Cleaning
Before a bearing can be checked it must be cleaned thoroughly to remove any dirt and the old lubricating fluid. The manual will dictate the cleaning process including use of any solvent, but a typical method is as follows.
Remove any excess grease with clothes and dry compressed air, whilst ensuring that the rolling elements remain stationary or only rotate slowly.
Soak the bearing in an approved solvent, such white spirit, to remove the remaining grease. The solvent may be applied as a forced jet if necessary.
Dry the bearing by using clean warm and dry compressed air, again avoiding fast rotation of any rolling elements.
Lightly lubricate all bearing surfaces with oil to prevent the onset of corrosion. Testing
Testing a bearing is usually restricted to rotational checks and excessive backlash or free play. Slow rotation of the rolling elements and raceway will highlight any roughness due to damage, corrosion or wear. A serviceable bearing should have a smooth actuation. Free play should be tested in both a radial and axial direction and is normally done by using a DTI. Some wear is usually permitted and will depend on the grade of fit, but any wear that leaves excessive backlash in the system is unsatisfactory.
The rate of this wear depends on the speed the bearing is rotated at, with high speed bearings failing quicker than those which rotate slowly or through distances of less than one complete revolution. A bearing that has any indication of a fault should be discarded immediately.
Due to their construction internal inspection of shielded bearings will be restricted. Taper bearings can be dismantled and a thorough inspection of the rolling elements and raceway surfaces can be completed.
JAR 66 CATEGORY B1 MODULE 7 MAINTENANCE PRACTICES (MECHANICAL)
uk
engineering
10.2 INSPECTION OF BEARINGSOnce clean the bearing should be inspected for signs of failure, some of the more common being:
1. Normal Fatigue 2. Excessive Loading
3. Installation and Misalignment 4. Loose Fitting or Spinning 5. Brinelling
6. Overheating and Lubrication Deficiency 7. Contamination and Corrosion
10.2.1 Normal Fatigue
Normal fatigue failure is often shown as a fracture of the running surface, with subsequent removal of small particles of metal and is commonly called spalling. (Refer Fig. 1)
It occurs on both rolling elements and raceways, and is always accompanied by an increase in vibration. Moderately spalled areas show the bearing has reached the end of its normal service life.
10.2.2 Excessive Loads
Excessive loading of a bearing is usually the same as normal fatigue, but the rolling elements wear path is usually heavier. There is also increased evidence of overheating with a widespread and deeper fatigue or spalled area. This often causes premature bearing failure. (Refer Fig. 1)
Ball Path Spalled Area Fig. 1 Spalled Areas
JAR 66 CATEGORY B1 MODULE 7 MAINTENANCE PRACTICES (MECHANICAL)
uk
engineering
10.2.3 Installation and Misalignment
Installation damage is usually the result of an impact that occurs when a bearing is fitted incorrectly. This may be due to a sharp strike from a drift or pressing the wrong raceway when mounting the bearing.
Misalignment damage can be seen on the raceway of the non-rotating ring because the rolling element wear path is not parallel to the raceway edge. Excessive misalignment can cause high temperatures as well as heavy wear of the cage.
10.2.4 Loose Fit
A bearing should always be mounted onto a shaft or housing with an interference fit. If the raceway becomes loose then it will rotate on these surfaces and cause fretting. This fretting will remove metal particles, which oxidise and leave a distinctive brown colour. It usually occurs when the bearing outer raceway rotates inside a worn housing. The external surface of the raceway will be scored and discoloured as a result of a loose fitting bearing. (Refer Fig. 2)
10.2.5 Brinelling
Brinelling marks on a bearing raceway resemble the indentations that result from a Brinell Hardness Test. They are described as being either True Brinell or False Brinell marks.
True Brinelling occurs when loads on the bearings raceway exceed the elastic limit of the raceway material. Brinell marks are indentations on the rolling element caused by an excessive static or dynamic loads.
The indentations can be seen on the raceways and will increase bearing noise and vibration, which leads to the bearings premature failure. The damage is often caused by dropping the bearing or installing it incorrectly. (Refer Fig. 3 Left)
Fig. 2
JAR 66 CATEGORY B1 MODULE 7 MAINTENANCE PRACTICES (MECHANICAL)
uk
engineering
False Brinelling occurs when there is only small relative motion between the rolling elements and raceways during non-rotation periods. It is characterised by elliptical wear marks in the axial direction at each rolling element position (Refer Fig. 3 Right).
If the bearing is not turning then an oil film cannot be formed to prevent raceway wear. False Brinelling marks are normally perpendicular to the line of motion, well defined and maybe surrounded by debris.
10.2.6 Overheating and Lubrication Failure
Excessive heating of a bearing manifests itself as discoloration of the rings, rolling elements and cages from gold to blue. Excessive temperatures will usually be in excess of 400°C. In extreme cases the rolling elements and raceways will deform. A blue/black colour indicates an area close to the heat source and changes to a silver/gold discoloration the further you move away.
Failure or lack of lubrication often has similar signs as overheating because good lubrication should cool the material and transfer away any heat produced during rotation. Restricted flow and excessive temperatures can also degrade the chemical composition of the oil, making it ineffective and increase wear rates. The outcome of either overheating or lubrication failure will always result in the eventual failure of the bearing.
Fig. 3
True Brinelling (Left) False Brinelling (Right)
JAR 66 CATEGORY B1 MODULE 7 MAINTENANCE PRACTICES (MECHANICAL)
uk
engineering
10.2.7 Contamination and Corrosion
Contamination is one of the leading causes of premature bearing failure. The symptoms are dents or scratches embedded in the bearing raceway and rolling elements, resulting in bearing vibration and wear. (Refer Fig. 4 Left)
The contaminant would be an abrasive substance that gets into the bearing, such as sand, grit or dust. The principal sources are dirty tools, contaminated work areas, dirty hands and foreign matter in the lubricant or cleaning solutions.
Corrosion is usually the result of a chemical attack on the bearing material by an incompatible fluid such as moisture. It manifests itself as either black pitting marks or red/brown rust coloured areas on the rolling elements, raceways, or cages. It usually results in increased vibration followed by wear. (Refer Fig. 4 Right).