Failure forms and calculation criteria of rolling bearings
1. Load Analysis of Rolling Bearings
Take deep groove ball bearing as an example for analysis. As shown in Figure 12-19, when the bearing is subjected to the radial load Fr, the loads on each rolling element are different, and the rolling element at the lowest position bears the largest load. According to theoretical analysis, the load on the rolling element with the largest load is F0≈(5/z)Fr, where z is the number of rolling elements.
When the outer ring does not move and the inner ring rotates, the rolling elements both rotate and revolve around the axis of the bearing, so the contact points between the inner and outer rings and the rolling elements change constantly, and the contact stress at a certain point on the contact surface between the raceway and the rolling elements Also with the periodic change, the rolling element and the rotating ring are subjected to the periodically changing pulsating cycle contact stress, and point A on the fixed ring is subjected to the largest stable pulsating cycle contact stress.
2. Failure form
There are three main failure modes of rolling bearings:
(1) Fatigue pitting: Under the contact stress of the pulsating cycle, the contact fatigue pitting will appear on the contact surface when the stress value or the number of stress cycles exceeds a certain value. Pitting corrosion causes the bearing to generate vibration and noise during operation, the rotation accuracy is reduced and the working temperature rises, which makes the bearing lose its normal working ability. Contact fatigue pitting is the most important failure mode of rolling bearings.
(2) Plastic deformation Under the action of excessive static load or impact load, the ferrule raceway or rolling body may undergo plastic deformation, pits appear in the raceway or the rolling body is flattened, which reduces the running accuracy and generates vibration and noise, causing the bearing not to work properly.
(3) Abrasion In the case of poor lubrication, unreliable sealing and dusty conditions, the rolling elements or ring raceways are prone to abrasive wear, thermal bonding wear will occur at high speeds, and the overheating of the bearing will also cause the rolling elements to temper.
In addition, due to improper fit, installation, disassembly and use and maintenance of rolling bearings, other forms of failure such as cracking of bearing components will also be caused, and corresponding measures should be taken to prevent them.
3. Calculation criteria
For the main failure modes mentioned above, the calculation criteria for rolling bearings are:
(1) For bearings with general speed (n>10r/min), fatigue pitting is the main failure mode, and life calculation should be carried out.
(2) For bearings operating under low speed (n≤10r/min) heavy load or high impact conditions, the main failure mode is plastic deformation, and static strength calculation should be carried out.
(3) For high-speed bearings, in addition to fatigue pitting, adhesive wear is also an important failure mode, so in addition to life calculation, the limit speed should be checked.