Noise grades for motor bearings: ZV1 to ZV4, EMQ and CM
What ZV1 to ZV4, EMQ and CM actually specify, how bearing vibration is measured, and why grease fill, cage and seal choice change the sound of a motor bearing.
What a noise grade really specifies
A noise grade is a vibration acceptance class. The bearing is run on a measuring spindle under defined conditions and its vibration is measured; the grade fixes the limit the measured value must not exceed. It says nothing directly about the sound pressure level of the motor, fan or appliance that will contain the bearing.
That distinction matters because the finished machine is a system. Structure-borne vibration, resonance, rotor balance, the housing and the mounting surface all dominate what a user hears. A bearing with a tight vibration grade is a necessary condition for a quiet appliance, not a sufficient one. When a noise complaint appears, the bearing is one candidate among several, and it should be tested before it is blamed.
The ZV scale: ZV1 to ZV4
The ZV numbers form a scale used in the Chinese bearing industry for low-noise motor and appliance bearings. The common ordering runs from ZV1 upward, with the higher number denoting a tighter requirement and a lower permitted vibration value. The exact limits, the measured quantity and the test conditions are set by the maker, and different makers do not always use the same assignments.
This is why a grade label cannot be compared across suppliers without the underlying specification. Two bearings described as ZV3 may have been measured at different speeds, with different measuring loads or on different instruments, and their limit values may not be the same. A technical comparison should always use the measured quantity and the limit table, not the grade name.
Vibration grading usually distinguishes velocity from acceleration. Velocity is sensitive to lower-frequency effects such as unbalance, misalignment and loose fits. Acceleration is more sensitive to the high-frequency events that a listener perceives as hiss or roughness: ball and raceway waviness, contamination, cage impacts. A low-noise motor grade is typically defined in the quantity that exposes those short-wavelength defects.
EMQ and CM
EMQ stands for Electric Motor Quality; CM is a motor-bearing designation used by several makers. Both are trade designations rather than a published standard: they indicate a bearing configured for electric-motor duty, and in practice usually imply a controlled vibration level, a defined clearance group, a suitable cage and a controlled grease fill.
What each designation covers is set by the maker: ask which of vibration limit, clearance, cage, grease and closure it fixes, and ask for the values. A buyer who compares EMQ against ZV4 without knowing what each covers is comparing labels, not bearings.
How bearing vibration is measured
The measurement method is standardised: a single bearing is mounted on a spindle, rotated at a defined speed, and outer-ring vibration is sensed radially or axially. ISO 15242-2 and GB/T 24610.2 describe the method for radial ball bearings with a cylindrical bore.
The result depends on the whole set-up: speed, measuring load, lubrication, sensor position, spindle condition and bearing cleanliness. This has practical consequences. A limit value is meaningful only together with the method that produced it, so the purchase specification should state the standard, the measured quantity and the acceptance value. Comparison between two suppliers is only valid on the same basis. And the bearing must be clean and correctly lubricated at the time of measurement; a fingerprint or a dry raceway changes the reading.
Grease, cage and seals: what changes the sound
The rings and balls set the baseline, but the peripheral parts decide whether that baseline is audible.
- Grease: type, base oil viscosity and fill quantity affect damping and churning. Too little leaves the raceway under-lubricated and noisy; too much can raise friction and temperature in small bearings. A custom grease is a legitimate way to tune noise.
- Cage: steel and nylon cages differ in mass, pocket clearance and damping. A lighter cage with controlled pocket clearance produces less impact noise; a nylon cage also runs quieter when lubrication is marginal.
- Seals and shields: a contacting rubber seal adds friction and can contribute its own sound; a shield or non-contacting seal runs with lower friction. Closure choice therefore trades protection against noise.
None of these parts can compensate for a poor raceway. They change the measured result at the margin, which is exactly the margin a low-noise application is buying.
Writing a noise requirement into an RFQ
A noise requirement is only useful if it is testable. State the following:
- The grade you want, for example ZV3, EMQ or CM, as the starting point.
- The measurement standard and the measured quantity, velocity or acceleration.
- The test speed, measuring load and sensor direction, if your application is sensitive to them.
- The acceptance limit and the sample size for approval.
- The grease, cage and closure, since each affects the result.
- The assembled test you will apply to the motor or appliance, if noise is a final acceptance criterion.
Then ask the supplier to confirm the values behind the grade in writing, and approve a sample on your own test bench. If two quotations use different noise schemes, normalise them to the same measurement basis before comparing price.
Sources
- ISO 15242-2:2015, Rolling bearings: measuring methods for vibration, part 2, radial ball bearings with cylindrical bore and outside surface.
- GB/T 24610.2, Rolling bearings: measuring methods for vibration, part 2, radial ball bearings with cylindrical bore and outside surface.