True brinelling is a surface failure caused by a constant repeating overload. “False brinelling” on the other hand is a type of surface damage caused by fretting corrosion. The two damage modesdisplay many similarities, they only appear by different mechanisms.
The main cause of “false brinelling” is the fact that the lubricant is pressed away at the loaded zone. Without lubricant film between the rolling element and the raceway zal er accelerated wear will take place in this area. The wear particles will oxidate and form a grindingpaste which accelerate the wear process further on.
During normal operation a lubricant film will be build up between rolling element and the raceways. Because of this the roughness peaks will be sepearted of each other. In the sketch below mentioned as full lubrication.
Situation false brinelling
If the lubricant film is pushed away, a metal to metal contact will arise between the rolling elements and the raceways. At the sketch below this phenomena is the situation “false brinelling”.
Due to this the load will be distributed to a much smaller surface, the Hertzian pressures will increase to extremely high values. These extremely high pressures are causing loosening of small particles at a microscopic scale on the surfaces of the rolling elements and the raceways.
During the overrolling these particles are welded to the raceways, which take place under high pressure, and afterwards will loose again.
Wear fracture Overrolling wear particle
The broken out wear particles,due to there irregular shape, will take care for more zones with extremely high Hertzian pressures.
During normal operation, due to the rolling movements, a constantly building up of a new lubrication film take place. Of course, during a stationary situation this mechanism wiil not arise. Vibrations and oscillating movements of the loaded rolling elements will push away the lubricant film. These movements may result in degredation of the grease. The vibration energy is converted into heat, causing a increase of the internal grease friction. The grease is bleeding oil, thickener and oil are seperated from each other. The thickener skeleton persists, but this skeleton without oil has no lubricating properties .
As the lubricant is fully disappeared, the above described process strengthen themself time and time again. The wear will accelerate in the time.
The highest wear will take place on the raceways, because here we can find the pressure surfaces.
The problem “false brinelling” arises very often at stand-by machines in a system.
If these stand-by units are mounted on the same structure or situated near to the operating machine, vibrations from the operating unit can cause bearing damage on the stand-by unit. There are two solutions to avoid “false brinelling” in this situations:
Increase the distance between the two machines or use proper flexible mountings to eliminate the vibrations.
Turning the stand-by unit on a weekly base, to build up a new lubrication film, also can be sufficient.
A second possible situation in which “false brinelling” can occur is during transportation and stocking before mounting. In this situation a proper solution could be to support the shaft in a sufficient way, to eleiminate the shaftload.
If the machine is stored for a longer period of time, it is recommended to turn the shaft periodically by hand, to build up a new lubricantion film between rolling element and raceway.
“False brinelling” gives the typical failure mode, shown on the picture below. Eventually these damages wiil, during a period of time, cause cracks.
Influence “false brinelling” on housing fits
False brinelling is caused by external vibrations. These vibrations are causing a kind of friction corrosion inside the bearing. At wet environments this corrosion process accelerates. Friction corrosion causes wear.
The micro movements which are arising inside the bearing (due to the radial or axial play) are transferred to the outerring.
On this point so called “fretting corrosion” or friction corrosion will arise.
Friction corrosion si in fatc a damage by mechanical wear of the mating surfaces. This wear will be mostly increased by corrosion in wet or oxygenated environments. It arises through a lot of micro movements at the loaded zones, in this particular case the point where the balls are touching the raceways, by the lack of a lubricant film. The housing is mostly made of aluminium or grey cast iron, and has a much lower hardness than the bearing outerring, with his minimum hardness of HRc 60 (the red line at the graph)
Grey cast iron has a hardness of appr. HV 200. (the green line)
The outerdiameter of a current insulated bearing has a plasma sprayed Al2O3 coating with a minimum hardness of HV 1.900. (the yellow line at the graph)
Due to the friction wear between outrring and housing seat, it is logical consequence that the weakest part, in this particular case the grey cast iron housing seat, material will be taken away. This phenomena will earlier appear at the housing fit than on the shaft fit. Because in most cases the shaft fit is a press fit and the housing fit a loose, most common H7, fit.
At a grey cast iron housing seat brown discolorations will appear. At aluminium housing seats the dicoloration will be black.
The speed of the wear depends on the possible interference fit, the kind of cast iron (real hardness) and the height of the vibrationlevel.
Mostly the housing get an out of roundness, because the loaded zone of the bearing is only located at the lower part. (see sketch below)
In case of “false brinelling” we talk about a static load. At the housing seat theoretically a surface of 70°- 90° with more wear should be visible or measurable.
With a three point measurement, every time shifted for 120°, this difference in wear should be measurable.