Bearings are a commonly used component in engineering systems and you will encounter them throughout your career. Bearings are central for the operation of many devices/systems and because of this, when they fail it can be detrimental. There are a number of reasons why bearings fails and here we will cover the main 7 reasons bearings fail.
1. Lubrication Failure
Amazingly, approximately 80% of all bearing failures are a result of lubrication failure. This includes: using the incorrect lubricant, insufficient lubrication and excessive heat resulting in lubrication degradation. Lubricants serve the main functions of reducing friction and wear whilst also carrying away heat generated by the bearing.
The two lubricants used for bearings are grease and oil. Grease is very similar to oil although it has a thickener added to it. Additionally, at room temperature the main way to tell the difference is that grease is a solid at room temperature whereas oil is a liquid. Determining factors for the amount of lubricant to apply to a bearing include: size, speed of operation, viscosity and type of bearing.
When you are inspecting a bearing, discoloured lubricant is a sign of excessive heat in the bearing. Also, excessive heating will leave ‘bluing’ on the bearing.
2. Contamination
Contamination is when particles of other substances e.g dirt enter the bearing lubricant and become contained within the bearing system. When the particles are hard in nature, they will inflict plastic deformation on the bearing raceways due to three body abrasive wear. This type of wear will leave scratches, gouges and ploughing marks which are visible.
Contamination can occur extremely easily so it is important that surrounding areas are cleaned and that bearings are handled with care. This will largely reduce the chance of contamination occurring. Although, contamination could occur from solid particles remaining in the bearing from manufacturing.
3. Fatigue
The failure mode as a result of bearing fatigue is via spalling. Spalling is caused from surface or sub surface fatigue resulting in cracking in the raceways. After the repeated motion of the rolling elements travelling over these cracks, flakes form and the material breaks away. Once spalling has initiated, the damage will progress quickly and the bearing will become unserviceable.
4. Improper Mounting
The common method of fitting bearings is by a press fit or shrink fit. If the fit between the shaft and inner race too large, it will result in a greater expansion of the inner race giving more stress and removing any clearance required. Fitting issues are also associated with the outer race in a housing. A fit that is too tight will result in excessive contraction of the outer ring and remove any clearance required. It is important that when fitting bearings, they are pressed up to an adjacent component to help introduce the correct bearing pre-load.
In addition, improper mounting can result in damage being inflicted on the bearing during the fitting process.
5. Excessive Loading
Excessive loading will reduce the life of a bearing as a result of pre mature failure. Excessive loads witnessed on bearings will initiate spalling sooner and may also lead to over heating of the bearing. Both these causes can result in a detrimental failure of a bearing.
6. Overheating
Overheating is a result of excessive operating temperatures which is primarily a result of inadequate lubrication. This goes for both too much and too little. If there is too much grease in a bearing, the bearing elements will begin churning the grease resulting in a lot of energy being used to move the grease, resulting in elevated temperatures. If there is not enough lubricant, metal on metal contact may occur giving local hot spots but additionally, the lubricant will not be able to take away excess heat due to the small amount.
7. Misalignment
Misalignment of the shaft relative to the bearing housing can cause non contact seals to rub. The rubbing will result in elevated noise levels during operation and may give rise to excessive heating. Additionally, accelerated wear on the seal will occur, compromising its integrity.
If two housings or shafts are not lined aligned in a straight line, additional loading can be seen on the bearing resulting in early failure.
Summary
We have covered the 7 common reasons why bearings fail and explained the effects they will have on the operation of a bearing. When investigating bearings, they commonly leave marks and paint a picture of how they were operating. An example of this will be a wear path, any bluing of the races etc.
Learning from these key failure modes will help your understanding of how bearings should be set up and how they like to operate.
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