Electric Motor Bearing: Bearing Number, Replacement Timing and Noise Symptoms

An electric motor usually has two bearings: one in the front end shield on the shaft side and one in the rear end shield on the fan side. On many motors the bearing number is printed on the nameplate; if not, it can be read on the outer ring or on the shield of the bearing itself. Replacement timing is set by symptoms rather than by the calendar: if the character of the noise has changed, if the bearing area is clearly hotter than the rest of the frame or if vibration has increased, the cause should be found before the motor is dismantled.

Front bearing, rear bearing: where is each and what load does it carry?

Technical documents abbreviate the shaft side as DE (drive end) and the fan side as NDE (non-drive end). The drive-end bearing takes most of the force coming through the pulley, coupling or pinion, which is why it is usually the first to wear in belt drives. The non-drive-end bearing works under lighter load but helps keep the rotor in its axial position. In many designs a wave washer on this side preloads the bearing slightly to take up play.

The bearings sit in housings machined into the end shields. The part that people in the field call the "bearing cover" or end cap is in fact the shield carrying this housing. If the outer ring has started to turn inside the housing, the bore becomes polished and oval, and a new bearing fitted into such a shield soon suffers the same fate. During replacement, checking whether the housing is scored, shiny or loose is therefore at least as important as the bearing itself.

How do you read the bearing number?

There are three places to look for the number: the motor nameplate (some manufacturers list the front and rear bearing on separate lines), the bearing itself and the manufacturer's catalogue. At HEM Motor, bearing types are listed separately for the drive end and the fan end, by frame size and number of poles, in the 2026 technical and product catalogue. Even if the nameplate has worn off, the IEC frame number and the number of poles are a sufficient starting point for looking it up.

To show how the reading works, take a number such as "6205-2Z C3". This is only an example, not the bearing of a particular model:

• 6: Single-row deep groove ball bearing. It is the most common type in small and medium-power motors.
• 2: Dimension series. Bearings of the 62 and 63 series exist with the same bore; the 63 series is larger and has a higher load capacity. Both fit the shaft, but only the correct series fits the housing.
• 05: Bore code. As a general rule, codes from 04 upwards are multiplied by 5 to give the bore diameter in millimetres.
• 2Z or 2RS: Bearing closed on both sides. Z means a metal shield, RS a rubber seal. These bearings are greased at the factory and take no grease from outside.
• C3: Internal clearance larger than normal. It is common in motor bearings that run warm; fitting a bearing without a clearance suffix in place of one marked C3 can cause heating and noise.

The most frequent mistake is to skip the suffixes and give only the four-digit number. A part requested as "6205" may arrive as an open type, with a different seal or with a different clearance. When ordering, write the number with all its suffixes, letter for letter.

Noise, heat and vibration: when should a bearing be replaced?

Bearing life is used up not by hours but by load, speed, temperature and the state of lubrication. The same bearing may run for years in a smoothly running pump yet fatigue much sooner in a fan with a tight belt. The replacement decision is therefore based on symptoms. Your ears, your hand and a simple infrared thermometer are often enough for a first diagnosis.

There is a simple way to locate the source of the noise: switch off the supply while the motor is running and listen as it slows down. If the noise disappears the moment the power is cut, the source is most likely electrical, and phase imbalance or magnetic noise is suspected. If the noise continues while the motor coasts freely, the source is mechanical: the bearing, the fan or the driven machine. Then disconnect the motor from the load and turn the shaft by hand; a rough, catching or loose feel points to the bearing.

In motors driven by a frequency inverter, electrical wear that leaves regular lines on the ring surface can also appear. In that case renewing the bearing alone is not a lasting fix; the cable, shield and earthing arrangement should also be examined.

The effect of belt tension and axial load

In a belt drive the drive-end bearing's worst enemy is excessive tension. Tightening the adjusting bolt all the way so the belt does not slip puts a constant side force on the bearing. Fitting the pulley pushed out towards the end of the shaft lengthens the lever arm of this force and increases the load the bearing sees. The pulley should sit as close to the shaft shoulder as possible, and the belt should be kept at the tension recommended by its manufacturer.

Axial load is force acting along the shaft. Helical gears, some pump impellers and, in vertical mounting, the weight of the rotor itself create this force. A deep groove ball bearing carries limited axial load; if the machine applies a noticeable push or pull to the shaft, this information should be given when ordering. In a coupled drive, two shafts that are not on the same axis also create both radial and axial strain. Measuring the alignment before connecting after a bearing change directly determines how long the new bearing will last.

