Which bearing does my electric motor need? Size, C3 and sealing explained
Most motors use a deep groove ball bearing at each end, usually with C3 clearance. Here is how to find yours and order the right one the first time.
When an electric motor gets noisy or hot, the bearings are the first place to look. The good news is that most motors use ordinary, standard bearings, so the right replacement is usually easy to find once you know where to read the number.
Most motors use deep groove ball bearings
Small and medium induction motors very often carry a deep groove ball bearing at each end of the shaft, commonly from the 62 and 63 series, such as the 6203, 6205 or 6306. Larger motors, and motors that drive belts or heavy couplings, sometimes use a cylindrical roller bearing (for example an NU 2xx) at the drive end, because it carries a heavier radial load.
The two ends do not always use the same bearing. The drive end, where the shaft comes out, is often a larger bearing than the non-drive end.
How to find the size
- Read the number on the old bearing. It is stamped on the face of the ring, for example 6205-2Z/C3. Our guide to reading a bearing number explains each part, and the bearing number decoder does it for you.
- If the number is worn off, measure it. You need the bore, the outside diameter and the width in millimetres. Our guide to measuring a bearing shows how, and the size chart turns the three numbers into a bearing number.
- Check the motor’s own paperwork. The nameplate gives the model and frame size, and the maker’s catalogue lists the bearing for each end.
Why most motors use C3 clearance
A running motor warms up, and the inner ring, which sits on the hot shaft, expands more than the outer ring. A bearing with normal clearance can end up with too little clearance when hot, which makes it run hotter still. The extra internal clearance of a C3 bearing leaves room for that expansion, which is why C3 is the usual choice for motors. Read what C3 means for the full picture.
If the old bearing says C3, order C3. If it says nothing, it was normal clearance, and you should ask the motor maker before changing.
Shields, seals or open
- ZZ or 2Z (metal shields): greased for life, for clean, dry places. The most common type in small motors. See ZZ or 2RS?
- 2RS (rubber seals): better against dust and moisture, but slightly more friction.
- Open: used where the motor has grease nipples, so the bearing can be re-greased in service.
If your motor has grease nipples, keep an open or re-greasable bearing. Fitting a sealed-for-life bearing would stop the grease reaching it.
Replace both ends, and fit them properly
When one bearing has failed, the other has usually seen the same heat and wear, so it is good practice to replace both. Push the new bearing on by the ring that has the tight fit, never through the balls, and keep it clean until it is on the shaft. The usual causes of early failure are covered in why bearings fail.
Motors driven by a variable frequency drive can pass small electric currents through the bearings, which pits the raceways over time. If yours is one, ask us about insulated bearings.
Need a motor bearing?
Send us the number from the old bearing, or its three measurements, with the quantity, on WhatsApp. We supply SKF, FAG, NTN, NSK and KOYO bearings for motors, as single pieces for repairs or in quantity, across India, with same or next-day delivery in Mumbai.