Noise grade, internal clearance and grease are one decision, not three catalogue options. A motor bearing specified as C3, Z2 and a high temperature grease behaves as one system, and a mismatch shows up as heat, noise or early failure rather than a fault on any single datasheet line. Here is how the three interact and what to fix before the order.
Quick answer
- Specify the clearance group against the fit, not the noise. A tight shaft fit can remove approximately 70 to 90 percent of the effective interference, so a CN bearing can run at negative clearance in a hot motor.
- C3 is the usual motor answer, CM the compromise, C4 is not a free upgrade. C3 is the most common upgraded clearance in practice, used in electric motors, fans and pumps.
- Noise grade Z1, Z2 and Z3 is a separate axis from clearance. The published 6203 offers precision P0, P6 and P5, clearance C0, C2 and C3, vibration V1, V2 and V3 and noise Z1, Z2 and Z3 as four independent axes.
- A sealed bearing runs slower than an open one. The contact seal generates heat, so take the speed limit from the sealed variant data, not the open catalogue.
- Grease has to be stated as a material with a temperature range and a fill quantity. Our in-house process checks grease application on every batch, alongside noise, sealing, hardness and dimensional checks.
Our bearing range covers deep groove ball bearings in open and sealed versions, ceramic and hybrid series and track rollers, and the 6002 ZZ page describes the family our motor customers order from most often. What follows works from published data rather than preference, because the three specifications that get discussed separately are the three that interact most.
Internal clearance is decided by the fit, not by the noise
The single most consequential misunderstanding in motor bearing selection is treating catalogue clearance as operating clearance. They are three different quantities. Initial clearance is what the datasheet lists, measured before mounting. Residual clearance is what survives the fit. Operating clearance is what the bearing runs with once temperature, load and fit have acted on it.
Interference fits reduce clearance, and the published engineering estimate is that radial clearance may reduce by approximately 70 to 90 percent of the effective interference. That is why a motor designed around CN clearance can end up with negative operating clearance once the inner ring is pressed onto the shaft and the housing heats up. That is the mechanical reason C3 is the most common upgraded clearance in industrial practice, used in electric motors, fans and pumps: it carries the space the fit and the temperature take away.
| Bore diameter d | CN | C3 | C4 |
|---|---|---|---|
| 10 to 18 mm | 3 to 18 | 11 to 25 | 18 to 33 |
| 30 to 40 mm | 5 to 20 | 15 to 33 | 28 to 46 |
| 50 to 65 mm | 8 to 28 | 23 to 43 | 38 to 61 |
| 80 to 100 mm | 12 to 36 | 30 to 58 | 53 to 84 |
These are indicative values for unmounted bearings under zero measuring load, and should be confirmed against the applicable standard or catalogue. The governing reference is ISO 492:2023, Rolling bearings, Radial bearings, Geometrical product specifications (GPS) and tolerance values, with ISO 15:2017 fixing the boundary dimensions.
A caution worth stating plainly, because it is the mistake we see most: C4 is not a universal upgrade. In a low-speed, low-temperature application it may increase noise and reduce life, and excessive clearance increases vibration, rolling element slip and fatigue loss. C4 belongs where the temperature difference between rings is large, shock loads are heavy, or very tight fits are unavoidable. For electric motors, CM is sometimes specified between CN and C3 to balance noise against heat.
Noise grade and vibration class are their own decision
The confusion here is that buyers expect the noise grade to follow from the clearance, and it does not. Our published 6203 lists four axes, combined independently.