Where do you find the lubrication interval on regreasable motors?

Motors with bearings sealed on both sides are considered lubricated for life; when the bearing fatigues, it is replaced. On large-frame motors with a grease nipple on the bearing cap, however, the lubrication interval and the amount of grease to apply each time are given on the motor nameplate, on a separate lubrication plate or in the catalogue. Do not change these values by guesswork; too little grease dries the bearing out, while too much fills the housing, churns and generates heat.

• Note the date and amount of each lubrication on a maintenance card kept next to the motor; only then can the interval be tracked.
• If you need to change the grease type, have the old grease cleaned out; greases that are not compatible may lose their consistency when mixed.
• If there is a drain opening for the old grease, make sure it is not blocked.
• If lubrication is done on a running machine, do it only in line with occupational safety rules and the manufacturer's instructions.

For spare motors kept on the shelf for a long time, the condition of the grease and bearing matters as well; we covered this in our article on keeping a spare motor in storage.

Points to watch when replacing a bearing

• Remove it with a puller: The puller's legs should press on the inner ring, which is tight on the shaft. Forcing the bearing by its outer ring or hitting it with a hammer can damage the shaft and the housing.
• Fit it hot: The new bearing is expanded with an induction heater and slides onto the shaft by itself. An open flame damages the grease and the seal. If cold fitting is unavoidable, force should be applied only to the inner ring through a suitable tube.
• Check the housing and the shaft: If the housing in the end shield is worn or the bearing seat on the shaft is scored, the new bearing will sit loose. In that case the shield or shaft must be repaired first.
• Do not forget the wave washer and the seal: If the washer on the fan side is not refitted, the rotor moves axially; a hardened seal opens the way to dust and moisture.
• Measure the winding: If the bearing has collapsed and the rotor has rubbed against the stator, the winding may also be damaged. The motor must not be energised again before its insulation resistance has been measured.

Replace the bearing or the motor?

In a motor where only the bearing is worn and the winding is sound, a bearing change is usually a sensible repair. If the rotor has rubbed against the stator, the winding has been affected by overheating or the housings in the end shields have become oval, the repair cost and downtime approach those of a new motor. In such a case, switching to a new motor of the same power, speed and frame can be considered. Browse suitable HEM Motor models and current prices.

Example models from different power ratings (three-phase, cast iron frame, IP55, class F insulation, 50 Hz; current price on the product page):

• 3 kW, 4-pole, 1,450 rpm, IEC 100L frame – 100L2-4: 230/400 V (Δ/Y), rated current 6.02 A at 400 V.
• 11 kW, 4-pole, 1,465 rpm, IEC 160M frame – 160M-4: 400/690 V (Δ/Y), rated current 20.4 A at 400 V.
• 30 kW, 4-pole, 1,475 rpm, IEC 200L frame – 200L-4: 400/690 V (Δ/Y), rated torque 194.2 Nm.

You can see the whole series on the cast iron frame models page.

Conclusion: also fix the conditions that wear the bearing

A bearing failure usually stems not from the bearing itself but from the conditions that wear it: excessive belt tension, a misaligned coupling, the wrong amount of grease or a part bought with a missing suffix. Read the number in full, monitor noise and temperature regularly and replace the bearing with the right tools. If you are going to renew the motor, have the power, speed, mounting arrangement, voltage and application ready, and if possible a photo of the nameplate.

Browse suitable models and current prices in the shop. Send the nameplate photo, speed and mounting details of your current motor through the nameplate submission page and ask for a quote for a suitable model.

Frequently Asked Questions

Where can the bearing number be found if it is not on the nameplate?

A number is printed on the outer ring or shield of the bearing. If it cannot be read, the bearing table in the manufacturer's catalogue is checked using the IEC frame number and the number of poles.

Can 2Z and 2RS bearings be used interchangeably?

Their dimensions are the same; the difference lies in the sealing. The rubber-sealed 2RS keeps dust out better but creates more friction; on high-speed motors it is better to stay with the type specified by the motor manufacturer.

Can a motor with a noisy bearing keep running?

A light, steady noise can be monitored for a short time. If the noise grows or turns into clicking, the bearing is close to collapsing; since a collapsed bearing can let the rotor rub against the stator and damage the winding, scheduling the change for a planned stop costs less.

Should the front and rear bearings be replaced together?

Since the motor is already dismantled, renewing both bearings together is common practice. If one has worn, the other has completed a similar running time, and a second stop shortly afterwards is avoided.