Why bearings fail: the seven most common causes and how to prevent them
A bearing that fails early is nearly always telling you something. Here are the seven most common causes, what they look like and how to stop them.
A bearing that is chosen well, fitted well and looked after can run for years. When one fails early, the cause is rarely the bearing itself. Reading the damage on the old bearing tells you what went wrong, and fixing that cause is what keeps the replacement alive. Here are seven common ones.
1. Poor lubrication
Too little grease or oil, the wrong grease, or grease that has aged lets metal touch metal. The rolling surfaces turn dull or discoloured, and the bearing runs hot. Too much grease also does harm, because it churns and heats. Use the grease the maker specifies, in the right amount, and mix greases only if they are compatible.
2. Dirt and contamination
Dust, grit and metal chips get between the balls or rollers and the rings and dent the surfaces, which makes the bearing noisy and rough. Keep the bearing in its wrapping until you fit it, work on a clean bench, and choose seals where the machine is dusty. See ZZ or 2RS.
3. Moisture and corrosion
Water in the grease causes rust, which pits the raceways, and the pits grow into damage. Sealed bearings, stainless bearings and good housing seals help where a machine is washed down or stands in damp air. Never leave a bearing unwrapped in a damp store.
4. The wrong fit
A bearing ring that is too loose on the shaft or in the housing can slowly turn there, which wears the seat and the ring. One that is too tight squeezes out the internal clearance and the bearing overheats. This is one reason clearance such as C3 matters.
5. Misalignment
If the shaft and housing are not in line, the load falls on one edge of the raceway, which wears an uneven track and shortens life. Check alignment when you fit the bearing. If misalignment cannot be avoided, a self-aligning ball bearing or a spherical roller bearing tolerates some of it.
6. Overload
A load beyond what the bearing is rated for causes early fatigue, where flakes break away from the raceway. The rating, shown in the data for each size, is a guide to what a bearing can carry. If a machine has changed, a bigger bearing or a different type may be needed. Axial load on a bearing built for radial load is a common mistake, and an angular contact bearing is made for it.
7. Bad mounting
Hammering a bearing on, or pushing it through the wrong ring, sends the force through the balls and dents the raceway, which later makes the bearing hum. Press or heat the bearing on by the ring that has the tight fit, use the right sleeve, and do not heat it above the maker’s limit.
Other causes worth knowing
- Electric current: motors on variable frequency drives can pass current through the bearing, which leaves a washboard pattern in the raceway. Insulated bearings prevent it.
- Vibration while standing still: a machine that shakes without turning can mark the raceways.
- Normal end of life: a bearing that has run its full life shows fatigue flaking. This is expected, not a fault.
- Counterfeits: fake bearings fail early because of poor steel and finish. See how to spot fake bearings.
Warning signs to act on
A bearing is telling you something when you notice a new humming or grinding noise, heat you can feel on the housing, extra vibration, or grease leaking or turning dark. Plants that keep a few spares of the critical sizes avoid long stops. See bearing stock for plants.
Bring us the failed bearing
Send a photo of the old bearing, or bring it to our counter, with the machine it came from. We can identify it, quote the right replacement, and tell you what the damage suggests so the new one lasts.