| Parameter | Published value | What it changes |
|---|---|---|
| Size | 17 x 40 x 12 mm | The envelope that has to fit the housing |
| Weight | 0.064 kg | Handling and rotor balance |
| Precision rating | P0, P6, P5 | Running accuracy, and so acoustic behaviour |
| Clearance | C0, C2, C3 | Behaviour once fitted and hot |
| Vibration | V1, V2, V3 | Surface finish class of rings and elements |
| Noise | Z1, Z2, Z3 | The acoustic band the motor may make |
| Service | OEM | Whether the part is built to your drawing |
| Minimum order quantity | 100 pieces | Planning against the build schedule |
| Supply ability | 10,000 pieces per month | Whether the part can support your volume |
Smaller clearance does improve rotational accuracy and reduce axial play, which is why C2 appears in precision spindles and measuring instruments. But that benefit has a boundary: with heat or tight fits, C2 can fail prematurely. And in a motor the two effects pull against each other, because the same interference fit that tightens the running clearance is what generates the heat that removes it. That is why CM exists between CN and C3.
Noise measurement has its own reference framework in ISO 15242-1:2015, Rolling bearings, Measuring methods for vibration, Part 1: Fundamentals, alongside the vibration classes in the published table. The point for a buyer is that noise grade and vibration class are measured separately from clearance, so they belong in the specification as separate lines.
The fit is what makes the clearance real
Clearance selection is not free-standing, and the load direction determines the fit that consumes it. In most applications the rotating ring receives an interference fit and the non-rotating ring a clearance or transition fit, but the final selection also weighs load magnitude, speed, temperature, and whether the shaft and housing are steel, cast iron or aluminium.
| Application condition | Typical shaft tolerance | Reason |
|---|---|---|
| Light load, stationary inner ring | g6, h6 | Easier mounting, less clearance loss |
| Normal load, rotating inner ring | j6, k6 | Secure seating, moderate interference |
| Heavy load, high vibration or impact | m6, n6 | Prevents inner ring creep |
| Very heavy interference required | p6, r6 | Only where clearance allows |
Housing tolerance follows the same logic from the other direction: H7 or J7 for a stationary outer ring under normal load, allowing slight movement and supporting heat expansion; K7 or M7 where the outer ring must not rotate; H7 or G7 for a split housing; and a tighter class for aluminium. That last line catches motor designers out, because an aluminium end housing expands more than cast iron and eats clearance at the same time as the interference you were counting on.
Grease, seal and speed belong in the same conversation
Grease is chosen against temperature and duty, and the seal choice changes what the bearing can do before grease enters the discussion. A contact seal adds friction and heat, so the sealed version has a lower speed limit than the open or shielded one. The rating has to be taken from the data for the sealed configuration being ordered, which is what most motor programmes ship, and treating the open catalogue limit as the sealed limit is a routine and expensive error.
Two more published points. The quantity and type of grease matter as much as the seal type, and a specification saying only “greased” is not a specification: our in-house process checks grease application on every batch, and a build should quote the grease type, its temperature range and the fill quantity. And where the duty runs hot, the seal has to match: we publish a special rubber seal developed with Japanese technology, tougher at high temperature than ordinary NBR, and with high temperature grease above 250 degrees Celsius we warrant a bearing life of at least 12 months.
Life is calculated, not looked up
The operating point decides the bearing, and the catalogue number is an input to that decision rather than a substitute for it. Load rating and rating life are calculated from the dynamic equivalent load, the speed factor and the reliability requirement, and ISO 281:2007, Rolling bearings, Dynamic load ratings and rating life, is the framework for it.
For high-speed duty the published speed metric is the dmN value, the product of bearing pitch diameter and rotational speed, because a higher dmN means more demanding kinematic and thermal conditions. Our published guidance for very high speed electric motors is that deep groove and angular contact ball bearings remain the practical starting point, with hybrid ceramic versions chosen where thermal control, electrical insulation or extended grease life is required. Ceramic balls have about 40 percent of the mass of steel balls, lowering centrifugal force on the raceways and cage, rolling friction and grease aging and, being non-conductive, erosion from stray shaft currents in inverter drives.
Over-greasing is worth one more line, because it is counter-intuitive. At high speed, excess grease increases churning losses and raises temperature, so a generously packed bearing is not a safer one. That, with heat buildup, lubricant degradation, cage instability, skidding and electrical erosion, is where most high-speed failures originate.
