Electric Motor Bearing

An electric motor bearing supports the rotor at both ends of the shaft and keeps the air gap between rotor and stator even. In most induction motors up to medium frame sizes, both bearings are single row deep groove ball bearings from the 6200 or 6300 series, usually with C3 clearance and either ZZ shields or 2RS seals. This page covers why motors use C3, how the two bearings share the work, the shield and seal choice, noise, grease, sizes by shaft diameter, and what to check before you order.

Drive end and non-drive end: two bearings, two jobs

A motor shaft has a drive-end (DE) bearing next to the pulley or coupling and a non-drive-end (NDE) bearing near the cooling fan. They are often different sizes. The DE bearing carries the belt pull or coupling load and is usually the larger one, for example a 6306 at the drive end and a 6205 at the fan end.

One of the two bearings locates the shaft axially. The other is allowed to float, so the shaft can grow with heat without loading the bearings against each other. In many small and medium motors the floating bearing sits behind a wave spring washer. The spring pushes lightly on the outer ring, takes up the axial play and keeps the balls rolling instead of skidding. If you find a wave washer when you take the motor apart, put it back on the same side.

The same 6200 and 6300 sizes appear on gearbox input shafts, where they usually run open in oil; see gearbox bearings. Belt-driven motors with heavy radial load sometimes use a cylindrical roller bearing at the drive end. Keep the type of the original. Changing a roller bearing to a ball bearing reduces the load rating; changing a ball bearing to a roller bearing at low load can cause skidding and noise.

Why motor bearings use C3 clearance

Two things reduce the internal clearance of a motor bearing after it leaves the box.

1. The fit. The inner ring is pressed onto the shaft with interference. It expands slightly, and the clearance between balls and raceways shrinks.
2. Temperature. The rotor heats the shaft and inner ring more than the end shield cools the outer ring. The inner ring grows more than the outer ring, and the clearance shrinks again.

A bearing that starts at normal clearance (C0) can end up with almost none, which raises temperature and noise. C3 clearance starts larger and leaves the right amount after both effects. That is why motor bearings are normally ordered as C3, for example 6205-2Z/C3. The bearing internal clearance guide explains the groups from C2 to C5.

If the old bearing has no clearance mark at all, it is C0. Some small motors with light fits do use C0. Read the marking on the old bearing and order the same; when in doubt, ask the motor maker.

ZZ or 2RS for a motor bearing

Closure	Suffix	Friction	Speed	Protection	Typical motor use
Metal shields	ZZ (2Z)	Lowest	Highest	Keeps out larger particles, not water	Enclosed fan-cooled motors in clean indoor duty
Rubber seals, contact	2RS (2RS1)	Higher	Lower	Keeps out dust and splashes	Pumps, washdown areas, outdoor and agricultural motors
Open	none	Lowest	Highest	None of its own	Larger motors with grease chambers and nipples

Both ZZ and 2RS bearings leave the factory greased, and in normal use they usually do not need regreasing. If the motor has grease nipples, the bearings are open and the end caps hold the grease. Do not replace an open bearing in a re-greasable motor with a sealed one unless the motor maker allows it; the fresh grease will not reach it.

Noise and vibration grade

Motor makers care about bearing noise because the motor housing amplifies it. A small dent on the raceway that would be unnoticed in a conveyor roller becomes an audible tone in a fan motor.

FULI deep groove ball bearings are supplied at vibration grade Z3V3 as standard. Our factory tests bearings on vibration and noise instruments, and for orders with special noise or vibration requirements, every bearing is tested before packing. If your motor needs a quieter grade, tell us the grade and we will confirm whether we can supply it. The noise grade vs precision class article explains why a P6 tolerance class does not by itself make a bearing quieter.

In our experience, a noisy bearing has usually left the bearing factory that way. Knocks between rings and balls during assembly are the main source, which is why bearings with a noise or vibration requirement are tested one at a time before packing. On the customer side, the most common cause is a lower-grade bearing fitted to a motor that was designed for a quieter one. Fitting still matters: pushing on the outer ring to press the bearing onto the shaft, or tapping it on with a hammer, sends the force through the balls and can dent the raceways. Press on the inner ring when fitting to the shaft.

