Electric Motor Bearings: How to Choose Size, Clearance, Seals and Preload

Choose an electric motor bearing by its complete operating specification: dimensions, load capacity, clearance, closures, lubricant and bearing arrangement. For a replacement, start with the motor manufacturer's bearing designation for each shaft end.

A bearing can fit the shaft and still be wrong for the motor. A different seal can change friction; an unsuitable clearance can cause heating after mounting. An inverter-fed motor may also need protection against bearing currents.

Start with the motor specification and duty

Record the motor model and the complete drive-end (DE) and non-drive-end (NDE) bearing specifications. Include operating speed, mounting orientation, duty cycle, temperature, coupling or belt drive, and any thrust from the driven equipment.

Do not select a replacement from motor power alone. Two motors with the same kW rating can use different bearing sizes, shaft arrangements and lubrication systems.

1. Choose the size and type for the loads

Single-row deep groove ball bearings are common in small and medium motors. They combine low friction with radial and bidirectional axial load capability. Their permissible axial load still requires checking.

Heavier radial duty can require cylindrical roller bearings. Significant thrust, particularly in vertical machines, can require angular contact or dedicated thrust-bearing arrangements. Follow the motor design rather than substituting a different type during a routine rebuild.

Bearing-type reference: NSK, “Rolling Bearings for Industrial Motors.”

First check the three boundary dimensions. A 6006 measures 30 × 55 × 13 mm; a 6206 measures 30 × 62 × 16 mm. Both fit a nominal 30 mm shaft diameter, but their housing and axial space requirements differ.

For a new design, calculate bearing reactions from the rotor, belt or gear forces and any applied thrust. Then check equivalent dynamic load, required fatigue life and static safety against the exact bearing data.

For ball bearings, basic rating life varies with (C/P)³ under the calculation's assumptions. Doubling equivalent load reduces calculated life to one-eighth. Belt tension therefore deserves a calculation, even when the shaft dimensions already fix the available bearing envelope.

Life relationship: NSK, Bearing ABCs, “Dynamic Load Ratings and Fatigue Life.”

2. Select clearance for the mounted, warm bearing

C3 means radial internal clearance greater than Normal. It is not a precision grade, shaft-fit class or preload setting.

Some motors require C3 because interference fits and a hotter inner ring reduce clearance in service. That does not make C3 the correct choice for every motor. Use the specified group or verify the required operating clearance.

A useful first check is:

Operating clearance ≈ initial clearance − fit-induced reduction − temperature-induced reduction.

This is a simplified relationship. Ring, shaft and housing temperatures, material expansion and elastic effects require the manufacturer's calculation method for a design decision.

For a 30 mm bore deep groove ball bearing, NSK's table lists CN as 5–20 µm and C3 as 13–28 µm. Those are unmounted radial values under the catalog conditions, not guaranteed warm operating gaps.

The overlap matters: a C3 bearing does not have one fixed clearance value. Also, the correct row includes the upper bore boundary; 30 mm belongs in the “over 24 through 30 mm” row.

Clearance references: NSK Online Catalog Internal Clearance; NSK, “Handling Instructions for Bearings.”

Check the fits at the same time

An inner ring under a rotating load generally needs an interference fit to resist creep. Excessive interference can remove too much internal clearance and cause heating.

Choose shaft and housing tolerances from the applicable fit tables. Do not apply one fit class to every motor size, load and housing material.

Thermal expansion also needs somewhere to go. In a locating/non-locating arrangement, one bearing positions the shaft axially while the other allows thermal movement. Some smaller motors use a different arrangement; preserve the actual design.

Fit reference: NTN, “Bearings With Shaft & Housing Fits.” Arrangement reference: SKF, “Rolling Bearings and Seals in Electric Motors and Generators.”

3. Match seals or shields to the environment

A shielded bearing is not waterproof. A contact-sealed bearing also does not establish a motor's washdown or IP rating. The whole enclosure and shaft-sealing system determine that protection.