How to specify the motor bearing
- State the speed, temperature and duty before any part number, and use dmN to describe the speed demand.
- Specify the clearance group against the fit, starting from C3 for a motor with a real interference fit on the shaft.
- Give the shaft and housing tolerances on the drawing, because the fit consumes your specified clearance.
- Ask for the noise grade and vibration class as separate lines, chosen against the acoustic requirement, not inherited by default.
- Name the grease with its temperature range and fill quantity, and take the speed limit from the sealed variant data.
- Ask for the life calculation and batch test records, so the part is traceable to evidence rather than to a datasheet.
Frequently Asked Questions About Motor Bearing Selection
What clearance should I specify for an electric motor bearing?
C3 is the most common upgraded clearance in industrial practice and is widely used in electric motors, fans, pumps and compressors, because it provides space to compensate for the clearance lost to interference fits and thermal expansion. CN remains the default for general machinery, and CM is sometimes specified in electric motors between the two when noise performance and thermal compensation have to be balanced.
What is the difference between noise grade Z1, Z2 and Z3?
They are the published noise grade options on the 6203, alongside vibration class V1, V2 and V3 and precision rating P0, P6 and P5. Noise grade is a separate specification from clearance and from precision, so it should be chosen against the acoustic requirement of the application rather than treated as part of the clearance decision.
Does a tighter bearing clearance reduce motor noise?
Smaller clearance can improve rotational accuracy and reduce axial play, which benefits precision spindles and measuring instruments. But with heat or tight fits, C2 may fail prematurely, and a bearing that runs correctly with C3 clearance may overheat if CN clearance becomes negative after mounting.
Can a sealed (2RS) bearing run at the same speed as an open one?
No. A contact seal adds friction and heat, so the sealed version has a lower speed limit than the open or shielded version. The limit has to be taken from the sealed variant data sheet rather than the open catalogue, which is a routine miss on high-speed motor drives.
How much load capacity do I lose by switching a motor bearing to stainless?
Expect roughly 20 to 30 percent less dynamic capacity than the chrome steel equivalent for 440C, and more again for 316. A 6203 in chrome steel is rated at roughly 10 kN dynamic, while stainless versions of the same envelope typically sit in the 7 to 8 kN range depending on grade and maker, so the substitution changes the life calculation rather than being a drop-in swap.
What grease should I specify for a motor bearing?
The grease has to survive the running temperature and the duty, and the specification should quote the grease type, its temperature range and the fill quantity rather than a generic word. Our in-house quality process checks grease application on every batch, and we publish a high temperature seal variant using Japanese technology that we warrant for at least 12 months when paired with high temperature grease above 250 degrees Celsius.
Specifying bearings for a motor programme?
Send the speed, load, temperature, shaft and housing tolerances and the acoustic requirement, and we will come back with the size, clearance group, noise grade and grease specification, with the calculation and test records behind it.
This article was prepared by the Juding engineering and export team at Ningbo Giant Bearings Manufacturing Co., Ltd., a bearing manufacturer in Yuyao, Ningbo, producing deep groove ball bearings, ceramic and hybrid series, track rollers and OEM bearing programmes for motor, pump, automotive and industrial applications. The team supports customers from drawing review and material selection through to inspection records and shipment, and published inquiries are answered within 24 hours.
Related reading
Last verified: 2026-11-13. Bearing specification data, clearance ranges, fit tolerance classes, load rating references, quality process and company details are as published by Ningbo Giant Bearings and Ningbo Demy (D&M) Bearings. Clearance values are indicative ranges for unmounted bearings under zero measuring load and should be confirmed against the applicable standard or catalogue. ISO 492:2023, ISO 281:2007, ISO 15:2017 and ISO 15242-1:2015 are cited as published standards. No unit price, market size figure or project cost figure appears here, because none is published, and load rating, rating life, speed limit and the final selection must be calculated for the actual operating conditions.
Post time: Nov-13-2026