Grease in sealed motor bearings

A sealed motor bearing runs on the grease it was filled with at the factory. Our default fill is about 30% of the free internal space. The grease quantity is set on the greasing machine after weighing sample bearings before and after filling, so each bearing gets the same amount.

Too much grease churns and heats the bearing. Too little runs out early. The grease problem we see most is too little fill: a little grease in the cage pockets on one side and none visible on the other.

The temperature range of the grease matters for motors that start in the cold or run in hot plant rooms. Buyers in North America often ask for grease rated from about −40 °C to 120 °C. If you need a particular grease or temperature range, tell us and we will confirm whether we can supply it. The bearing lubrication basics article and the bearing grease fill calculator cover open bearings that you grease yourself.

Motors on variable frequency drives

Motors fed by inverters can develop shaft voltages that discharge through the bearing. The result is a pattern of fine grey lines across the raceway, often called fluting, and a bearing that grows noisy within months. The fixes are on the motor side (shaft grounding rings, insulated end shields) or insulated bearings. If your motor needs an insulated bearing, send the full designation from the old bearing and we will confirm whether we can supply it before quoting. Do not replace an insulated bearing with a standard one.

Electric motor bearing sizes by shaft diameter

The bore is the shaft diameter at the bearing seat. The 6200 series is the most common motor bearing; the 6300 series is heavier and often sits at the drive end.

Shaft d (mm)	6200 series	d x D x B (mm)	Cr (kN)	6300 series	d x D x B (mm)	Cr (kN)
10	6200	10 x 30 x 9	4.59	6300	10 x 35 x 11	7.38
12	6201	12 x 32 x 10	5.49	6301	12 x 37 x 12	8.73
15	6202	15 x 35 x 11	6.98	6302	15 x 42 x 13	10.26
17	6203	17 x 40 x 12	8.64	6303	17 x 47 x 14	12.15
20	6204	20 x 47 x 14	11.52	6304	20 x 52 x 15	14.31
25	6205	25 x 52 x 15	on request	6305	25 x 62 x 17	19.08
30	6206	30 x 62 x 16	17.55	6306	30 x 72 x 19	24.03
35	6207	35 x 72 x 17	23.13	6307	35 x 80 x 21	30.15
40	6208	40 x 80 x 18	26.19	6308	40 x 90 x 23	36.45
45	6209	45 x 85 x 19	29.25	6309	45 x 100 x 25	47.70
50	6210	50 x 90 x 20	31.50	6310	50 x 110 x 27	on request

Cr is the basic dynamic load rating. Values from FULI’s catalog; confirm for your application. The 6205 and 6310 load ratings are being checked against FULI’s catalog. Ask for them with the quote.

Close-coupled pumps run on their motor bearings, so the water pump bearing page uses the same sizes, and fan motors are covered on the fan and blower bearing page. Small fractional-horsepower motors in fans and appliances often use the 6000 series or miniature 600 series sizes. The full lists are on the 6000 series, 6200 series and 6300 series pages.

How to order electric motor bearings

1. The full number from the old bearing, including the closure and clearance suffix, for example 6306-2RS/C3. Suffix letters are easy to leave out, and they change what we make.
2. Whether it is the DE or NDE bearing, if you are ordering a set.
3. The quantity per model.
4. Any noise or vibration requirement beyond Z3V3, or a grease temperature range. We confirm whether we can supply it before quoting.
5. Whether the bearing is insulated or has any other special feature.
6. Ring marking and box printing: your authorized brand, the factory brand or neutral.

Send the list to sales@fuli-bearings.com or through the contact page; the quotation request template shows the format. Before shipment we check rotation smoothness, dimensions, grease fill, rust and appearance. An inspection report can cover dimensions, radial clearance, rotation noise, vibration, appearance, material certificate and batch traceability. Rings and balls are GCr15 chrome bearing steel, and the steel maker’s material certificate is available on request for OEM and industrial orders. Export documents are the commercial invoice, packing list and CCPIT certificate of origin, plus an FTA Form COO where the destination country qualifies.

Backed by 20+ years of bearing export experience in Linqing, Shandong. FULI ships to 50+ countries & regions.