1 Rated calculation life and bearing life

Misunderstanding between bearing manufacturers and bearing users on technical issues is quite common.

The bearing user always wants a reliable bearing so that he can avoid unexpected stops. Taking a step back, it is also acceptable to know when a bearing fails. In other words, you just want to know the service life of the bearing.

I'm sorry I can't say what the bearing life will be. I can only calculate the rated life of the bearing. The two are completely different, and the calculated bearing rated life does not take into account all the parameters that may affect the service life. Moreover, the calculation method for calculating the rated life is also different, and different choices can give different results.

2 Bearing life

The so-called bearing service life refers to the actual number of revolutions or time that the bearing runs in the machine, such as:

If the centrifugal pump bearing is removed and scrapped after 5 years as part of preventive maintenance, its service life is 5 years, even if the bearing is still in good condition.

If the bearings damaged during transportation are installed on a large centrifugal pump and then removed after 3 hours due to excessive vibration of the equipment, the service life is 3 hours.

If the centrifugal pump bearing is removed after 20 years due to vibration caused by raceway stripping, the service life is 20 years.

If we take the same type of bearings, manufacture them at the same time, and run them under the same load and lubricant, we will also get different results. In other words, they won't have the same lifespan.

3 Real life of bearing

Usually, the bearing life ends when a small piece of the bearing is peeled off. Spalling is caused by the alternating stress in the steel structure due to the passage of the rolling element through the raceway. Under normal circumstances, that is, there is no pollution, to ensure sufficient lubrication film thickness, the greatest stress is on the surface of the bearing raceway. Under the action of alternating stress, the steel structure changes and micro-cracks occur near the weak points. Figure 1 below shows the microcracks at the sub-surface.

图1 次表面裂纹

The predecessors of the bearing industry have found in the bearing research and development, experiments, and tests that through a large number of bearing life tests:

1, under the same conditions, the same bearings will not reach the same life.

2, it is completely impossible to predict the life of a particular bearing.

However, when the amount of experimental data is large enough, the bearing life can be simulated into Weibull function curve, that is, the percentage of bearings that reach a specific life.

Through this curve, the probability of non-failure can be predicted for the same bearing operating under exactly the same conditions. This is very helpful for engineers to select the right bearing for the application equipment.

4 Bearing rated calculated life (probabilistic life)

In the bearing industry, 90% reliability is usually used. In other words, when calculating life, 90% of bearings should meet or exceed the expected life. As a result:

When the equipment manufacturer requires the selection of bearings for equipment with a life of 80,000 hours or more. This means that we will select a bearing according to the theoretical operating conditions provided by the customer, which should theoretically operate for 80,000 hours or more in 90% of cases. It is obvious that in 10% of cases, the bearing will not reach the required life.

Assume that there are 100 identical equipment bearings in operation under exactly the same operating conditions. If the calculated life of 90% reliability is 200,000 hours, then in theory:

Bearings with 99% reliability have a rated calculated life of 50,000 hours or more

Bearings with 95% reliability have a rated calculated life of 128,000 hours or more

The rated calculated life of bearings with 50% reliability reaches or exceeds 1 000 000 hours

The calculated basic rated life of the bearing L10h(L represents the life, 10 indicates that the failure probability is 10%, the reliability is 90%) only considers the bearing load and speed. The revised bearing rated life is L10mh, taking into account solid contamination, oil viscosity ratio and fatigue load limit. However, even the corrected life does not include all the operating parameters of the bearing.