Why does the condition of your rolling bearings matter so much? Because if a bearing fails, your machine immediately comes to a stop. Your work comes to a halt. Finding the cause of bearing failure can help you prevent problems in the future. In association with bearing specialist Schaeffler we have identified the most common causes of bearing failure.
It is possible to calculate the fatigue life for each rolling bearing. But, in practice, not every bearing achieves this service life. The following are the main causes of premature failure, from corrosion and electrical erosion to the use of “fake” bearings: how can you identify and prevent different forms of failure?
1. Material fatigue
Material fatigue finds its origin just beneath the raceway surface of the bearing. Variable loads with each passage of a rolling element eventually lead to changes in the structure of the material. Hairline cracks break through to the surface. If the load on the bearing is known, the fatigue life can be calculated. Of course this is a statistic process, so it’s always possible that a bearing doesn’t reach the calculated lifetime. There is another factor to consider: lubricant. Choosing a lubricant with the correct viscosity for the application can greatly increase service life. If this parameter is known, a more accurate fatigue life can be calculated than when only the load on the bearing and the speed of the shaft are taken into account.
2. Wear and tear
Wear and tear can occur when foreign material is present in the bearing. This may come from outside, when the seal is insufficient or consists of wear particles of components in the immediate vicinity of the bearing, such as gears, for example. However, wear and tear can also occur without material from outside entering the bearing. If a bearing has insufficient load the rolling elements may slide on the raceways rather than roll. In this way no lubricant film can be built up between the components and, through direct steel-on-steel sliding contact, wear occurs which can cause the bearing to fail very quickly.
3. Corrosion
Corrosion occurs when water or aggressive substances find their way into the bearing. If this happens, the lubricant often does not provide sufficient protection and rust will form. Choosing the correct seal for the application is therefore very important. In addition to this form of corrosion, there is a phenomenon known as fretting corrosion. Micro-movements can occur between the contact surfaces, as a result of an incorrect fit of the shaft or of the bearing housing. This causes small particles to come loose from the surface. Exposure to oxygen causes these particles to oxidise, causing damage to the bearing. Something similar may also occur in the bearing if the bearing does not rotate but only makes small oscillating movements. As a result, the lubricant between the rolling elements and raceways is ‘brushed away’ and damage occurs on these contact surfaces.
4. Electrical erosion
Electrical erosion can occur when electric current passes through the bearing. Sparks then jump between raceways and rolling elements, leading to crater formation in the surfaces. To prevent this, bearings with an electrically insulating coating around the outer ring or inside the inner ring are available. These are often used in larger electric motors and generators. During welding operations on machines containing bearings, careful attention must always be paid to the earth connection of the welding machine, in order to prevent welding currents from flowing through the bearings.
5. Plastic deformation
Plastic deformation in a bearing may be caused by a variety of factors, such as overloading as a result of static or shock loads, or indentations due to dirt particles or incorrect mounting. For example, when a bearing is pressed into the housing, mounting forces are frequently transmitted via the balls. The bearing may be destroyed before it has made a single revolution. In order to prevent overloading, it is of course important to properly determine all load situations in the design phase and to ensure that there is sufficient safety in the static load capacity (denoted by ‘Co’) of the bearing in relation to the maximum load.
6. Fracture of the bearing
Fractures of bearing rings can have many different causes. A fracture can, for example, be caused by incorrect mounting (such as hammer blows on the bearing). A fit that offers insufficient support can lead to fracture, but a very heavy shaft fit that results in an excessively high material stress in the inner ring of the bearing can also be a cause. A fracture of a rib is often caused by too much axial load or insufficient support of this rib. Consequential damage can also often occur, where a defective bearing continues to run until the material finally breaks, leading to blockage.
7. Fake bearings
The term ‘fake bearing’ has, unfortunately, cropped up more often recently. These are bearings brought onto the market that can hardly be distinguished from branded bearings. The bearings have the correct codes and logos and the packaging may also look original. In practice, however, these bearings function very poorly, which has already led to enormous damage in many cases. By purchasing bearings exclusively through the official Schaeffler dealer network, you are guaranteed of original quality. If there is any doubt about the origin of the bearings, Schaeffler can give you a definitive answer.
Establishing the right lubricant to use for bearings maintenance is easier when you follow our guide. You will also find a handy checklist for the maintenance of bearings here.
