 34 | 30 |
| >24–30 | 45 | 40 | 36 | 32 |
| >30–40 | 47 | 42 | 38 | 34 |
| >40–50 | 49 | 44 | 40 | 36 |
| >50–60 | 51 | 46 | 41 | 37 |
| >60–80 | 54 | 49 | 44 | 40 |
| >80–100 | 57 | 52 | 47 | 43 |
| >100–120 | 60 | 55 | 50 | 46 |
How to choose the grade quickly
- General machinery, transmission, power tools: Z1 is sufficient and the most cost-effective
- Motors, fans, pumps: Z2 minimum — the standard motor grade
- Air conditioners, refrigerators, washing machines: Z3 and up — the user's ear is next to the machine; quiet is the selling point
- Premium silent equipment, precision instruments: Z4, with batch test reports required from the supplier
Every grade step costs money and lead time. Specify what the application needs, not the highest number available.
Three inspection points that protect the buyer
- Write it into the contract: part number + grade + standard — e.g.
6203-2RS Z3 per JB/T 7047. "Low noise" without a grade number is unenforceable. - Require test data: a legitimate factory tests every batch at Z2 and above on the vibration tester, with results traceable by batch number.
- Isolate the variables: if the assembled motor is noisy, the bearing grade is only one suspect — fit tolerances, rotor balance and grease all contribute. Test components separately before assigning blame.
Z grades vs V grades — two parallel systems
Z measures vibration acceleration (dB); V measures vibration velocity (µm/s). They are parallel systems with different instruments and different sensitivities:
- Z is sensitive to impact-type defects — pits, spalling, contamination on the raceway produce sharp acceleration peaks
- V is sensitive to continuous-wave defects — roundness and waviness show up in velocity values
Domestic Chinese motor industry traditionally specifies Z; some OEMs and export orders specify V (per the ISO 15242 velocity framework). Precision motor orders often require both.
What NLHB measures in-house
NLHB operates its own vibration testers in Cixi — grinding, superfinishing and assembly are all under our control. Z2 and Z3 are regular production grades; Z4 is run by project. For RFQs, send part number + quantity + noise grade requirement, and we quote with the test data we can provide per batch.
See also our Quality & Sourcing Process for everything we verify before quoting.
FAQ
What is Z3 in bearing noise?
Z3 is the extra-low-noise grade on the Z1–Z4 vibration acceleration scale — the typical grade for air conditioner motors, washing machines and precision motors. For a 6204 (bore 20 mm), the Z3 limit is 34 dB.
Is Z4 quieter than Z1?
Yes. On the Z scale, higher numbers are quieter: Z4 is the quietest grade, Z1 the standard grade. The same applies to the V (velocity) scale — V4 is quietest.
How is bearing noise tested?
On a bearing vibration tester (BVT): the inner ring rotates at a standard speed under a defined load while a sensor reads outer-ring vibration. Acceleration results in dB map to Z1–Z4 grades.
Does a quiet bearing grade guarantee a quiet motor?
No. Bearing grade controls the bearing only. Fit tolerances, rotor unbalance, grease and housing stiffness all affect assembled motor noise. Test components separately when tracing noise.
Which grade do electric motors need?
Z2 is the standard motor grade. Appliance and HVAC motors typically specify Z3. Premium silent equipment goes to Z4 with batch test reports.
Related resources
- Bearing Precision Grades P0 to P5 — precision is a separate axis from noise
- Bearing Clearance Explained: C2, CN, C3, C4
- 6204 Bearing Guide — what NLHB verifies before quoting