Check the actual suffix. For example, NSK DDU identifies two contact seals, while VV identifies two non-contact seals. Generic “2RS” descriptions do not capture every manufacturer's closure design. Our shielded vs sealed bearing guide compares the general closure trade-offs.

Closure reference: NSK, “Bearing and Linear Replacement Guide.”

Use the speed data for the exact closure and lubricant. A catalog reference speed is not a universal safe maximum; operating temperature, load and cooling still matter.

Speed reference: SKF, “Bearing Handbook for Electric Motors,” speed limitations.

4. Specify the grease and maintenance method

Follow the motor specification for grease type, base-oil viscosity, quantity and relubrication interval. Temperature, speed, bearing size, orientation and contamination affect lubrication needs.

For a pregreased sealed bearing intended to operate without relubrication, specify the correct grease from the start. “Lubricated for life” describes the intended maintenance arrangement; it does not promise unlimited service.

For relubricatable motors, keep the intended grease inlet and discharge paths functional. Do not assume a double-sealed bearing can replace an open bearing without affecting grease delivery.

More grease is not automatically better. Overfilling can cause heat and grease migration, while incompatible greases can separate or harden. Use the approved quantity and confirm compatibility before changing grease.

The U.S. Department of Energy's Energy Management for Motor-Driven Systems, Chapter 9, discusses these lubrication risks and recommends starting with the motor manufacturer's guidance.

5. Apply preload only when the arrangement calls for it

Some lightly loaded motor bearing systems use axial spring preload to improve running behavior and reduce noise. Preload belongs to the bearing arrangement; it is not specified by choosing C3.

One common arrangement applies a spring to the non-locating bearing's outer ring. That ring must be able to move axially in its seat. Spring travel helps accommodate thermal movement while controlling load.

Motor arrangement reference: SKF, “Rolling Bearings and Seals in Electric Motors and Generators,” bearing systems and spring preload.

A spring's force changes with deflection, so its part number alone may not establish the installed force. Check the installed height, available travel and tolerance stack against the motor drawing.

Excessive preload increases internal contact load, friction and heat, and can reduce fatigue life. Do not add shims, tighten an end cover or substitute a stronger spring simply to remove all detectable end movement.

For a rebuild, restore the specified spring and its position. For a new design, establish preload from the bearing arrangement and validate it through the operating temperature range. There is no universal preload force for every motor using a 6206.

Preload principles: NSK, “Preload,” constant-pressure preload and effects of excessive preload.

6. Check bearing-current risk on inverter-fed motors

Variable-frequency drives can create electrical conditions that allow current to pass through bearings. Electrical discharges can damage rolling surfaces even when the mechanical selection is correct.

Where that risk exists, assess the drive, grounding, cabling, motor and driven machine together. Insulated bearings, hybrid bearings and shaft-grounding measures address different current paths; they are not automatic substitutes for one another.

A C3 clearance or a rubber seal does not provide specified electrical insulation. Preserve any insulation requirement in the original bearing designation.

Electrical-damage reference: SKF Evolution, “Prevention of Electric Erosion in Bearings.”

Record the complete specification before ordering

For each shaft end, record the bearing number, closures, clearance, grease, cage and precision requirements, plus any insulation specification. Keep the arrangement drawing with the spring and retaining-part details.

LILY supplies deep groove ball bearings in different configurations. To review a motor application, contact us with the motor model, full bearing markings, speed, drive type and service conditions.

FAQ

Does “electric motor quality” identify a complete bearing specification?

No. Ask for the supplier's actual noise, vibration, clearance, grease and tolerance requirements. An EMQ label alone does not define every feature needed for the motor.

Must the drive-end and non-drive-end bearings be identical?

No. Their loads and locating functions can differ. Check each position separately rather than ordering two bearings from the marking on one shaft end.

What should be recorded during the first run after replacement?

Record speed, load, ambient temperature, bearing-housing temperatures and vibration as the motor warms up. Compare the trends with the motor's acceptance criteria; one housing-temperature reading cannot identify the cause of heating.